Simple AMQP client in Scala/Akka based on the RabbitMQ java client.
This client provides a simple API for
- publishing and consuming messages over AMQP
- setting up RPC clients and servers
- automatic reconnection
It is based on the Akka 2.0 framework.
- This client is compatible with AMQP 0.9.1, not AMQP 1.0.
- This client is most probably not easily usable from Java
This very simple library is being used in production in a few projects now, either directly or through the Akka AMQP Proxies pattern, and so far so good.... So it kind of works and will be maintained for some time :-)
<repositories>
<repository>
<id>sstone snapshots</id>
<url>http://sstone.github.com/sstone-mvn-repo/snapshots</url>
</repository>
<repository>
<id>sstone releases</id>
<url>http://sstone.github.com/sstone-mvn-repo/releases</url>
</repository>
</repositories>
<dependencies>
<dependency>
<groupId>com.aphelia</groupId>
<artifactId>amqp-client_2.9.2</artifactId>
<version>1.1-SNAPSHOT</version>
</dependency>
<dependency>
<groupId>com.typesafe.akka</groupId>
<artifactId>akka-actor</artifactId>
<version>2.0.3</version>
</dependency>
</dependencies>Please note that the Akka dependency is now in the "provided" scope which means that you'll have to define it explicitely in your maven/sbt projects.
The latest snapshot (development) version is 1.1-SNAPSHOT, the latest released version is 1.0
This is a thin wrapper over the RabbitMQ java client, which tries to take advantage of the nice actor model provided by the Akka library. There is no effort to "hide/encapsulate" the RabbitMQ library (and I don't really see the point anyway since AMQP is a binary protocol spec, not an API spec). So to make the most of this library you should first check the documentation for the RabbitMQ client, and learn a bit about AMQP 0.9.1. There are very nice tutorial on the RabbitMQ website, and also there, and probably many other...
- AMQP connections are equivalent to "physical" connections. They are managed by ConnectionOwner objects. Each ConnectionOwner object manages a single connection and will try and reconnect when the connection is lost.
- AMQP channels are multiplexed over AMQP connections. You use channels to publish and consume messages. Channels are managed by ChannelOwner objects.
ConnectionOwner and ChannelOwner are implemened as Akka actors, using Akka supervision and the very useful Akka FSM:
- channel owners are created by connection owners
- when a connection is lost, the connection owner will create a new connection and provide each of its children with a new channel
- connection owners and channel owners are implemented as Finite State Machines, with 2 possible states: Connected and Disconnected
- For a connection owner, "connected" means that it owns a valid connection to the AMQP broker
- For a channel owner, "connected" means that it owns a valid AMQP channel
YMMV, but using few connections (one per JVM) and many channels per connection (one per thread) is a common practice.
To create a connection, and a channel owner that you can use to publish message:
val connFactory = new ConnectionFactory()
connFactory.setHost("rabbit")
val conn = system.actorOf(Props(new ConnectionOwner(connFactory)), name = "conn")
val producer = ConnectionOwner.createActor(conn, Props(new ChannelOwner()))
waitForConnection(system, consumer, producer).await()
producer ! Publish("amq.direct", "my_key", "yo!".getBytes)
To consume messages:
val connFactory = new ConnectionFactory()
connFactory.setHost("localhost")
val conn = system.actorOf(Props(new ConnectionOwner(connFactory)), name = "conn")
// use the standard direct exchange
val exchange = ExchangeParameters(name = "amq.direct", exchangeType = "", passive = true)
// and a shared queue
val queue = QueueParameters(name = "my_queue", passive = false, exclusive = true, autodelete = true)
val foo = system.actorOf(Props(new Actor {
def receive = {
case Delivery(tag, envelope, properties, body) => {
println("got a message")
println(properties)
sender ! Ack(envelope.getDeliveryTag)
}
}
}))
// create a consumer that will pass all messages to the foo Actor; the consumer will declare the bindings
val consumer = ConnectionOwner.createActor(conn, Props(new Consumer(List(Binding(exchange, queue, "my_key", autoack = false)), foo)), 5000 millis)
val producer = ConnectionOwner.createActor(conn, Props(new ChannelOwner()))
waitForConnection(system, consumer, producer).await()
producer ! Publish("amq.direct", "my_key", "yo!".getBytes, Some(new BasicProperties.Builder().contentType("my content").build()))
Typical RPC with AMQP follows this pattern:
- client sets up a private, exclusive response queue
- client sends message and set their 'replyTo' property to the name of this response queue
- server processes the message and replies to its 'replyTo' queue by publishing the response to the default exchange using the queue name as routing key (all queues are bound to their name on the default exchange)
Usually, you would set up several RPC servers which all use the same shared queue. The broker will load-balance messsages between these consumers using round-robin distribution, which can be combined with 'prefetch' channel settings. Setting 'prefetch' to 1 is very useful if you need resource-based (CPU, ...) load-balancing.
But you can also extend this pattern by setting up RPC servers which all use private exclusive queues bound to the same key. In this case, each server will receive the same request and will send back a response. This is very useful if you want to break a single operation into multiple, parallel steps. For example, if you want to decrypt things, you could divide the key space into N parts, set up one RPC server for each part, publish a single RPC request and wait for N responses.
This could be further extended with a simple 'workflow' pattern where each server publishes its results to the shared queue used by the next step. For example, if you want to chain steps A, B and C, set up a shared queue for each step, have 'A' processors publish to queue 'B', 'B' processors publish to queue 'C' ....
Please check ChannelOwnerSpec.scala in src/test/scala/com/aphelia/amqp/ChannelOwnerSpec.scala for more comprehensive samples
/**
* basic consumer/producer test
*/
def testConsumer() {
val system = ActorSystem("MySystem")
val connFactory = new ConnectionFactory()
connFactory.setHost("localhost")
// create a "connection owner" actor, which will try and reconnect automatically if the connection ins lost
val conn = system.actorOf(Props(new ConnectionOwner(connFactory)), name = "conn")
// use the standard direct exchange
val exchange = ExchangeParameters(name = "amq.direct", exchangeType = "", passive = true)
// and an exclusive, private queue (name = "" means that the broker will generate a random name)
val queue = QueueParameters(name = "", passive = false, exclusive = true)
val foo = system.actorOf(Props(new Actor {
def receive = {
case Delivery(tag, envelope, properties, body) => {
println("got a message")
sender ! Ack(envelope.getDeliveryTag)
}
}
}))
// create a consumer that will pass all messages to the foo Actor; the consumer will declare the bindings
val consumer = ConnectionOwner.createActor(conn, Props(new Consumer(List(Binding(exchange, queue, "my_key", autoack = false)), foo)), 5000 millis)
val producer = ConnectionOwner.createActor(conn, Props(new ChannelOwner()))
waitForConnection(system, consumer, producer).await()
producer ! Publish("amq.direct", "my_key", "yo!".getBytes)
consumer ! PoisonPill
producer ! PoisonPill
system.shutdown()
}
/**
* basic transaction test
*/
def testTransactions() {
val system = ActorSystem("MySystem")
val connFactory = new ConnectionFactory()
connFactory.setHost("localhost")
// create a "connection owner" actor, which will try and reconnect automatically if the connection ins lost
val conn = system.actorOf(Props(new ConnectionOwner(connFactory)), name = "conn")
val exchange = ExchangeParameters(name = "amq.direct", exchangeType = "", passive = true)
val queue = QueueParameters(name = "queue", passive = false, exclusive = false)
val foo = system.actorOf(Props(new Actor {
def receive = {
case Delivery(tag, envelope, properties, body) => println("got a message")
}
}))
val producer = ConnectionOwner.createActor(conn, Props(new ChannelOwner()))
val consumer = ConnectionOwner.createActor(conn, Props(new Consumer(Binding(exchange, queue, "my_key", true) :: Nil, foo)))
waitForConnection(system, producer, consumer).await()
for(i <- 0 to 10) producer ! Transaction(Publish("amq.direct", "my_key", "yo".getBytes, true, false) :: Nil)
consumer ! PoisonPill
producer ! PoisonPill
foo ! PoisonPill
system.shutdown()
}
/**
* RPC sample where each request is picked up by 2 different server and results in 2 responses
*/
def testMultipleResponses() {
val system = ActorSystem("MySystem")
val connFactory = new ConnectionFactory()
connFactory.setHost("localhost")
// create a "connection owner" actor, which will try and reconnect automatically if the connection ins lost
val conn = system.actorOf(Props(new ConnectionOwner(connFactory)), name = "conn")
// basic processor
val proc = new RpcServer.IProcessor() {
def process(delivery : Delivery) = {
println("processing")
Some(delivery.body)
}
def onFailure(delivery : Delivery, e: Exception) = Some(e.toString.getBytes)
}
// amq.direct is one of the standard AMQP exchanges
val exchange = ExchangeParameters(name = "amq.direct", exchangeType = "", passive = true)
// this is how you define an exclusive, private response queue. The name is empty
// which means that the broker will generate a unique, random name when the queue is declared
val queue = QueueParameters(name = "", passive = false, exclusive = true)
// create 2 servers, each with its own private queue bound to the same key
val server1 = ConnectionOwner.createActor(conn, Props(new RpcServer(queue, exchange, "my_key", proc)), 2000 millis)
val server2 = ConnectionOwner.createActor(conn, Props(new RpcServer(queue, exchange, "my_key", proc)), 2000 millis)
val client = ConnectionOwner.createActor(conn, Props(new RpcClient()), 2000 millis)
waitForConnection(system, server1, server2, client).await()
for (i <-0 to 10) {
try {
// send one request and wait for 2 responses
val future = client.ask(Request(Publish("amq.direct", "my_key", "client1".getBytes) :: Nil, 2))(1000 millis)
val result = Await.result(future, 1000 millis).asInstanceOf[Response]
println("result : " + result)
Thread.sleep(100)
}
catch {
case e: Exception => println(e.toString)
}
}
client ! PoisonPill
server1 ! PoisonPill
server2 ! PoisonPill
system.shutdown()
}
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