Mainflux.mainflux/vendor/github.com/rabbitmq/amqp091-go/doc.go

// Copyright (c) 2021 VMware, Inc. or its affiliates. All Rights Reserved.
// Copyright (c) 2012-2021, Sean Treadway, SoundCloud Ltd.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

/*
Package amqp091 is an AMQP 0.9.1 client with RabbitMQ extensions

Understand the AMQP 0.9.1 messaging model by reviewing these links first. Much
of the terminology in this library directly relates to AMQP concepts.

  Resources

  http://www.rabbitmq.com/tutorials/amqp-concepts.html
  http://www.rabbitmq.com/getstarted.html
  http://www.rabbitmq.com/amqp-0-9-1-reference.html

Design

Most other broker clients publish to queues, but in AMQP, clients publish
Exchanges instead.  AMQP is programmable, meaning that both the producers and
consumers agree on the configuration of the broker, instead of requiring an
operator or system configuration that declares the logical topology in the
broker.  The routing between producers and consumer queues is via Bindings.
These bindings form the logical topology of the broker.

In this library, a message sent from publisher is called a "Publishing" and a
message received to a consumer is called a "Delivery".  The fields of
Publishings and Deliveries are close but not exact mappings to the underlying
wire format to maintain stronger types.  Many other libraries will combine
message properties with message headers.  In this library, the message well
known properties are strongly typed fields on the Publishings and Deliveries,
whereas the user defined headers are in the Headers field.

The method naming closely matches the protocol's method name with positional
parameters mapping to named protocol message fields.  The motivation here is to
present a comprehensive view over all possible interactions with the server.

Generally, methods that map to protocol methods of the "basic" class will be
elided in this interface, and "select" methods of various channel mode selectors
will be elided for example Channel.Confirm and Channel.Tx.

The library is intentionally designed to be synchronous, where responses for
each protocol message are required to be received in an RPC manner.  Some
methods have a noWait parameter like Channel.QueueDeclare, and some methods are
asynchronous like Channel.Publish.  The error values should still be checked for
these methods as they will indicate IO failures like when the underlying
connection closes.

Asynchronous Events

Clients of this library may be interested in receiving some of the protocol
messages other than Deliveries like basic.ack methods while a channel is in
confirm mode.

The Notify* methods with Connection and Channel receivers model the pattern of
asynchronous events like closes due to exceptions, or messages that are sent out
of band from an RPC call like basic.ack or basic.flow.

Any asynchronous events, including Deliveries and Publishings must always have
a receiver until the corresponding chans are closed.  Without asynchronous
receivers, the synchronous methods will block.

Use Case

It's important as a client to an AMQP topology to ensure the state of the
broker matches your expectations.  For both publish and consume use cases,
make sure you declare the queues, exchanges and bindings you expect to exist
prior to calling Channel.Publish or Channel.Consume.

  // Connections start with amqp.Dial() typically from a command line argument
  // or environment variable.
  connection, err := amqp.Dial(os.Getenv("AMQP_URL"))

  // To cleanly shutdown by flushing kernel buffers, make sure to close and
  // wait for the response.
  defer connection.Close()

  // Most operations happen on a channel.  If any error is returned on a
  // channel, the channel will no longer be valid, throw it away and try with
  // a different channel.  If you use many channels, it's useful for the
  // server to
  channel, err := connection.Channel()

  // Declare your topology here, if it doesn't exist, it will be created, if
  // it existed already and is not what you expect, then that's considered an
  // error.

  // Use your connection on this topology with either Publish or Consume, or
  // inspect your queues with QueueInspect.  It's unwise to mix Publish and
  // Consume to let TCP do its job well.

SSL/TLS - Secure connections

When Dial encounters an amqps:// scheme, it will use the zero value of a
tls.Config.  This will only perform server certificate and host verification.

Use DialTLS when you wish to provide a client certificate (recommended),
include a private certificate authority's certificate in the cert chain for
server validity, or run insecure by not verifying the server certificate dial
your own connection.  DialTLS will use the provided tls.Config when it
encounters an amqps:// scheme and will dial a plain connection when it
encounters an amqp:// scheme.

SSL/TLS in RabbitMQ is documented here: http://www.rabbitmq.com/ssl.html

Best practises to handle library notifications.

Best practises for Connections and Channels notifications:

In order to be notified when a connection or channel gets closed both the structures offer the possibility to register channels using the `notifyClose` function like:
notifyConnClose := make(chan *amqp.Error)
conn.NotifyClose(notifyConnClose)
No errors will be sent in case of a graceful connection close.
In case of a non-graceful close, because of a network issue of forced disconnection from the UI, the error will be notified synchronously by the library.
You can see that in the shutdown function of connection and channel (see connection.go and channel.go)

 if err != nil {
      for _, c := range c.closes {
        c <- err
      }
    }

The error is sent synchronously to the channel so that the flow will wait until the channel will be consumed by the caller.
To avoid deadlocks it is necessary to consume the messages from the channels.
This could be done inside a different goroutine with a select listening on the two channels inside a for loop like:

  go func() {
    for notifyConnClose != nil || notifyChanClose != nil {
      select {
        case err, ok := <-notifyConnClose:
          if !(ok) {
            notifyConnClose = nil
          } else {
            fmt.Printf("connection closed, error %s", err)
          }
        case err, ok := <-notifyChanClose:
          if !(ok) {
            notifyChanClose = nil
          } else {
            fmt.Printf("channel closed, error %s", err)
          }
        }
    }
  }()

Best practises for NotifyPublish notifications:

Similary to the previous sceneario using the NotifyPublish method allows the caller of the library to be notified through a go channel when a message has been received
from the broker after Channel.Confirm has been set.
It's advisable to wait for all Confirmations to arrive before calling Channel.Close() or Connection.Close().
It is also necessary for the caller to always consume from this channel till it get closed from the library to avoid possible deadlocks.
Confirmations go channel are indeed notified inside the confirm function of the Confirm struct synchronously:

  // confirm confirms one publishing, increments the expecting delivery tag, and
  // removes bookkeeping for that delivery tag.
  func (c *confirms) confirm(confirmation Confirmation) {
	  delete(c.sequencer, c.expecting)
	  c.expecting++
	  for _, l := range c.listeners {
		  l <- confirmation
	  }
  }

It is so necessary to have a goroutine consuming from this channel till it get closed.

*/
package amqp091