[rabbitmq-discuss] RabbitMQ-shovel

Mon Feb 1 10:56:48 GMT 2010

Morning,

Several people have recently been asking about a shovel for Rabbit,
which can consume messages from one consumer and deliver them to
another. Several of our clients have also been asking for such message
relocation equipment, and thus we have built one: the RabbitMQ-shovel
which is available from http://hg.rabbitmq.com/rabbitmq-shovel/

Documentation is in the form of a README, which is duplicated below.

This code has not yet been through QA, so there may be bugs in it.
We are, as always, very happy to receive bug reports. But certainly, our
testing suggests it works for us ;)

It is licensed under the MPL v1.1

Have fun!

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

Currently, you must build the server from source, under branch
bug16653. You must also have checked out the rabbitmq-public-umbrella
hg repository, and have the rabbitmq-erlang-client built. From
scratch, the following commands should build RabbitMQ with the shovel
plug-in:

hg clone http://hg.rabbitmq.com/rabbitmq-public-umbrella
cd rabbitmq-public-umbrella
hg clone http://hg.rabbitmq.com/rabbitmq-codegen
hg clone http://hg.rabbitmq.com/rabbitmq-erlang-client
hg clone http://hg.rabbitmq.com/rabbitmq-server
hg clone http://hg.rabbitmq.com/rabbitmq-shovel
cd rabbitmq-server
hg up -C bug16653
make -j
mkdir -p plugins
cd plugins
ln -s ../../rabbitmq-erlang-client
ln -s ../../rabbitmq-shovel
cd ../../rabbitmq-erlang-client
make
cd ../rabbitmq-shovel
make
cd ../rabbitmq-server
./scripts/rabbitmq-activate-plugins
make cleandb run

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:

{rabbit_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, <<>>},
       {qos, 10},
       {auto_ack, false},
       {tx_size, 0},
       {delivery_mode, keep},
       {publish_fields, [{exchange, <<"my_exchange">>},
                         {routing_key, <<"from_shovel">>}]},
       {reconnect, 5}
      ]}
     ]
   }]
}

Firstly, all shovels are named. Here we have one shovel, called
'my_first_shovel'. We can have multiple shovels if you wish. Every
shovel must have all sub-fields specified: sources, destinations, qos,
auto_ack, delivery_mode, publish_fields, reconnect.

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

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 to using SSL to connect to the same node
that's running the shovel.

Resource Declarations
---------------------

Both sources and destinations can have an optional 'declarations'
clause. The value of this is a key, 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 feature 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.

qos :: non-negative-integer
---------------------------

The shovel consumes from a queue. The QoS controls how many messages
are sent to the shovel in advance of the message the shovel is
currently processing.

auto_ack :: boolean
-------------------

Setting this to 'true' turns on the no_ack flag when subscribing to
the source queue.

tx_size :: non-negative-integer
-------------------------------

When set to 0, transactions are not used. Other values make publishes
transactional, with a commit every N messages. In lieu of the auto-ack
option, when transactions are not used, messages are acknowledged to
the source immediately after every publish. When transactions are
used, acks are only issued to the source on receipt of the commit-ok
message from the destination. This can thus be used to guarantee that
messages are only acknowledged (and thus forgotten about by the source
broker) when they are guaranteed to have been received by the
destination broker.

delivery_mode :: 'keep' | 0 | 2
-------------------------------

This affects the delivery_mode field when publishing to the
destination. A value of 'keep' means that the same delivery_mode
should be used as when the message was originally published to the
source broker. 0 and 2 override the original setting.

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 routing key of the message as
it is received by the shovel is passed through, but this can be
overridden as necessary.

reconnect :: non-negative-integer
---------------------------------

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 the number of seconds to wait
between each connection attempt.

Note that if set to 0, the shovel will never try to reconnect: it'll
stop after the first error.

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': The shovel is up and running, shovelling messages.

{'terminated', Reason}: Something's gone wrong. The Reason should give
a further indication of where the fault lies.