Merge pull request #14335 from vespa-engine/revert-14330-revert-14308-hmusum/refactor-RemoteSession-take-2

Move code from RemoteSession to SessionRepository, take 3

15 files changed, 453 insertions, 466 deletions

diff --git a/configserver/src/main/java/com/yahoo/vespa/config/server/ApplicationRepository.java b/configserver/src/main/java/com/yahoo/vespa/config/server/ApplicationRepository.java
index fb07bf626f3..8aaf896cb72 100644
--- a/configserver/src/main/java/com/yahoo/vespa/config/server/ApplicationRepository.java
+++ b/configserver/src/main/java/com/yahoo/vespa/config/server/ApplicationRepository.java
@@ -607,9 +607,10 @@ public class ApplicationRepository implements com.yahoo.config.provision.Deploye
         try {
             Tenant tenant = getTenant(applicationId);
             if (tenant == null) throw new NotFoundException("Tenant '" + applicationId.tenant() + "' not found");
-            long sessionId = getSessionIdForApplication(tenant, applicationId);
-            RemoteSession session = getRemoteSession(tenant, sessionId);
-            return session.ensureApplicationLoaded().getForVersionOrLatest(version, clock.instant());
+            RemoteSession session = getActiveSession(applicationId);
+            if (session == null) throw new NotFoundException("No active session found for '" + applicationId + "'");
+            SessionRepository sessionRepository = tenant.getSessionRepository();
+            return sessionRepository.ensureApplicationLoaded(session).getForVersionOrLatest(version, clock.instant());
         } catch (NotFoundException e) {
             log.log(Level.WARNING, "Failed getting application for '" + applicationId + "': " + e.getMessage());
             throw e;
@@ -948,7 +949,7 @@ public class ApplicationRepository implements com.yahoo.config.provision.Deploye
         try {
             long currentActiveSessionId = applicationRepo.requireActiveSessionOf(appId);
             RemoteSession currentActiveSession = getRemoteSession(tenant, currentActiveSessionId);
-            currentActiveApplicationSet = Optional.ofNullable(currentActiveSession.ensureApplicationLoaded());
+            currentActiveApplicationSet = Optional.ofNullable(tenant.getSessionRepository().ensureApplicationLoaded(currentActiveSession));
         } catch (IllegalArgumentException e) {
             // Do nothing if we have no currently active session
         }
diff --git a/configserver/src/main/java/com/yahoo/vespa/config/server/modelfactory/ActivatedModelsBuilder.java b/configserver/src/main/java/com/yahoo/vespa/config/server/modelfactory/ActivatedModelsBuilder.java
index 7fc6b35722f..6d2ef4028c6 100644
--- a/configserver/src/main/java/com/yahoo/vespa/config/server/modelfactory/ActivatedModelsBuilder.java
+++ b/configserver/src/main/java/com/yahoo/vespa/config/server/modelfactory/ActivatedModelsBuilder.java
@@ -4,7 +4,6 @@ package com.yahoo.vespa.config.server.modelfactory;
 import com.google.common.collect.ImmutableSet;
 import com.yahoo.component.Version;
 import com.yahoo.config.application.api.ApplicationPackage;
-import com.yahoo.config.application.api.DeployLogger;
 import com.yahoo.config.model.api.ConfigDefinitionRepo;
 import com.yahoo.config.model.api.ModelContext;
 import com.yahoo.config.model.api.ModelFactory;
@@ -57,7 +56,6 @@ public class ActivatedModelsBuilder extends ModelsBuilder<Application> {
     private final ConfigDefinitionRepo configDefinitionRepo;
     private final Metrics metrics;
     private final Curator curator;
-    private final DeployLogger logger;
     private final FlagSource flagSource;
     private final SecretStore secretStore;
 
@@ -76,7 +74,6 @@ public class ActivatedModelsBuilder extends ModelsBuilder<Application> {
         this.configDefinitionRepo = globalComponentRegistry.getStaticConfigDefinitionRepo();
         this.metrics = globalComponentRegistry.getMetrics();
         this.curator = globalComponentRegistry.getCurator();
-        this.logger = new SilentDeployLogger();
         this.flagSource = globalComponentRegistry.getFlagSource();
         this.secretStore = globalComponentRegistry.getSecretStore();
     }
@@ -90,14 +87,14 @@ public class ActivatedModelsBuilder extends ModelsBuilder<Application> {
                                             Optional<AllocatedHosts> ignored // Ignored since we have this in the app package for activated models
     ) {
         log.log(Level.FINE, String.format("Loading model version %s for session %s application %s",
-                                              modelFactory.version(), appGeneration, applicationId));
+                                          modelFactory.version(), appGeneration, applicationId));
         ModelContext.Properties modelContextProperties = createModelContextProperties(applicationId);
         Provisioned provisioned = new Provisioned();
         ModelContext modelContext = new ModelContextImpl(
                 applicationPackage,
                 Optional.empty(),
                 permanentApplicationPackage.applicationPackage(),
-                logger,
+                new SilentDeployLogger(),
                 configDefinitionRepo,
                 getForVersionOrLatest(applicationPackage.getFileRegistries(), modelFactory.version()).orElse(new MockFileRegistry()),
                 createStaticProvisioner(applicationPackage.getAllocatedHosts(),
diff --git a/configserver/src/main/java/com/yahoo/vespa/config/server/session/RemoteSession.java b/configserver/src/main/java/com/yahoo/vespa/config/server/session/RemoteSession.java
index 36cac87a326..78a37499cef 100644
--- a/configserver/src/main/java/com/yahoo/vespa/config/server/session/RemoteSession.java
+++ b/configserver/src/main/java/com/yahoo/vespa/config/server/session/RemoteSession.java
@@ -1,22 +1,12 @@
 // Copyright 2017 Yahoo Holdings. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
 package com.yahoo.vespa.config.server.session;
 
-import com.yahoo.config.application.api.ApplicationPackage;
-import com.yahoo.config.provision.AllocatedHosts;
 import com.yahoo.config.provision.TenantName;
-import com.yahoo.lang.SettableOptional;
 import com.yahoo.transaction.Transaction;
-import com.yahoo.vespa.config.server.GlobalComponentRegistry;
 import com.yahoo.vespa.config.server.application.ApplicationSet;
-import com.yahoo.vespa.config.server.modelfactory.ActivatedModelsBuilder;
-import com.yahoo.vespa.curator.Curator;
-import org.apache.zookeeper.KeeperException;
 
-import java.time.Clock;
+import java.util.Objects;
 import java.util.Optional;
-import java.util.Set;
-import java.util.logging.Level;
-import java.util.logging.Logger;
 
 /**
  * A RemoteSession represents a session created on another config server. This session can
@@ -26,95 +16,55 @@ import java.util.logging.Logger;
  */
 public class RemoteSession extends Session {
 
-    private static final Logger log = Logger.getLogger(RemoteSession.class.getName());
-    private ApplicationSet applicationSet = null;
-    private final ActivatedModelsBuilder applicationLoader;
-    private final Clock clock;
+    private final Optional<ApplicationSet> applicationSet;
 
     /**
-     * Creates a session. This involves loading the application, validating it and distributing it.
+     * Creates a remote session, no application set loaded
      *
      * @param tenant The name of the tenant creating session
      * @param sessionId The session id for this session.
-     * @param componentRegistry a registry of global components
      * @param zooKeeperClient a SessionZooKeeperClient instance
      */
-    public RemoteSession(TenantName tenant,
-                         long sessionId,
-                         GlobalComponentRegistry componentRegistry,
-                         SessionZooKeeperClient zooKeeperClient) {
-        super(tenant, sessionId, zooKeeperClient);
-        this.applicationLoader = new ActivatedModelsBuilder(tenant, sessionId, zooKeeperClient, componentRegistry);
-        this.clock = componentRegistry.getClock();
+    RemoteSession(TenantName tenant, long sessionId, SessionZooKeeperClient zooKeeperClient) {
+        this(tenant, sessionId, zooKeeperClient, Optional.empty());
     }
 
-    void prepare() {
-        Curator.CompletionWaiter waiter = sessionZooKeeperClient.getPrepareWaiter();
-        ensureApplicationLoaded();
-        notifyCompletion(waiter);
+    /**
+     * Creates a remote session, with application set
+     *
+     * @param tenant The name of the tenant creating session
+     * @param sessionId The session id for this session.
+     * @param zooKeeperClient a SessionZooKeeperClient instance
+     * @param applicationSet current application set for this session
+     */
+    RemoteSession(TenantName tenant,
+                  long sessionId,
+                  SessionZooKeeperClient zooKeeperClient,
+                  Optional<ApplicationSet> applicationSet) {
+        super(tenant, sessionId, zooKeeperClient);
+        this.applicationSet = applicationSet;
     }
 
-    private ApplicationSet loadApplication() {
-        ApplicationPackage applicationPackage = sessionZooKeeperClient.loadApplicationPackage();
-
-        // Read hosts allocated on the config server instance which created this
-        Optional<AllocatedHosts> allocatedHosts = applicationPackage.getAllocatedHosts();
-
-        return ApplicationSet.fromList(applicationLoader.buildModels(getApplicationId(),
-                                                                     sessionZooKeeperClient.readDockerImageRepository(),
-                                                                     sessionZooKeeperClient.readVespaVersion(),
-                                                                     applicationPackage,
-                                                                     new SettableOptional<>(allocatedHosts),
-                                                                     clock.instant()));
+    Optional<ApplicationSet> applicationSet() {
+        return applicationSet;
     }
 
-    public synchronized ApplicationSet ensureApplicationLoaded() {
-        return applicationSet == null ? applicationSet = loadApplication() : applicationSet;
+    public synchronized RemoteSession activated(ApplicationSet applicationSet) {
+        Objects.requireNonNull(applicationSet, "applicationSet cannot be null");
+        return new RemoteSession(tenant, sessionId, sessionZooKeeperClient, Optional.of(applicationSet));
     }
 
-    public synchronized void deactivate() {
-        applicationSet = null;
+    public synchronized RemoteSession deactivated() {
+        return new RemoteSession(tenant, sessionId, sessionZooKeeperClient, Optional.empty());
     }
 
     public Transaction createDeleteTransaction() {
         return sessionZooKeeperClient.createWriteStatusTransaction(Status.DELETE);
     }
-    
-    void confirmUpload() {
-        Curator.CompletionWaiter waiter = sessionZooKeeperClient.getUploadWaiter();
-        log.log(Level.FINE, "Notifying upload waiter for session " + getSessionId());
-        notifyCompletion(waiter);
-        log.log(Level.FINE, "Done notifying upload for session " + getSessionId());
-    }
-
-    void notifyCompletion(Curator.CompletionWaiter completionWaiter) {
-        try {
-            completionWaiter.notifyCompletion();
-        } catch (RuntimeException e) {
-            // Throw only if we get something else than NoNodeException or NodeExistsException.
-            // NoNodeException might happen when the session is no longer in use (e.g. the app using this session
-            // has been deleted) and this method has not been called yet for the previous session operation on a
-            // minority of the config servers.
-            // NodeExistsException might happen if an event for this node is delivered more than once, in that case
-            // this is a no-op
-            Set<Class<? extends KeeperException>> acceptedExceptions = Set.of(KeeperException.NoNodeException.class,
-                                                                              KeeperException.NodeExistsException.class);
-            Class<? extends Throwable> exceptionClass = e.getCause().getClass();
-            if (acceptedExceptions.contains(exceptionClass))
-                log.log(Level.FINE, "Not able to notify completion for session " + getSessionId() +
-                                    " (" + completionWaiter + ")," +
-                                    " node " + (exceptionClass.equals(KeeperException.NoNodeException.class)
-                                    ? "has been deleted"
-                                    : "already exists"));
-            else
-                throw e;
-        }
-    }
 
-    public void delete() {
-        Transaction transaction = sessionZooKeeperClient.deleteTransaction();
-        transaction.commit();
-        transaction.close();
+    @Override
+    public String toString() {
+        return super.toString() + ",application set=" + applicationSet;
     }
 
 }
diff --git a/configserver/src/main/java/com/yahoo/vespa/config/server/session/Session.java b/configserver/src/main/java/com/yahoo/vespa/config/server/session/Session.java
index b3e35e955de..85f9b575942 100644
--- a/configserver/src/main/java/com/yahoo/vespa/config/server/session/Session.java
+++ b/configserver/src/main/java/com/yahoo/vespa/config/server/session/Session.java
@@ -28,7 +28,7 @@ import java.util.Optional;
  */
 public abstract class Session implements Comparable<Session>  {
 
-    private final long sessionId;
+    protected final long sessionId;
     protected final TenantName tenant;
     protected final SessionZooKeeperClient sessionZooKeeperClient;
     protected final Optional<ApplicationPackage> applicationPackage;
diff --git a/configserver/src/main/java/com/yahoo/vespa/config/server/session/SessionCache.java b/configserver/src/main/java/com/yahoo/vespa/config/server/session/SessionCache.java
index b7d78f11201..37de857a251 100644
--- a/configserver/src/main/java/com/yahoo/vespa/config/server/session/SessionCache.java
+++ b/configserver/src/main/java/com/yahoo/vespa/config/server/session/SessionCache.java
@@ -15,23 +15,13 @@ public class SessionCache<SESSIONTYPE extends Session> {
 
     private final HashMap<Long, SESSIONTYPE> sessions = new HashMap<>();
 
-    public synchronized void addSession(SESSIONTYPE session) {
-        sessions.putIfAbsent(session.getSessionId(), session);
-    }
+    public synchronized void putSession(SESSIONTYPE session) { sessions.put(session.getSessionId(), session); }
 
     synchronized void removeSession(long id) {
         sessions.remove(id);
     }
 
-    /**
-     * Gets a Session
-     *
-     * @param id session id
-     * @return a session belonging to the id supplied, or null if no session with the id was found
-     */
-    public synchronized SESSIONTYPE getSession(long id) {
-        return sessions.get(id);
-    }
+    public synchronized SESSIONTYPE getSession(long id) { return sessions.get(id); }
 
     public synchronized List<SESSIONTYPE> getSessions() {
         return new ArrayList<>(sessions.values());
diff --git a/configserver/src/main/java/com/yahoo/vespa/config/server/session/SessionRepository.java b/configserver/src/main/java/com/yahoo/vespa/config/server/session/SessionRepository.java
index cbfa59b26e4..b251e2c6564 100644
--- a/configserver/src/main/java/com/yahoo/vespa/config/server/session/SessionRepository.java
+++ b/configserver/src/main/java/com/yahoo/vespa/config/server/session/SessionRepository.java
@@ -8,9 +8,11 @@ import com.yahoo.config.application.api.ApplicationPackage;
 import com.yahoo.config.application.api.DeployLogger;
 import com.yahoo.config.model.application.provider.DeployData;
 import com.yahoo.config.model.application.provider.FilesApplicationPackage;
+import com.yahoo.config.provision.AllocatedHosts;
 import com.yahoo.config.provision.ApplicationId;
 import com.yahoo.config.provision.TenantName;
 import com.yahoo.io.IOUtils;
+import com.yahoo.lang.SettableOptional;
 import com.yahoo.path.Path;
 import com.yahoo.transaction.AbstractTransaction;
 import com.yahoo.transaction.NestedTransaction;
@@ -22,6 +24,7 @@ import com.yahoo.vespa.config.server.application.TenantApplications;
 import com.yahoo.vespa.config.server.configchange.ConfigChangeActions;
 import com.yahoo.vespa.config.server.deploy.TenantFileSystemDirs;
 import com.yahoo.vespa.config.server.filedistribution.FileDirectory;
+import com.yahoo.vespa.config.server.modelfactory.ActivatedModelsBuilder;
 import com.yahoo.vespa.config.server.monitoring.MetricUpdater;
 import com.yahoo.vespa.config.server.monitoring.Metrics;
 import com.yahoo.vespa.config.server.tenant.TenantRepository;
@@ -36,6 +39,7 @@ import com.yahoo.vespa.flags.Flags;
 import org.apache.curator.framework.CuratorFramework;
 import org.apache.curator.framework.recipes.cache.ChildData;
 import org.apache.curator.framework.recipes.cache.PathChildrenCacheEvent;
+import org.apache.zookeeper.KeeperException;
 
 import java.io.File;
 import java.io.FilenameFilter;
@@ -51,6 +55,7 @@ import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.Optional;
+import java.util.Set;
 import java.util.concurrent.Executor;
 import java.util.logging.Level;
 import java.util.logging.Logger;
@@ -121,10 +126,8 @@ public class SessionRepository {
     // ---------------- Local sessions ----------------------------------------------------------------
 
     public synchronized void addLocalSession(LocalSession session) {
-        localSessionCache.addSession(session);
-        long sessionId = session.getSessionId();
-        RemoteSession remoteSession = createRemoteSession(sessionId);
-        addSessionStateWatcher(sessionId, remoteSession, Optional.of(session));
+        localSessionCache.putSession(session);
+        createRemoteSession(session.getSessionId());
     }
 
     public LocalSession getLocalSession(long sessionId) {
@@ -262,11 +265,6 @@ public class SessionRepository {
         return getSessionList(curator.getChildren(sessionsPath));
     }
 
-    public void addRemoteSession(RemoteSession session) {
-        remoteSessionCache.addSession(session);
-        metrics.incAddedSessions();
-    }
-
     public int deleteExpiredRemoteSessions(Clock clock, Duration expiryTime) {
         int deleted = 0;
         for (long sessionId : getRemoteSessions()) {
@@ -275,13 +273,26 @@ public class SessionRepository {
             if (session.getStatus() == Session.Status.ACTIVATE) continue;
             if (sessionHasExpired(session.getCreateTime(), expiryTime, clock)) {
                 log.log(Level.FINE, () -> "Remote session " + sessionId + " for " + tenantName + " has expired, deleting it");
-                session.delete();
+                deleteRemoteSession(session);
                 deleted++;
             }
         }
         return deleted;
     }
 
+    public void deactivate(RemoteSession remoteSession) {
+        RemoteSession session = remoteSession.deactivated();
+        remoteSessionCache.putSession(session);
+        updateSessionStateWatcher(session);
+    }
+
+    public void deleteRemoteSession(RemoteSession session) {
+        SessionZooKeeperClient sessionZooKeeperClient = createSessionZooKeeperClient(session.getSessionId());
+        Transaction transaction = sessionZooKeeperClient.deleteTransaction();
+        transaction.commit();
+        transaction.close();
+    }
+
     public int deleteExpiredLocks(Clock clock, Duration expiryTime) {
         int deleted = 0;
         for (var lock : curator.getChildren(locksPath)) {
@@ -347,36 +358,27 @@ public class SessionRepository {
         log.log(Level.FINE, () -> "Adding remote session to SessionRepository: " + sessionId);
         RemoteSession remoteSession = createRemoteSession(sessionId);
         loadSessionIfActive(remoteSession);
-        addRemoteSession(remoteSession);
-        Optional<LocalSession> localSession = Optional.empty();
         if (distributeApplicationPackage())
-            localSession = createLocalSessionUsingDistributedApplicationPackage(sessionId);
-        addSessionStateWatcher(sessionId, remoteSession, localSession);
+            createLocalSessionUsingDistributedApplicationPackage(sessionId);
     }
 
     void activate(RemoteSession session) {
         long sessionId = session.getSessionId();
         Curator.CompletionWaiter waiter = createSessionZooKeeperClient(sessionId).getActiveWaiter();
         log.log(Level.FINE, () -> session.logPre() + "Getting session from repo: " + sessionId);
-        ApplicationSet app = session.ensureApplicationLoaded();
+        ApplicationSet app = ensureApplicationLoaded(session);
         log.log(Level.FINE, () -> session.logPre() + "Reloading config for " + sessionId);
         applicationRepo.reloadConfig(app);
         log.log(Level.FINE, () -> session.logPre() + "Notifying " + waiter);
-        session.notifyCompletion(waiter);
+        notifyCompletion(waiter, session);
         log.log(Level.INFO, session.logPre() + "Session activated: " + sessionId);
     }
 
-    public void deactivate(RemoteSession remoteSession) {
-        remoteSession.deactivate();
-    }
-
-    public void delete(RemoteSession remoteSession, Optional<LocalSession> localSession) {
-        localSession.ifPresent(this::deleteLocalSession);
-        remoteSession.deactivate();
-    }
-
-    void prepare(RemoteSession session) {
-        session.prepare();
+    void deleteSession(RemoteSession remoteSession) {
+        LocalSession localSession = getLocalSession(remoteSession.getSessionId());
+        if (localSession != null)
+            deleteLocalSession(localSession);
+        deactivate(remoteSession);
     }
 
     boolean distributeApplicationPackage() {
@@ -394,13 +396,85 @@ public class SessionRepository {
         for (ApplicationId applicationId : applicationRepo.activeApplications()) {
             if (applicationRepo.requireActiveSessionOf(applicationId) == session.getSessionId()) {
                 log.log(Level.FINE, () -> "Found active application for session " + session.getSessionId() + " , loading it");
-                applicationRepo.reloadConfig(session.ensureApplicationLoaded());
+                applicationRepo.reloadConfig(ensureApplicationLoaded(session));
                 log.log(Level.INFO, session.logPre() + "Application activated successfully: " + applicationId + " (generation " + session.getSessionId() + ")");
                 return;
             }
         }
     }
 
+    void prepareRemoteSession(RemoteSession session) {
+        SessionZooKeeperClient sessionZooKeeperClient = createSessionZooKeeperClient(session.getSessionId());
+        Curator.CompletionWaiter waiter = sessionZooKeeperClient.getPrepareWaiter();
+        ensureApplicationLoaded(session);
+        notifyCompletion(waiter, session);
+    }
+
+    public ApplicationSet ensureApplicationLoaded(RemoteSession session) {
+        try (var lock = lock(session.sessionId)) {
+            if (session.applicationSet().isPresent()) {
+                return session.applicationSet().get();
+            }
+
+            ApplicationSet applicationSet = loadApplication(session);
+            RemoteSession activated = session.activated(applicationSet);
+            remoteSessionCache.putSession(activated);
+            updateSessionStateWatcher(activated);
+
+            return applicationSet;
+        }
+    }
+
+    void confirmUpload(RemoteSession session) {
+        Curator.CompletionWaiter waiter = session.getSessionZooKeeperClient().getUploadWaiter();
+        long sessionId = session.getSessionId();
+        log.log(Level.FINE, "Notifying upload waiter for session " + sessionId);
+        notifyCompletion(waiter, session);
+        log.log(Level.FINE, "Done notifying upload for session " + sessionId);
+    }
+
+    void notifyCompletion(Curator.CompletionWaiter completionWaiter, RemoteSession session) {
+        try {
+            completionWaiter.notifyCompletion();
+        } catch (RuntimeException e) {
+            // Throw only if we get something else than NoNodeException or NodeExistsException.
+            // NoNodeException might happen when the session is no longer in use (e.g. the app using this session
+            // has been deleted) and this method has not been called yet for the previous session operation on a
+            // minority of the config servers.
+            // NodeExistsException might happen if an event for this node is delivered more than once, in that case
+            // this is a no-op
+            Set<Class<? extends KeeperException>> acceptedExceptions = Set.of(KeeperException.NoNodeException.class,
+                                                                              KeeperException.NodeExistsException.class);
+            Class<? extends Throwable> exceptionClass = e.getCause().getClass();
+            if (acceptedExceptions.contains(exceptionClass))
+                log.log(Level.FINE, "Not able to notify completion for session " + session.getSessionId() +
+                                    " (" + completionWaiter + ")," +
+                                    " node " + (exceptionClass.equals(KeeperException.NoNodeException.class)
+                        ? "has been deleted"
+                        : "already exists"));
+            else
+                throw e;
+        }
+    }
+
+    private ApplicationSet loadApplication(RemoteSession session) {
+        log.log(Level.FINE, () -> "Loading application for " + session);
+        SessionZooKeeperClient sessionZooKeeperClient = createSessionZooKeeperClient(session.getSessionId());
+        ApplicationPackage applicationPackage = sessionZooKeeperClient.loadApplicationPackage();
+        ActivatedModelsBuilder builder = new ActivatedModelsBuilder(session.getTenantName(),
+                                                                    session.getSessionId(),
+                                                                    sessionZooKeeperClient,
+                                                                    componentRegistry);
+        // Read hosts allocated on the config server instance which created this
+        Optional<AllocatedHosts> allocatedHosts = applicationPackage.getAllocatedHosts();
+        return ApplicationSet.fromList(builder.buildModels(session.getApplicationId(),
+                                                           sessionZooKeeperClient.readDockerImageRepository(),
+                                                           sessionZooKeeperClient.readVespaVersion(),
+                                                           applicationPackage,
+                                                           new SettableOptional<>(allocatedHosts),
+                                                           clock.instant()));
+    }
+
     private void nodeChanged() {
         zkWatcherExecutor.execute(() -> {
             Multiset<Session.Status> sessionMetrics = HashMultiset.create();
@@ -436,7 +510,7 @@ public class SessionRepository {
             RemoteSession session = remoteSessionCache.getSession(sessionId);
             if (session == null) continue; // session might have been deleted after getting session list
             log.log(Level.FINE, () -> session.logPre() + "Confirming upload for session " + sessionId);
-            session.confirmUpload();
+            confirmUpload(session);
         }
     }
 
@@ -454,8 +528,16 @@ public class SessionRepository {
     }
 
     public RemoteSession createRemoteSession(long sessionId) {
+        return createRemoteSession(sessionId, Optional.empty());
+    }
+
+    public RemoteSession createRemoteSession(long sessionId, Optional<ApplicationSet> applicationSet) {
         SessionZooKeeperClient sessionZKClient = createSessionZooKeeperClient(sessionId);
-        return new RemoteSession(tenantName, sessionId, componentRegistry, sessionZKClient);
+        RemoteSession session = new RemoteSession(tenantName, sessionId, sessionZKClient, applicationSet);
+        remoteSessionCache.putSession(session);
+        updateSessionStateWatcher(session);
+        metrics.incAddedSessions();
+        return session;
     }
 
     private void ensureSessionPathDoesNotExist(long sessionId) {
@@ -603,10 +685,11 @@ public class SessionRepository {
      * Returns a new local session for the given session id if it does not already exist.
      * Will also add the session to the local session cache if necessary
      */
-    public Optional<LocalSession> createLocalSessionUsingDistributedApplicationPackage(long sessionId) {
+    public void createLocalSessionUsingDistributedApplicationPackage(long sessionId) {
         if (applicationRepo.hasLocalSession(sessionId)) {
             log.log(Level.FINE, () -> "Local session for session id " + sessionId + " already exists");
-            return Optional.of(createSessionFromId(sessionId));
+            createSessionFromId(sessionId);
+            return;
         }
 
         SessionZooKeeperClient sessionZKClient = createSessionZooKeeperClient(sessionId);
@@ -621,17 +704,15 @@ public class SessionRepository {
             } catch (IllegalArgumentException e) {
                 // We cannot be guaranteed that the file reference exists (it could be that it has not
                 // been downloaded yet), and e.g when bootstrapping we cannot throw an exception in that case
-                log.log(Level.INFO, "File reference for session id " + sessionId + ": " + fileReference + " not found in " + fileDirectory);
-                return Optional.empty();
+                log.log(Level.FINE, () -> "File reference for session id " + sessionId + ": " + fileReference + " not found in " + fileDirectory);
+                return;
             }
             ApplicationId applicationId = sessionZKClient.readApplicationId()
                     .orElseThrow(() -> new RuntimeException("Could not find application id for session " + sessionId));
             log.log(Level.FINE, () -> "Creating local session for tenant '" + tenantName + "' with session id " + sessionId);
             LocalSession localSession = createLocalSession(sessionDir, applicationId, sessionId);
             addLocalSession(localSession);
-            return Optional.of(localSession);
         }
-        return Optional.empty();
     }
 
     private Optional<Long> getActiveSessionId(ApplicationId applicationId) {
@@ -670,15 +751,15 @@ public class SessionRepository {
         return new TenantFileSystemDirs(componentRegistry.getConfigServerDB(), tenantName).getUserApplicationDir(sessionId);
     }
 
-    private void addSessionStateWatcher(long sessionId, RemoteSession remoteSession, Optional<LocalSession> localSession) {
-        // Remote session will always be present in an existing state watcher, but local session might not
-        if (sessionStateWatchers.containsKey(sessionId)) {
-            localSession.ifPresent(session -> sessionStateWatchers.get(sessionId).addLocalSession(session));
-        } else {
+    private void updateSessionStateWatcher(RemoteSession session) {
+        long sessionId = session.getSessionId();
+        SessionStateWatcher sessionStateWatcher = sessionStateWatchers.get(sessionId);
+        if (sessionStateWatcher == null) {
             Curator.FileCache fileCache = curator.createFileCache(getSessionStatePath(sessionId).getAbsolute(), false);
             fileCache.addListener(this::nodeChanged);
-            sessionStateWatchers.put(sessionId, new SessionStateWatcher(fileCache, remoteSession, localSession,
-                                                                        metrics, zkWatcherExecutor, this));
+            sessionStateWatchers.put(sessionId, new SessionStateWatcher(fileCache, session, metrics, zkWatcherExecutor, this));
+        } else {
+            sessionStateWatcher.setSession(session);
         }
     }
 
diff --git a/configserver/src/main/java/com/yahoo/vespa/config/server/session/SessionStateWatcher.java b/configserver/src/main/java/com/yahoo/vespa/config/server/session/SessionStateWatcher.java
index 57d9f027447..2fe0f9d6cbc 100644
--- a/configserver/src/main/java/com/yahoo/vespa/config/server/session/SessionStateWatcher.java
+++ b/configserver/src/main/java/com/yahoo/vespa/config/server/session/SessionStateWatcher.java
@@ -6,7 +6,6 @@ import com.yahoo.vespa.config.server.monitoring.MetricUpdater;
 import com.yahoo.vespa.curator.Curator;
 import org.apache.curator.framework.recipes.cache.ChildData;
 
-import java.util.Optional;
 import java.util.concurrent.Executor;
 import java.util.logging.Level;
 import java.util.logging.Logger;
@@ -25,21 +24,18 @@ public class SessionStateWatcher {
     private static final Logger log = Logger.getLogger(SessionStateWatcher.class.getName());
 
     private final Curator.FileCache fileCache;
-    private final RemoteSession remoteSession;
+    private RemoteSession session;
     private final MetricUpdater metrics;
     private final Executor zkWatcherExecutor;
     private final SessionRepository sessionRepository;
-    private Optional<LocalSession> localSession;
 
     SessionStateWatcher(Curator.FileCache fileCache,
-                        RemoteSession remoteSession,
-                        Optional<LocalSession> localSession,
+                        RemoteSession session,
                         MetricUpdater metrics,
                         Executor zkWatcherExecutor,
                         SessionRepository sessionRepository) {
         this.fileCache = fileCache;
-        this.remoteSession = remoteSession;
-        this.localSession = localSession;
+        this.session = session;
         this.metrics = metrics;
         this.fileCache.addListener(this::nodeChanged);
         this.fileCache.start();
@@ -48,24 +44,24 @@ public class SessionStateWatcher {
     }
 
     private void sessionStatusChanged(Status newStatus) {
-        long sessionId = remoteSession.getSessionId();
+        long sessionId = session.getSessionId();
         switch (newStatus) {
             case NEW:
             case NONE:
                 break;
             case PREPARE:
                 createLocalSession(sessionId);
-                sessionRepository.prepare(remoteSession);
+                sessionRepository.prepareRemoteSession(session);
                 break;
             case ACTIVATE:
                 createLocalSession(sessionId);
-                sessionRepository.activate(remoteSession);
+                sessionRepository.activate(session);
                 break;
             case DEACTIVATE:
-                sessionRepository.deactivate(remoteSession);
+                sessionRepository.deactivate(session);
                 break;
             case DELETE:
-                sessionRepository.delete(remoteSession, localSession);
+                sessionRepository.deleteSession(session);
                 break;
             default:
                 throw new IllegalStateException("Unknown status " + newStatus);
@@ -73,13 +69,13 @@ public class SessionStateWatcher {
     }
 
     private void createLocalSession(long sessionId) {
-        if (sessionRepository.distributeApplicationPackage() && localSession.isEmpty()) {
-            localSession = sessionRepository.createLocalSessionUsingDistributedApplicationPackage(sessionId);
+        if (sessionRepository.distributeApplicationPackage()) {
+            sessionRepository.createLocalSessionUsingDistributedApplicationPackage(sessionId);
         }
     }
 
     public long getSessionId() {
-        return remoteSession.getSessionId();
+        return session.getSessionId();
     }
 
     public void close() {
@@ -97,20 +93,21 @@ public class SessionStateWatcher {
                 ChildData node = fileCache.getCurrentData();
                 if (node != null) {
                     newStatus = Status.parse(Utf8.toString(node.getData()));
-                    log.log(Level.FINE, remoteSession.logPre() + "Session change: Session "
-                                        + remoteSession.getSessionId() + " changed status to " + newStatus.name());
+                    final String statusName = newStatus.name();
+                    log.log(Level.FINE, () -> session.logPre() + "Session change: Session "
+                                              + getSessionId() + " changed status to " + statusName);
                     sessionStatusChanged(newStatus);
                 }
             } catch (Exception e) {
-                log.log(Level.WARNING, remoteSession.logPre() + "Error handling session change to " +
+                log.log(Level.WARNING, session.logPre() + "Error handling session change to " +
                                        newStatus.name() + " for session " + getSessionId(), e);
                 metrics.incSessionChangeErrors();
             }
         });
     }
 
-    void addLocalSession(LocalSession session) {
-        localSession = Optional.of(session);
+    void setSession(RemoteSession session) {
+        this.session = session;
     }
 
 }
diff --git a/configserver/src/test/apps/app-major-version-2/deployment.xml b/configserver/src/test/apps/app-major-version-2/deployment.xml
new file mode 100644
index 00000000000..7523c104b7e
--- /dev/null
+++ b/configserver/src/test/apps/app-major-version-2/deployment.xml
@@ -0,0 +1 @@
+<deployment version='1.0' major-version='2'/>
diff --git a/configserver/src/test/apps/app-major-version-2/hosts.xml b/configserver/src/test/apps/app-major-version-2/hosts.xml
new file mode 100644
index 00000000000..f4256c9fc81
--- /dev/null
+++ b/configserver/src/test/apps/app-major-version-2/hosts.xml
@@ -0,0 +1,7 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<!-- Copyright 2017 Yahoo Holdings. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root. -->
+<hosts>
+  <host name="mytesthost">
+    <alias>node1</alias>
+  </host>
+</hosts>
diff --git a/configserver/src/test/apps/app-major-version-2/searchdefinitions/music.sd b/configserver/src/test/apps/app-major-version-2/searchdefinitions/music.sd
new file mode 100644
index 00000000000..7670e78f22b
--- /dev/null
+++ b/configserver/src/test/apps/app-major-version-2/searchdefinitions/music.sd
@@ -0,0 +1,50 @@
+# Copyright 2017 Yahoo Holdings. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
+# A basic search definition - called music, should be saved to music.sd
+search music {
+
+  # It contains one document type only - called music as well
+  document music {
+
+    field title type string {
+      indexing: summary | index   # How this field should be indexed
+      # index-to: title, default    # Create two indexes
+      weight: 75 # Ranking importancy of this field, used by the built in nativeRank feature
+    }
+
+    field artist type string {
+      indexing: summary | attribute | index
+      # index-to: artist, default
+
+      weight: 25 
+    }
+
+    field year type int {
+      indexing: summary | attribute
+    }
+
+    # Increase query
+    field popularity type int {
+      indexing: summary | attribute
+    }
+
+    field url type uri {
+      indexing: summary | index
+    }
+
+  }
+
+  rank-profile default inherits default {
+    first-phase {
+      expression: nativeRank(title,artist) + attribute(popularity)
+    }
+
+  }
+
+  rank-profile textmatch inherits default {
+    first-phase {
+      expression: nativeRank(title,artist) 
+    }
+
+  }
+
+}
diff --git a/configserver/src/test/apps/app-major-version-2/services.xml b/configserver/src/test/apps/app-major-version-2/services.xml
new file mode 100644
index 00000000000..509d7786be0
--- /dev/null
+++ b/configserver/src/test/apps/app-major-version-2/services.xml
@@ -0,0 +1,38 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<!-- Copyright 2017 Yahoo Holdings. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root. -->
+<services version="1.0">
+
+    <admin version="2.0">
+        <adminserver hostalias="node1"/>
+        <logserver hostalias="node1" />
+    </admin>
+
+    <content version="1.0">
+        <redundancy>2</redundancy>
+        <documents>
+            <document type="music" mode="index"/>
+        </documents>
+        <nodes>
+            <node hostalias="node1" distribution-key="0"/>
+        </nodes>
+
+    </content>
+
+    <container version="1.0">
+        <document-processing compressdocuments="true">
+            <chain id="ContainerWrapperTest">
+                <documentprocessor id="com.yahoo.vespa.config.AppleDocProc"/>
+            </chain>
+        </document-processing>
+
+        <config name="project.specific">
+            <value>someval</value>
+        </config>
+
+        <nodes>
+            <node hostalias="node1" />
+        </nodes>
+
+    </container>
+
+</services>
diff --git a/configserver/src/test/java/com/yahoo/vespa/config/server/http/v2/ApplicationHandlerTest.java b/configserver/src/test/java/com/yahoo/vespa/config/server/http/v2/ApplicationHandlerTest.java
index d84b81bd8e7..b7310917449 100644
--- a/configserver/src/test/java/com/yahoo/vespa/config/server/http/v2/ApplicationHandlerTest.java
+++ b/configserver/src/test/java/com/yahoo/vespa/config/server/http/v2/ApplicationHandlerTest.java
@@ -226,7 +226,7 @@ public class ApplicationHandlerTest {
         ApplicationId unknown = new ApplicationId.Builder().applicationName("unknown").tenant("default").build();
         HttpResponse responseForUnknown = fileDistributionStatus(unknown, zone);
         assertEquals(404, responseForUnknown.getStatus());
-        assertEquals("{\"error-code\":\"NOT_FOUND\",\"message\":\"Unknown application id 'default.unknown'\"}",
+        assertEquals("{\"error-code\":\"NOT_FOUND\",\"message\":\"No active session found for 'default.unknown'\"}",
                      getRenderedString(responseForUnknown));
     }
 
diff --git a/configserver/src/test/java/com/yahoo/vespa/config/server/rpc/RpcServerTest.java b/configserver/src/test/java/com/yahoo/vespa/config/server/rpc/RpcServerTest.java
index ae6bd5feeab..d8d20d37551 100644
--- a/configserver/src/test/java/com/yahoo/vespa/config/server/rpc/RpcServerTest.java
+++ b/configserver/src/test/java/com/yahoo/vespa/config/server/rpc/RpcServerTest.java
@@ -64,7 +64,8 @@ public class RpcServerTest {
             ApplicationRepository applicationRepository = tester.applicationRepository();
             applicationRepository.deploy(testApp, new PrepareParams.Builder().applicationId(applicationId).build());
             TenantApplications applicationRepo = tester.tenant().getApplicationRepo();
-            applicationRepo.reloadConfig(applicationRepository.getActiveSession(applicationId).ensureApplicationLoaded());
+            ApplicationSet applicationSet = tester.tenant().getSessionRepository().ensureApplicationLoaded(applicationRepository.getActiveSession(applicationId));
+            applicationRepo.reloadConfig(applicationSet);
             testPrintStatistics(tester);
             testGetConfig(tester);
             testEnabled(tester);
diff --git a/configserver/src/test/java/com/yahoo/vespa/config/server/session/RemoteSessionTest.java b/configserver/src/test/java/com/yahoo/vespa/config/server/session/RemoteSessionTest.java
deleted file mode 100644
index cda3e09d3a8..00000000000
--- a/configserver/src/test/java/com/yahoo/vespa/config/server/session/RemoteSessionTest.java
+++ /dev/null
@@ -1,299 +0,0 @@
-// Copyright 2017 Yahoo Holdings. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
-package com.yahoo.vespa.config.server.session;
-
-import com.yahoo.cloud.config.ConfigserverConfig;
-import com.yahoo.component.Version;
-import com.yahoo.config.application.api.ApplicationPackage;
-import com.yahoo.config.model.NullConfigModelRegistry;
-import com.yahoo.config.model.api.Model;
-import com.yahoo.config.model.api.ModelContext;
-import com.yahoo.config.model.api.ModelCreateResult;
-import com.yahoo.config.model.api.ModelFactory;
-import com.yahoo.config.model.api.ValidationParameters;
-import com.yahoo.config.model.deploy.DeployState;
-import com.yahoo.config.model.test.MockApplicationPackage;
-import com.yahoo.config.provision.ApplicationId;
-import com.yahoo.config.provision.TenantName;
-import com.yahoo.vespa.config.server.TestComponentRegistry;
-import com.yahoo.vespa.config.server.application.ApplicationSet;
-import com.yahoo.vespa.config.server.application.PermanentApplicationPackage;
-import com.yahoo.vespa.config.server.modelfactory.ModelFactoryRegistry;
-import com.yahoo.vespa.curator.Curator;
-import com.yahoo.vespa.curator.mock.MockCurator;
-import com.yahoo.vespa.model.VespaModel;
-import com.yahoo.vespa.model.VespaModelFactory;
-import org.junit.Before;
-import org.junit.Rule;
-import org.junit.Test;
-import org.junit.rules.TemporaryFolder;
-
-import java.io.IOException;
-import java.time.Clock;
-import java.time.Instant;
-import java.time.LocalDate;
-import java.time.ZoneOffset;
-import java.time.format.DateTimeFormatter;
-import java.util.ArrayList;
-import java.util.Arrays;
-import java.util.Collections;
-import java.util.List;
-import java.util.Optional;
-
-import static org.hamcrest.core.Is.is;
-import static org.junit.Assert.assertEquals;
-import static org.junit.Assert.assertNotNull;
-import static org.junit.Assert.assertThat;
-import static org.junit.Assert.assertTrue;
-
-/**
- * @author Ulf Lilleengen
- * @author bratseth
- */
-public class RemoteSessionTest {
-
-    private static final TenantName tenantName = TenantName.from("default");
-
-    private Curator curator;
-
-    @Rule
-    public TemporaryFolder temporaryFolder = new TemporaryFolder();
-
-    @Before
-    public void setupTest() {
-        curator = new MockCurator();
-    }
-
-    @Test
-    public void require_that_session_is_initialized() {
-        Clock clock = Clock.systemUTC();
-        Session session = createSession(2, clock);
-        assertThat(session.getSessionId(), is(2L));
-        session = createSession(Long.MAX_VALUE, clock);
-        assertThat(session.getSessionId(), is(Long.MAX_VALUE));
-    }
-
-    @Test
-    public void require_that_applications_are_loaded() {
-        RemoteSession session = createSession(3, Arrays.asList(new MockModelFactory(), new VespaModelFactory(new NullConfigModelRegistry())));
-        session.prepare();
-        ApplicationSet applicationSet = session.ensureApplicationLoaded();
-        assertNotNull(applicationSet);
-        assertThat(applicationSet.getApplicationGeneration(), is(3L));
-        assertThat(applicationSet.getForVersionOrLatest(Optional.empty(), Instant.now()).getId().application().value(), is("foo"));
-        assertNotNull(applicationSet.getForVersionOrLatest(Optional.empty(), Instant.now()).getModel());
-        session.deactivate();
-
-        applicationSet = session.ensureApplicationLoaded();
-        assertNotNull(applicationSet);
-        assertThat(applicationSet.getApplicationGeneration(), is(3L));
-        assertThat(applicationSet.getForVersionOrLatest(Optional.empty(), Instant.now()).getId().application().value(), is("foo"));
-        assertNotNull(applicationSet.getForVersionOrLatest(Optional.empty(), Instant.now()).getModel());
-    }
-
-    @Test(expected = IllegalArgumentException.class)
-    public void require_that_new_invalid_application_throws_exception() {
-        MockModelFactory failingFactory = new MockModelFactory();
-        failingFactory.vespaVersion = new Version(1, 2, 0);
-        failingFactory.throwOnLoad = true;
-
-        MockModelFactory okFactory = new MockModelFactory();
-        okFactory.vespaVersion = new Version(1, 1, 0);
-        okFactory.throwOnLoad = false;
-
-        RemoteSession session = createSession(3, Arrays.asList(okFactory, failingFactory));
-        session.prepare();
-    }
-
-    @Test
-    public void require_that_old_invalid_application_does_not_throw_exception_if_skipped_also_across_major_versions() {
-        MockModelFactory failingFactory = new MockModelFactory();
-        failingFactory.vespaVersion = new Version(1, 0, 0);
-        failingFactory.throwOnLoad = true;
-
-        MockModelFactory okFactory =
-                new MockModelFactory("<validation-overrides><allow until='2000-01-30'>skip-old-config-models</allow></validation-overrides>");
-        okFactory.vespaVersion = new Version(2, 0, 0);
-        okFactory.throwOnLoad = false;
-
-        RemoteSession session = createSession(3, Arrays.asList(okFactory, failingFactory), failingFactory.clock());
-        session.prepare();
-    }
-
-    @Test
-    public void require_that_an_application_package_can_limit_to_one_major_version() {
-        ApplicationPackage application =
-                new MockApplicationPackage.Builder().withServices("<services version='1.0'/>")
-                                                    .withDeploymentSpec("<deployment version='1.0' major-version='2'/>")
-                                                    .build();
-        assertTrue(application.getMajorVersion().isPresent());
-        assertEquals(2, (int)application.getMajorVersion().get());
-
-        MockModelFactory failingFactory = new MockModelFactory();
-        failingFactory.vespaVersion = new Version(3, 0, 0);
-        failingFactory.throwErrorOnLoad = true;
-
-        MockModelFactory okFactory = new MockModelFactory();
-        okFactory.vespaVersion = new Version(2, 0, 0);
-        okFactory.throwErrorOnLoad = false;
-
-        SessionZooKeeperClient zkc = new MockSessionZKClient(curator, tenantName, 3, application);
-        RemoteSession session = createSession(3, zkc, Arrays.asList(okFactory, failingFactory));
-        session.prepare();
-
-        // Does not cause an error because model version 3 is skipped
-    }
-
-    @Test
-    public void require_that_an_application_package_can_limit_to_one_higher_major_version() {
-        ApplicationPackage application =
-                new MockApplicationPackage.Builder().withServices("<services version='1.0'/>")
-                                                    .withDeploymentSpec("<deployment version='1.0' major-version='3'/>")
-                                                    .build();
-        assertTrue(application.getMajorVersion().isPresent());
-        assertEquals(3, (int)application.getMajorVersion().get());
-
-        MockModelFactory failingFactory = new MockModelFactory();
-        failingFactory.vespaVersion = new Version(4, 0, 0);
-        failingFactory.throwErrorOnLoad = true;
-
-        MockModelFactory okFactory = new MockModelFactory();
-        okFactory.vespaVersion = new Version(2, 0, 0);
-        okFactory.throwErrorOnLoad = false;
-
-        SessionZooKeeperClient zkc = new MockSessionZKClient(curator, tenantName, 3, application);
-        RemoteSession session = createSession(4, zkc, Arrays.asList(okFactory, failingFactory));
-        session.prepare();
-
-        // Does not cause an error because model version 4 is skipped
-    }
-
-    @Test
-    public void require_that_session_status_is_updated() {
-        SessionZooKeeperClient zkc = new MockSessionZKClient(curator, tenantName, 3);
-        RemoteSession session = createSession(3, zkc, Clock.systemUTC());
-        assertThat(session.getStatus(), is(Session.Status.NEW));
-        zkc.writeStatus(Session.Status.PREPARE);
-        assertThat(session.getStatus(), is(Session.Status.PREPARE));
-    }
-
-    @Test
-    public void require_that_permanent_app_is_used() throws IOException {
-        Optional<PermanentApplicationPackage> permanentApp = Optional.of(new PermanentApplicationPackage(
-                new ConfigserverConfig(new ConfigserverConfig.Builder()
-                                               .applicationDirectory(temporaryFolder.newFolder("appdir").getAbsolutePath()))));
-        MockModelFactory mockModelFactory = new MockModelFactory();
-        try {
-            int sessionId = 3;
-            SessionZooKeeperClient zkc = new MockSessionZKClient(curator, tenantName, sessionId);
-            createSession(sessionId, zkc, Collections.singletonList(mockModelFactory), permanentApp, Clock.systemUTC()).ensureApplicationLoaded();
-        } catch (Exception e) {
-            e.printStackTrace();
-            // ignore, we're not interested in deploy errors as long as the below state is OK.
-        }
-        assertNotNull(mockModelFactory.modelContext);
-        assertTrue(mockModelFactory.modelContext.permanentApplicationPackage().isPresent());
-    }
-
-    private RemoteSession createSession(long sessionId, Clock clock) {
-        return createSession(sessionId, Collections.singletonList(new VespaModelFactory(new NullConfigModelRegistry())), clock);
-    }
-
-    private RemoteSession createSession(long sessionId, SessionZooKeeperClient zkc, Clock clock) {
-        return createSession(sessionId, zkc, Collections.singletonList(new VespaModelFactory(new NullConfigModelRegistry())), clock);
-    }
-
-    private RemoteSession createSession(long sessionId, List<ModelFactory> modelFactories) {
-        SessionZooKeeperClient zkc = new MockSessionZKClient(curator, tenantName, sessionId);
-        return createSession(sessionId, zkc, modelFactories, Clock.systemUTC());
-    }
-
-    private RemoteSession createSession(long sessionId, List<ModelFactory> modelFactories, Clock clock) {
-        SessionZooKeeperClient zkc = new MockSessionZKClient(curator, tenantName, sessionId);
-        return createSession(sessionId, zkc, modelFactories, clock);
-    }
-
-    private RemoteSession createSession(long sessionId, SessionZooKeeperClient zkc, List<ModelFactory> modelFactories) {
-        return createSession(sessionId, zkc, modelFactories, Optional.empty(), Clock.systemUTC());
-    }
-
-    private RemoteSession createSession(long sessionId, SessionZooKeeperClient zkc, List<ModelFactory> modelFactories, Clock clock) {
-        return createSession(sessionId, zkc, modelFactories, Optional.empty(), clock);
-    }
-
-    private RemoteSession createSession(long sessionId, SessionZooKeeperClient zkc,
-                                        List<ModelFactory> modelFactories,
-                                        Optional<PermanentApplicationPackage> permanentApplicationPackage,
-                                        Clock clock) {
-        zkc.writeStatus(Session.Status.NEW);
-        zkc.writeApplicationId(new ApplicationId.Builder().applicationName("foo").instanceName("bim").build());
-        TestComponentRegistry.Builder registryBuilder = new TestComponentRegistry.Builder()
-                .curator(curator)
-                .clock(clock)
-                .modelFactoryRegistry(new ModelFactoryRegistry(modelFactories));
-        permanentApplicationPackage.ifPresent(registryBuilder::permanentApplicationPackage);
-
-        return new RemoteSession(tenantName, sessionId, registryBuilder.build(), zkc);
-    }
-
-    private static class MockModelFactory implements ModelFactory {
-
-        /** Throw a RuntimeException on load - this is handled gracefully during model building */
-        boolean throwOnLoad = false;
-
-        /** Throw an Error on load - this is useful to propagate this condition all the way to the test */
-        boolean throwErrorOnLoad = false;
-
-        ModelContext modelContext;
-        public Version vespaVersion = new Version(1, 2, 3);
-
-        /** The validation overrides of this, or null if none */
-        private final String validationOverrides;
-        
-        private final Clock clock = Clock.fixed(LocalDate.parse("2000-01-01", DateTimeFormatter.ISO_DATE).atStartOfDay().atZone(ZoneOffset.UTC).toInstant(), ZoneOffset.UTC);
-
-        MockModelFactory() { this(null); }
-
-        MockModelFactory(String validationOverrides) {
-            this.validationOverrides = validationOverrides;
-        }
-
-        @Override
-        public Version version() {
-            return vespaVersion;
-        }
-        
-        /** Returns the clock used by this, which is fixed at the instant 2000-01-01T00:00:00 */
-        public Clock clock() { return clock; }
-
-        @Override
-        public Model createModel(ModelContext modelContext) {
-            if (throwErrorOnLoad)
-                throw new Error("Foo");
-            if (throwOnLoad)
-                throw new IllegalArgumentException("Foo");
-            this.modelContext = modelContext;
-            return loadModel();
-        }
-
-        Model loadModel() {
-            try {
-                ApplicationPackage application = new MockApplicationPackage.Builder().withEmptyHosts().withEmptyServices().withValidationOverrides(validationOverrides).build();
-                DeployState deployState = new DeployState.Builder().applicationPackage(application).now(clock.instant()).build();
-                return new VespaModel(deployState);
-            } catch (Exception e) {
-                throw new RuntimeException(e);
-            }
-        }
-
-        @Override
-        public ModelCreateResult createAndValidateModel(ModelContext modelContext, ValidationParameters validationParameters) {
-            if (throwErrorOnLoad)
-                throw new Error("Foo");
-            if (throwOnLoad)
-                throw new IllegalArgumentException("Foo");
-            this.modelContext = modelContext;
-            return new ModelCreateResult(loadModel(), new ArrayList<>());
-        }
-    }
-
-}
diff --git a/configserver/src/test/java/com/yahoo/vespa/config/server/session/SessionRepositoryTest.java b/configserver/src/test/java/com/yahoo/vespa/config/server/session/SessionRepositoryTest.java
index b89b63aed46..d8adbd398d1 100644
--- a/configserver/src/test/java/com/yahoo/vespa/config/server/session/SessionRepositoryTest.java
+++ b/configserver/src/test/java/com/yahoo/vespa/config/server/session/SessionRepositoryTest.java
@@ -2,14 +2,26 @@
 package com.yahoo.vespa.config.server.session;
 
 import com.yahoo.cloud.config.ConfigserverConfig;
+import com.yahoo.component.Version;
+import com.yahoo.config.application.api.ApplicationPackage;
+import com.yahoo.config.model.api.Model;
+import com.yahoo.config.model.api.ModelContext;
+import com.yahoo.config.model.api.ModelCreateResult;
+import com.yahoo.config.model.api.ModelFactory;
+import com.yahoo.config.model.api.ValidationParameters;
+import com.yahoo.config.model.deploy.DeployState;
+import com.yahoo.config.model.test.MockApplicationPackage;
 import com.yahoo.config.provision.ApplicationId;
 import com.yahoo.config.provision.TenantName;
 import com.yahoo.text.Utf8;
 import com.yahoo.vespa.config.server.ApplicationRepository;
 import com.yahoo.vespa.config.server.GlobalComponentRegistry;
 import com.yahoo.vespa.config.server.TestComponentRegistry;
+import com.yahoo.vespa.config.server.application.ApplicationSet;
 import com.yahoo.vespa.config.server.application.OrchestratorMock;
+import com.yahoo.vespa.config.server.http.InvalidApplicationException;
 import com.yahoo.vespa.config.server.http.SessionHandlerTest;
+import com.yahoo.vespa.config.server.modelfactory.ModelFactoryRegistry;
 import com.yahoo.vespa.config.server.tenant.TenantRepository;
 import com.yahoo.vespa.config.server.zookeeper.ConfigCurator;
 import com.yahoo.vespa.config.util.ConfigUtils;
@@ -17,16 +29,25 @@ import com.yahoo.vespa.curator.Curator;
 import com.yahoo.vespa.curator.mock.MockCurator;
 import com.yahoo.vespa.flags.FlagSource;
 import com.yahoo.vespa.flags.InMemoryFlagSource;
+import com.yahoo.vespa.model.VespaModel;
 import org.junit.Rule;
 import org.junit.Test;
 import org.junit.rules.TemporaryFolder;
 
 import java.io.File;
+import java.time.Clock;
 import java.time.Duration;
 import java.time.Instant;
+import java.time.LocalDate;
+import java.time.ZoneOffset;
+import java.time.format.DateTimeFormatter;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Optional;
 import java.util.function.LongPredicate;
 
 import static org.hamcrest.CoreMatchers.is;
+import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.assertNull;
 import static org.junit.Assert.assertThat;
@@ -53,9 +74,13 @@ public class SessionRepositoryTest {
     }
 
     private void setup(FlagSource flagSource) throws Exception {
+        setup(flagSource, new TestComponentRegistry.Builder());
+    }
+
+    private void setup(FlagSource flagSource, TestComponentRegistry.Builder componentRegistryBuilder) throws Exception {
         curator = new MockCurator();
         File configserverDbDir = temporaryFolder.newFolder().getAbsoluteFile();
-        GlobalComponentRegistry globalComponentRegistry = new TestComponentRegistry.Builder()
+        GlobalComponentRegistry globalComponentRegistry = componentRegistryBuilder
                 .curator(curator)
                 .configServerConfig(new ConfigserverConfig.Builder()
                                             .configServerDBDir(configserverDbDir.getAbsolutePath())
@@ -75,6 +100,7 @@ public class SessionRepositoryTest {
         sessionRepository = tenantRepository.getTenant(tenantName).getSessionRepository();
     }
 
+
     @Test
     public void require_that_local_sessions_are_created_and_deleted() throws Exception {
         setup();
@@ -84,6 +110,12 @@ public class SessionRepositoryTest {
         assertNotNull(sessionRepository.getLocalSession(secondSessionId));
         assertNull(sessionRepository.getLocalSession(secondSessionId + 1));
 
+        ApplicationSet applicationSet = sessionRepository.ensureApplicationLoaded(sessionRepository.getRemoteSession(firstSessionId));
+        assertNotNull(applicationSet);
+        assertEquals(2, applicationSet.getApplicationGeneration());
+        assertEquals(applicationId.application(), applicationSet.getForVersionOrLatest(Optional.empty(), Instant.now()).getId().application());
+        assertNotNull(applicationSet.getForVersionOrLatest(Optional.empty(), Instant.now()).getModel());
+
         sessionRepository.close();
         // All created sessions are deleted
         assertNull(sessionRepository.getLocalSession(firstSessionId));
@@ -150,6 +182,81 @@ public class SessionRepositoryTest {
         assertThat(sessionRepository.getRemoteSessions().size(), is(1));
     }
 
+    @Test(expected = InvalidApplicationException.class)
+    public void require_that_new_invalid_application_throws_exception() throws Exception {
+        MockModelFactory failingFactory = new MockModelFactory();
+        failingFactory.vespaVersion = new Version(1, 2, 0);
+        failingFactory.throwOnLoad = true;
+
+        MockModelFactory okFactory = new MockModelFactory();
+        okFactory.vespaVersion = new Version(1, 1, 0);
+        okFactory.throwOnLoad = false;
+
+        TestComponentRegistry.Builder registryBuilder = new TestComponentRegistry.Builder()
+                .modelFactoryRegistry(new ModelFactoryRegistry(List.of(okFactory, failingFactory)));
+        setup(new InMemoryFlagSource(), registryBuilder);
+
+        deploy();
+    }
+
+    @Test
+    public void require_that_old_invalid_application_does_not_throw_exception_if_skipped_also_across_major_versions() throws Exception {
+        MockModelFactory failingFactory = new MockModelFactory();
+        failingFactory.vespaVersion = new Version(1, 0, 0);
+        failingFactory.throwOnLoad = true;
+
+        MockModelFactory okFactory =
+                new MockModelFactory("<validation-overrides><allow until='2000-01-30'>skip-old-config-models</allow></validation-overrides>");
+        okFactory.vespaVersion = new Version(2, 0, 0);
+        okFactory.throwOnLoad = false;
+
+        TestComponentRegistry.Builder registryBuilder = new TestComponentRegistry.Builder()
+                .modelFactoryRegistry(new ModelFactoryRegistry(List.of(okFactory, failingFactory)));
+        setup(new InMemoryFlagSource(), registryBuilder);
+
+        deploy();
+    }
+
+    @Test
+    public void require_that_an_application_package_can_limit_to_one_major_version() throws Exception {
+        MockModelFactory failingFactory = new MockModelFactory();
+        failingFactory.vespaVersion = new Version(3, 0, 0);
+        failingFactory.throwErrorOnLoad = true;
+
+        MockModelFactory okFactory = new MockModelFactory();
+        okFactory.vespaVersion = new Version(2, 0, 0);
+        okFactory.throwErrorOnLoad = false;
+
+        TestComponentRegistry.Builder registryBuilder = new TestComponentRegistry.Builder()
+                .modelFactoryRegistry(new ModelFactoryRegistry(List.of(okFactory, failingFactory)));
+        setup(new InMemoryFlagSource(), registryBuilder);
+
+        File testApp = new File("src/test/apps/app-major-version-2");
+        deploy(applicationId, testApp);
+
+        // Does not cause an error because model version 3 is skipped
+    }
+
+    @Test
+    public void require_that_an_application_package_can_limit_to_one_higher_major_version() throws Exception {
+       MockModelFactory failingFactory = new MockModelFactory();
+        failingFactory.vespaVersion = new Version(3, 0, 0);
+        failingFactory.throwErrorOnLoad = true;
+
+        MockModelFactory okFactory = new MockModelFactory();
+        okFactory.vespaVersion = new Version(1, 0, 0);
+        okFactory.throwErrorOnLoad = false;
+
+        TestComponentRegistry.Builder registryBuilder = new TestComponentRegistry.Builder()
+                .modelFactoryRegistry(new ModelFactoryRegistry(List.of(okFactory, failingFactory)));
+        setup(new InMemoryFlagSource(), registryBuilder);
+
+        File testApp = new File("src/test/apps/app-major-version-2");
+        deploy(applicationId, testApp);
+
+        // Does not cause an error because model version 3 is skipped
+    }
+
     private void createSession(long sessionId, boolean wait) {
         SessionZooKeeperClient zkc = new SessionZooKeeperClient(curator,
                                                                 ConfigCurator.create(curator),
@@ -204,8 +311,74 @@ public class SessionRepositoryTest {
     }
 
     private long deploy(ApplicationId applicationId) {
+        return deploy(applicationId, testApp);
+    }
+
+    private long deploy(ApplicationId applicationId, File testApp) {
         applicationRepository.deploy(testApp, new PrepareParams.Builder().applicationId(applicationId).build());
         return applicationRepository.getActiveSession(applicationId).getSessionId();
     }
 
+    private static class MockModelFactory implements ModelFactory {
+
+        /** Throw a RuntimeException on load - this is handled gracefully during model building */
+        boolean throwOnLoad = false;
+
+        /** Throw an Error on load - this is useful to propagate this condition all the way to the test */
+        boolean throwErrorOnLoad = false;
+
+        ModelContext modelContext;
+        public Version vespaVersion = new Version(1, 2, 3);
+
+        /** The validation overrides of this, or null if none */
+        private final String validationOverrides;
+
+        private final Clock clock = Clock.fixed(LocalDate.parse("2000-01-01", DateTimeFormatter.ISO_DATE).atStartOfDay().atZone(ZoneOffset.UTC).toInstant(), ZoneOffset.UTC);
+
+        MockModelFactory() { this(null); }
+
+        MockModelFactory(String validationOverrides) {
+            this.validationOverrides = validationOverrides;
+        }
+
+        @Override
+        public Version version() {
+            return vespaVersion;
+        }
+
+        /** Returns the clock used by this, which is fixed at the instant 2000-01-01T00:00:00 */
+        public Clock clock() { return clock; }
+
+        @Override
+        public Model createModel(ModelContext modelContext) {
+            if (throwErrorOnLoad)
+                throw new Error("error on load");
+            if (throwOnLoad)
+                throw new IllegalArgumentException("exception on load");
+            this.modelContext = modelContext;
+            return loadModel();
+        }
+
+        Model loadModel() {
+            try {
+                ApplicationPackage application = new MockApplicationPackage.Builder().withEmptyHosts().withEmptyServices().withValidationOverrides(validationOverrides).build();
+                DeployState deployState = new DeployState.Builder().applicationPackage(application).now(clock.instant()).build();
+                return new VespaModel(deployState);
+            } catch (Exception e) {
+                throw new RuntimeException(e);
+            }
+        }
+
+        @Override
+        public ModelCreateResult createAndValidateModel(ModelContext modelContext, ValidationParameters validationParameters) {
+            if (throwErrorOnLoad)
+                throw new Error("error on load");
+            if (throwOnLoad)
+                throw new IllegalArgumentException("exception on load");
+            this.modelContext = modelContext;
+            return new ModelCreateResult(loadModel(), new ArrayList<>());
+        }
+    }
+
+
 }