How to pass extra arguments to RabbitMQ connection in Erlang client

Question

I have written some extension modules for eJabberd most of which pass pieces of information to RabbitMQ for various reasons. All has been fine until we brought the server up in staging where we have a Rabbit cluster rather than a single box.

In order to utilize the cluster you need to pass "x-ha-policy" parameter to Rabbit with either the "all" or "nodes" value. This works fine for the Java and Python Producers and Consumers, but the eJabberd (using the Erlang AMQP client of course) has me a bit stumped. The x-ha-policy parameter needs to be passed into the "client_properties" parameter which is just the "catchall" for extra parameters.

In Python with pika I can do:

client_params = {"x-ha-policy": "all"}
queue.declare(host, vhost, username, password, arguments=client_params)

and that works. However the doc for the Erlang client says the arguments should be passed in as a list per:

[{binary(), atom(), binary()}]

If it were just [{binary(), binary()}] I could see the relationship with key/value but not sure what the atom would be there.

Just to be clear, I am a novice Erlang programmer so this may be a common construct that I am not familiar with, so no answer would be too obvious.

Emil Vikström Emil Vikström · Accepted Answer · 2012-06-17T14:31:37

I found this in amqp_network_connection.erl, which looks like a wrapper to set some default values:

client_properties(UserProperties) ->
    {ok, Vsn} = application:get_key(amqp_client, vsn),
    Default = [{<<"product">>,   longstr, <<"RabbitMQ">>},
               {<<"version">>,   longstr, list_to_binary(Vsn)},
               {<<"platform">>,  longstr, <<"Erlang">>},
               {<<"copyright">>, longstr,
                <<"Copyright (c) 2007-2012 VMware, Inc.">>},
               {<<"information">>, longstr,
                <<"Licensed under the MPL.  "
                  "See http://www.rabbitmq.com/">>},
               {<<"capabilities">>, table, ?CLIENT_CAPABILITIES}],
    lists:foldl(fun({K, _, _} = Tuple, Acc) ->
                    lists:keystore(K, 1, Acc, Tuple)
                end, Default, UserProperties).

Apparently the atom describes the value type. I don't know the available types, but there's a chance that longstr will work in your case.

How to pass extra arguments to RabbitMQ connection in Erlang client

1 Answers