我要评论概要
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;
}
总结
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