什么是Metadata？ Topic/Partion与broker的映射关系：每一个Topic的每一个Partion的Leader、Follower的信息。
它存在哪里？持久化在Zookeeper中；运行时存储在Broker的内存中。
1 Metadata的2种更新机制
-
周期性的更新: 每隔一段时间更新一次。，这个通过 Metadata的lastRefreshMs, lastSuccessfulRefreshMs 这2个字段来实现。对应的ProducerConfig配置项为：
- metadata.max.age.ms //缺省300000，即10分钟1次
-
失效检测，强制更新：检查到metadata失效以后，调用
metadata.requestUpdate()强制更新。 requestUpdate()函数里面其实什么都没做，就是把needUpdate置成了false
每次Sender.poll的时候，都检查这2种更新机制，达到了，就触发更新。
那如何判定Metadata失效了呢？这个在代码中很分散，有很多地方，会判定Metadata失效。
2. Metadata失效检测
条件1：initConnect的时候 – NetworkClient.java
private void initiateConnect(Node node, long now) {
String nodeConnectionId = node.idString();
try {
log.debug("Initiating connection to node {} at {}:{}.", node.id(), node.host(), node.port());
this.connectionStates.connecting(nodeConnectionId, now);
selector.connect(nodeConnectionId,
new InetSocketAddress(node.host(), node.port()),
this.socketSendBuffer,
this.socketReceiveBuffer);
} catch (IOException e) {
/* attempt failed, we'll try again after the backoff */
connectionStates.disconnected(nodeConnectionId, now);
/* maybe the problem is our metadata, update it */
metadataUpdater.requestUpdate(); //判定metadata失效
log.debug("Error connecting to node {} at {}:{}:", node.id(), node.host(), node.port(), e);
}
}
条件2：poll里面IO的时候，连接断掉了 – NetworkClient.java
private void handleDisconnections(List<ClientResponse> responses, long now) {
for (String node : this.selector.disconnected()) {
log.debug("Node {} disconnected.", node);
processDisconnection(responses, node, now);
}
if (this.selector.disconnected().size() > 0)
metadataUpdater.requestUpdate(); //判定metadata失效
}
条件3：有请求超时 – NetworkClient.java
private void handleTimedOutRequests(List<ClientResponse> responses, long now) {
List<String> nodeIds = this.inFlightRequests.getNodesWithTimedOutRequests(now, this.requestTimeoutMs);
for (String nodeId : nodeIds) {
this.selector.close(nodeId);
log.debug("Disconnecting from node {} due to request timeout.", nodeId);
processDisconnection(responses, nodeId, now);
} if (nodeIds.size() > 0)
metadataUpdater.requestUpdate(); //判定metadata失效
}
条件4：发消息的时候，有partition的leader没找到 – Sender.java
public void run(long now) {
Cluster cluster = metadata.fetch();
RecordAccumulator.ReadyCheckResult result = this.accumulator.ready(cluster, now); if (result.unknownLeadersExist)
this.metadata.requestUpdate();
条件5：返回的response和请求对不上的时候
private void handleProduceResponse(ClientResponse response, Map<TopicPartition, RecordBatch> batches, long now) {
int correlationId = response.request().request().header().correlationId();
if (response.wasDisconnected()) {
log.trace("Cancelled request {} due to node {} being disconnected", response, response.request()
.request()
.destination());
for (RecordBatch batch : batches.values())
completeBatch(batch, Errors.NETWORK_EXCEPTION, -1L, correlationId, now);
总之：发生各式各样的异常，数据不同步，都认为metadata可能出问题了，要求更新。
3.Metadata更新特点
Metadata的更新，还有以下几个特点：
- 更新请求MetadataRequest是nio异步发送的，在
Sender.poll的返回中，处理MetadataResponse的时候，才真正更新Metadata。
这里有个关键点：Metadata的cluster对象，每次是整个覆盖的，而不是局部更新。所以cluster内部不用加锁。
- 更新的时候，是从metadata保存的所有Node，或者说Broker中，选负载最小的那个，也就是当前接收请求最少的那个。向其发送MetadataRequest请求，获取新的Cluster对象。
4 Sender poll()更新Metadata
从下面可以看出，Metadata的更新，是在while循环，每次调用client.poll()的时候更新的。
Sender 是KafkaProducer的一个线程类。
public void run() {
// main loop, runs until close is called
while (running) {
try {
run(time.milliseconds());
} catch (Exception e) {
log.error("Uncaught error in kafka producer I/O thread: ", e);
}
}
。。。
} public void run(long now) {
Cluster cluster = metadata.fetch();
。。。
RecordAccumulator.ReadyCheckResult result = this.accumulator.ready(cluster, now); //遍历消息队列中所有的消息，找出对应的，已经ready的Node if (result.unknownLeadersExist) //如果一个ready的node都没有，请求更新metadata
this.metadata.requestUpdate(); 。。。 //client的2个关键函数，一个发送ClientRequest，一个接收ClientResponse。底层调用的是NIO的poll。关于nio, 后面会详细介绍
for (ClientRequest request : requests)
client.send(request, now); this.client.poll(pollTimeout, now);
}//NetworkClient
public List<ClientResponse> poll(long timeout, long now) {
long metadataTimeout = metadataUpdater.maybeUpdate(now); //关键点：每次poll的时候判断是否要更新metadata try {
this.selector.poll(Utils.min(timeout, metadataTimeout, requestTimeoutMs));
} catch (IOException e) {
log.error("Unexpected error during I/O", e);
} // process completed actions
long updatedNow = this.time.milliseconds();
List<ClientResponse> responses = new ArrayList<>();
handleCompletedSends(responses, updatedNow);
handleCompletedReceives(responses, updatedNow); //在返回的handler中，会处理metadata的更新
handleDisconnections(responses, updatedNow);
handleConnections();
handleTimedOutRequests(responses, updatedNow); // invoke callbacks
for (ClientResponse response : responses) {
if (response.request().hasCallback()) {
try {
response.request().callback().onComplete(response);
} catch (Exception e) {
log.error("Uncaught error in request completion:", e);
}
}
} return responses;
} //DefaultMetadataUpdater
@Override
public long maybeUpdate(long now) {
// should we update our metadata?
long timeToNextMetadataUpdate = metadata.timeToNextUpdate(now);
long timeToNextReconnectAttempt = Math.max(this.lastNoNodeAvailableMs + metadata.refreshBackoff() - now, 0);
long waitForMetadataFetch = this.metadataFetchInProgress ? Integer.MAX_VALUE : 0;
// if there is no node available to connect, back off refreshing metadata
long metadataTimeout = Math.max(Math.max(timeToNextMetadataUpdate, timeToNextReconnectAttempt),
waitForMetadataFetch); if (metadataTimeout == 0) {
// highly dependent on the behavior of leastLoadedNode.
Node node = leastLoadedNode(now); //找到负载最小的Node
maybeUpdate(now, node); //把更新Metadata的请求，发给这个Node
} return metadataTimeout;
} private void maybeUpdate(long now, Node node) {
if (node == null) {
log.debug("Give up sending metadata request since no node is available");
// mark the timestamp for no node available to connect
this.lastNoNodeAvailableMs = now;
return;
}
String nodeConnectionId = node.idString(); if (canSendRequest(nodeConnectionId)) {
Set<String> topics = metadata.needMetadataForAllTopics() ? new HashSet<String>() : metadata.topics();
this.metadataFetchInProgress = true;
ClientRequest metadataRequest = request(now, nodeConnectionId, topics); //关键点：发送更新Metadata的Request
log.debug("Sending metadata request {} to node {}", metadataRequest, node.id());
doSend(metadataRequest, now); //这里只是异步发送，返回的response在上面的handleCompletedReceives里面处理
} else if (connectionStates.canConnect(nodeConnectionId, now)) {
log.debug("Initialize connection to node {} for sending metadata request", node.id());
initiateConnect(node, now); } else { // connected, but can't send more OR connecting
this.lastNoNodeAvailableMs = now;
}
} private void handleCompletedReceives(List<ClientResponse> responses, long now) {
for (NetworkReceive receive : this.selector.completedReceives()) {
String source = receive.source();
ClientRequest req = inFlightRequests.completeNext(source);
ResponseHeader header = ResponseHeader.parse(receive.payload());
// Always expect the response version id to be the same as the request version id
short apiKey = req.request().header().apiKey();
short apiVer = req.request().header().apiVersion();
Struct body = (Struct) ProtoUtils.responseSchema(apiKey, apiVer).read(receive.payload());
correlate(req.request().header(), header);
if (!metadataUpdater.maybeHandleCompletedReceive(req, now, body))
responses.add(new ClientResponse(req, now, false, body));
}
} @Override
public boolean maybeHandleCompletedReceive(ClientRequest req, long now, Struct body) {
short apiKey = req.request().header().apiKey();
if (apiKey == ApiKeys.METADATA.id && req.isInitiatedByNetworkClient()) {
handleResponse(req.request().header(), body, now);
return true;
}
return false;
}//关键函数
private void handleResponse(RequestHeader header, Struct body, long now) {
this.metadataFetchInProgress = false;
MetadataResponse response = new MetadataResponse(body);
Cluster cluster = response.cluster(); //从response中，拿到一个新的cluster对象
if (response.errors().size() > 0) {
log.warn("Error while fetching metadata with correlation id {} : {}", header.correlationId(), response.errors());
} if (cluster.nodes().size() > 0) {
this.metadata.update(cluster, now); //更新metadata，用新的cluster覆盖旧的cluster
} else {
log.trace("Ignoring empty metadata response with correlation id {}.", header.correlationId());
this.metadata.failedUpdate(now); //更新metadata失败，做失败处理逻辑
}
}//更新成功，version+1, 同时更新其它字段
public synchronized void update(Cluster cluster, long now) {
this.needUpdate = false;
this.lastRefreshMs = now;
this.lastSuccessfulRefreshMs = now;
this.version += 1; for (Listener listener: listeners)
listener.onMetadataUpdate(cluster); //如果有人监听了metadata的更新，通知他们 this.cluster = this.needMetadataForAllTopics ? getClusterForCurrentTopics(cluster) : cluster; //新的cluster覆盖旧的cluster notifyAll(); //通知所有的阻塞的producer线程 log.debug("Updated cluster metadata version {} to {}", this.version, this.cluster);
}//更新失败，只更新lastRefreshMs
public synchronized void failedUpdate(long now) {
this.lastRefreshMs = now;
}
