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小编给大家分享一下RocketMQ如何获取指定消息，相信大部分人都还不怎么了解，因此分享这篇文章给大家参考一下，希望大家阅读完这篇文章后大有收获，下面让我们一起去了解一下吧！
概要
消息查询是什么？
消息查询就是根据用户提供的msgId从MQ中取出该消息
RocketMQ如果有多个节点如何查询？
问题：RocketMQ分布式结构中，数据分散在各个节点，即便是同一Topic的数据，也未必都在一个broker上。客户端怎么知道数据该去哪个节点上查？
猜想1：逐个访问broker节点查询数据
猜想2：有某种数据中心存在，该中心知道所有消息存储的位置，只要向该中心查询即可得到消息具体位置，进而取得消息内容
实际：
1.消息Id中含有消息所在的broker的地址信息（IP\Port）以及该消息在CommitLog中的偏移量。
2.客户端实现会从msgId字符串中解析出broker地址，向指定broker节查询消息。
问题：CommitLog文件有多个，只有偏移量估计不能确定在哪个文件吧？
实际：单个Broker节点内offset是全局唯一的，不是每个CommitLog文件的偏移量都是从0开始的。单个节点内所有CommitLog文件共用一套偏移量，每个文件的文件名为其第一个消息的偏移量。所以可以根据偏移量和文件名确定CommitLog文件。
源码阅读
0.使用方式
MessageExt msg = consumer.viewMessage(msgId);
1.消息ID解析
这个了解下就可以了
public class MessageId {
private SocketAddress address;
private long offset;
public MessageId(SocketAddress address, long offset) {
this.address = address;
this.offset = offset;
}
//get-set
}
//from MQAdminImpl.java
public MessageExt viewMessage(
String msgId) throws RemotingException, MQBrokerException, InterruptedException, MQClientException {
MessageId messageId = null;
try {
//从msgId字符串中解析出address和offset
//address = ip:port
//offset为消息在CommitLog文件中的偏移量
messageId = MessageDecoder.decodeMessageId(msgId);
} catch (Exception e) {
throw new MQClientException(ResponseCode.NO_MESSAGE, "query message by id finished, but no message.");
}
return this.mQClientFactory.getMQClientAPIImpl().viewMessage(RemotingUtil.socketAddress2String(messageId.getAddress()),
messageId.getOffset(), timeoutMillis);
}
//from MessageDecoder.java
public static MessageId decodeMessageId(final String msgId) throws UnknownHostException {
SocketAddress address;
long offset;
//ipv4和ipv6的区别
//如果msgId总长度超过32字符，则为ipv6
int ipLength = msgId.length() == 32 ? 4 * 2 : 16 * 2;
byte[] ip = UtilAll.string2bytes(msgId.substring(0, ipLength));
byte[] port = UtilAll.string2bytes(msgId.substring(ipLength, ipLength + 8));
ByteBuffer bb = ByteBuffer.wrap(port);
int portInt = bb.getInt(0);
address = new InetSocketAddress(InetAddress.getByAddress(ip), portInt);
// offset
byte[] data = UtilAll.string2bytes(msgId.substring(ipLength + 8, ipLength + 8 + 16));
bb = ByteBuffer.wrap(data);
offset = bb.getLong(0);
return new MessageId(address, offset);
}2.长连接客户端RPC实现
要发请求首先得先建立连接，这里方法可以看到创建连接相关的操作。值得注意的是，第一次访问的时候可能连接还没建立，建立连接需要消耗一段时间。代码中对这个时间也做了判断，如果连接建立完成后，发现已经超时，则不再发出请求。目的应该是尽可能减少请求线程的阻塞时间。
//from NettyRemotingClient.java
@Override
public RemotingCommand invokeSync(String addr, final RemotingCommand request, long timeoutMillis)
throws InterruptedException, RemotingConnectException, RemotingSendRequestException, RemotingTimeoutException {
long beginStartTime = System.currentTimeMillis();
//这里会先检查有无该地址的通道，有则返回，无则创建
final Channel channel = this.getAndCreateChannel(addr);
if (channel != null && channel.isActive()) {
try {
//前置钩子
doBeforeRpcHooks(addr, request);
//判断通道建立完成时是否已到达超时时间，如果超时直接抛出异常。不发请求
long costTime = System.currentTimeMillis() - beginStartTime;
if (timeoutMillis < costTime) {
throw new RemotingTimeoutException("invokeSync call timeout");
}
//同步调用
RemotingCommand response = this.invokeSyncImpl(channel, request, timeoutMillis - costTime);
//后置钩子
doAfterRpcHooks(RemotingHelper.parseChannelRemoteAddr(channel), request, response); //后置钩子
return response;
} catch (RemotingSendRequestException e) {
log.warn("invokeSync: send request exception, so close the channel[{}]", addr);
this.closeChannel(addr, channel);
throw e;
} catch (RemotingTimeoutException e) {
if (nettyClientConfig.isClientCloseSocketIfTimeout()) {
this.closeChannel(addr, channel);
log.warn("invokeSync: close socket because of timeout, {}ms, {}", timeoutMillis, addr);
}
log.warn("invokeSync: wait response timeout exception, the channel[{}]", addr);
throw e;
}
} else {
this.closeChannel(addr, channel);
throw new RemotingConnectException(addr);
}
}下一步看看它的同步调用做了什么处理。注意到它会构建一个Future对象加入待响应池，发出请求报文后就挂起线程，然后等待唤醒（waitResponse内部使用CountDownLatch等待）。
//from NettyRemotingAbstract.javapublic RemotingCommand invokeSyncImpl(final Channel channel, final RemotingCommand request,
final long timeoutMillis)
throws InterruptedException, RemotingSendRequestException, RemotingTimeoutException {
//请求id
final int opaque = request.getOpaque();
try {
//请求存根
final ResponseFuture responseFuture = new ResponseFuture(channel, opaque, timeoutMillis, null, null);
//加入待响应的请求池
this.responseTable.put(opaque, responseFuture);
final SocketAddress addr = channel.remoteAddress();
//将请求发出，成功发出时更新状态
channel.writeAndFlush(request).addListener(new ChannelFutureListener() {
@Override
public void operationComplete(ChannelFuture f) throws Exception {
if (f.isSuccess()) { //若成功发出，更新请求状态为“已发出”
responseFuture.setSendRequestOK(true);
return;
} else {
responseFuture.setSendRequestOK(false);
}
//若发出失败，则从池中移除（没用了，释放资源）
responseTable.remove(opaque);
responseFuture.setCause(f.cause());
//putResponse的时候会唤醒等待的线程
responseFuture.putResponse(null);
log.warn("send a request command to channel <" + addr + "> failed.");
}
});
//只等待一段时间，不会一直等下去
//若正常响应，则收到响应后，此线程会被唤醒，继续执行下去
//若超时，则到达该时间后线程苏醒，继续执行
RemotingCommand responseCommand = responseFuture.waitResponse(timeoutMillis);
if (null == responseCommand) {
if (responseFuture.isSendRequestOK()) {
throw new RemotingTimeoutException(RemotingHelper.parseSocketAddressAddr(addr), timeoutMillis,
responseFuture.getCause());
} else {
throw new RemotingSendRequestException(RemotingHelper.parseSocketAddressAddr(addr), responseFuture.getCause());
}
}
return responseCommand;
} finally {
//正常响应完成时，将future释放（正常逻辑）
//超时时，将future释放。这个请求已经作废了，后面如果再收到响应，就可以直接丢弃了（由于找不到相关的响应钩子，就不处理了）
this.responseTable.remove(opaque);
}
}好，我们再来看看收到报文的时候是怎么处理的。我们都了解JDK中的Future的原理，大概就是将这个任务提交给其他线程处理，该线程处理完毕后会将结果写入到Future对象中，写入时如果有线程在等待该结果，则唤醒这些线程。这里也差不多，只不过执行线程在服务端，服务执行完毕后会将结果通过长连接发送给客户端，客户端收到后根据报文中的ID信息从待响应池中找到Future对象，然后就是类似的处理了。
class NettyClientHandler extends SimpleChannelInboundHandler<RemotingCommand> {
//底层解码完毕得到RemotingCommand的报文
@Override
protected void channelRead0(ChannelHandlerContext ctx, RemotingCommand msg) throws Exception {
processMessageReceived(ctx, msg);
}
}
public void processMessageReceived(ChannelHandlerContext ctx, RemotingCommand msg) throws Exception {
final RemotingCommand cmd = msg;
if (cmd != null) {
//判断类型
switch (cmd.getType()) {
case REQUEST_COMMAND:
processRequestCommand(ctx, cmd);
break;
case RESPONSE_COMMAND:
processResponseCommand(ctx, cmd);
break;
default:
break;
}
}
}
public void processResponseCommand(ChannelHandlerContext ctx, RemotingCommand cmd) {
//取得消息id
final int opaque = cmd.getOpaque();
//从待响应池中取得对应请求
final ResponseFuture responseFuture = responseTable.get(opaque);
if (responseFuture != null) {
//将响应值注入到ResponseFuture对象中，等待线程可从这个对象获取结果
responseFuture.setResponseCommand(cmd);
//请求已处理完毕，释放该请求
responseTable.remove(opaque);
//如果有回调函数的话则回调（由当前线程处理）
if (responseFuture.getInvokeCallback() != null) {
executeInvokeCallback(responseFuture);
} else {
//没有的话，则唤醒等待线程（由等待线程做处理）
responseFuture.putResponse(cmd);
responseFuture.release();
}
} else {
log.warn("receive response, but not matched any request, " + RemotingHelper.parseChannelRemoteAddr(ctx.channel()));
log.warn(cmd.toString());
}
}总结一下，客户端的处理时序大概是这样的：
结构大概是这样的：
3.服务端的处理
//todo 服务端待补充CommitLog文件映射相关内容
class NettyServerHandler extends SimpleChannelInboundHandler<RemotingCommand> {
@Override
protected void channelRead0(ChannelHandlerContext ctx, RemotingCommand msg) throws Exception {
processMessageReceived(ctx, msg);
}
}
//from NettyRemotingAbscract.java
public void processMessageReceived(ChannelHandlerContext ctx, RemotingCommand msg) throws Exception {
final RemotingCommand cmd = msg;
if (cmd != null) {
switch (cmd.getType()) {
case REQUEST_COMMAND: //服务端走这里
processRequestCommand(ctx, cmd);
break;
case RESPONSE_COMMAND:
processResponseCommand(ctx, cmd);
break;
default:
break;
}
}
}
//from NettyRemotingAbscract.java
public void processRequestCommand(final ChannelHandlerContext ctx, final RemotingCommand cmd) {
//查看有无该请求code相关的处理器
final Pair<NettyRequestProcessor, ExecutorService> matched = this.processorTable.get(cmd.getCode());
//如果没有，则使用默认处理器（可能没有默认处理器）
final Pair<NettyRequestProcessor, ExecutorService> pair = null == matched ? this.defaultRequestProcessor : matched;
final int opaque = cmd.getOpaque();
if (pair != null) {
Runnable run = new Runnable() {
@Override
public void run() {
try {
doBeforeRpcHooks(RemotingHelper.parseChannelRemoteAddr(ctx.channel()), cmd);
final RemotingResponseCallback callback = new RemotingResponseCallback() {
@Override
public void callback(RemotingCommand response) {
doAfterRpcHooks(RemotingHelper.parseChannelRemoteAddr(ctx.channel()), cmd, response);
if (!cmd.isOnewayRPC()) {
if (response != null) { //不为null，则由本类将响应值写会给请求方
response.setOpaque(opaque);
response.markResponseType();
try {
ctx.writeAndFlush(response);
} catch (Throwable e) {
log.error("process request over, but response failed", e);
log.error(cmd.toString());
log.error(response.toString());
}
} else { //为null，意味着processor内部已经将响应处理了，这里无需再处理。
}
}
}
};
if (pair.getObject1() instanceof AsyncNettyRequestProcessor) {//QueryMessageProcessor为异步处理器
AsyncNettyRequestProcessor processor = (AsyncNettyRequestProcessor)pair.getObject1();
processor.asyncProcessRequest(ctx, cmd, callback);
} else {
NettyRequestProcessor processor = pair.getObject1();
RemotingCommand response = processor.processRequest(ctx, cmd);
doAfterRpcHooks(RemotingHelper.parseChannelRemoteAddr(ctx.channel()), cmd, response);
callback.callback(response);
}
} catch (Throwable e) {
log.error("process request exception", e);
log.error(cmd.toString());
if (!cmd.isOnewayRPC()) {
final RemotingCommand response = RemotingCommand.createResponseCommand(RemotingSysResponseCode.SYSTEM_ERROR,
RemotingHelper.exceptionSimpleDesc(e));
response.setOpaque(opaque);
ctx.writeAndFlush(response);
}
}
}
};
if (pair.getObject1().rejectRequest()) {
final RemotingCommand response = RemotingCommand.createResponseCommand(RemotingSysResponseCode.SYSTEM_BUSY,
"[REJECTREQUEST]system busy, start flow control for a while");
response.setOpaque(opaque);
ctx.writeAndFlush(response);
return;
}
try {
final RequestTask requestTask = new RequestTask(run, ctx.channel(), cmd);
pair.getObject2().submit(requestTask);
} catch (RejectedExecutionException e) {
if ((System.currentTimeMillis() % 10000) == 0) {
log.warn(RemotingHelper.parseChannelRemoteAddr(ctx.channel())
+ ", too many requests and system thread pool busy, RejectedExecutionException "
+ pair.getObject2().toString()
+ " request code: " + cmd.getCode());
}
if (!cmd.isOnewayRPC()) {
final RemotingCommand response = RemotingCommand.createResponseCommand(RemotingSysResponseCode.SYSTEM_BUSY,
"[OVERLOAD]system busy, start flow control for a while");
response.setOpaque(opaque);
ctx.writeAndFlush(response);
}
}
} else {
String error = " request type " + cmd.getCode() + " not supported";
final RemotingCommand response =
RemotingCommand.createResponseCommand(RemotingSysResponseCode.REQUEST_CODE_NOT_SUPPORTED, error);
response.setOpaque(opaque);
ctx.writeAndFlush(response);
log.error(RemotingHelper.parseChannelRemoteAddr(ctx.channel()) + error);
}
}
//from QueryMessageProcesor.java
@Override
public RemotingCommand processRequest(ChannelHandlerContext ctx, RemotingCommand request)
throws RemotingCommandException {
switch (request.getCode()) {
case RequestCode.QUERY_MESSAGE:
return this.queryMessage(ctx, request);
case RequestCode.VIEW_MESSAGE_BY_ID: //通过msgId查询消息
return this.viewMessageById(ctx, request);
default:
break;
}
return null;
}
public RemotingCommand viewMessageById(ChannelHandlerContext ctx, RemotingCommand request)
throws RemotingCommandException {
final RemotingCommand response = RemotingCommand.createResponseCommand(null);
final ViewMessageRequestHeader requestHeader =
(ViewMessageRequestHeader) request.decodeCommandCustomHeader(ViewMessageRequestHeader.class);
response.setOpaque(request.getOpaque());
//getMessagetStore得到当前映射到内存中的CommitLog文件，然后根据偏移量取得数据
final SelectMappedBufferResult selectMappedBufferResult =
this.brokerController.getMessageStore().selectOneMessageByOffset(requestHeader.getOffset());
if (selectMappedBufferResult != null) {
response.setCode(ResponseCode.SUCCESS);
response.setRemark(null);
//将响应通过socket写回给客户端
try {
//response对象的数据作为header
//消息内容作为body
FileRegion fileRegion =
new OneMessageTransfer(response.encodeHeader(selectMappedBufferResult.getSize()),
selectMappedBufferResult);
ctx.channel().writeAndFlush(fileRegion).addListener(new ChannelFutureListener() {
@Override
public void operationComplete(ChannelFuture future) throws Exception {
selectMappedBufferResult.release();
if (!future.isSuccess()) {
log.error("Transfer one message from page cache failed, ", future.cause());
}
}
});
} catch (Throwable e) {
log.error("", e);
selectMappedBufferResult.release();
}
return null; //如果有值，则直接写回给请求方。这里返回null是不需要由外层处理响应。
} else {
response.setCode(ResponseCode.SYSTEM_ERROR);
response.setRemark("can not find message by the offset, " + requestHeader.getOffset());
}
return response;
}以上是RocketMQ如何获取指定消息的所有内容，感谢各位的阅读！相信大家都有了一定的了解，希望分享的内容对大家有所帮助，如果还想学习更多知识，欢迎关注亿速云行业资讯频道！
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