cache: respond to watches in parallel v2 (envoyproxy#85)

Add ExecutorGroup to allow to respond watches in parallel

Signed-off-by: Sebastian Schepens <sebastian.schepens@mercadolibre.com>

server/src/main/java/io/envoyproxy/controlplane/server/DefaultExecutorGroup.java

@@ -0,0 +1,20 @@
+package io.envoyproxy.controlplane.server;
+
+import com.google.common.util.concurrent.MoreExecutors;
+
+import java.util.concurrent.Executor;
+
+/**
+ * Default implementation of {@link ExecutorGroup} which
+ * always returns {@link MoreExecutors#directExecutor}.
+ */
+public class DefaultExecutorGroup implements ExecutorGroup {
+  /**
+   * Returns the next {@link Executor} to use, which in this case is
+   * always {@link MoreExecutors#directExecutor}.
+   */
+  @Override
+  public Executor next() {
+    return MoreExecutors.directExecutor();
+  }
+}

server/src/main/java/io/envoyproxy/controlplane/server/DiscoveryServer.java

@@ -25,6 +25,7 @@
 import java.util.Map;
 import java.util.Set;
 import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.Executor;
 import java.util.concurrent.atomic.AtomicLong;
 import java.util.stream.Collectors;
 import org.slf4j.Logger;
@@ -38,6 +39,7 @@ public class DiscoveryServer {
 
   private final List<DiscoveryServerCallbacks> callbacks;
   private final ConfigWatcher configWatcher;
+  private final ExecutorGroup executorGroup;
   private final AtomicLong streamCount = new AtomicLong();
 
   public DiscoveryServer(ConfigWatcher configWatcher) {
@@ -54,11 +56,25 @@ public DiscoveryServer(DiscoveryServerCallbacks callbacks, ConfigWatcher configW
    * @param configWatcher source of configuration updates
    */
   public DiscoveryServer(List<DiscoveryServerCallbacks> callbacks, ConfigWatcher configWatcher) {
+    this(callbacks, configWatcher, new DefaultExecutorGroup());
+  }
+
+  /**
+   * Creates the server.
+   * @param callbacks server callbacks
+   * @param configWatcher source of configuration updates
+   * @param executorGroup executor group to use for responding stream requests
+   */
+  public DiscoveryServer(List<DiscoveryServerCallbacks> callbacks,
+                         ConfigWatcher configWatcher,
+                         ExecutorGroup executorGroup) {
     Preconditions.checkNotNull(callbacks, "callbacks cannot be null");
     Preconditions.checkNotNull(configWatcher, "configWatcher cannot be null");
+    Preconditions.checkNotNull(executorGroup, "executorGroup cannot be null");
 
     this.callbacks = callbacks;
     this.configWatcher = configWatcher;
+    this.executorGroup = executorGroup;
   }
 
   /**
@@ -149,13 +165,14 @@ private StreamObserver<DiscoveryRequest> createRequestHandler(
       String defaultTypeUrl) {
 
     long streamId = streamCount.getAndIncrement();
+    Executor executor = executorGroup.next();
 
     LOGGER.info("[{}] open stream from {}", streamId, defaultTypeUrl);
 
     callbacks.forEach(cb -> cb.onStreamOpen(streamId, defaultTypeUrl));
 
     final DiscoveryRequestStreamObserver requestStreamObserver =
-        new DiscoveryRequestStreamObserver(defaultTypeUrl, responseObserver, streamId, ads);
+        new DiscoveryRequestStreamObserver(defaultTypeUrl, responseObserver, streamId, ads, executor);
 
     if (responseObserver instanceof ServerCallStreamObserver) {
       ((ServerCallStreamObserver) responseObserver).setOnCancelHandler(requestStreamObserver::onCancelled);
@@ -173,11 +190,15 @@ private class DiscoveryRequestStreamObserver implements StreamObserver<Discovery
     private final StreamObserver<DiscoveryResponse> responseObserver;
     private final long streamId;
     private final boolean ads;
+    private final Executor executor;
 
     private AtomicLong streamNonce;
 
-    public DiscoveryRequestStreamObserver(String defaultTypeUrl, StreamObserver<DiscoveryResponse> responseObserver,
-                                          long streamId, boolean ads) {
+    public DiscoveryRequestStreamObserver(String defaultTypeUrl,
+                                          StreamObserver<DiscoveryResponse> responseObserver,
+                                          long streamId,
+                                          boolean ads,
+                                          Executor executor) {
       this.defaultTypeUrl = defaultTypeUrl;
       this.responseObserver = responseObserver;
       this.streamId = streamId;
@@ -186,6 +207,7 @@ public DiscoveryRequestStreamObserver(String defaultTypeUrl, StreamObserver<Disc
       latestResponse = new ConcurrentHashMap<>(Resources.TYPE_URLS.size());
       ackedResources = new ConcurrentHashMap<>(Resources.TYPE_URLS.size());
       streamNonce = new AtomicLong();
+      this.executor = executor;
     }
 
     @Override
@@ -238,7 +260,7 @@ public void onNext(DiscoveryRequest request) {
                 ads,
                 request,
                 ackedResources.getOrDefault(typeUrl, Collections.emptySet()),
-                r -> send(r, typeUrl));
+                r -> executor.execute(() -> send(r, typeUrl)));
           });
 
           return;

server/src/main/java/io/envoyproxy/controlplane/server/ExecutorGroup.java

@@ -0,0 +1,14 @@
+package io.envoyproxy.controlplane.server;
+
+import java.util.concurrent.Executor;
+
+/**
+ * The {@link ExecutorGroup} is responsible for providing the {@link Executor}'s to use
+ * via its {@link #next()} method.
+ */
+public interface ExecutorGroup {
+  /**
+   * Returns the next {@link Executor} to use.
+   */
+  Executor next();
+}