rascal

Rascal

Rascal is a config driven wrapper around amqplib with mostly safe defaults

tl;dr

var rascal = require('rascal')
var definitions = require('./definitions.json')

var config = rascal.withDefaultConfig(definitions)

rascal.createBroker(config, function(err, broker) {
    if (err) bail(err)

    broker.subscribe('s1', function(err, subscription) {
        if (err) bail(err)

        subscription.on('message', function(message, content, ackOrNack) {
            console.log(content)
            ackOrNack()
        }).on('error', bail)
    })
    setInterval(function() {
        broker.publish('p1', 'This is a test message', function(err, publication) {
            if (err) bail(err)
        })
    }, 100).unref()
})

function bail(err) {
    console.error(err)
    process.exit(1)
}

definitions.json

{
  "vhosts": {
    "/": {
      "exchanges": {
        "e1": {}
      },
      "queues": {
        "q1": {}
      },
      "bindings": {
        "b1": {
          "source": "e1",
          "destination": "q1"
        }
      }
    }
  },
  "publications": {
    "p1": {
      "exchange": "e1"
    }
  },
  "subscriptions": {
    "s1": {
      "queue": "q1"
    }
  }
}

About

Rascal is a wrapper for the excellent amqplib. One of the best things about amqplib is that it doesn't make assumptions about how you use it. Another is that it doesn't attempt to abstract away AMQP Concepts. As a result the library offers a great deal of control and flexibility, but the onus is on you adopt appropriate patterns and configuration. You need to be aware that:

• Messages are not persistent by default and will be lost if your broker restarts
• Messages that crash your app will be infinitely retried
• Without prefetch a sudden flood of messages may bust your event loop
• Dropped connections and borked channels will not be automatically recovered
• Any connection or channel errors are emitted as "error" events. Unless you handle them or use domains these will cause your application to crash

Rascal seeks to solve these problems.

Caveats

• Rascal currently implements only a small subset of the amqplib api. It was written with a strong bias towards moderate volume pub/sub systems for a project with some quite agressive timescales. If you need one of the missing api calls, then your best approach is to submit a PR.

• Rascal deliberately uses a new channel per publish operation. This is because any time a channel operation encounters an error, the channel becomes unusable and must be replaced. In an asynchronous environment such as node you are likely to have passed the channel reference to multiple callbacks, meaning that for every channel error, multiple publish operations will fail. The negative of the new channel per publish operation, is a little extra overhead and the chance of busting the maxium number of channels (the default is 65K). We urge you to test Rascal with realistic peak production loads to ensure this isn't the case.

• Rascal has plenty of automated tests, but is by no means battle hardened (yet).

VERY IMPORTANT SECTION ABOUT EVENT HANDLING

amqplib emits error events when a connection or channel encounters a problem. Rascal will listen for these and provided you use the default configuration will attempt automatic recovery (reconnection etc), however these events can indicate errors in your code, so it's also important to bring them to your attention. Rascal does this by re-emitting the error event, which means if you don't handle them, they will bubble up to the uncaught error handler and crash your application. There are three places where you should do this

1. Immediately after obtaining a broker instance

  broker.on('error', function(err) {
    console.error('Broker error', err)
  })

1. After subscribing to a channel

  broker.subscribe('s1', function(err, subscription) {
    subscription.on('message', function(message, content, ackOrNack) {
      // Do stuff with message
    }).on('error', function(err) {
      console.error('Subscriber error', err)
    })

1. After publishing a message

  broker.publish('p1', message, function(err, publication) {
    publication.on('error', function(err) {
      console.error('Publisher error', err)
    })
  })

Installation

npm install rascal

Configuration

Rascal provides what we consider to be sensible defaults (optimised for reliability rather than speed) for production and test environments.

var rascal = require('rascal')
var definitions = require('./your-config.json')
var config = rascal.withDefaultConfig(definitions)

or

var rascal = require('rascal')
var definitions = require('./your-test-config.json')
var config = rascal.withTestConfig(definitions)

We advise you to review these defaults before using them in an environment you care about.

Vhosts

namespace

Running automated tests against shared queues and exchanges is problematic. Messages left over from a previous test run can cause assertions to fail. Rascal has several strategies which help you cope with this problem, one of which is to namespace your queues and exchange.

{
  "vhosts": {
      "v1": {
          "namespace": true
      }
  }
}

If you specify "namespace" :true Rascal will prefix the queues and exchanges it creates with a uuid. Alternatively you can specify your own namespace, "namespace": "foo". Namespaces are also if you want to use a single vhost locally but multiple vhosts in other environments.

connection

The simplest way to specify a connection is with a url

{
  "vhosts": {
      "v1": {
          "connection": {
              "url":  "amqp://guest:[e-mail]:5672/v1?heartbeat=10"
          }
      }
  }
}

Alternatively you can specify the individual connection details

{
  "vhosts": {
      "v1": {
          "connection": {
              "slashes": true,
              "protocol": "amqp",
              "hostname": "localhost",
              "user": "guest",
              "password": "guest",
              "port": 5672,
              "vhost": "v1",
              "options": {
                  "heartbeat": 5
              }
          }
      }
  }
}

Any attributes you add to the "options" sub document will be converted to query parameters. Providing you merge your configuration with the default configuration rascal.withDefaultConfig(config) you need only specify the attributes you want to override

{
  "vhosts": {
      "v1": {
          "connection": {
              "hostname": "example.com",
              "user": "bob",
              "password": "secret"
          }
      }
  }
}

Rascal also supports automatic connection retries. It's enabled in the default config, or you want enable it specifically as follows.

{
  "vhosts": {
      "v1": {
          "connection": {
              "retry": {
                  "delay": 1000
              }
          }
      }
  }
}

Exchanges

assert

Setting assert to true will cause Rascal to create the exchange on initialisation. If the exchange already exists and has the same configuration (type, durability, etc) everything will be fine, however if the existing exchange has a different configuration an error will be returned. Assert is enabled in the default configuration.

check

If you don't want to create exchanges on initialisation, but still want to validate that they exist set assert to false and check to true

{
  "vhosts": {
      "v1": {
          "exchanges": {
              "e1": {
                  "assert": false,
                  "check": true
              }
          }
      }
  }
}

type

Declares the exchange type. Must be one of direct, topic, headers or fanout. The default configuration sets the exchange type to "topic" unless overriden.

options

Define any further configuration in an options block

{
  "vhosts": {
      "v1": {
          "exchanges": {
              "e1": {
                  "type": "fanout",
                  "options": {
                      "durable": false
                  }
              }
          }
      }
  }
}

Refer to the amqplib documentation for further exchange options.

Queues

assert

Setting assert to true will cause Rascal to create the queue on initialisation. If the queue already exists and has the same configuration (durability, etc) everything will be fine, however if the existing queue has a different configuration an error will be returned. Assert is enabled in the default configuration.

check

If you don't want to create queues on initialisation, but still want to validate that they exist set assert to false and check to true

{
  "vhosts": {
      "v1": {
          "queues": {
              "q1": {
                  "assert": false,
                  "check": true
              }
          }
      }
  }
}

purge

Enable to purge the queue during initialisation. Useful when running automated tests

{
  "vhosts": {
      "v1": {
          "queues": {
              "q1": {
                  "purge": true
              }
          }
      }
  }
}

options

Define any further configuration in an options block

{
  "queues": {
      "q1": {
          "options": {
              "durable": false,
              "exclusive": true
          }
      }
  }
}

Refer to the amqplib documentation for further queue options.

bindings

You can bind exchanges to exchanges, or exchanges to queues.

{
  "vhosts": {
    "v1": {
      "exchanges": {
        "e1": {
        }
      },
      "queues": {
        "q1": {
        }
      },
      "bindings": {
        "b1": {
          "source": "e1",
          "destination": "q1",
          "destinationType": "queue",
          "bindingKey": "foo"
        }
      }
    }
  }
}

When using Rascals defaults, destinationType will default to "queue" and "bindingKey" will default to "#" (although this is only applicable for topics anyway)

Publications

Now that you've bound your queues and exchanges, you need to start sending them messages. This is where publications come in.

{
  "publications": {
    "p1": {
      "exchange": "e1",
      "vhost": "v1",
      "routingKey": "foo"
    }
  }
}

broker.publish("p1", "some message")

If you prefer to send messages to a queue

{
  "publications": {
    "p1": {
      "exchange": "e1",
      "vhost": "v1"
    }
  }
}

Rascal supports text, buffers and anything it can JSON.stringify. When publish a message Rascal sets the contentType message property to "text/plain", "application/json" (it uses this when reading the message too). The broker.publish method is overloaded to accept a runtime routing key or options.

broker.publish("p1", "some message", callback)
broker.publish("p1", "some message", "some.routing.key", callback)
broker.publish("p1", "some message", { routingKey: "some.routing.key", options: { "expiration": 5000 } })

The callback parameters are err (indicating the publication could not be found) and publication. Listen to the publication's "success" event to obtain the Rascal generated message id and the "error" event to handle errors

broker.publish("p1", "some message", function(err, publication) {
  publication.on("success", function(messageId) {
     console.log("Message id was", messageId)
  }).on("error", function(err) {
     console.error("Error was", err.message)
  })
})

On publish option you should be aware of is the "persistent". Unless persistent is true, your messages will be discarded when you restart Rabbit. Despite having an impact on performance Rascal sets this in it's default configuration.

Refer to the amqplib documentation for further exchange options.

It's important to realise that even though publication emits a "success" event, this offers no guarantee that the message has been sent UNLESS you use a confirm channel.

{
  "publications": {
    "p1": {
      "exchange": "e1",
      "vhost": "v1",
      "confirm": true
    }
  }
}

Subscriptions

The real fun begins with subscriptions

{
  "subscriptions": {
    "s1": {
      "queue": "e1",
      "vhost": "v1"
    }
  }
}

broker.subscribe('s1', function(err, subscription) {
  subscription.on('message', function(message, content, ackOrNack) {
    // Do stuff with message
  }).on('error', function(err) {
    console.error('Subscriber error', err)
  })
})

It's very important that you handle errors emitted by the subscriber. If not an underlying channel error will bubble up to the uncaught error handler and crash your node process.

Rascal supports text, buffers and anything it can JSON.parse, providing the contentType message property is set correctly. Text messages should be set to "text/plain" and JSON messages to "application/json". Other content types will be returned as a Buffer. If the publisher doesn't set the contentType or you want to override it you can do so in the subscriber configuration.

{
  "subscriptions": {
    "s1": {
      "queue": "e1",
      "vhost": "v1",
      "contentType": "application/json"
    }
  }
}

The broker.subscribe method also accepts an options parameter which will override options specified in config

broker.subscribe("s1", { prefetch: 10, retry: false }, callback)

The arguments to the on message event handler are function(message, content, ackOrNack), where message is the raw message, the content (a buffer, text, or object) and an ackOrNack callback. This callback should only be used for messages which were not { "options": { "noAck": true } } by the subscription configuration or the options passed to broker.subscribe.

For messages which are not auto-acknowledged (the default) calling ackOrNack() with no error argument will acknowledge it. Calling ackOrNack(err) will nack the message causing it to be discarded (or potentially sent to a dead letter exchange). If you want to requeue the message call ackOrNack(err, { requeue: true }). You can also delay the requeue by specifying a defer argument, ackOrNack(err, { requeue: true, defer: 1000 })

An alternative to requeueing to republish the message back to the queue it was consumed from. Not only does this give all other messages on the queue a better chance of being processed, but it also enables rascal to set a republished count in the message header. This allows you to configure a republish limit per message. ackOrNack(err, { republish: true, defer: 1000, attempts: 5 })

There are some subtle implications to republishing that you should be aware of.

1. Rascal will copy messages properties from the original message to the republished one. If you set an expiration time on the original message this will also be recopied, effectively resetting it.

2. Rascal wil ack the original message after successfully publishing the copy. This does not take place in a distributed transaction so there is a potential of the original message being rolled back after the copy has been published.

3. Rascal will publish the copy using a confirm channel, and nack the message with requeue true if the publish fails.

prefetch

Prefetch limits the number of unacknowledged messages your application can have outstanding. It's a great way to ensure that you don't overload your event loop or a downstream service. Rascal's default configuration sets the prefetch to 10 which may seem low, but we've managed to knock out firewalls, breach AWS thresholds and all sorts of other things by setting it to higher values.

retry

If an error occurs on the channel (which will happen if you accidentally acknowledge a message twice), then it becomes unusable and no more messages will be delivered. Rascal listens to the channel's error even and assuming you are using its defaults will automatically attempt to resubscribe to a new channel after a one second delay. You can disable or customise this in your configuration or in the call to subscribe.

// Does not retry. This will cause an error to be emitted which unhandled will crash your process. See [Subscriber Events](#subscriber-events)
broker.subscribe("s1", { prefetch: 10, retry: false }, callback)

// Retries without delay.
broker.subscribe("s1", { prefetch: 10, retry: true }, callback)

// Retries after a one second interval.
broker.subscribe("s1", { prefetch: 10, retry: { delay: 1000 } }, callback)

Subscriber Events

amqplib emits error events from the channel. These can happen for a number of reasons, but a common cause is because you have acknowledged the message twice. The subscriber will listen for channel errors so it can automatically re-subscribe but still emits them so they can be reported by your application. If you don not listen to these events or handle them in a domain they will cause your application to crash.

Defaults

Configuring each vhost, exchange, queue, binding, publication and subscription explicitly wouldn't be much fun. Not only does Rascal ship with default production and test configuration files, but you can also specify your own defaults in your configuration files by adding a "defaults" sub document.

{
  "defaults": {
      "vhosts": {
          "exchanges": {
              "assert": true,
              "type": "topic"
          },
          "queues": {
              "assert": true
          },
          "bindings": {
              "destinationType": "queue",
              "bindingKey": "#"
          }
      },
      "publications": {
          "vhost": "/",
          "routingKey": "",
          "options": {
              "persistent": true
          }
      },
      "subscriptions": {
          "vhost": "/",
          "prefetch": 10,
          "retry": {
              "delay": 1000
          }
      }
  }
}

Cancelling subscriptions

You can cancel subscriptions as follows

broker.subscribe('s1', function(err, subscription) {

  subscription.cancel(function(err) {
    console.err(err)
  })
})

Bonus Features

Nuke

In a test environment it's useful to be able to nuke your setup between tests. The specifics will vary based on your test runner, but assuming you were using Mocha...

afterEach(function(done) {
    broker ? broker.nuke(done) : done()
})

Bounce

Bounce disconnects and reinistialises the broker. We're hoping to use it for some automated reconnection tests

Running the tests

npm test

You'll need a RabbitMQ server running locally with default configuration. If that's too much trouble try installing docker and running the following

docker run -d -p 5672:5672 -p 15672:15672 dockerfile/rabbitmq