mongo/transport/service_state_machine.cpp已经分析startSession创建ASIOSession过程，并且验证connection是否超过限制。
现在继续研究ASIOSession和connection是怎么接受客户端命令的？
mongo/transport/service_state_machine.cpp核心方法有：
cpp
enum class State {
Created, // The session has been created, but no operations have been performed yet
Source, // Request a new Message from the network to handle
SourceWait, // Wait for the new Message to arrive from the network
Process, // Run the Message through the database
SinkWait, // Wait for the database result to be sent by the network
EndSession, // End the session - the ServiceStateMachine will be invalid after this
Ended // The session has ended. It is illegal to call any method besides
// state() if this is the current state.
};
void _processMessage()
void _sinkCallback()
void _sourceCallback()
void _sourceMessage()
void _sinkMessage()
void _runNextInGuard()mongo第一条命令状态转变流程是：State::Created 》 State::Source 》State::SourceWait 》 State::Process 》 State::SinkWait 》 State::Source 》State::SourceWait
状态解释：State::Created, //session刚刚创建，但是还没有接受任何命令
State::Source, //去接受客户端新的命令
State::SourceWait, // 等待客户端新的命令
State::Process, // 将接受的命令发送给mongodb数据库
State:: SinkWait, // 等待将命令的执行结果返回给客户端
mongo/transport/service_state_machine.cpp核心方法循环调用的流程图下：
mongo/transport/service_state_machine.cpp核心_runNextInGuard方法主要判断状态State::Source和State::Process，State::Source主要session等待客户端请求find命令 ，State::Process将命令发送给mongodb数据库， _runNextInGuard代码如下：
cpp
void ServiceStateMachine::_runNextInGuard(ThreadGuard guard) {
auto curState = state();
dassert(curState != State::Ended);
// If this is the first run of the SSM, then update its state to Source
if (curState == State::Created) {
curState = State::Source;
_state.store(curState);
}
// Make sure the current Client got set correctly
dassert(Client::getCurrent() == _dbClientPtr);
try {
switch (curState) {
case State::Source:
LOG(1) << "conca _runNextInGuard State::Source" ;
_sourceMessage(std::move(guard));
break;
case State::Process:
LOG(1) << "conca _runNextInGuard State::Process" ;
_processMessage(std::move(guard));
break;
case State::EndSession:
LOG(1) << "conca _runNextInGuard State::EndSession" ;
_cleanupSession(std::move(guard));
break;
default:
MONGO_UNREACHABLE;
}
return;
} catch (const DBException& e) {
log() << "DBException handling request, closing client connection: " << redact(e);
}
if (!guard) {
guard = ThreadGuard(this);
}
_state.store(State::EndSession);
_cleanupSession(std::move(guard));
}session等待connection请求，状态转变流程：State::Created 》 State::Source 》State::SourceWait，
cpp
void ServiceStateMachine::_sourceMessage(ThreadGuard guard) {
invariant(_inMessage.empty());
invariant(_state.load() == State::Source);
LOG(1) << "conca _sourceMessage State::Source";
_state.store(State::SourceWait);
LOG(1) << "conca _sourceMessage store State::SourceWait";
guard.release();
auto sourceMsgImpl = [&] {
if (_transportMode == transport::Mode::kSynchronous) {
MONGO_IDLE_THREAD_BLOCK;
return Future<Message>::makeReady(_session()->sourceMessage());
} else {
invariant(_transportMode == transport::Mode::kAsynchronous);
return _session()->asyncSourceMessage();
}
};
sourceMsgImpl().getAsync([this](StatusWith<Message> msg) {
if (msg.isOK()) {
_inMessage = std::move(msg.getValue());
invariant(!_inMessage.empty());
}
_sourceCallback(msg.getStatus());
});
}_sourceMessage主要处理状态State::Source 》State::SourceWait，等待session接受消息。_session()->asyncSourceMessage()方法session异步等待客户端发送的find命令消息。
如果有find命令到来则调用_sourceCallback(msg.getStatus());_sourceCallback方法代码如下：
cpp
void ServiceStateMachine::_sourceCallback(Status status) {
// The first thing to do is create a ThreadGuard which will take ownership of the SSM in this
// thread.
ThreadGuard guard(this);
// Make sure we just called sourceMessage();
LOG(1) << "conca _sinkMessage State::SinkWait";
dassert(state() == State::SourceWait);
auto remote = _session()->remote();
if (status.isOK()) {
_state.store(State::Process);
LOG(1) << "conca _sinkMessage store State::Process";
// Since we know that we're going to process a message, call scheduleNext() immediately
// to schedule the call to processMessage() on the serviceExecutor (or just unwind the
// stack)
// If this callback doesn't own the ThreadGuard, then we're being called recursively,
// and the executor shouldn't start a new thread to process the message - it can use this
// one just after this returns.
return _scheduleNextWithGuard(std::move(guard),
ServiceExecutor::kMayRecurse,
transport::ServiceExecutorTaskName::kSSMProcessMessage);
} else if (ErrorCodes::isInterruption(status.code()) ||
ErrorCodes::isNetworkError(status.code())) {
LOG(1) << "Session from " << remote
<< " encountered a network error during SourceMessage: " << status;
_state.store(State::EndSession);
} else if (status == TransportLayer::TicketSessionClosedStatus) {
// Our session may have been closed internally.
LOG(1) << "Session from " << remote << " was closed internally during SourceMessage";
_state.store(State::EndSession);
} else {
log() << "Error receiving request from client: " << status << ". Ending connection from "
<< remote << " (connection id: " << _session()->id() << ")";
_state.store(State::EndSession);
}
// There was an error receiving a message from the client and we've already printed the error
// so call runNextInGuard() to clean up the session without waiting.
_runNextInGuard(std::move(guard));
}收到find命令转给mongodb执行find命令，状态转变：State::SourceWait》 State::Process，继续调用_scheduleNextWithGuard 》 schedule 调度 》 _runNextInGuard（上面已经存在，反复调用这个方法）。
现在_runNextInGuard方法里面状态State::Process，所以继续调用的方法是_processMessage执行mongodb数据库命令，接受mongodb数据库返回的数据，_processMessage详细代码如下：
cpp
void ServiceStateMachine::_processMessage(ThreadGuard guard) {
invariant(!_inMessage.empty());
LOG(1) << "conca _processMessage";
TrafficRecorder::get(_serviceContext)
.observe(_sessionHandle, _serviceContext->getPreciseClockSource()->now(), _inMessage);
auto& compressorMgr = MessageCompressorManager::forSession(_session());
_compressorId = boost::none;
if (_inMessage.operation() == dbCompressed) {
MessageCompressorId compressorId;
auto swm = compressorMgr.decompressMessage(_inMessage, &compressorId);
uassertStatusOK(swm.getStatus());
_inMessage = swm.getValue();
_compressorId = compressorId;
}
networkCounter.hitLogicalIn(_inMessage.size());
// Pass sourced Message to handler to generate response.
auto opCtx = Client::getCurrent()->makeOperationContext();
// The handleRequest is implemented in a subclass for mongod/mongos and actually all the
// database work for this request.
DbResponse dbresponse = _sep->handleRequest(opCtx.get(), _inMessage);
// opCtx must be destroyed here so that the operation cannot show
// up in currentOp results after the response reaches the client
opCtx.reset();
// Format our response, if we have one
Message& toSink = dbresponse.response;
if (!toSink.empty()) {
invariant(!OpMsg::isFlagSet(_inMessage, OpMsg::kMoreToCome));
invariant(!OpMsg::isFlagSet(toSink, OpMsg::kChecksumPresent));
// Update the header for the response message.
toSink.header().setId(nextMessageId());
toSink.header().setResponseToMsgId(_inMessage.header().getId());
if (OpMsg::isFlagSet(_inMessage, OpMsg::kChecksumPresent)) {
#ifdef MONGO_CONFIG_SSL
if (!SSLPeerInfo::forSession(_session()).isTLS) {
OpMsg::appendChecksum(&toSink);
}
#else
OpMsg::appendChecksum(&toSink);
#endif
}
// If the incoming message has the exhaust flag set and is a 'getMore' command, then we
// bypass the normal RPC behavior. We will sink the response to the network, but we also
// synthesize a new 'getMore' request, as if we sourced a new message from the network. This
// new request is sent to the database once again to be processed. This cycle repeats as
// long as the associated cursor is not exhausted. Once it is exhausted, we will send a
// final response, terminating the exhaust stream.
_inMessage = makeExhaustMessage(_inMessage, &dbresponse);
_inExhaust = !_inMessage.empty();
networkCounter.hitLogicalOut(toSink.size());
if (_compressorId) {
auto swm = compressorMgr.compressMessage(toSink, &_compressorId.value());
uassertStatusOK(swm.getStatus());
toSink = swm.getValue();
}
TrafficRecorder::get(_serviceContext)
.observe(_sessionHandle, _serviceContext->getPreciseClockSource()->now(), toSink);
_sinkMessage(std::move(guard), std::move(toSink));
LOG(1) << "conca _processMessage _sinkMessage";
} else {
_state.store(State::Source);
_inMessage.reset();
LOG(1) << "conca _processMessage store(State::Source)";
return _scheduleNextWithGuard(std::move(guard),
ServiceExecutor::kDeferredTask,
transport::ServiceExecutorTaskName::kSSMSourceMessage);
}
}DbResponse dbresponse = _sep->handleRequest(opCtx.get(), _inMessage)将接受的find命令发送给mongodb数据库，mongodb数据库执行逻辑，返回响应结果。
最后调用_sinkMessage，将mongodb结果数据发送给客户端。状态转变流程：State::Process 》 State::SinkWait
cpp
void ServiceStateMachine::_sinkMessage(ThreadGuard guard, Message toSink) {
// Sink our response to the client
invariant(_state.load() == State::Process);
LOG(1) << "conca _sinkMessage State::Source";
_state.store(State::SinkWait);
LOG(1) << "conca _sinkMessage store State::SinkWait";
guard.release();
auto sinkMsgImpl = [&] {
if (_transportMode == transport::Mode::kSynchronous) {
// We don't consider ourselves idle while sending the reply since we are still doing
// work on behalf of the client. Contrast that with sourceMessage() where we are waiting
// for the client to send us more work to do.
return Future<void>::makeReady(_session()->sinkMessage(std::move(toSink)));
} else {
invariant(_transportMode == transport::Mode::kAsynchronous);
return _session()->asyncSinkMessage(std::move(toSink));
}
};
sinkMsgImpl().getAsync([this](Status status) { _sinkCallback(std::move(status)); });
}_session()->asyncSinkMessage(std::move(toSink))发送结果给客户端，成功之后继续调用_sinkCallback。
cpp
void ServiceStateMachine::_sinkCallback(Status status) {
// The first thing to do is create a ThreadGuard which will take ownership of the SSM in this
// thread.
ThreadGuard guard(this);
LOG(1) << "conca _sinkCallback State::SinkWait";
dassert(state() == State::SinkWait);
// If there was an error sinking the message to the client, then we should print an error and
// end the session. No need to unwind the stack, so this will runNextInGuard() and return.
//
// Otherwise, update the current state depending on whether we're in exhaust or not, and call
// scheduleNext() to unwind the stack and do the next step.
if (!status.isOK()) {
log() << "Error sending response to client: " << status << ". Ending connection from "
<< _session()->remote() << " (connection id: " << _session()->id() << ")";
_state.store(State::EndSession);
return _runNextInGuard(std::move(guard));
} else if (_inExhaust) {
_state.store(State::Process);
LOG(1) << "conca _sinkCallback store State::Process";
return _scheduleNextWithGuard(std::move(guard),
ServiceExecutor::kDeferredTask |
ServiceExecutor::kMayYieldBeforeSchedule,
transport::ServiceExecutorTaskName::kSSMExhaustMessage);
} else {
_state.store(State::Source);
LOG(1) << "conca _sinkCallback store State::Source";
return _scheduleNextWithGuard(std::move(guard),
ServiceExecutor::kDeferredTask |
ServiceExecutor::kMayYieldBeforeSchedule,
transport::ServiceExecutorTaskName::kSSMSourceMessage);
}
}session发完消息之后， State::SinkWait 》 State::Source 到此位置这个find命令已经完成，_scheduleNextWithGuard后面继续等待新的命令到来。
上面find命令打印日志如下：
cpp
2025-04-24T21:24:02.580+0800 D1 NETWORK [conn1] conca _sourceMessage State::Source
2025-04-24T21:24:02.581+0800 D1 NETWORK [conn1] conca _sourceMessage store State::SourceWait
2025-04-24T21:24:09.957+0800 D1 NETWORK [conn1] conca _sinkMessage State::SinkWait
2025-04-24T21:24:09.957+0800 D1 NETWORK [conn1] conca _sinkMessage store State::Process
2025-04-24T21:24:09.958+0800 D1 NETWORK [conn1] conca func end
2025-04-24T21:24:09.961+0800 D1 NETWORK [conn1] conca guard.release() end
2025-04-24T21:24:09.962+0800 D1 NETWORK [conn1] conca _runNextInGuard State::Process
2025-04-24T21:24:09.963+0800 D1 NETWORK [conn1] conca _processMessage
2025-04-24T21:24:09.964+0800 D1 NETWORK [conn1] conca _processMessage _sep->handleRequest
2025-04-24T21:24:09.965+0800 I SHARDING [conn1] Marking collection db.user as collection version: <unsharded>
2025-04-24T21:24:09.969+0800 I COMMAND [conn1] command db.user appName: "MongoDB Shell" command: find { find: "user", filter: {}, lsid: { id: UUID("7ae50c73-fcc6-4d15-8a80-7f2bf2192e0f") }, $db: "db" } planSummary: COLLSCAN keysExamined:0 docsExamined:6 cursorExhausted:1 numYields:0 nreturned:6 reslen:421 locks:{ ReplicationStateTransition: { acquireCount: { w: 1 } }, Global: { acquireCount: { r: 1 } }, Database: { acquireCount: { r: 1 } }, Collection: { acquireCount: { r: 1 } }, Mutex: { acquireCount: { r: 1 } } } storage:{ data: { bytesRead: 416 } } protocol:op_msg 3ms
2025-04-24T21:24:09.975+0800 D1 NETWORK [conn1] conca _processMessage _sep->handleRequest dbresponse
2025-04-24T21:24:09.978+0800 D1 NETWORK [conn1] conca _processMessage !toSink.empty()
2025-04-24T21:24:09.978+0800 D1 NETWORK [conn1] conca _processMessage makeExhaustMessage
2025-04-24T21:24:09.979+0800 D1 NETWORK [conn1] conca _processMessage TrafficRecorder::get(_serviceContext) .observe
2025-04-24T21:24:09.980+0800 D1 NETWORK [conn1] conca _processMessage _sinkMessage
2025-04-24T21:24:09.981+0800 D1 NETWORK [conn1] conca _sinkMessage State::Source
2025-04-24T21:24:09.986+0800 D1 NETWORK [conn1] conca _sinkMessage store State::SinkWait
2025-04-24T21:24:09.986+0800 D1 NETWORK [conn1] conca _sinkCallback State::SinkWait
2025-04-24T21:24:09.987+0800 D1 NETWORK [conn1] conca _sinkCallback store State::Source
2025-04-24T21:24:09.988+0800 D1 NETWORK [conn1] conca func end
2025-04-24T21:24:09.989+0800 D1 NETWORK [conn1] conca guard.release() end
2025-04-24T21:24:09.990+0800 D1 NETWORK [conn1] conca_serviceExecutor->schedule(std::move(func), flags, taskName)
2025-04-24T21:24:09.991+0800 D1 NETWORK [conn1] conca_serviceExecutor->schedule(std::move(func), flags, taskName)
2025-04-24T21:24:09.992+0800 D1 NETWORK [conn1] conca _runNextInGuard State::Source
2025-04-24T21:24:09.993+0800 D1 NETWORK [conn1] conca _sourceMessage State::Source
2025-04-24T21:24:09.994+0800 D1 NETWORK [conn1] conca _sourceMessage store State::SourceWait总结：
mongo第一条命令流程是：State::Created 》 State::Source 》State::SourceWait 》 State::Process 》 State::SinkWait 》 State::Source 》State::SourceWait
mongo第二条命令流程是： State::Process 》 State::SinkWait 》 State::Source 》State::SourceWait
mongo第三条命令流程是： State::Process 》 State::SinkWait 》 State::Source 》State::SourceWait