# AMQP
[](https://travis-ci.org/pma/amqp)
[](https://hex.pm/packages/amqp)
[](http://inch-ci.org/github/pma/amqp)
Simple Elixir wrapper for the Erlang RabbitMQ client.
The API is based on Langohr, a Clojure client for RabbitMQ.
## WARNING - this branch is under the development
We are preparing 1.0.0 release which will have some backward imcompatibilities
against the current stable version - 0.3.x.
If you are looking for a document for 0.3.x please visit to [v0.3 branch](https://github.com/pma/amqp/tree/v0.3).
If you want to check the plan and progress for 1.0.0 please check out [this github issue](https://github.com/pma/amqp/issues/69).
## Usage
Add AMQP as a dependency in your `mix.exs` file.
(If you want to use the stable version, set `~> 0.2.3` to the version instead)
```elixir
def deps do
[{:amqp, "~> 1.0.0-pre.4"}]
end
```
You should also update your application list to include `:amqp`:
```elixir
def application do
[applications: [:amqp]]
end
```
After you are done, run `mix deps.get` in your shell to fetch and compile AMQP. Start an interactive Elixir shell with `iex -S mix`.
```iex
iex> {:ok, conn} = AMQP.Connection.open
{:ok, %AMQP.Connection{pid: #PID<0.165.0>}}
iex> {:ok, chan} = AMQP.Channel.open(conn)
{:ok, %AMQP.Channel{conn: %AMQP.Connection{pid: #PID<0.165.0>}, pid: #PID<0.177.0>}
iex> AMQP.Queue.declare chan, "test_queue"
{:ok, %{consumer_count: 0, message_count: 0, queue: "test_queue"}}
iex> AMQP.Exchange.declare chan, "test_exchange"
:ok
iex> AMQP.Queue.bind chan, "test_queue", "test_exchange"
:ok
iex> AMQP.Basic.publish chan, "test_exchange", "", "Hello, World!"
:ok
iex> {:ok, payload, meta} = AMQP.Basic.get chan, "test_queue"
iex> payload
"Hello, World!"
iex> AMQP.Queue.subscribe chan, "test_queue", fn(payload, _meta) -> IO.puts("Received: #{payload}") end
{:ok, "amq.ctag-5L8U-n0HU5doEsNTQpaXWg"}
iex> AMQP.Basic.publish chan, "test_exchange", "", "Hello, World!"
:ok
Received: Hello, World!
```
### Setup a consumer GenServer
```elixir
defmodule Consumer do
use GenServer
use AMQP
def start_link do
GenServer.start_link(__MODULE__, [], [])
end
@exchange "gen_server_test_exchange"
@queue "gen_server_test_queue"
@queue_error "#{@queue}_error"
def init(_opts) do
{:ok, conn} = Connection.open("amqp://guest:guest@localhost")
{:ok, chan} = Channel.open(conn)
setup_queue(chan)
# Limit unacknowledged messages to 10
:ok = Basic.qos(chan, prefetch_count: 10)
# Register the GenServer process as a consumer
{:ok, _consumer_tag} = Basic.consume(chan, @queue)
{:ok, chan}
end
# Confirmation sent by the broker after registering this process as a consumer
def handle_info({:basic_consume_ok, %{consumer_tag: consumer_tag}}, chan) do
{:noreply, chan}
end
# Sent by the broker when the consumer is unexpectedly cancelled (such as after a queue deletion)
def handle_info({:basic_cancel, %{consumer_tag: consumer_tag}}, chan) do
{:stop, :normal, chan}
end
# Confirmation sent by the broker to the consumer process after a Basic.cancel
def handle_info({:basic_cancel_ok, %{consumer_tag: consumer_tag}}, chan) do
{:noreply, chan}
end
def handle_info({:basic_deliver, payload, %{delivery_tag: tag, redelivered: redelivered}}, chan) do
spawn fn -> consume(chan, tag, redelivered, payload) end
{:noreply, chan}
end
defp setup_queue(chan) do
{:ok, _} = Queue.declare(chan, @queue_error, durable: true)
# Messages that cannot be delivered to any consumer in the main queue will be routed to the error queue
{:ok, _} = Queue.declare(chan, @queue,
durable: true,
arguments: [
{"x-dead-letter-exchange", :longstr, ""},
{"x-dead-letter-routing-key", :longstr, @queue_error}
]
)
:ok = Exchange.fanout(chan, @exchange, durable: true)
:ok = Queue.bind(chan, @queue, @exchange)
end
defp consume(channel, tag, redelivered, payload) do
number = String.to_integer(payload)
if number <= 10 do
:ok = Basic.ack channel, tag
IO.puts "Consumed a #{number}."
else
:ok = Basic.reject channel, tag, requeue: false
IO.puts "#{number} is too big and was rejected."
end
rescue
# Requeue unless it's a redelivered message.
# This means we will retry consuming a message once in case of exception
# before we give up and have it moved to the error queue
#
# You might also want to catch :exit signal in production code.
# Make sure you call ack, nack or reject otherwise comsumer will stop
# receiving messages.
exception ->
:ok = Basic.reject channel, tag, requeue: not redelivered
IO.puts "Error converting #{payload} to integer"
end
end
```
```iex
iex> Consumer.start_link
{:ok, #PID<0.261.0>}
iex> {:ok, conn} = AMQP.Connection.open
{:ok, %AMQP.Connection{pid: #PID<0.165.0>}}
iex> {:ok, chan} = AMQP.Channel.open(conn)
{:ok, %AMQP.Channel{conn: %AMQP.Connection{pid: #PID<0.165.0>}, pid: #PID<0.177.0>}
iex> AMQP.Basic.publish chan, "gen_server_test_exchange", "", "5"
:ok
Consumed a 5.
iex> AMQP.Basic.publish chan, "gen_server_test_exchange", "", "42"
:ok
42 is too big and was rejected.
iex> AMQP.Basic.publish chan, "gen_server_test_exchange", "", "Hello, World!"
:ok
Error converting Hello, World! to integer
Error converting Hello, World! to integer
```
## Stable RabbitMQ Connection
While the above example works, it does nothing to handle RabbitMQ connection
outages. In case of an outage your Genserver will remain stale and won't
receive any messages from the broker as the connection is never restarted.
Luckily, implementing a reconnection logic is quite straight forward. Since the
connection record holds the pid of the connection itself, we can monitor it
and get a notification when it goes down.
Example implementation (only changes from the last example):
```elixir
# 1. Extract your connect logic into a private method rabbitmq_connect
def init(_opts) do
rabbitmq_connect
end
defp rabbitmq_connect do
case Connection.open("amqp://guest:guest@localhost") do
{:ok, conn} ->
# Get notifications when the connection goes down
Process.monitor(conn.pid)
# Everything else remains the same
{:ok, chan} = Channel.open(conn)
setup_queue(chan)
Basic.qos(chan, prefetch_count: 10)
{:ok, _consumer_tag} = Basic.consume(chan, @queue)
{:ok, chan}
{:error, _} ->
# Reconnection loop
:timer.sleep(10000)
rabbitmq_connect
end
end
# 2. Implement a callback to handle DOWN notifications from the system
# This callback should try to reconnect to the server
def handle_info({:DOWN, _, :process, _pid, _reason}, _) do
{:ok, chan} = rabbitmq_connect
{:noreply, chan}
end
```
Now, when the connection drops, or if the server is down when your application
starts, it will try to reconnect indefinitely until it succeeds.
## Types of arguments and headers
The parameter `arguments` in `Queue.declare`, `Exchange.declare`, `Basic.consume` and the parameter `headers` in `Basic.publish` are a list of tuples in the form `{name, type, value}`, where `name` is a binary containing the argument/header name, `type` is an atom describing the AMQP field type and `value` a term compatible with the AMQP field type.
The valid AMQP field types are:
`:longstr` | `:signedint` | `:decimal` | `:timestamp` | `:table` | `:byte` | `:double` | `:float` | `:long` | `:short` | `:bool` | `:binary` | `:void` | `:array`
Valid argument names in `Queue.declare` include:
* "x-expires"
* "x-message-ttl"
* "x-dead-letter-routing-key"
* "x-dead-letter-exchange"
* "x-max-length"
* "x-max-length-bytes"
Valid argument names in `Basic.consume` include:
* "x-priority"
* "x-cancel-on-ha-failover"
Valid argument names in `Exchange.declare` include:
* "alternate-exchange"
## Troubleshooting
#### Consumer stops receiving messages
Most popular cause is your code not sending acknowledgement(ack, nack or reject)
after receiving a message.
You want to investigate if...
- an exception was raised and how it would be handled
- :exit signal was thrown and how it would be handled
- a message processing took long time.
If you use GenServer in consumer, try storing number of messages the server is
currently processing to the GenServer state.
If the number equals `prefetch_count`, those messages were left without
acknowledgements and that's why consumer have stopped receiving more
messages.
#### Old version of Elixir or OTP
OTP 17 and 18 are supported only on [version 0.1.x](https://github.com/pma/amqp/tree/v0.1).
Please understand that we won't make further changes to 0.1 except for major security issues.
