From 2520774b35d528e06722fbd1232c4d7f91672d5f Mon Sep 17 00:00:00 2001
From: Tor Brede Vekterli <vekterli@verizonmedia.com>
Date: Tue, 2 Jul 2019 15:36:31 +0200
Subject: Defer MBus DocumentAccess creation until time of first use

Avoids static init at class loading time, which may throw if config subscription
fails and render the container partially inoperable. Downside is that if no
warmup is used, initial queries to a newly started container may have increased
latencies if config subscription is slow.
---
 .../yahoo/vespa/streamingvisitors/VdsVisitor.java  | 37 ++++++++++++++++++++--
 1 file changed, 34 insertions(+), 3 deletions(-)

diff --git a/container-search/src/main/java/com/yahoo/vespa/streamingvisitors/VdsVisitor.java b/container-search/src/main/java/com/yahoo/vespa/streamingvisitors/VdsVisitor.java
index 5288b28cad1..32b48f0f8ae 100644
--- a/container-search/src/main/java/com/yahoo/vespa/streamingvisitors/VdsVisitor.java
+++ b/container-search/src/main/java/com/yahoo/vespa/streamingvisitors/VdsVisitor.java
@@ -41,6 +41,7 @@ import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.atomic.AtomicReference;
 import java.util.logging.Logger;
 
 /**
@@ -90,8 +91,16 @@ class VdsVisitor extends VisitorDataHandler implements Visitor {
     }
 
     private static class MessageBusVisitorSessionFactory implements VisitorSessionFactory {
-        private static final LoadTypeSet loadTypes = new LoadTypeSet("client");
-        private static final DocumentAccess access = new MessageBusDocumentAccess(new MessageBusParams(loadTypes));
+        private static final Object initMonitor = new Object();
+        private static final AtomicReference<MessageBusVisitorSessionFactory> instance = new AtomicReference<>();
+
+        private final LoadTypeSet loadTypes;
+        private final DocumentAccess access;
+
+        private MessageBusVisitorSessionFactory() {
+            loadTypes = new LoadTypeSet("client");
+            access = new MessageBusDocumentAccess(new MessageBusParams(loadTypes));
+        }
 
         @Override
         public VisitorSession createVisitorSession(VisitorParameters params) throws ParseException {
@@ -102,10 +111,32 @@ class VdsVisitor extends VisitorDataHandler implements Visitor {
         public LoadTypeSet getLoadTypeSet() {
             return loadTypes;
         }
+
+        /**
+         * Returns a single, shared instance of this class which is lazily created in a thread-safe
+         * manner the first time this method is invoked.
+         *
+         * May throw any config-related exception if subscription fails.
+         */
+        static MessageBusVisitorSessionFactory sharedInstance() {
+            var ref = instance.getAcquire();
+            if (ref != null) {
+                return ref;
+            }
+            synchronized (initMonitor) {
+                ref = instance.getAcquire();
+                if (ref != null) {
+                    return ref;
+                }
+                ref = new MessageBusVisitorSessionFactory();
+                instance.setRelease(ref);
+            }
+            return ref;
+        }
     }
 
     public VdsVisitor(Query query, String searchCluster, Route route, String documentType) {
-        this(query, searchCluster, route, documentType, new MessageBusVisitorSessionFactory());
+        this(query, searchCluster, route, documentType, MessageBusVisitorSessionFactory.sharedInstance());
     }
 
     public VdsVisitor(Query query, String searchCluster, Route route,
-- 
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