KAFKA-14247: Consumer background thread base implementation (apache#1…

…2672) (#76)

Adds skeleton of the background thread.

1-pager: https://cwiki.apache.org/confluence/display/KAFKA/Proposal%3A+Consumer+Threading+Model+Refactor
Continuation of apache#12663

Reviewers: Guozhang Wang <guozhang@apache.org>, Kirk True <kirk@mustardgrain.com>, John Roesler <vvcephei@apache.org>

Co-authored-by: Philip Nee <pnee@confluent.io>

clients/src/main/java/org/apache/kafka/clients/consumer/internals/DefaultBackgroundThread.java

@@ -0,0 +1,230 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.kafka.clients.consumer.internals;
+
+import org.apache.kafka.clients.CommonClientConfigs;
+import org.apache.kafka.clients.consumer.ConsumerConfig;
+import org.apache.kafka.clients.consumer.internals.events.ApplicationEvent;
+import org.apache.kafka.clients.consumer.internals.events.BackgroundEvent;
+import org.apache.kafka.clients.consumer.internals.events.NoopApplicationEvent;
+import org.apache.kafka.common.KafkaException;
+import org.apache.kafka.common.errors.WakeupException;
+import org.apache.kafka.common.metrics.Metrics;
+import org.apache.kafka.common.utils.KafkaThread;
+import org.apache.kafka.common.utils.LogContext;
+import org.apache.kafka.common.utils.Time;
+import org.apache.kafka.common.utils.Utils;
+import org.slf4j.Logger;
+
+import java.util.Objects;
+import java.util.Optional;
+import java.util.concurrent.BlockingQueue;
+import java.util.concurrent.atomic.AtomicReference;
+
+/**
+ * Background thread runnable that consumes {@code ApplicationEvent} and
+ * produces {@code BackgroundEvent}. It uses an event loop to consume and
+ * produce events, and poll the network client to handle network IO.
+ * <p>
+ * It holds a reference to the {@link SubscriptionState}, which is
+ * initialized by the polling thread.
+ */
+public class DefaultBackgroundThread extends KafkaThread {
+    private static final String BACKGROUND_THREAD_NAME =
+        "consumer_background_thread";
+    private final Time time;
+    private final Logger log;
+    private final BlockingQueue<ApplicationEvent> applicationEventQueue;
+    private final BlockingQueue<BackgroundEvent> backgroundEventQueue;
+    private final ConsumerNetworkClient networkClient;
+    private final SubscriptionState subscriptions;
+    private final ConsumerMetadata metadata;
+    private final Metrics metrics;
+    private final ConsumerConfig config;
+
+    private String clientId;
+    private long retryBackoffMs;
+    private int heartbeatIntervalMs;
+    private boolean running;
+    private Optional<ApplicationEvent> inflightEvent = Optional.empty();
+    private final AtomicReference<Optional<RuntimeException>> exception =
+        new AtomicReference<>(Optional.empty());
+
+    public DefaultBackgroundThread(final ConsumerConfig config,
+                                   final LogContext logContext,
+                                   final BlockingQueue<ApplicationEvent> applicationEventQueue,
+                                   final BlockingQueue<BackgroundEvent> backgroundEventQueue,
+                                   final SubscriptionState subscriptions,
+                                   final ConsumerMetadata metadata,
+                                   final ConsumerNetworkClient networkClient,
+                                   final Metrics metrics) {
+        this(
+            Time.SYSTEM,
+            config,
+            logContext,
+            applicationEventQueue,
+            backgroundEventQueue,
+            subscriptions,
+            metadata,
+            networkClient,
+            metrics
+        );
+    }
+
+    public DefaultBackgroundThread(final Time time,
+                                   final ConsumerConfig config,
+                                   final LogContext logContext,
+                                   final BlockingQueue<ApplicationEvent> applicationEventQueue,
+                                   final BlockingQueue<BackgroundEvent> backgroundEventQueue,
+                                   final SubscriptionState subscriptions,
+                                   final ConsumerMetadata metadata,
+                                   final ConsumerNetworkClient networkClient,
+                                   final Metrics metrics) {
+        super(BACKGROUND_THREAD_NAME, true);
+        try {
+            this.time = time;
+            this.log = logContext.logger(DefaultBackgroundThread.class);
+            this.applicationEventQueue = applicationEventQueue;
+            this.backgroundEventQueue = backgroundEventQueue;
+            this.config = config;
+            setConfig();
+            this.inflightEvent = Optional.empty();
+            // subscriptionState is initialized by the polling thread
+            this.subscriptions = subscriptions;
+            this.metadata = metadata;
+            this.networkClient = networkClient;
+            this.metrics = metrics;
+            this.running = true;
+        } catch (final Exception e) {
+            // now propagate the exception
+            close();
+            throw new KafkaException("Failed to construct background processor", e);
+        }
+    }
+
+    private void setConfig() {
+        this.retryBackoffMs = this.config.getLong(ConsumerConfig.RETRY_BACKOFF_MS_CONFIG);
+        this.clientId = config.getString(CommonClientConfigs.CLIENT_ID_CONFIG);
+        this.heartbeatIntervalMs = config.getInt(ConsumerConfig.HEARTBEAT_INTERVAL_MS_CONFIG);
+    }
+
+    @Override
+    public void run() {
+        try {
+            log.debug("Background thread started");
+            while (running) {
+                try {
+                    runOnce();
+                } catch (final WakeupException e) {
+                    log.debug(
+                        "Exception thrown, background thread won't terminate",
+                        e
+                    );
+                    // swallow the wakeup exception to prevent killing the
+                    // background thread.
+                }
+            }
+        } catch (final Throwable t) {
+            log.error(
+                "The background thread failed due to unexpected error",
+                t
+            );
+            if (t instanceof RuntimeException)
+                this.exception.set(Optional.of((RuntimeException) t));
+            else
+                this.exception.set(Optional.of(new RuntimeException(t)));
+        } finally {
+            close();
+            log.debug("{} closed", getClass());
+        }
+    }
+
+    /**
+     * Process event from a single poll
+     */
+    void runOnce() {
+        this.inflightEvent = maybePollEvent();
+        if (this.inflightEvent.isPresent()) {
+            log.debug("processing application event: {}", this.inflightEvent);
+        }
+        if (this.inflightEvent.isPresent() && maybeConsumeInflightEvent(this.inflightEvent.get())) {
+            // clear inflight event upon successful consumption
+            this.inflightEvent = Optional.empty();
+        }
+
+        // if there are pending events to process, poll then continue without
+        // blocking.
+        if (!applicationEventQueue.isEmpty() || inflightEvent.isPresent()) {
+            networkClient.poll(time.timer(0));
+            return;
+        }
+        // if there are no events to process, poll until timeout. The timeout
+        // will be the minimum of the requestTimeoutMs, nextHeartBeatMs, and
+        // nextMetadataUpdate. See NetworkClient.poll impl.
+        networkClient.poll(time.timer(timeToNextHeartbeatMs(time.milliseconds())));
+    }
+
+    private long timeToNextHeartbeatMs(final long nowMs) {
+        // TODO: implemented when heartbeat is added to the impl
+        return 100;
+    }
+
+    private Optional<ApplicationEvent> maybePollEvent() {
+        if (this.inflightEvent.isPresent() || this.applicationEventQueue.isEmpty()) {
+            return this.inflightEvent;
+        }
+        return Optional.ofNullable(this.applicationEventQueue.poll());
+    }
+
+    /**
+     * ApplicationEvent are consumed here.
+     *
+     * @param event an {@link ApplicationEvent}
+     * @return true when successfully consumed the event.
+     */
+    private boolean maybeConsumeInflightEvent(final ApplicationEvent event) {
+        log.debug("try consuming event: {}", Optional.ofNullable(event));
+        Objects.requireNonNull(event);
+        return event.process();
+    }
+
+    /**
+     * Processes {@link NoopApplicationEvent} and equeue a
+     * {@link NoopBackgroundEvent}. This is intentionally left here for
+     * demonstration purpose.
+     *
+     * @param event a {@link NoopApplicationEvent}
+     */
+    private void process(final NoopApplicationEvent event) {
+        backgroundEventQueue.add(new NoopBackgroundEvent(event.message));
+    }
+
+    public boolean isRunning() {
+        return this.running;
+    }
+
+    public void wakeup() {
+        networkClient.wakeup();
+    }
+
+    public void close() {
+        this.running = false;
+        this.wakeup();
+        Utils.closeQuietly(networkClient, "consumer network client");
+        Utils.closeQuietly(metadata, "consumer metadata client");
+    }
+}