RabbitMQ-shovel

 ===============

 Introduction

 ------------

 This is a plug-in for RabbitMQ that shovels messages from a queue on

 one broker to an exchange on another broker. The two brokers may be

 the same. The plug-in allows several shovels to be specified at the

 same time. Each shovel may have a number of source and destination

 brokers specified, and one of each is chosen whenever the shovel

 attempts to make a connection: this permits simple round-rabbit load

 balancing.

 Resources can be declared upon connection to both the source and

 destination brokers, and parameters can be specified for both the

 reception and publishing of messages.

 Requirements

 ------------

 You can build and install it like any other plugin (see

 http://www.rabbitmq.com/plugin-development.html).

 Configuration

 -------------

 The RabbitMQ configuration file specifies the shovel

 configurations. This exists by default, in

 /etc/rabbitmq/rabbitmq.config under Linux systems,

 %RABBITMQ_BASE%\rabbitmq.config under Windows or somewhere else under

 OS X. This file configures both RabbitMQ-server and all the plugins

 installed in it. It is an Erlang-syntax file of the form:

 [{section1, [section1-config]},

  {section2, [section2-config]},

  ...

  {sectionN, [sectionN-config]}

 ].

 thus a list of tuples, where the left element of each tuple names the

 applications being configured. Don't forget the last element of the

 list doesn't have a trailing comma, and don't forget the full-stop is

 needed after closing the list. Hence if you configure RabbitMQ-server

 and the RabbitMQ-shovel, then the configuration file may have a

 structure like this:

 [{rabbit,        [configuration-for-RabbitMQ-server]},

  {rabbit-shovel, [configuration-for-RabbitMQ-shovel]}

 ].

 A full example of the shovel configuration is:

 {rabbitmq_shovel,

   [{shovels,

     [{my_first_shovel,

       [{sources,      [{brokers,

                           ["amqp://fred:secret@host1.domain/my_vhost",

                            "amqp://john:secret@host2.domain/my_vhost"

                           ]},

                        {declarations,

                           ['queue.declare',

                            {'queue.bind',

                                   [{exchange, <<"my_exchange">>},

                                    {queue,    <<>>}]}

                           ]}]},

        {destinations, [{broker, "amqp://"},

                        {declarations,

                           [{'exchange.declare',

                                   [{exchange, <<"my_exchange">>},

                                    {type, <<"direct">>},

                                    durable]}

                           ]}]},

        {queue, <<>>},

        {prefetch_count, 10},

        {ack_mode, on_confirm},

        {publish_properties, [{delivery_mode, 2}]},

        {publish_fields, [{exchange, <<"my_exchange">>},

                          {routing_key, <<"from_shovel">>}]},

        {reconnect_delay, 5}

       ]}

      ]

    }]

 }

 Firstly, all shovels are named. Here we have one shovel, called

 'my_first_shovel'. We can have multiple shovels if you wish. Secondly,

 every shovel must have the subfields sources, destinations and queue

 specified, whereas the other fields (prefetch_count, ack_mode,

 publish_properties, publish_fields, reconnect_delay) are optional and

 have defaults as follows:

 {prefetch_count,      0}

 {ack_mode,            on_confirm}

 {publish_properties,  []}

 {publish_fields,      []}

 {reconnect_delay,     5}

 Sources and Destinations

 ------------------------

 Sources and destinations specify respectively where messages are

 fetched from and delivered too. One of 'broker' and 'brokers' must be

 specified, and 'broker' is simply shorthand for when only one broker

 needs specifying. Using 'brokers' allows a list of brokers to be

 specified: whenever the connection to a broker is lost, another one is

 chosen at random from the list and a connection attempt is made to

 that. The syntax for broker URIs is:

 amqp://username:password@host:port/vhost?key1=value1&key2=value2...

 If username or password are omitted, the default values of guest and

 guest are used. If the vhost is omitted, the default value of / is

 used. If the host is omitted, then the plugin uses the "direct"

 connection internally rather than a network connection: this means it

 connects to the RabbitMQ-server node on which it is running without

 going through the network stack. This is much more efficient. If port

 is omitted then the default value is used (5672 or 5671 if SSL is

 used).

 SSL is implemented, for which additional parameters are needed:

 amqps://username:password@host:port/vhost?cacertfile=/path/to/cacert.pem&certfile=/path/to/certfile.pem&keyfile=/path/to/keyfile.pem&verify=verifyOption&fail_if_no_peer_cert=failOption

 All five parameters (3 paths: cacertfile, certfile and keyfile; 2

 options: verify, fail_if_no_peer_cert) must be specified. See the SSL

 guide at http://www.rabbitmq.com/ssl.html#configure-erlang for details

 of SSL in RabbitMQ in general and specifically for the Erlang client

 (on which the shovel is built).

 Note that SSL cannot be used with the direct connection (i.e. a host

 must be specified when using SSL), and that it is preferable to use

 the non-SSL direct connection when connecting to the same node that's

 running the shovel.

 The query part of the URI permits the configuration of additional

 connection parameters, permitting heartbeat, channel_max, and

 frame_max to be specified. These can be given in any order, and

 omitted fields assume default values. For example:

 amqp://myhost?heartbeat=5&frame_max=8192

 Specifies a non-encrypted network connection to the host 'myhost',

 using default username, password, port, vhost and channel_max, but

 specifying the heartbeat interval of 5 seconds, and the maximum frame

 size of 8192 bytes.

 Resource Declarations

 ---------------------

 Both sources and destinations can have an optional 'declarations'

 clause. The value of this is a list, consisting of AMQP Methods. If

 default values are sufficient, then the method name alone can be

 specified - e.g. 'queue.declare'. If parameters need to be set then

 the method should be given as a tuple, with the right hand side a

 proplist specifying which fields need altering from their default

 values. E.g:

     {'exchange.declare',[{exchange, <<"my_exchange">>},

                          {type, <<"direct">>},

                          durable]},

 One very useful feature here is the Most-Recently-Declared-Queue

 feature, in which RabbitMQ remembers the name of the most recently

 declared queue. This means that you can declare a private queue, and

 then bind it to exchanges without ever needing to know its name.

 queue :: binary

 ---------------

 This parameter specifies the name of the queue on the source brokers

 to consume from. This queue must exist. Use the resource declarations

 to create the queue (or ensure it exists) first. Note again that the

 Most-Recently-Declared-Queue feature can be used here, thus an

 anonymous queue can be used: use <<>> to indicate the

 Most-Recently-Declared-Queue.

 prefetch_count :: non-negative-integer

 --------------------------------------

 The shovel consumes from a queue. This parameter imposes a limit on

 the number of messages which are sent to the shovel in advance of the

 message the shovel is currently processing.

 ack_mode :: 'no_ack' | 'on_publish' | 'on_confirm'

 --------------------------------------------------

 This setting controls if or when acknowledgements to the source broker

 are issued by the shovel. The default is 'on_confirm'.

 'no_ack' - The shovel consumes from its queue with no_ack = 'true':

 i.e. the shovel does not issue explicit acks for messages it receives,

 and the source broker considers messages acknowledged as soon as it

 has sent them to the shovel.

 'on_publish' - the shovel consumes from its queue with no_ack =

 'false', and a message is acknowledged to the source broker as soon as

 it has been published to the destination broker.

 'on_confirm' - the shovel consumes from its queue with no_ack =

 'false' and it requests publisher confirmations from the destination

 broker. It only issues acknowledgements to the source broker when it

 has received confirmation from the destination broker that each

 message has been successfully received. This setting gives a guarantee

 that messages will not be discarded from the source broker until after

 the destination broker has confirmed receipt of the message.

 'on_confirm' is strongly recommended. However the destination broker

 must support publish confirms, meaning it can only be used with

 RabbitMQ 2.3.1 and up.

 publish_properties

 ------------------

 This is a list of tuples which override fields in the basic class

 properties when publishing to the destination. This can be used to

 override any of the following fields: content_type, content_encoding,

 headers, delivery_mode, priority, correlation_id, reply_to,

 expiration, message_id, timestamp, type, user_id, app_id, cluster_id.

 By default, all the properties of the basic class that are received

 with the delivery are passed through to the destination, but this

 field can be used to override them.

 publish_fields

 --------------

 This is a list of tuples which override fields in the publish method

 when publishing to the destination. This can be used to direct

 messages to a particular exchange on the destination, for example, or

 change the routing key. By default, the exchange and the routing key

 of the message as it is received by the shovel is passed through, but

 this can be overridden as necessary.

 reconnect_delay :: non-negative-number

 --------------------------------------

 When an error occurs, the shovel will disconnect from both the source

 and destination broker immediately. This will force uncommitted

 transactions at the destination to be rolled back, and delivered but

 unacknowledged messages from the source to be requeued. The shovel

 will then try connecting again. If this is unsuccessful, then it's not

 a good idea for the shovel to very quickly and repeatedly try to

 reconnect. The value specified here is determines the delay, in

 seconds, between each connection attempt.

 Note that if set to 0, the shovel will never try to reconnect: it'll

 stop after the first error.

 Also, the value can be a floating point number, thus permitting the

 specification of delays at a granularity smaller than whole seconds.

 Obtaining shovel statuses

 -------------------------

 From the broker Erlang prompt, call

 rabbit_shovel_status:status(). This will return a list, with one row

 for each configured shovel. Each row has three fields: the shovel

 name, the shovel status, and the timestamp (a local calendar time of

 {{YYYY,MM,DD},{HH,MM,SS}}). There are 3 possible statuses:

 'starting': The shovel is starting up, connecting and creating

     resources.

 {'running'|'blocked', {'source', Source},

                       {'destination', Destination}}:

     When 'running', the shovel is up and running, shovelling

     messages. When 'blocked', the destination has raised channel.flow,

     preventing the shovel from sending messages to the

     destination. The shovel will raise channel.flow to the source,

     asking the source to stop sending further messages to the

     shovel. Any messages that are received by the shovel before the

     source observes the channel.flow are correctly buffered and

     maintained in order, and are published to the destination as soon

     as the destination drops the channel.flow block.

     Source and Destination give the connection parameters used to

     connect to the endpoints.

 {'terminated', Reason}: Something's gone wrong. The Reason should give

     a further indication of where the fault lies.