# `AMQP.Basic`
[🔗](https://github.com/pma/amqp/blob/v4.2.1/lib/amqp/basic.ex#L1)

Functions to publish, consume and acknowledge messages.

# `consumer_tag`

```elixir
@type consumer_tag() :: String.t()
```

# `delivery_tag`

```elixir
@type delivery_tag() :: integer()
```

# `error`

```elixir
@type error() :: {:error, reason :: :blocked | :closing}
```

# `exchange`

```elixir
@type exchange() :: String.t()
```

# `payload`

```elixir
@type payload() :: String.t()
```

# `queue`

```elixir
@type queue() :: String.t()
```

# `routing_key`

```elixir
@type routing_key() :: String.t()
```

# `ack`

```elixir
@spec ack(AMQP.Channel.t(), delivery_tag(), keyword()) :: :ok | error()
```

Acknowledges one or more messages.

## Options

  * `:multiple` - If set, all messages up to the one specified by `delivery_tag`
    are acknowledged (default `false`)

# `cancel`

```elixir
@spec cancel(AMQP.Channel.t(), String.t(), keyword()) :: {:ok, String.t()} | error()
```

Stops the given consumer from consuming.

This method cancels a consumer. This does not affect already delivered
messages, but it does mean the server will not send any more messages for
that consumer. The client may receive an arbitrary number of messages in
between sending the cancel method and receiving the reply.

`consumer_tag` identifies the "subscription" to cancel, that is, the
subscription of a consumer to a specific queue. The consumer tag is returned
by `consume/4`.

## Options

  * `:nowait` - If set, the cancel operation is asynchronous (default
    `false`)

# `cancel_return`

```elixir
@spec cancel_return(AMQP.Channel.t()) :: :ok
```

Removes the return handler, if it exists. Does nothing if there is no
such handler.

# `consume`

```elixir
@spec consume(AMQP.Channel.t(), String.t(), pid() | nil, keyword()) ::
  {:ok, String.t()} | error()
```

Registers a queue consumer process. The `pid` of the process can be set using
the `consumer_pid` argument and defaults to the calling process.

The consumer process will receive the following data structures:

  * `{:basic_deliver, payload, meta}` - This is sent for each message
    consumed, where `payload` contains the message content and `meta`
    contains all the metadata set when sending with `AMQP.Basic.publish/5` or
    additional info set by the broker

  * `{:basic_consume_ok, %{consumer_tag: consumer_tag}}` - Sent when the
    consumer process is registered with Basic.consume. The caller receives
    the same information as the return of `AMQP.Basic.consume/4`

  * `{:basic_cancel, %{consumer_tag: consumer_tag, nowait: nowait}}` - Sent
    by the broker when the consumer is unexpectedly cancelled (such as after a
    queue deletion)

  * `{:basic_cancel_ok, %{consumer_tag: consumer_tag}}` - Sent to the
    consumer process after a call to `AMQP.Basic.cancel/3`

## Options

  * `:consumer_tag` - Specifies the consumer tag for this consumer (as a
    string).  This tag is local to the given channel `chan`, so different
    channels can have consumers that use the same consumer tag. If the given
    consumer tag is `""`, then the server autogenerates the tag (default `""`)

  * `:no_local` - If set, the server won't send messages to the connection
    that published them (default `false`)

  * `:no_ack` - If set, the server will not expect message acks from the
    consumer and will consider every message that it believes was delivered to
    the consumer as acknowledged. Defaults to `false`, meaning that messages
    need to be acked explicitly through `ack/3`

  * `:exclusive` - If set, requests exclusive consumer access, meaning that
    only this consumer can consume from the given `queue`. Note that the client
    cannot have exclusive access to a queue that already has consumers

  * `:nowait` - If set, the consume operation is asynchronous (default
    `false`)

  * `:arguments` - A list of arguments to pass when consuming
    (of type `t:AMQP.arguments/0`). See the [README](readme.html)
    for more information (default `[]`)

# `get`

```elixir
@spec get(AMQP.Channel.t(), queue(), keyword()) ::
  {:ok, String.t(), map()} | {:empty, map()} | error()
```

Polls a queue for an existing message.

Returns the tuple `{:empty, meta}` if the queue is empty or the tuple `{:ok,
payload, meta}` if at least one message exists in the queue. The returned
`meta` map includes the entry `:message_count` with the current number of
messages in the queue.

Receiving messages by polling a queue is not as as efficient as subscribing a
consumer to a queue, so consideration should be taken when receiving large
volumes of messages.

## Options

  * `:no_ack` - If set, the broker is told that the received will not send an
    acknowledgement of the message. Once the broker believes it has delivered
    the message, then it's free to assume that the consuming application has
    taken responsibility for it. In general, a lot of applications will not
    want these semantics, rather, they will want to explicitly acknowledge the
    receipt of a message (through `ack/3`) (default `false`, meaning
    explicit acks)

# `nack`

```elixir
@spec nack(AMQP.Channel.t(), delivery_tag(), keyword()) :: :ok | error()
```

Negative acknowledges of one or more messages.

This is a RabbitMQ specific extension to AMQP 0.9.1. It is equivalent to `reject/3`, but allows
rejecting multiple messages using the `:multiple` option.

## Options

  * `:multiple` - If set, all messages up to the one specified by
    `delivery_tag` are considered as not acknowledged by the server (default
    `false`)

  * `:requeue` - If set, the message will be returned to the queue and
    redelivered to the next available consumer (default `true`)

# `publish`

```elixir
@spec publish(AMQP.Channel.t(), exchange(), routing_key(), payload(), keyword()) ::
  :ok | error()
```

Publishes a message to an Exchange.

This method publishes a message to a specific exchange. The message will be
routed to queues as defined by the exchange configuration and distributed to
any subscribers.

The parameter `exchange` specifies the name of the exchange to publish to. If
set to empty string, it publishes to the default exchange.

The `routing_key` parameter specifies the routing key for the message.

The `payload` parameter specifies the message content as a binary.

In addition to the previous parameters, the following options can be used:

## Options

  * `:mandatory` - If set, returns an error if the broker can't route the
    message to a queue (default `false`)

  * `:immediate` - If set, returns an error if the broker can't deliver the
    message to a consumer immediately (default `false`)

  * `:content_type` - MIME Content type

  * `:content_encoding` - MIME Content encoding

  * `:headers` - Message headers of type `t:AMQP.arguments/0`. Can be used
    with headers Exchanges

  * `:persistent` - If set, uses persistent delivery mode. Messages marked as
    `persistent` that are delivered to `durable` queues will be logged to disk

  * `:correlation_id` - application correlation identifier

  * `:priority` - message priority, ranging from 0 to 9

  * `:reply_to` - name of the reply queue

  * `:expiration` - how long the message is valid (in milliseconds)

  * `:message_id` - message identifier

  * `:timestamp` - timestamp associated with this message (epoch time)

  * `:type` - message type as a string

  * `:user_id` - creating user ID. RabbitMQ will validate this against the
    active connection user

  * `:app_id` - publishing application ID

## Examples

    iex> AMQP.Basic.publish chan, "my_exchange", "my_routing_key", "Hello World!", persistent: true
    :ok

# `qos`

```elixir
@spec qos(
  AMQP.Channel.t(),
  keyword()
) :: :ok | error()
```

Sets the message prefetch count or prefetch size (in bytes).

This allows you to limit the number of unacknowledged messages.

## Options

  * `:prefetch_size` - the prefetch size in bytes (default `0`)

  * `:prefetch_count` - the prefetch count (default `0`)

  * `:global` - If true, this applies to the entire Channel.
    Otherwise, it applies to each consumer (default `false`)

# `recover`

```elixir
@spec recover(
  AMQP.Channel.t(),
  keyword()
) :: :ok | error()
```

Asks the server to redeliver all unacknowledged messages on a specified channel.

## Options

  * `:requeue` - If set, the server will attempt to requeue the message,
    potentially delivering it to another subscriber. Otherwise it will be
    redelivered to the original recipient (default `false`)

# `reject`

```elixir
@spec reject(AMQP.Channel.t(), delivery_tag(), keyword()) :: :ok | error()
```

Rejects (and, optionally, requeues) a message.

## Options

  * `:requeue` - If set, the message is requeued by the server, otherwise
    it's discarded (default `true`)

# `return`

```elixir
@spec return(AMQP.Channel.t(), pid()) :: :ok
```

Registers a handler to deal with returned messages.

The registered process will receive `{:basic_return, payload, meta}` tuples.

---

*Consult [api-reference.md](api-reference.md) for complete listing*