Cache flags DB

Up until now every lookup of a flag on ZooKeeperFlagSource would hit ZooKeeper.
Flags are ideal for caching: Changes seldom, little data, clients should handle
short-lived inconsistencies.

This PR will make the backing FlagsDbImpl cache the /flags/v1 ZK directory and
completes the optimization of ConfigServerFlagSource.

4 files changed, 19 insertions, 0 deletions

diff --git a/zkfacade/abi-spec.json b/zkfacade/abi-spec.json
index 5610fb2e9cf..eb8edaa5b97 100644
--- a/zkfacade/abi-spec.json
+++ b/zkfacade/abi-spec.json
@@ -36,6 +36,7 @@
       "public abstract void start()",
       "public abstract void addListener(org.apache.curator.framework.recipes.cache.PathChildrenCacheListener)",
       "public abstract java.util.List getCurrentData()",
+      "public abstract org.apache.curator.framework.recipes.cache.ChildData getCurrentData(com.yahoo.path.Path)",
       "public abstract void close()"
     ],
     "fields": []
diff --git a/zkfacade/src/main/java/com/yahoo/vespa/curator/Curator.java b/zkfacade/src/main/java/com/yahoo/vespa/curator/Curator.java
index 6ba394c340d..91140fcf713 100644
--- a/zkfacade/src/main/java/com/yahoo/vespa/curator/Curator.java
+++ b/zkfacade/src/main/java/com/yahoo/vespa/curator/Curator.java
@@ -358,6 +358,9 @@ public class Curator implements AutoCloseable {
 
         List<ChildData> getCurrentData();
 
+        /** Returns the ChildData, or null if it does not exist. */
+        ChildData getCurrentData(Path absolutePath);
+
         void close();
 
     }
diff --git a/zkfacade/src/main/java/com/yahoo/vespa/curator/PathChildrenCacheWrapper.java b/zkfacade/src/main/java/com/yahoo/vespa/curator/PathChildrenCacheWrapper.java
index 2340fb1b5a8..e069034e1d8 100644
--- a/zkfacade/src/main/java/com/yahoo/vespa/curator/PathChildrenCacheWrapper.java
+++ b/zkfacade/src/main/java/com/yahoo/vespa/curator/PathChildrenCacheWrapper.java
@@ -1,6 +1,7 @@
 // Copyright 2017 Yahoo Holdings. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
 package com.yahoo.vespa.curator;
 
+import com.yahoo.path.Path;
 import org.apache.curator.framework.CuratorFramework;
 import org.apache.curator.framework.recipes.cache.ChildData;
 import org.apache.curator.framework.recipes.cache.PathChildrenCache;
@@ -44,6 +45,11 @@ class PathChildrenCacheWrapper implements Curator.DirectoryCache {
     }
 
     @Override
+    public ChildData getCurrentData(Path absolutePath) {
+        return wrapped.getCurrentData(absolutePath.getAbsolute());
+    }
+
+    @Override
     public void close() {
         try {
             wrapped.close();
diff --git a/zkfacade/src/main/java/com/yahoo/vespa/curator/mock/MockCurator.java b/zkfacade/src/main/java/com/yahoo/vespa/curator/mock/MockCurator.java
index 4013cf1d649..85fa0ded838 100644
--- a/zkfacade/src/main/java/com/yahoo/vespa/curator/mock/MockCurator.java
+++ b/zkfacade/src/main/java/com/yahoo/vespa/curator/mock/MockCurator.java
@@ -572,6 +572,15 @@ public class MockCurator extends Curator {
             return childData;
         }
 
+        @Override
+        public ChildData getCurrentData(Path fullPath) {
+            if (!fullPath.getParentPath().equals(path)) {
+                throw new IllegalArgumentException("Path '" + fullPath + "' is not a child path of '" + path + "'");
+            }
+
+            return getData(fullPath).map(bytes -> new ChildData(fullPath.getAbsolute(), null, bytes)).orElse(null);
+        }
+
         private void collectData(Node parent, Path parentPath, List<ChildData> data) {
             for (Node child : parent.children().values()) {
                 Path childPath = parentPath.append(child.name());