AMQP as Promised

     

A high-level promise-based API built on amqplib extended with functions for AMQP-based RPC.

• amqplib API docs
• Old versions of this package were based on node-amqp.

Table of contents

• Configuration
• Examples
• API
  • The amqpc object
  • The exchange object
  • The queue object
  • RPC functions
• Changelog

Version Notes

5.0

Syntax to access the library has been changed in 5.0 to improve connection management. See the Running-section for instructions.

3.0

The underlying amqp library was changed from node-amqp to amqplib. Efforts have been made to keep everything as backwards compatible as possible, but some things have changed:

• Local mode is no longer supported.
• queue.shift() is no longer supported.
• Q has been dropped in favor of native promises. As a result, support for promise progress notifications over RPC is no longer supported.

Installing

npm install amqp-as-promised

Running

5.0+

    conf = require './myconf.json' # see example conf below
    ((require 'amqp-as-promised') conf.amqp).then (amqpc) ->

Earlier versions

    conf = require './myconf.json' # see example conf below
    amqpc = (require 'amqp-as-promised') conf.amqp

Configuration

As of version 0.1.0, the following config parameters are accepted, although we also try to keep backwards compatibility with the older format.

connection

Connection settings accepted by node-amqp. You need to at minimum specify either

• host
• vhost
• login
• password

or

• url.

rpc

• timeout: timeout in ms for rpc calls. Default: 1000ms

logLevel

• logLevel: sets the log level. Defaults to INFO. Possible levels are DEBUG, INFO, WARN, ERROR

errorHandler

Since 2.0.0 connection errors are rethrown to crash process.

• errorHandler: sets a handler function to receive the error instead of throwing to process. This option is deprecated, as a better way to do this is to attach an error event handler.

waitForConnection

Since 4.1.0

• waitForConnection: on startup, keeps retrying to connect until successful. Will not attempt reconnect after established connection.

Example config

{
    "connection": {
        "host": "192.168.0.10",
        "vhost": "test",
        "login": "test",
        "password": "supersecret"
    },
    "logLevel": "warn",
    "rpc": {
        "timeout": 2000
    }
}

Or with url:

{
    "connection": {
        "url": "amqp://myuser:supersecret@192.168.0.10/test"
    },
    "logLevel": "warn"
}

Events

Amqp-as-promised emits error events on unexpected network errors, for example then the connection to the server has been lost. It is up to the client to handle these errors, as amqp-as-promised doesn't reconnect automatically. Keep in mind that error recovery can be tricky, and the best option might be to just crash and restart the application on error.

This is a simple but effective error handler:

amqpc.on 'error', (err) ->
    console.log err
    process.exit 1

Unhandled errors

If there are no error handlers attached (either using amqp.on() or setting the errorHandler in the configuration), amqp-as-promised will as a last resort throw the error. This will most likely result in an application crash unless there is an uncaught exception handler set on the process.

Examples

Using amqpc to publish

amqpc.exchange('myexchange').then (ex) ->
    msg = {}
    msg.domain = domain
    ex.publish('mytopic.foo', msg).then ->
        console.log 'published message!'

Using amqpc to bind

This is shorthand for binding and subscribing.

amqpc.bind 'myexchange', 'myqueue', 'mytopic.#', (msg, headers, del) ->
    console.log 'received message', msg

To bind an anonymous queue.

amqpc.bind 'myexchange', '', 'mytopic.#', (msg, headers, del) ->
    console.log 'received message', msg

Or even shorter

amqpc.bind 'myexchange', 'mytopic.#', (msg, headers, del) ->
    console.log 'received message', msg

To bind the queue to the exchange without subscribing to it, skip the last parameter (the subscription callback). This is essentially the same as queue.bind myexchange, 'mytopic', except the exchange and queue are specified by their names:

amqpc.bind 'myexchange', 'myqueue', 'mytopic.#'

Using amqpc to get an anomymous queue

To create an anomymous queue.

amqpc.queue().then (q) -> console.log 'my queue', q

Using amqpc to do RPC-style calls

to send a message to a service that honors the replyTo/correlationId contract:

amqpc.rpc('myexchange', 'routing.key', msg, [headers], [options]).then (response) ->
    console.log 'received message', response

• headers is an optional parameter holding any custom headers to be passed on the RPC service.
• options supports the following settings
  • timeout - the timeout in ms for this call

Note! In earlier versions the response was an array that included the response headers. As of version 0.1.0, this is no longer the case.

Using amqpc to serve RPC-style calls

To set up a message consumer that automatically honors the replyTo/correlationId contract:

amqpc.serve 'myexchange', 'mytopic.#', (msg, headers, del) ->
    return { result: 'ok' }

The value returned from the handler will be sent back on the queue specified by the replyTo header, with the correlationId set.

If an exception is thrown by the handler, it will be propagated back to the client as an object:

{
  "error": {
    "message": <exception.message>,
    [ "code": <exception.code>, ]
    [ "errno": <exception.errno> ]
  }
}

Serve with prefetchCount/ack

To rate limit the rpc calls to 5 concurrent, we use an options object to set {ack:true, prefetchCount:5}.

Notice that the message acking is handled by the rpc backend wrapper.

amqpc.serve 'myexchange', 'mytopic.#', {ack:true, prefetchCount:5}, (msg, headers, del) ->
    return { result: 'ok' }

Shutting down

graceful = (opts) ->
    log.info 'Shutting down'
    amqpc.shutdown().then ->
        process.exit 0

process.on 'SIGINT', graceful
process.on 'SIGTERM', graceful

API

The amqpc object

amqpc.on(event, handler)

Attach an event handler. Currently only error events are supported.

amqpc.exchange(name, opts)

A promise for an exchange. If opts is omitted, then passive:true is assumed.

amqpc.queue(qname, opts)

A promise for a queue. If qname is omitted, "" is used. If opts is omitted, then exclusive:true is assumed if the name is empty, or passive:true if not.

Thus, amqpc.queue() will create a new exclusive, anonymous, queue that is automatically deleted on disconnect, while amqpc.queue('my-queue') will try to passively declare the existing queue my-queue.

See queue.* below.

amqpc.bind(exchange, queue, topic[, callback])

Shorthand for

1. If exchange is a string, then look up the existing exchange with that name.
2. If queue is a string, then look up the existing queue with that name.
3. Bind queue to exchange/topic.
4. Subscribe callback to queue (optional).

Parameters

• exchange - an exchange object or a string with the name of an exchange
• queue - a queue object or a string with the name of a queue
• topic - a string with the topic name.
• callback - see queue.subscribe below.

amqpc.shutdown()

Will unbind all queues and unsubscribe all callbacks then gracefully shut down the socket connection.

The exchange object

exchange.publish(routingKey, msg, options)

Publishes a message, returning a promise.

The queue object

queue.bind(exchange, topic)

Binds the queue to the given exchange (object, or string). Will unbind if queue was already bound.

queue.unbind()

Unbinds the queue (if currently bound).

queue.subscribe(opts, callback)

Subscribes the callback to this queue. Will unsubscribe any previous callback. If opts is omitted, defaults to ack: false, prefetchCount: 1

The callback will be called with arguments (msg, headers, deliveryinfo, actions), where actions is an object that holds these methods:

• acknowledge(): returns a Promise to acknowledge the message. This is only relevant if opts.ack is false (which is the default).

queue.unsubscribe()

Unsubscribes current callback (if any).

queue.name

Read only property with the queue name.

RPC functions

amqpc.rpc(exchange, routingKey, msg, [headers], [options])

Perform an AMQP-based remote procedure call, and returns a promise for the return value:

1. Creates an exlusive, anonymous, return queue if doesn't already exist.
2. Publishes an RPC-style message on the given exchange, with the specified routingkey, headers and options. The replyTo and correlationId headers are set automatically.
3. Waits for a reply on the return queue, and resolves the promise with the contents of the reply. If no reply is received before the timeout, the promise is instead rejected. Replies that are JSON objects that have an error property set are assumed to be remote errors, and will result in a rejected promise.

Parameters

• exchange - the name of an exchange, or an exchange object
• routingkey
• headers - AMQP headers to be sent with the message. See exchange.publish().
• options - valid options are:
  • timeout - timeout in milliseconds. If none is specified, the default value specified when creating the client is used.
  • compress - set to true to use payload compression

Compression

Since 0.4.0

The RPC mechanism has a transparent payload gzip compression of JSON objects Buffer. When activated both request and response are compressed. To activate, the rpc client must ask for compression by setting the compress option.

Example

amqpc.rpc('myexchange', 'routing.key', msg, [headers], {compress:true}).then (response) ->
    console.log 'received message', response