上一篇博客我们介绍了remote模式下Actor的创建,其实与local的创建并没有太大区别,一般情况下还是使用LocalActorRef创建了Actor。那么发消息是否意味着也是相同的呢?
既然actorOf还是委托给了LocalActorRef,那么在本地创建的Actor发消息还是跟以前一样的,那么如果如何给远程的Actor发消息呢?我们一般是通过actorSelection或者给远程Actor发送一个Identify消息,来接收对应的ActorRef,然后再发消息。我们来分析一下这两者的区别。
首先来看actorSelection,不管是用ActorSystem或者ActorContext的actorSelection方法,最终都是调用了ActorRefFactory对应的方法。
/** * Construct an [[akka.actor.ActorSelection]] from the given path, which is * parsed for wildcards (these are replaced by regular expressions * internally). No attempt is made to verify the existence of any part of * the supplied path, it is recommended to send a message and gather the * replies in order to resolve the matching set of actors. */ def actorSelection(path: String): ActorSelection = path match { case RelativeActorPath(elems) ⇒ if (elems.isEmpty) ActorSelection(provider.deadLetters, "") else if (elems.head.isEmpty) ActorSelection(provider.rootGuardian, elems.tail) else ActorSelection(lookupRoot, elems) case ActorPathExtractor(address, elems) ⇒ ActorSelection(provider.rootGuardianAt(address), elems) case _ ⇒ ActorSelection(provider.deadLetters, "") }
我们发现它支持两种类型的path:RelativeActorPath、ActorPathExtractor。
/** * Extractor for so-called “relative actor paths” as in “relative URI”, not in * “relative to some actor”. Examples: * * * "grand/child" * * "/user/hello/world" */ object RelativeActorPath extends PathUtils { def unapply(addr: String): Option[immutable.Seq[String]] = { try { val uri = new URI(addr) if (uri.isAbsolute) None else Some(split(uri.getRawPath, uri.getRawFragment)) } catch { case _: URISyntaxException ⇒ None } } }
RelativeActorPath提取器比较简单,就是创建了一个URI对象,然后判断其是否为Absolute,如果是就返回None,如果不是就返回对应的elemes。对于远程Actor,我们一般会指定主机名、端口号,例如akka.tcp://actorSystemName@10.0.0.1:2552/user/actorName,根据URI的定义,这个URI的schema是akka.tcp,很显然是Absolute,那就会返回None。
/** * Given an ActorPath it returns the Address and the path elements if the path is well-formed */ object ActorPathExtractor extends PathUtils { def unapply(addr: String): Option[(Address, immutable.Iterable[String])] = try { val uri = new URI(addr) uri.getRawPath match { case null ⇒ None case path ⇒ AddressFromURIString.unapply(uri).map((_, split(path, uri.getRawFragment).drop(1))) } } catch { case _: URISyntaxException ⇒ None } }
ActorPathExtractor这个提取器的名称定义的是有问题的,既然actorSelection只支持两种类型的路径选择:本地和远程。第一个解析器定义成相对路径,那么后面一个就直接是绝对路径好了啊,为啥用ActorPathExtractor这样蹩脚的命名?难道本地模式下,就不是ActorPath提取器了?我们来看看对于akka.tcp://actorSystemName@10.0.0.1:2552/user/actorName提取出了什么。经调试,address是akka.tcp://actorSystemName@10.0.0.1:2552,elems就是后面的user、actorName了。
也就是说remote模式下,如果有host、prot等信息就会返回ActorSelection(provider.rootGuardianAt(address), elems)这个类。不过好像无论哪种情况都返回这个类,好尴尬啊,但传入的第一个参数是不同的:provider.rootGuardianAt(address)。也就是说actorSelection这个函数是不区分当前的模式的,只要含有host/port就会传入provider.rootGuardianAt(address),否则就传入provider.rootGuardian。如果在local模式下,也强制用actorSelection查找远程Actor会发生什么呢?我们来看看LocalActorRefProvider。
override def rootGuardianAt(address: Address): ActorRef = if (address == rootPath.address) rootGuardian else deadLetters
local模式下,如果待查询actor的地址就是本地地址,则直接在本地返回查找;否则就返回deadLetters。其实是无法查找远程actor的。那么RemoteActorRefProvider呢?
def rootGuardianAt(address: Address): ActorRef = { if (hasAddress(address)) rootGuardian else try { new RemoteActorRef(transport, transport.localAddressForRemote(address), RootActorPath(address), Nobody, props = None, deploy = None) } catch { case NonFatal(e) ⇒ log.error(e, "No root guardian at [{}]", address) new EmptyLocalActorRef(this, RootActorPath(address), eventStream) } }
当然了,它也会判断一下本地地址是否包含待查询地址(防止多网卡或其他特殊情况),如果包含,则意味着是本地Actor交给rootGuardian;否则就创建RemoteActorRef。
分析到这里我们知道了,其实在remote模式下,actorSelection返回了一个RemoteActorRef,还记得这个类的作用嘛?我们之前简单分析过,它其实是对远程Acotor的一个本地网络代理,也就是说所有通过actorSelection发送给远程actor的消息,都会经过他中转。
我们继续分析ActorSelection的源码
/** * Construct an ActorSelection from the given string representing a path * relative to the given target. This operation has to create all the * matching magic, so it is preferable to cache its result if the * intention is to send messages frequently. */ def apply(anchorRef: ActorRef, elements: Iterable[String]): ActorSelection = { val compiled: immutable.IndexedSeq[SelectionPathElement] = elements.collect({ case x if !x.isEmpty ⇒ if ((x.indexOf('?') != -1) || (x.indexOf('*') != -1)) SelectChildPattern(x) else if (x == "..") SelectParent else SelectChildName(x) })(scala.collection.breakOut) new ActorSelection with ScalaActorSelection { override val anchor = anchorRef override val path = compiled } }
很显然这里的anchorRef是上面创建的RemoteActorRef实例,其中ActorSelection的anchor(锚定)是anchorRef。至此,一个ActorSelection创建完毕。那么如何发消息呢?这就需要分析tell或者!方法了。
def tell(msg: Any, sender: ActorRef): Unit = ActorSelection.deliverSelection(anchor.asInstanceOf[InternalActorRef], sender, ActorSelectionMessage(msg, path, wildcardFanOut = false))
其实乍一看,我们应该明白,这就是在deliverSelection函数内部,把消息封装成ActorSelectionMessage发送给了anchor。
该函数首先判断sel的elements是否为空,很显然不为空,进入rec函数。该函数比较复杂而且还是一个尾递归函数,但我们知道此处的ref就是RemoteActorRef,那么RemoteActorRef是不是一个ActorRefWithCell呢?
private[akka] class RemoteActorRef private[akka] ( remote: RemoteTransport, val localAddressToUse: Address, val path: ActorPath, val getParent: InternalActorRef, props: Option[Props], deploy: Option[Deploy]) extends InternalActorRef with RemoteRef
那么rec就会走到case _的逻辑,也就是把消息转发给了前面创建的RemoteActorRef,我们来看看这个示例是如何实现tell的。
override def !(message: Any)(implicit sender: ActorRef = Actor.noSender): Unit = { if (message == null) throw InvalidMessageException("Message is null") try remote.send(message, OptionVal(sender), this) catch handleException(message, sender) }
RemoteActorRef这个类,通过remote把消息发送出去了,那么remote是什么呢?RemoteTransport是不是很熟悉?在ActorSystem启动的时候我们分析过这个对象,它是Remoting类的实例,Remoting里面send方法是怎样的呢?
override def send(message: Any, senderOption: OptionVal[ActorRef], recipient: RemoteActorRef): Unit = endpointManager match { case Some(manager) ⇒ manager.tell(Send(message, senderOption, recipient), sender = senderOption getOrElse Actor.noSender) case None ⇒ throw new RemoteTransportExceptionNoStackTrace("Attempted to send remote message but Remoting is not running.", null) }
它又把消息转发给了manager,而manager就是endpointManager。endpointManager是不是也比较眼熟呢?前面文章中我们也见到过,这是一个EndpointManager实例,而EndpointManager是一个Actor。请注意这里用Send又对message进行了封装。EndpointManager是如何对Send消息进行反应的呢?
case s @ Send(message, senderOption, recipientRef, _) ⇒ val recipientAddress = recipientRef.path.address def createAndRegisterWritingEndpoint(): ActorRef = { endpoints.registerWritableEndpoint( recipientAddress, uid = None, createEndpoint( recipientAddress, recipientRef.localAddressToUse, transportMapping(recipientRef.localAddressToUse), settings, handleOption = None, writing = true)) } endpoints.writableEndpointWithPolicyFor(recipientAddress) match { case Some(Pass(endpoint, _)) ⇒ endpoint ! s case Some(Gated(timeOfRelease)) ⇒ if (timeOfRelease.isOverdue()) createAndRegisterWritingEndpoint() ! s else extendedSystem.deadLetters ! s case Some(Quarantined(uid, _)) ⇒ // timeOfRelease is only used for garbage collection reasons, therefore it is ignored here. We still have // the Quarantined tombstone and we know what UID we don't want to accept, so use it. createAndRegisterWritingEndpoint() ! s case None ⇒ createAndRegisterWritingEndpoint() ! s }
分析以上逻辑,简单来看,会先判断是不是存在一个endpoint,如果存在说明链接已经建立,可以直接发送,否则出于其他状态,就重新创建endpoint,然后把消息转发给该endpoint。
def registerWritableEndpoint(address: Address, uid: Option[Int], endpoint: ActorRef): ActorRef = addressToWritable.get(address) match { case Some(Pass(e, _)) ⇒ throw new IllegalArgumentException(s"Attempting to overwrite existing endpoint [$e] with [$endpoint]") case _ ⇒ // note that this overwrites Quarantine marker, // but that is ok since we keep the quarantined uid in addressToRefuseUid addressToWritable += address → Pass(endpoint, uid) writableToAddress += endpoint → address endpoint }
registerWritableEndpoint没有太复杂的逻辑,就是查询addressToWritable这个HashMap,如果不存在则把对应的endpoint加入缓存,并返回endpoint。而endpoint是通过createEndpoint创建的。
private def createEndpoint( remoteAddress: Address, localAddress: Address, transport: AkkaProtocolTransport, endpointSettings: RemoteSettings, handleOption: Option[AkkaProtocolHandle], writing: Boolean): ActorRef = { require(transportMapping contains localAddress, "Transport mapping is not defined for the address") // refuseUid is ignored for read-only endpoints since the UID of the remote system is already known and has passed // quarantine checks val refuseUid = endpoints.refuseUid(remoteAddress) if (writing) context.watch(context.actorOf( RARP(extendedSystem).configureDispatcher(ReliableDeliverySupervisor.props( handleOption, localAddress, remoteAddress, refuseUid, transport, endpointSettings, AkkaPduProtobufCodec, receiveBuffers)).withDeploy(Deploy.local), "reliableEndpointWriter-" + AddressUrlEncoder(remoteAddress) + "-" + endpointId.next())) else context.watch(context.actorOf( RARP(extendedSystem).configureDispatcher(EndpointWriter.props( handleOption, localAddress, remoteAddress, refuseUid, transport, endpointSettings, AkkaPduProtobufCodec, receiveBuffers, reliableDeliverySupervisor = None)).withDeploy(Deploy.local), "endpointWriter-" + AddressUrlEncoder(remoteAddress) + "-" + endpointId.next())) }
createEndpoint最终创建了ReliableDeliverySupervisor这个Actor,也就是说RemoteActorRef最终又把消息发送给了ReliableDeliverySupervisor,ReliableDeliverySupervisor收到消息去调用handleSend方法。
private def handleSend(send: Send): Unit = if (send.message.isInstanceOf[SystemMessage]) { val sequencedSend = send.copy(seqOpt = Some(nextSeq())) tryBuffer(sequencedSend) // If we have not confirmed the remote UID we cannot transfer the system message at this point just buffer it. // GotUid will kick resendAll() causing the messages to be properly written. // Flow control by not sending more when we already have many outstanding. if (uidConfirmed && resendBuffer.nonAcked.size <= settings.SysResendLimit) writer ! sequencedSend } else writer ! send
除去特殊情况,用户发的普通消息又发送给了writer,艾玛我去,真是绕啊。writer是什么呢?
var writer: ActorRef = createWriter()
private def createWriter(): ActorRef = { context.watch(context.actorOf(RARP(context.system).configureDispatcher(EndpointWriter.props( handleOrActive = currentHandle, localAddress = localAddress, remoteAddress = remoteAddress, refuseUid, transport = transport, settings = settings, AkkaPduProtobufCodec, receiveBuffers = receiveBuffers, reliableDeliverySupervisor = Some(self))).withDeploy(Deploy.local), "endpointWriter")) }
很显然这又是一个ACor!!!哎,继续查找EndpointWriter这个Actor喽
def receive = if (handle.isEmpty) initializing else writing
val writing: Receive = { case s: Send ⇒ if (!writeSend(s)) { enqueueInBuffer(s) scheduleBackoffTimer() context.become(buffering) } // We are in Writing state, so buffer is empty, safe to stop here case FlushAndStop ⇒ flushAndStop() case AckIdleCheckTimer if ackDeadline.isOverdue() ⇒ trySendPureAck() }
这个Actor会先判断是否已经初始化,这里就假设初始化吧,初始化之后就会进入writing这个偏函数,对send类型的消息,又调用了writeSend函数。
这个函数简单来看,就是调用codec对消息进行序列化,然后创建了一个pdu,最终把pdu通过handle的write发送出去。handle又是什么呢?
var handle: Option[AkkaProtocolHandle] = handleOrActive
private[remote] class AkkaProtocolHandle( _localAddress: Address, _remoteAddress: Address, val readHandlerPromise: Promise[HandleEventListener], _wrappedHandle: AssociationHandle, val handshakeInfo: HandshakeInfo, private val stateActor: ActorRef, private val codec: AkkaPduCodec) extends AbstractTransportAdapterHandle(_localAddress, _remoteAddress, _wrappedHandle, AkkaScheme) { override def write(payload: ByteString): Boolean = wrappedHandle.write(codec.constructPayload(payload)) override def disassociate(): Unit = disassociate(Unknown) def disassociate(info: DisassociateInfo): Unit = stateActor ! DisassociateUnderlying(info) }
handle最终是一个AkkaProtocolHandle,这个对象我们不再具体分析,我们可以认为这是一个本地与远程地址链接的通道,通过这个通道就可以与远程actor发送消息了。
分析到这个地方,actorSelection与远程通信的过程大概就梳理清楚了。为了方便理解,作者特意辛苦的画了一个流程图,以供参考。细心的读者一定会问,那我的消息通过handle发送出去了,对方怎么接收呢?接收之后怎么发送到指定actor的邮箱呢?这一点我们后面再分析。
actorSelection分析清楚了,剩下的就是通过ActorRef发送消息了。那么如何得到远程Actor的ActorRef呢?当然是“问”它了啊,怎么“问”呢?发消息啊。发什么消息呢?
/** * A message all Actors will understand, that when processed will reply with * [[akka.actor.ActorIdentity]] containing the `ActorRef`. The `messageId` * is returned in the `ActorIdentity` message as `correlationId`. */ @SerialVersionUID(1L) final case class Identify(messageId: Any) extends AutoReceivedMessage with NotInfluenceReceiveTimeout
官网对Identify的注释非常清楚,这个消息继承了AutoReceivedMessage,所有的Actor都理解该消息,且受到该消息后会返回akka.actor.ActorIdentity消息,里面包含当前Actor的ActorRef。那么所有的Actor为啥都理解该消息呢?
//Memory consistency is handled by the Mailbox (reading mailbox status then processing messages, then writing mailbox status final def invoke(messageHandle: Envelope): Unit = { val influenceReceiveTimeout = !messageHandle.message.isInstanceOf[NotInfluenceReceiveTimeout] try { currentMessage = messageHandle if (influenceReceiveTimeout) cancelReceiveTimeout() messageHandle.message match { case msg: AutoReceivedMessage ⇒ autoReceiveMessage(messageHandle) case msg ⇒ receiveMessage(msg) } currentMessage = null // reset current message after successful invocation } catch handleNonFatalOrInterruptedException { e ⇒ handleInvokeFailure(Nil, e) } finally { if (influenceReceiveTimeout) checkReceiveTimeout // Reschedule receive timeout } } def autoReceiveMessage(msg: Envelope): Unit = { if (system.settings.DebugAutoReceive) publish(Debug(self.path.toString, clazz(actor), "received AutoReceiveMessage " + msg)) msg.message match { case t: Terminated ⇒ receivedTerminated(t) case AddressTerminated(address) ⇒ addressTerminated(address) case Kill ⇒ throw ActorKilledException("Kill") case PoisonPill ⇒ self.stop() case sel: ActorSelectionMessage ⇒ receiveSelection(sel) case Identify(messageId) ⇒ sender() ! ActorIdentity(messageId, Some(self)) } }
如果读者看过我之前分析的文章对上面的代码一定还有印象,它是ActorCell里面处理消息的两个函数,invoke会先判断消息类型是不是AutoReceivedMessage,如果是就自己处理了,不会去调用开发者自定义的receive函数。而Identify属于AutoReceivedMessage,收到后给sender发送了ActorIdentity消息,该消息的第二个参数是当前Actor的ActorFef变量。这样本地的actor收到远程actor返回的ActorIdentity,就可以通过对方的ActorRef给它发送消息了。当然本地actor收到的ActorIdentity消息中,第二个参数应该是一个RemoteActorRef类型。如何通过RemoteActorRef发送消息,上文已经分析清楚了,其实actorSelection最终也是通过远程actor的ActorPath创建了对应的RemoteActorRef,来发送消息的。
至此给远程actor发消息的两种方法就讲解完毕了。其实还有第三种方式,就是在本地创建一个远程Actor,当然了最终还是需要通过RemoteActorRef发消息的,这个具体就不再详细介绍了。