MetaQ技术内幕——源码分析(八)

上一篇以及上上篇基本介绍了MetaQ如何使用Gecko框架在网络上传输数据，今天将继续进一步介绍在Broker，各种命令的处理逻辑（暂时将不涉及到事务处理）。

依旧是在MetaMorphosisBroker的registerProcessors()方法中，我们可以注意到一点，每个Processor的实例在构造的时候都注入了一个brokerProcessor的变量，该变量的类型为CommandProcessor。其实，各个Processor的业务逻辑又委托给了CommandProcessor进行处理，比如我们看看其中的GetProcessor的源码：

public class GetProcessor implements RequestProcessor<GetCommand> {
public static final Logger log = LoggerFactory.getLogger(GetProcessor.class);

private final ThreadPoolExecutor executor;

private final CommandProcessor processor;

public GetProcessor(final CommandProcessor processor, final ThreadPoolExecutor executor) {
this.processor = processor;
this.executor = executor;
}

@Override
public ThreadPoolExecutor getExecutor() {
return this.executor;
}

@Override
public void handleRequest(final GetCommand request, final Connection conn) {
// Processor并没有处理具体的业务逻辑，而是将业务逻辑交给CommandProcessor的processGetCommand()进行处理，Processor只是将处理结果简单的返回给客户端
final ResponseCommand response = this.processor.processGetCommand(request, SessionContextHolder.getOrCreateSessionContext(conn, null));
if (response != null) {
RemotingUtils.response(conn, response);
}
}
}

CommandProcessor业务逻辑的处理模块采用责任链的处理方式，目前来说只有两个类型的业务逻辑处理单元：带有事务处理(TransactionalCommandProcessor)的和不带有事务处理(BrokerCommandProcessor)的。老习惯，先上类图：

CommandProcessor接口定义如下：

public interface CommandProcessor extends Service {
//处理Put命令，结果通过PutCallback的回调返回
public void processPutCommand(final PutCommand request, final SessionContext sessionContext, final PutCallback cb) throws Exception;
//处理Get命令
public ResponseCommand processGetCommand(GetCommand request, final SessionContext ctx);

/**
* Under conditions that cannot use notify-remoting directly.
*/
//处理Get命令，并根据条件zeroCopy是否使用zeroCopy
public ResponseCommand processGetCommand(GetCommand request, final SessionContext ctx, final boolean zeroCopy);
//处理查询最近可用offset位置请求
public ResponseCommand processOffsetCommand(OffsetCommand request, final SessionContext ctx);
//处理退出请求
public void processQuitCommand(QuitCommand request, final SessionContext ctx);

public ResponseCommand processVesionCommand(VersionCommand request, final SessionContext ctx);
//处理统计请求
public ResponseCommand processStatCommand(StatsCommand request, final SessionContext ctx);
//下面主要定义与事务相关的方法，暂时先不介绍
public void removeTransaction(final XATransactionId xid);

public Transaction getTransaction(final SessionContext context, final TransactionId xid) throws MetamorphosisException, XAException;

public void forgetTransaction(final SessionContext context, final TransactionId xid) throws Exception;

public void rollbackTransaction(final SessionContext context, final TransactionId xid) throws Exception;

public void commitTransaction(final SessionContext context, final TransactionId xid, final boolean onePhase) throws Exception;

public int prepareTransaction(final SessionContext context, final TransactionId xid) throws Exception;

public void beginTransaction(final SessionContext context, final TransactionId xid, final int seconds) throws Exception;

public TransactionId[] getPreparedTransactions(final SessionContext context, String uniqueQualifier) throws Exception;
}

细心的读者会发现，每个定义的方法的参数都有一个参数SessionContext，SessionContext携带了连接的信息，由Broker创建，具体代码见SessionContextHolder的getOrCreateSessionContext()方法，getOrCreateSessionContext()方法在Processor委托给CommandProcessor处理业务逻辑时被调用。

BrokerCommandProcessor和TransactionalCommandProcessor其实就是各模块的粘合剂，将各模块的功能统一协调形成整体对外提供功能。BrokerCommandProcessor的实现并不难理解，下面让我们来具体分析一下BrokerCommandProcessor这个类：

//Put请求的业务逻辑处理
@Override
public void processPutCommand(final PutCommand request, final SessionContext sessionContext, final PutCallback cb) {
final String partitionString = this.metaConfig.getBrokerId() + "-" + request.getPartition();
//统计计算
this.statsManager.statsPut(request.getTopic(), partitionString, 1);
this.statsManager.statsMessageSize(request.getTopic(), request.getData().length);
int partition = -1;
try {
//如果对应存储的分区已经关闭，则拒绝该消息
if (this.metaConfig.isClosedPartition(request.getTopic(), request.getPartition())) {
log.warn("Can not put message to partition " + request.getPartition() + " for topic=" + request.getTopic() + ",it was closed");
if (cb != null) {
cb.putComplete(new BooleanCommand(HttpStatus.Forbidden, this.genErrorMessage(request.getTopic(), request.getPartition()) + "Detail:partition[" + partitionString + "] has been
4000
closed", request.getOpaque()));
}
return;
}

partition = this.getPartition(request);
//获取对应Topic分区的MessageStore实例
final MessageStore store = this.storeManager.getOrCreateMessageStore(request.getTopic(), partition);
// 如果是动态添加的topic，需要注册到zk
//就到目前为止，我着实没想明白下面这句代码的用途是什么？
//如果topic没有在该Broker的配置中配置，在MessageStoreManager中的isLegalTopic()方法中检查就通不过而抛出异常，那么下面这句代码怎么样都不会被执行，而Client要向Broker发送消息，一定要先发布topic，保证topic在zk发布；
this.brokerZooKeeper.registerTopicInZk(request.getTopic(), false);
// 设置唯一id
final long messageId = this.idWorker.nextId();
//存储消息，之前的文章介绍过Broker的存储使用回调的方式，易于异步的实现，代码简单不分析
store.append(messageId, request, new StoreAppendCallback(partition, partitionString, request, messageId, cb));
} catch (final Exception e) {
//发生异常，统计计算回滚
this.statsManager.statsPutFailed(request.getTopic(), partitionString, 1);
log.error("Put message failed", e);
if (cb != null) {
//返回结果
cb.putComplete(new BooleanCommand(HttpStatus.InternalServerError, this.genErrorMessage(request.getTopic(), partition) + "Detail:" + e.getMessage(), request.getOpaque()));
}
}
}

@Override
// GET请求的业务逻辑处理
public ResponseCommand processGetCommand(final GetCommand request, final SessionContext ctx) {
//默认为zeroCopy
return this.processGetCommand(request, ctx, true);
}

@Override
public ResponseCommand processGetCommand(final GetCommand request, final SessionContext ctx, final boolean zeroCopy) {
//获取查询信息
final String group = request.getGroup();
final String topic = request.getTopic();
//统计计数（请求数统计）
this.statsManager.statsGet(topic, group, 1);

// 如果分区被关闭,禁止读数据 --wuhua
if (this.metaConfig.isClosedPartition(topic, request.getPartition())) {
log.warn("can not get message for topic=" + topic + " from partition " + request.getPartition() + ",it closed,");
return new BooleanCommand(HttpStatus.Forbidden, "Partition[" + this.metaConfig.getBrokerId() + "-" + request.getPartition() + "] has been closed", request.getOpaque());
}
//获取topic对应分区的MessageStore实例，如果实例不存在，则返回NotFound
final MessageStore store = this.storeManager.getMessageStore(topic, request.getPartition());
if (store == null) {
//统计计数
this.statsManager.statsGetMiss(topic, group, 1);
return new BooleanCommand(HttpStatus.NotFound, "The topic `" + topic + "` in partition `" + request.getPartition() + "` is not exists", request.getOpaque());
}
//如果请求的起始位置<0，判定该请求无效
if (request.getMaxSize() <= 0) {
return new BooleanCommand(HttpStatus.BadRequest, "Bad request,invalid max size:" + request.getMaxSize(), request.getOpaque());
}
try {
//读取由request.getOffset()开始的消息集合
final MessageSet set = store.slice(request.getOffset(), Math.min(this.metaConfig.getMaxTransferSize(), request.getMaxSize()));
//如果当前消息集不为空
if (set != null) {
//判断是否zeroCopy，如果是zeroCopy，则直接写；如果不是，则将消息集包装成DataCommand，这也就是前面为什么说DataCommand要实现encode()方法的缘故
if (zeroCopy) {
set.write(request, ctx);
return null;
} else {
// refer to the code of line 440 in MessageStore
// create two copies of byte array including the byteBuffer
// and new bytes
// this may not a good use case of Buffer
final ByteBuffer byteBuffer = ByteBuffer.allocate(Math.min(this.metaConfig.getMaxTransferSize(), request.getMaxSize()));
set.read(byteBuffer);
byteBuffer.flip();
final byte[] bytes = new byte[byteBuffer.remaining()];
byteBuffer.get(bytes);
return new DataCommand(bytes, request.getOpaque());
}
} else {
//如果为空消息集，则认为请求无效
//统计计数
this.statsManager.statsGetMiss(topic, group, 1);
this.statsManager.statsGetFailed(topic, group, 1);

// 当请求的偏移量大于实际最大值时,返回给客户端实际最大的偏移量.
final long maxOffset = store.getMaxOffset();
final long requestOffset = request.getOffset();
if (requestOffset > maxOffset && (this.metaConfig.isUpdateConsumerOffsets() || requestOffset == Long.MAX_VALUE)) {
log.info("offset[" + requestOffset + "] is exceeded,tell the client real max offset: " + maxOffset + ",topic=" + topic + ",group=" + group);
this.statsManager.statsOffset(topic, group, 1);
return new BooleanCommand(HttpStatus.Moved, String.valueOf(maxOffset), request.getOpaque());
} else {
return new BooleanCommand(HttpStatus.NotFound, "Could not find message at position " + requestOffset, request.getOpaque());
}
}
} catch (final ArrayIndexOutOfBoundsException e) {
log.error("Could not get message from position " + request.getOffset() + ",it is out of bounds,topic=" + topic);
// 告知最近可用的offset
this.statsManager.statsGetMiss(topic, group, 1);
this.statsManager.statsGetFailed(topic, group, 1);
final long validOffset = store.getNearestOffset(request.getOffset());
this.statsManager.statsOffset(topic, group, 1);
return new BooleanCommand(HttpStatus.Moved, String.valueOf(validOffset), request.getOpaque());
} catch (final Throwable e) {
log.error("Could not get message from position " + request.getOffset(), e);
this.statsManager.statsGetFailed(topic, group, 1);
return new BooleanCommand(HttpStatus.InternalServerError, this.genErrorMessage(request.getTopic(), request.getPartition()) + "Detail:" + e.getMessage(), request.getOpaque());
}
}

//查询最近可用offset请求的业务逻辑处理
@Override
public ResponseCommand processOffsetCommand(final OffsetCommand request, final SessionContext ctx) {
//统计计数
this.statsManager.statsOffset(request.getTopic(), request.getGroup(), 1);
//获取topic对应分区的MessageStore实例
final MessageStore store = this.storeManager.getMessageStore(request.getTopic(), request.getPartition());
//如果为空，则返回未找到
if (store == null) {
return new BooleanCommand(HttpStatus.NotFound, "The topic `" + request.getTopic() + "` in partition `" + request.getPartition() + "` is not exists", request.getOpaque());
}
//获取topic对应分区最近可用的offset
final long offset = store.getNearestOffset(request.getOffset());
return new BooleanCommand(HttpStatus.Success, String.valueOf(offset), request.getOpaque());
}

//退出请求业务逻辑处理
@Override
public void processQuitCommand(final QuitCommand request, final SessionContext ctx) {
try {
if (ctx.getConnection() != null) {
//关闭与客户端的连接
ctx.getConnection().close(false);
}
} catch (final NotifyRemotingException e) {
// ignore
}
}

//版本查询请求业务逻辑处理
@Override
public ResponseCommand processVesionCommand(final VersionCommand request, final SessionContext ctx) {
//返回当前Broker版本
return new BooleanCommand(HttpStatus.Success, BuildProperties.VERSION, request.getOpaque());
}

//统计请求查询业务逻辑处理
@Override
public ResponseCommand processStatCommand(final StatsCommand request, final SessionContext ctx) {
//判断类型，如果类型以config 开头，则传输整个配置文件
final String item = request.getItem();
if ("config".equals(item)) {
return this.processStatsConfig(request, ctx);
} else {
//如果是获取统计结果，则从统计模块获取响应结果并返回给客户端
final String statsInfo = this.statsManager.getStatsInfo(item);
return new BooleanCommand(HttpStatus.Success, statsInfo, request.getOpaque());
}
}

//获取配置文件内容，使用zeroCopy将文件内容发送到客户端，构造的响应用BooleanCommand
@SuppressWarnings("resource")
private ResponseCommand processStatsConfig(final StatsCommand request, final SessionContext ctx) {
try {
final FileChannel fc = new FileInputStream(this.metaConfig.getConfigFilePath()).getChannel();
// result code length opaque\r\n
IoBuffer buf = IoBuffer.allocate(11 + 3 + ByteUtils.stringSize(fc.size()) + ByteUtils.stringSize(request.getOpaque()));
ByteUtils.setArguments(buf, MetaEncodeCommand.RESULT_CMD, HttpStatus.Success, fc.size(), request.getOpaque());
buf.flip();
ctx.getConnection().transferFrom(buf, null, fc, 0, fc.size(), request.getOpaque(),
new SingleRequestCallBackListener() {
@Override
public void onResponse(ResponseCommand responseCommand, Connection conn) {
this.closeChannel();
}

@Override
public void onException(Exception e) {
this.closeChannel();
}

private void closeChannel() {
try {
fc.close();
} catch (IOException e) {
log.error("IOException while stats config", e);
}
}

@Override
public ThreadPoolExecutor getExecutor() {
return null;
}
}, 5000, TimeUnit.MILLISECONDS);
} catch (FileNotFoundException e) {
log.error("Config file not found:" + this.metaConfig.getConfigFilePath(), e);
return new BooleanCommand(HttpStatus.InternalServerError, "Config file not found:" + this.metaConfig.getConfigFilePath(), request.getOpaque());
} catch (IOException e) {
log.error("IOException while stats config", e);
return new BooleanCommand(HttpStatus.InternalServerError, "Read config file error:" + e.getMessage(), request.getOpaque());
} catch (NotifyRemotingException e) {
log.error("NotifyRemotingException while stats config", e);
}
return null;
}

如果不使用内容的事务，Broker已经完成了从网络接收数据—>处理请求(存储消息/查询结果等)—>返回结果的流程，Broker最基础的流程已经基本分析完毕。