RPC客户端工厂TransportClientFactory
TransportClientFactory是创建TransportClient的工厂类。TransportContext的createClientFactory方法可以创建TransportClientFactory的实例
/**
* Initializes a ClientFactory which runs the given TransportClientBootstraps prior to returning
* a new Client. Bootstraps will be executed synchronously, and must run successfully in order
* to create a Client.
*/
public TransportClientFactory createClientFactory(List bootstraps) {
return new TransportClientFactory(this, bootstraps);
}
public TransportClientFactory createClientFactory() {
return createClientFactory(Lists.newArrayList());
} 可以看到，TransportContext中有两个重载的createClientFactory方法，它们最终在构造TransportClientFactory时都会传递两个参数：TransportContext和TransportClientBootstrap列表。TransportClientFactory构造器的实现如代码所示。
public TransportClientFactory(
TransportContext context,
List clientBootstraps) {
this.context = Preconditions.checkNotNull(context);
this.conf = context.getConf();
this.clientBootstraps = Lists.newArrayList(Preconditions.checkNotNull(clientBootstraps));
this.connectionPool = new ConcurrentHashMap<>();
this.numConnectionsPerPeer = conf.numConnectionsPerPeer();
this.rand = new Random();
IOMode ioMode = IOMode.valueOf(conf.ioMode());
this.socketChannelClass = NettyUtils.getClientChannelClass(ioMode);
// TODO: Make thread pool name configurable.
this.workerGroup = NettyUtils.createEventLoop(ioMode, conf.clientThreads(), "shuffle-client");
this.pooledAllocator = NettyUtils.createPooledByteBufAllocator(
conf.preferDirectBufs(), false /* allowCache */, conf.clientThreads());
} TransportClientFactory构造器中的各个变量如下：
context：参数传递的TransportContext的引用。
conf：指TransportConf，这里通过调用TransportContext的getConf获取。
clientBootstraps：参数传递的TransportClientBootstrap列表。
connectionPool：针对每个Socket地址的连接池ClientPool的缓存。
connectionPool的数据结构较为复杂，为便于读者理解，这里以图来表示connectionPool的数据结构。
numConnectionsPerPeer：从TransportConf获 取 的key为“spark.+模 块 名+.io.num-ConnectionsPerPeer”的属性值。此属性值用于指定对等节点间的连接数。这里的模块名实际为TransportConf的module字段。Spark的很多组件都利用RPC框架构建，它们之间按照模块名区分，例如，RPC模块的key为“spark.rpc.io.numConnectionsPerPeer”。
#TransportConf中的 getConfKey 方法获取参数
private String getConfKey(String suffix) {
return "spark." + module + "." + suffix;
}rand：对Socket地址对应的连接池ClientPool中缓存的
TransportClient进行随机选择，对每个连接做负载均衡。
ioMode:IO模式，即从TransportConf获取key为“spark.+模块名+.io.mode”的属性值。默认值为NIO, Spark还支持EPOLL。
socketChannelClass：客户端Channel被创建时使用的类，通过ioMode来匹配，默认为NioSocketChannel, Spark还支持EpollEventLoopGroup。
workerGroup：根据Netty的规范，客户端只有worker组，所以此处创建worker-Group。workerGroup的实际类型是NioEventLoopGroup。
pooledAllocator ：汇集ByteBuf但对本地线程缓存禁用的分配器。
客户端引导程序TransportClientBootstrap
TransportClientFactory的clientBootstraps属性是TransportClientBootstrap的列表。Transport ClientBootstrap是在TransportClient上执行的客户端引导程序，主要对连接建立时进行一些初始化的准备（例如验证、加密）。TransportClientBootstrap所做的操作往往是昂贵的，好在建立的连接可以重用。TransportClientBootstrap的接口定义如代码清单3-10所示：
import io.netty.channel.Channel;
/**
* A bootstrap which is executed on a TransportClient before it is returned to the user.
* This enables an initial exchange of information (e.g., SASL authentication tokens) on a once-per-
* connection basis.
*
* Since connections (and TransportClients) are reused as much as possible, it is generally
* reasonable to perform an expensive bootstrapping operation, as they often share a lifespan with
* the JVM itself.
*/
public interface TransportClientBootstrap {
/** Performs the bootstrapping operation, throwing an exception on failure. */
void doBootstrap(TransportClient client, Channel channel) throws RuntimeException;
}
TransportClientBootstrap有两个实现类：EncryptionDisablerBootstrap和SaslClientBootstrap。
创建RPC客户端TransportClient
有了TransportClientFactory, Spark的各个模块就可以使用它创建RPC客户端TransportClient了。每个TransportClient实例只能和一个远端的RPC服务通信，所以Spark中的组件如果想要和多个RPC服务通信，就需要持有多个TransportClient实例。创建TransportClient的方法如代码所示（实际为从缓存中获取TransportClient）。
/**
* Create a {@link TransportClient} connecting to the given remote host / port.
*
* We maintains an array of clients (size determined by spark.shuffle.io.numConnectionsPerPeer)
* and randomly picks one to use. If no client was previously created in the randomly selected
* spot, this function creates a new client and places it there.
*
* Prior to the creation of a new TransportClient, we will execute all
* {@link TransportClientBootstrap}s that are registered with this factory.
*
* This blocks until a connection is successfully established and fully bootstrapped.
*
* Concurrency: This method is safe to call from multiple threads.
*/
public TransportClient createClient(String remoteHost, int remotePort) throws IOException {
// Get connection from the connection pool first.
// If it is not found or not active, create a new one.
// Use unresolved address here to avoid DNS resolution each time we creates a client.
final InetSocketAddress unresolvedAddress =
InetSocketAddress.createUnresolved(remoteHost, remotePort);
// Create the ClientPool if we don't have it yet.
ClientPool clientPool = connectionPool.get(unresolvedAddress);
if (clientPool == null) {
connectionPool.putIfAbsent(unresolvedAddress, new ClientPool(numConnectionsPerPeer));
clientPool = connectionPool.get(unresolvedAddress);
}
int clientIndex = rand.nextInt(numConnectionsPerPeer);
TransportClient cachedClient = clientPool.clients[clientIndex];
if (cachedClient != null && cachedClient.isActive()) {
// Make sure that the channel will not timeout by updating the last use time of the
// handler. Then check that the client is still alive, in case it timed out before
// this code was able to update things.
TransportChannelHandler handler = cachedClient.getChannel().pipeline()
.get(TransportChannelHandler.class);
synchronized (handler) {
handler.getResponseHandler().updateTimeOfLastRequest();
}
if (cachedClient.isActive()) {
logger.trace("Returning cached connection to {}: {}",
cachedClient.getSocketAddress(), cachedClient);
return cachedClient;
}
}
// If we reach here, we don't have an existing connection open. Let's create a new one.
// Multiple threads might race here to create new connections. Keep only one of them active.
final long preResolveHost = System.nanoTime();
final InetSocketAddress resolvedAddress = new InetSocketAddress(remoteHost, remotePort);
final long hostResolveTimeMs = (System.nanoTime() - preResolveHost) / 1000000;
if (hostResolveTimeMs > 2000) {
logger.warn("DNS resolution for {} took {} ms", resolvedAddress, hostResolveTimeMs);
} else {
logger.trace("DNS resolution for {} took {} ms", resolvedAddress, hostResolveTimeMs);
}
synchronized (clientPool.locks[clientIndex]) {
cachedClient = clientPool.clients[clientIndex];
if (cachedClient != null) {
if (cachedClient.isActive()) {
logger.trace("Returning cached connection to {}: {}", resolvedAddress, cachedClient);
return cachedClient;
} else {
logger.info("Found inactive connection to {}, creating a new one.", resolvedAddress);
}
}
clientPool.clients[clientIndex] = createClient(resolvedAddress);
return clientPool.clients[clientIndex];
}
}从代码得知，创建TransportClient的步骤如下。
1）调用InetSocketAddress的静态方法createUnresolved构建InetSocketAddress（这种方式创建InetSocketAddress，可以在缓存中已经有TransportClient时避免不必要的域名解析），然后从connectionPool中获取与此地址对应的ClientPool，如果没有，则需要新建ClientPool，并放入缓存connectionPool中。
2）根据numConnectionsPerPeer的大小（使用“spark.+模块名+.io.numConnections-PerPeer”属性配置），从ClientPool中随机选择一个TransportClient。
3）如果ClientPool的clients数组中在随机产生的索引位置不存在TransportClient或者TransportClient没有激活，则进入第5步，否则对此TransportClient进行第4步的检查。
4）更新TransportClient的channel中配置的TransportChannelHandler的最后一次使用时间，确保channel没有超时，然后检查TransportClient是否是激活状态，最后返回此TransportClient给调用方。
5）由于缓存中没有TransportClient可用，于是调用InetSocketAddress的构造器创建InetSocketAddress对象（直接使用InetSocketAddress的构造器创建InetSocketAddress会进行域名解析），在这一步骤多个线程可能会产生竞态条件（由于没有同步处理，所以多个线程极有可能同时执行到此处，都发现缓存中没有TransportClient可用，于是都使用InetSocketAddress的构造器创建InetSocketAddress）。
6）第5步创建InetSocketAddress的过程中产生的竞态条件如果不妥善处理，会产生线程安全问题，所以到了ClientPool的locks数组发挥作用的时候了。按照随机产生的数组索引，locks数组中的锁对象可以对clients数组中的TransportClient一对一进行同步。即便之前产生了竞态条件，但是在这一步只能有一个线程进入临界区。在临界区内，先进入的线程调用重载的createClient方法创建TransportClient对象并放入ClientPool的clients数组中。当率先进入临界区的线程退出临界区后，其他线程才能进入，此时发现ClientPool的clients数组中已经存在了TransportClient对象，那么将不再创建TransportClient，而是直接使用它。
下面代码的整个执行过程实际解决了TransportClient缓存的使用及createClient方法的线程安全问题，并没有涉及创建TransportClient的实现。TransportClient的创建过程在重载的createClient方法中实现。
/** Create a completely new {@link TransportClient} to the remote address. */
private TransportClient createClient(InetSocketAddress address) throws IOException {
logger.debug("Creating new connection to {}", address);
Bootstrap bootstrap = new Bootstrap();
bootstrap.group(workerGroup)
.channel(socketChannelClass)
// Disable Nagle's Algorithm since we don't want packets to wait
.option(ChannelOption.TCP_NODELAY, true)
.option(ChannelOption.SO_KEEPALIVE, true)
.option(ChannelOption.CONNECT_TIMEOUT_MILLIS, conf.connectionTimeoutMs())
.option(ChannelOption.ALLOCATOR, pooledAllocator);
final AtomicReference clientRef = new AtomicReference<>();
final AtomicReference channelRef = new AtomicReference<>();
bootstrap.handler(new ChannelInitializer() {
@Override
public void initChannel(SocketChannel ch) {
TransportChannelHandler clientHandler = context.initializePipeline(ch);
clientRef.set(clientHandler.getClient());
channelRef.set(ch);
}
});
// Connect to the remote server
long preConnect = System.nanoTime();
ChannelFuture cf = bootstrap.connect(address);
if (!cf.awaitUninterruptibly(conf.connectionTimeoutMs())) {
throw new IOException(
String.format("Connecting to %s timed out (%s ms)", address, conf.connectionTimeoutMs()));
} else if (cf.cause() != null) {
throw new IOException(String.format("Failed to connect to %s", address), cf.cause());
}
TransportClient client = clientRef.get();
Channel channel = channelRef.get();
assert client != null : "Channel future completed successfully with null client";
// Execute any client bootstraps synchronously before marking the Client as successful.
long preBootstrap = System.nanoTime();
logger.debug("Connection to {} successful, running bootstraps...", address);
try {
for (TransportClientBootstrap clientBootstrap : clientBootstraps) {
clientBootstrap.doBootstrap(client, channel);
}
} catch (Exception e) { // catch non-RuntimeExceptions too as bootstrap may be written in Scala
long bootstrapTimeMs = (System.nanoTime() - preBootstrap) / 1000000;
logger.error("Exception while bootstrapping client after " + bootstrapTimeMs + " ms", e);
client.close();
throw Throwables.propagate(e);
}
long postBootstrap = System.nanoTime();
logger.info("Successfully created connection to {} after {} ms ({} ms spent in bootstraps)",
address, (postBootstrap - preConnect) / 1000000, (postBootstrap - preBootstrap) / 1000000);
return client;
} 从代码得知，真正创建TransportClient的步骤如下。
1）构建根引导程序Bootstrap并对其进行配置。
2）为根引导程序设置管道初始化回调函数，此回调函数将调用TransportContext的initializePipeline方法初始化Channel的pipeline。
3）使用根引导程序连接远程服务器，当连接成功对管道初始化时会回调初始化回调函数，将TransportClient和Channel对象分别设置到原子引用clientRef与channelRef中。
4）给TransportClient设置客户端引导程序，即设置TransportClientFactory中的Transport-ClientBootstrap列表。
5）返回此TransportClient对象。
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