= Qpid Proton AMQP Library

This is a library for sending and receiving AMQP messages. It can be used to
build clients and servers.

== Installing

You can install the latest published Gem with

    gem install qpid_proton

*NOTE:* before installing the Gem, you must install the proton-C library.

The proton-C library can be installed by the package manager on many platforms,
    e.g.  yum install qpid-proton-c # Fedora < 25, RHEL < 7 dnf install
    qpid-proton-c # Fedora >= 25, RHEL >= 7

You can also download a source release or the latest development code from
http://qpid.apache.org/proton. To build from source:

    cmake -DBUILD_BINDINGS=ruby && make

This produces a Gem file at:

    ${CMAKE_BUILD_DIR}/proton-c/bindings/ruby/qpid_proton-${PN_VERSION}.gem

You can install the gem with +gem install+

== Overview

{Qpid::Proton::Message} represents a message that can be sent or received.  A
message body can be a string or byte sequence encoded any way you
choose. However, AMQP also provides standard, interoperable encodings for basic
data types like {Hash} and {Array}. The equivalent AMQP encodings can be
understood as maps or sequences in any programming langauge with an AMQP
library.

{Qpid::Proton::Link} allows messages to be transferred to or from a remote AMQP
process. The {Qpid::Proton::Sender} subclass sends messages, the
{Qpid::Proton::Receiver} subclass receives them. Links have a source and target
address, as explained below.

Links are grouped in a {Qpid::Proton::Session}. Messages in the same session are
sent sequentially, while those on different sessions can be interleaved. A large
message being sent on one session does not block messages being sent on another
session.

Sessions belong to a {Qpid::Proton::Connection}. If you don't need multiple
sessions, a connection will create links directly using a default session.

A {Qpid::Proton::Transfer} represents the transfer of a message, the
{Qpid::Proton::Delivery} subclass allows a receiver to accept or reject an
incoming message. The {Qpid::Proton::Tracker} subclass allows a sender to track
the status of a sent message and find out if it was accepted or rejected.

A transfer is _settled_ when both ends are done with it. Different settlement
methods give different levels of reliability: at-most-once, at-least-once, and
exactly-once. See below.

== Anatomy of a Proton application

{Qpid::Proton::Container} is the top-level object in a Proton application. A
client uses {Qpid::Proton::Container#connect} to establish connections. A server
uses {Qpid::Proton::Container#listen} to accept connections.

Proton is an event-driven library. You implement one or more _handlers_ which
subclass {Qpid::Proton::MessagingHandler MessagingHandler} and override
functions to handle AMQP events, such as {Qpid::Proton::MessagingHandler#on_message
#on_message}. Each connection is associated with a handler for its
events. {Qpid::Proton::Container#run} polls all connections and listeners and
calls the event handling functions on your handlers.

A multi-threaded application can call {Qpid::Proton::Container#run} in more than
one thread, the container will use all the {Qpid::Proton::Container#run #run}
threads as a thread pool to dispatch events.

== Sources and targets

Every link has two addresses, _source_ and _target_. The most common pattern for
using these addresses is as follows:

When a client creates a {Qpid::Proton::Receiver Receiver} link, it sets the
_source_ address. This means "I want to receive messages from this source". This
is often referred to as "subscribing" to the source. When a client creates a
{Qpid::Proton::Sender Sender} link, it sets the _target_ address. This means "I
want to send to this target".

In the case of a broker, the source or target usually refers to a queue or
topic. In general they can refer to any AMQP-capable node.

In the request-response pattern, a request message carries a reply-to address
for the response message. This can be any AMQP address, but it is often useful
to create a temporary address for the response message. The client creates a
receiver with no source address and the dynamic flag set. The server generates a
unique source address for the receiver, which is discarded when the link
closes. The client uses this source address as the reply-to when it sends the
request, so the response is delivered to the client's receiver.

The server_direct.cpp example shows how to implement a request-response server.

== Settling a Message Transfer

A message transfer is _settled_ at one end of a link when that end of the link
has finished with the message and forgotten it.

_Pre-settled_ messages are settled by the sender before sending. This gives _at
most once_ reliability(also known as _best effort_, _unreliable_ or _fire and
forget_) since the sender never knows for sure if the message arrived. If the
connection is lost before the message is received by the receiver, the message
will not be delivered.

If the sender does not pre-settle a message, then the receiver settles it once
it has been processed. The sender is informed of the settlement via the
{Qpid::Proton::Tracker Tracker}. If the connection is lost before the sender has
received notice of settlement, the delivery is considered in-doubt and the
sender can re-send it. This ensures it eventually gets delivered (provided the
connection and link can be reestablished) but it is possible for multiple copies
of the same message are delivered, so the receiver must be aware of that. This
is known as _at_least_once_ reliability.

== Multi-threaded applications

{Qpid::Proton::Container#run} can be called by multiple threads concurrently,
giving the container a thread-pool to execute handler methods in parallel.

Instances of {Qpid::Proton::Connection} and objects associated with it
({Qpid::Proton::Session}, {Qpid::Proton::Sender}, {Qpid::Proton::Receiver},
{Qpid::Proton::Delivery}, {Qpid::Proton::Tracker}) are not thread-safe and must
be used correctly when multiple threads call {Qpid::Proton::Container#run}

Calls to {Qpid::Proton::MessagingHandler} and {Qpid::Proton::Listener::Handler}
methods by the {Qpid::Proton::Container} are automatically serialized for each
connection instance.

Other threads may have code similarly serialized by adding it to the
{Qpid::Proton::Connection#work_queue} for the connection.  Each object related
to a {Qpid::Proton::Connection} also provides a +work_queue+ method.

You also need to use the {Qpid::Proton::WorkQueue} to communicate between a
{Qpid::Proton::MessagingHandler} method call for one connection instance, and a
different {Qpid::Proton::Connection} instance in the same container, as separate
connections can be processed in parallel.
