Andrew Smith e451a9e79d [AMQP 1.0] Add Acknowledgement and Batch Notification Topics

Explain difference in message acknowledgement for intermediary
types and relationship to use of batch notification.

Change-Id: I5877d143c55ae6ac23bf79856fef94e7b14cb722

2016-08-30 10:50:39 -04:00

21 KiB

Raw Blame History

AMQP 1.0 Protocol Driver Deployment Guide

oslo_messaging

Introduction

The AMQP 1.0 Protocol Driver is a messaging transport backend supported in oslo.messaging. The driver maps the base oslo.messaging capabilities for RPC and Notification message exchange onto version 1.0 of the Advanced Message Queuing Protocol (AMQP 1.0, ISO/IEC 19464). The driver is intended to support any messaging intermediary (e.g. broker or router) that implements version 1.0 of the AMQP protocol.

More detail regarding the AMQP 1.0 Protocol is available from the AMQP specification.

More detail regarding the driver's implementation is available from the oslo specification.

Abstract

The AMQP 1.0 driver is one of a family of oslo.messaging backend drivers. It currently supports two types of message intermediaries. The first type is an AMQP 1.0 messaging broker and the second type is an AMQP 1.0 message router. The driver should support additional intermediary types in the future but may require additions to driver configuration parameters in order to do so.

Intermediary Type	RPC Pattern	Notify Pattern	Message Treatment	Topology
Message Router	Yes	Limited	Direct Messaging	Single or Mesh
Message Broker	Yes	Yes	Store and Forward	Single or Cluster

Direct Messaging

The direct messaging capabilities provided by the message router intermediary type are recommended for the oslo.messaging RPC messaging pattern.

Additionally, the message router intermediary may be used for the Notification messaging pattern with the consideration that notification messages will be dropped when there is no active consumer. The message router does not provide durability or store-and-forward capabilities for notification messages.

The driver can readily scale operation from working with a single instances of a message router to working with a large scale routed mesh interconnect topology.

Store and Forward

The store and forward capabilities provide by the message broker intermediary type are recommended for the oslo.message Notification messaging pattern. This driver is able to work with a single instance of a message broker or a clustered broker deployment.

Oslo.messaging provides a mechanism to configure separate backends for RPC and Notification communications. This document provides deployment and configuration information for use of this driver in oslo.messaging.

Addressing

A new address syntax was added to the driver to support efficient direct message routing. This new syntax will also work with a broker intermediary backend but is not compatible with the address syntax previously used by the driver. In order to allow backward compatibility, the driver will attempt to identify the intermediary type for the backend in use and will automatically select the 'legacy' syntax for broker-based backends or the new 'routable' syntax for router-based backends. An address mode configuration option is provided to override this dynamic behavior and force the use of either the legacy or routable address syntax.

Message Acknowledgement

A primary functional difference between a router and a broker intermediary type is when message acknowledgement occurs.

The router does not "store" the message hence it does not generate an acknowledgement. Instead the consuming endpoint is responsible for message acknowledgement and the router forwards the acknowledgement back to the sender. This is known as 'end-to-end' acknowledgement. In contrast, a broker stores then forwards the message so that message acknowledgement is performed in two stages. In the first stage, a message acknowledgement occurs between the broker and the Sender. In the second stage, an acknowledgement occurs between the Server and the broker.

This difference affects how long the Sender waits for the message transfer to complete.

+dispatch+
|  (3)   |
|        |
|        v
+--------------+    (1)    +----------+    (2)     +--------------+
|    Client    |---------->|  Router  |----------->|    Server    |
|   (Sender)   |<----------| (Direct) |<-----------|  (Listener)  |
+--------------+    (5)    +----------+    (4)     +--------------+

For example when a router intermediary is used, the following sequence occurs:

The message is sent to the router
The router forwards the message to the Server
The Server dispatches the message to the application
The Server indicates the acknowledgement via the router
The router forwards the acknowledgement to the Sender

In this sequence, a Sender waits for the message acknowledgement until step (5) occurs.

+dispatch+
|  (4)   |
|        |
|        v
+--------------+    (1)    +----------+    (3)     +--------------+
|    Client    |---------->|  Broker  |----------->|    Server    |
|   (Sender)   |<----------| (Queue)  |<-----------|  (Listener)  |
+--------------+    (2)    +----------+    (5)     +--------------+

And when a broker intermediary is used, the following sequence occurs:

The message is sent to the broker
The broker stores the message and acknowledges the message to the Sender
The broker sends the message to the Server
The Server dispatches the message to the application
The Server indicates the acknowledgement to the broker

In this sequence, a Sender waits for the message acknowledgement until step (2) occurs.

Therefore the broker-based Sender receives the acknowledgement earlier in the transfer than the routed case. However in the brokered case receipt of the acknowledgement does not signify that the message has been (or will ever be) received by the Server.

Batched Notifications Note Well

While the use of a router intermediary for oslo.messaging Notification is currently not recommended, it should be noted that the use of a router intermediary with batched notifications may exacerbate the acknowledgement wait time for a Sender.

For example, when a batched notification configuration is used where batch size is set to 100, the Server will wait until 100 notification messages are buffered (or timeout occurs) before dispatching the notifications to the application for message acknowledgement. Since each notifier client can have at most one message outstanding (e.g. pending acknowledgement), then if the total number of notifying clients are less than 100 the batch limit will never be met. This will effectively pause all notifying clients until the batch timeout expires.

Prerequisites

Protocol Engine

This driver uses the Apache QPID Proton AMQP 1.0 protocol engine. This engine consists of a platform specific library and a python binding. The driver does not directly interface with the engine API, as the API is a very low-level interface to the AMQP protocol. Instead, the driver uses the pure python Pyngus client API, which is layered on top of the protocol engine.

In order to run the driver the Proton Python bindings, Proton library, Proton header files, and Pyngus must be installed. Pre-built packages for both Pyngus and the Proton protocol engine are available for various Linux distributions (see packages below). It is recommended to use the pre-built packages if they are available for your platform.

The Proton package includes a C extension that links to the Proton library. If this library is not installed, then the Proton install script will attempt to download the necessary Proton C source files from the Apache repository and build the library locally.

In order to build the Proton C source locally, there are a number of tools and libraries that need to be present:

The tools and library necessary for Python development
The SWIG wrapper generator
The OpenSSL development libraries and headers
The Cyrus SASL development libraries and headers

Note well: Currently the Proton Pypi package only supports building the C extension on Linux systems.

Router Intermediary

This driver supports a router intermediary that supports version 1.0 of the AMQP protocol. The direct messaging capabilities provided by this intermediary type are recommended for oslo.messaging RPC.

The driver has been tested with qpid-dispatch-router router in a devstack environment. The version of qpid-dispatch-router must be at least 0.6.0. The qpid-dispatch-router also uses the Proton engine for its AMQP 1.0 support, so the Proton library must be installed on the system hosting the qpid-dispatch-router daemon.

Pre-built packages for the router are available. See packages below.

Broker Intermediary

This driver supports a broker intermediary that supports version 1.0 of the AMQP protocol. The store and forward capabilities provided by this intermediary type are recommended for oslo.messaging Notifications.

The driver has been tested with the qpidd broker in a devstack environment. The version of qpidd must be at least 0.34. qpidd also uses the Proton engine for its AMQP 1.0 support, so the Proton library must be installed on the system hosting the qpidd daemon.

Pre-built packages for the broker are available. See packages below.

See the oslo specification for additional information regarding testing done on the driver.

Configuration

Transport URL Enable

In oslo.messaging, the transport_url parameters define the OpenStack service backends for RPC and Notify. The url is of the form:

transport://user:pass@host1:port[,hostN:portN]/virtual_host

Where the transport value specifies the rpc or notification backend as one of amqp, rabbit, zmq, etc.

To specify and enable the AMQP 1.0 driver for RPC, in the section [DEFAULT] of the service configuration file, specify the 'transport_url' parameter:

[DEFAULT]
transport_url = amqp://username:password@routerhostname:5672

To specify and enable the AMQP 1.0 driver for Notify, in the section [NOTIFICATIONS] of the service configuration file, specify the 'transport_url' parameter:

[NOTIFICATIONS]
transport_url = amqp://username:password@brokerhostname:5672

Note, that if a 'transport_url' parameter is not specified in the [NOTIFICATIONS] section, the [DEFAULT] transport_url will be used for both RPC and Notify backends.

Driver Options

It is recommended that the default configuration options provided by the AMQP 1.0 driver be used. The configuration options can be modified in the oslo_messaging_amqp section of the service configuration file.