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Clojure APIs for Message Queues

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README.md

Clamq - Clojure APIs for Message Queues - Version 0.4

Clamq is a Clojure adpater for interacting with message queues, providing simple APIs to connect to brokers and sending/consuming messages to/from message queues and topics.

Clamq supports JMS and AMQP brokers, more specifically:

  • JMS
    • Any JMS broker for which you're able to provide a javax.jmsConnectionFactory object.
    • ActiveMQ
  • AMQP

Configuring dependencies

Clamq comes with several sub-projects, depending on the broker/APIs you want to use: all dependencies are available on Clojars.

For working with generic JMS, import the following dependency:

[clamq/clamq-jms "0.4"]

For working with ActiveMQ, import the following dependency:

[clamq/clamq-activemq "0.4"]

For working with RabbitMQ, import the following dependency:

[clamq/clamq-rabbitmq "0.4"]

Connecting to brokers

Connection to your preferred broker is the first mandatory step in order to interact with your message queues.

Generic JMS

In order to connect to generic JMS brokers you need the following namespaces:

(use 'clamq.protocol.connection 'clamq.jms)

Now you can connect to a generic JMS broker as follows:

(jms-connection connection-factory close-fn)

Where:

  • connection-factory is a javax.jmsConnectionFactory object.
  • close-fn is a no-arg function invoked when closing the connection, whose implementation must be provided in order to properly close the connection factory and its connection.

ActiveMQ

In order to connect to an ActiveMQ broker you need the following namespaces:

(use 'clamq.protocol.connection 'clamq.activemq)

Now you can connect as follows:

(activemq-connection broker-uri)

Where:

  • broker-uri is the ActiveMQ broker URI.

It also supports the following optional keyed parameters:

  • :username : the username for broker authentication.
  • :password : the password for broker authentication.

RabbitMQ

In order to connect to a RabbitMQ broker you need the following namespaces:

(use 'clamq.protocol.connection 'clamq.rabbitmq)

Now you can connect as follows:

(rabbitmq-connection broker-host)

Where:

  • broker-host is the host where the RabbitMQ broker is running.

It also supports the following optional keyed parameters:

  • :username : the username for broker authentication (defaults to "guest").
  • :password : the password for broker authentication (defaults to "guest").

Producer/Consumer APIs

Message producers

In order to start producing messages, you need the following namespaces:

(use 'clamq.protocol.producer)

Now you can define a producer as follows:

(producer connection configuration)

Where:

  • connection is a broker connection obtained as previously described.
  • configuration is an optional map of configuration parameters.

The configuration map currently supports the following keys:

  • :pubSub : valid only for JMS brokers, defines (true/false) if produced messages are for publish/subscribe, that is, must be sent to a topic.

Once defined, the producer can be used to send messages as follows:

(publish producer endpoint message attributes)

Where:

  • producer is a producer obtained as previously described.
  • endpoint is the definition of a message queue endpoint.
  • message is the message to be sent, of type text or object.
  • attributes is only for JMS brokers, and defines an optional map of attributes to set into the message.

The endpoint definition depends on the actual broker: for JMS brokers, it is just the queue/topic name, for AMQP brokers it is a map containing two entries, :exchange and :routing-key.

Message consumers

Message consumers work by asynchronously pulling messages from the queue they listen to, and processing them through a user-defined function.

In order to start consuming messages, you need the following namespaces:

(use 'clamq.protocol.consumer)

Now you can define the consumer as follows:

(consumer connection configuration)

Where:

  • connection is a broker connection obtained as previously described.
  • configuration is a mandatory map of configuration parameters.

The configuration map currently supports the following keys (mandatory, except where differently noted):

  • :endpoint is the name of a message queue endpoint to consume from.
  • :on-message is the handler function called at each consumed message, accepting the message itself as unique argument.
  • :transacted defines (true/false) if the message consumption must be locally transactional.
  • :pubSub in only valid for JMS brokers, and defines if consumer messages are from publish/subscribe, that is, must be consumed from a topic (optional).
  • :limit defines the max number of consumed messages, after which the consumer stops itself (optional: defaults to 0, unlimited).
  • :on-failure defines a function called in case of exception during message handling (optional: by default the exception is just rethrown).

When setting an explicit :limit, you're strongly suggested to also set :transacted to true, in order to avoid losing messages when the consumption is interrupted due to the reached limit.

Once defined, asynchronous consumers can be started and closed as follows:

(start consumer)
(close consumer)

Message sequences

Message sequences are synchronous consumers based on Clojure lazy sequences, consuming messages only when actually requested to do so.

In order to establish a message sequence, you need the following namespaces:

(use 'clamq.protocol.seqable)

Then, you need to define a seqable consumer, which will act as the provider for the message sequence:

(def consumer (seqable connection configuration))

Where:

  • connection is a broker connection obtained as previously described.
  • configuration is a mandatory map of configuration parameters.

The configuration map currently supports the following keys (mandatory, except where differently noted):

  • :endpoint is the name of a message queue endpoint to consume from.
  • :timeout is the maximum time in milliseconds to wait for available message; if no new messages are found, the sequence returns nil.

Now you can obtain the message sequence:

(mseq consumer)

In order to advance with message consumption, you need to acknowledge the current message to the consumer, otherwise if trying to access another message without first acknowledging, the consumer will simply time out and return nil:

(ack consumer)

So here's a simple example about how to move all available messages from your sequable consumer to a vector:

(with-open [consumer (seqable connection {:endpoint "queue" :timeout 1000})]
    (reduce into [] 
        (map #(do (ack consumer) [%1]) (mseq consumer))))

Finally, seqable consumers (with related message sequence) must be closed as follows:

(close consumer)

Pipes and Filters APIs

Pipes

Pipes define a conduit between source and destination endpoints, which can be different queues/topics belonging to different brokers. Each message flowing between endpoints in a pipe is filtered by a filter function, and eventually processed by a failure function in case of errors.

Clamq provides different kind of pipes:

Unicast pipes, connecting two single endpoints:

(use 'clamq.pipes)
(def pipe (single-pipe {
  :from {:connection connection :endpoint source :pubSub pubSub}
  :to {:connection connection :endpoint destination} :transacted true :pubSub pubSub :limit limit :filter-by filter-fn :on-failure failure-fn}))

Multicast pipes, connecting a source endpoint with multiple destination endpoints:

(use 'clamq.pipes)
(def pipe (multi-pipe {
  :from {:connection connection :endpoint source :pubSub pubSub}
  :to [{:connection connection :endpoint destination1 :pubSub pubSub :filter-by filter-fn }] :transacted true :limit limit :on-failure failure-fn}))

Router pipes, connecting a source endpoint with one or more destination endpoints dynamically defined by a router function:

(use 'clamq.pipes)
(def pipe (router-pipe {
  :from {:connection connection :endpoint source :pubSub pubSub}
  :route-with router-fn :transacted true :limit limit :on-failure failure-fn}))

Where:

  • :connection is a broker connection obtained as previously described.
  • :endpoint is the definition of a JMS or AMQP message queue endpoint as previously described.
  • :transacted defines (true/false) if the message consumption must be transactional.
  • :limit defines the max number of flowing messages, after which the pipe stops consuming messages (optional: defaults to 0, unlimited).
  • :pubSub defines if messages are from/to a topic (valid only for JMS brokers, defaults to false).
  • :filter-by defines the name of the filter function (optional, defaults to identity function).
  • :on-failure defines a function called in case of exceptions during message handling (optional: by default the exception is just rethrown).
  • :route-with defines the name of the router function (only for router pipes, and mandatory).

Once defined, you can open/close pipes to let messages flow:

(open pipe)
(close pipe)

Filters

Pipes come with three types of filter functions, as previously cited:

Standard filter functions, configured under the :filter-by key, which takes the consumed message and returns an eventually processed message:

(defn my-identity-filter [message] message)

Router filters, configured under the :route-with key, which takes the consumed message and returns a vector of maps, each containing the message to route, the destination endpoint and the destination connection:

(defn my-router [message] {:message message :endpoint destination :connection connection-object})

Finally, failure filters, configured under the :on-failure key, fired in case of exceptions during message handling and taking a map containing the handled message (:message) and the raised exception (:exception):

(defn my-failure-handler [failure] (println "Message: " (failure :message)) (println "Exception: " (failure :exception)))

Runner

Clamq Runner is a quick and easy way to run Clojure code containing Clamq instructions, which comes in handy when you want to quickly interact with message queues, i.e. for processing messages on the fly or just moving them around.

In order to use it, you have to write a Clojure file containing some Clamq code, for example:

(with-open [c1 (activemq/activemq-connection "tcp://localhost:61616") c2 (rabbitmq/rabbitmq-connection "localhost")]
  (println "Opening pipe...")
  (pipes/open (pipes/single-pipe {:from {:connection c1 :endpoint "q1"} :to {:connection c2 :endpoint {:routing-key "q2"}} :transacted true}))
  (println "Done!")
  (.join (Thread/currentThread)))

You have access to all Clamq namespaces and functions, more specifically:

  • clamq.protocol.connection can be accessed with the "connection" prefix.
  • clamq.protocol.consumer can be accessed with the "consumer" prefix.
  • clamq.protocol.seqable can be accessed with the "seqable" prefix.
  • clamq.protocol.producer can be accessed with the "producer" prefix.
  • clamq.pipes can be accessed with the "pipes" prefix.
  • clamq.activemq can be accessed with the "activemq" prefix.
  • clamq.rabbitmq can be accessed with the "rabbitmq" prefix.

Then, download the Clamq Runner from the Downloads section and simply run it as follows:

java -jar clamq-runner-version-standalone.jar your.clj

A note about resource management

As you may have noted, connections, consumers and pipes provide a close function to properly free resources. So, you can use all of them in idiomatic "with-open" blocks, as follows:

(with-open [connection (activemq-connection "tcp://localhost:61616")]
    (do-something-with connection)
    )

Examples

Clamq tests work good as examples too:

Feedback

Feel free to open issues on the project tracker, and/or contact me on twitter: sbtourist

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