Message Queue

Concepts

Message queues provide an asynchronous communications protocol, meaning that the sender and the receiver of the message do not need to interact with the message queue at the same time. Messages placed onto the queue are stored until the recipient retrieves them. A message does not have information about previous and next messages.

Therefore, a message queues should be used if:

• A process can be executed asynchronously.

• A process does not affect user experience.

• Processes need to be executed in parallel for faster performance.

• You need a guarantee of processing.

• You need scalability.

For more information, see the following external resources:

• What is Message Queue

• Message Queue Benefits (most of them are applicable to Oro Message Queue Component)

• RabbitMQ Introduction

DBAL Transport

DBAL transport options

DBAL Broker

The DBAL broker is implemented by the OroMessageQueueBundle. It is part of OroPlatform which means that this broker is available in all Oro applications out-of-the-box.

For message storage, like the message queue, the DBAL broker uses application database tables.

Advantages of this broker are an easy installation and configuration, and out-of-the-box availability in every Oro application.

However, since RDBMS is not designed to work as a message queue, the DBAL broker type has some limitations:

• There is no way to use event-driven model and listen to new inserts into DB. We use a polling model to ask the DB if it has new messages. We run such queries ones per second by default which means that every consumer receives only one message per second. Use the polling_interval option to change this value but keep in mind that low interval values may cause DB load.

• When the consumer receives a message, it updates a DB record with a unique identifier so that a different consumer could not receive this message. Once the job is done and the message is acknowledged, the consumer removes this record from the DB. This is a success story but errors can happen. For instance, when a fatal error informs that the consumer is dead and the message is locked and remains in the DB. To handle such cases, RedeliverOrphanMessagesExtension periodically searches for messages which are consumed but not acknowledged and redelivers these messages.

DBAL Transport Options and Limitations

Options

oro_message_queue:
  transport:
    default: 'dbal'
    dbal:
      connection: default                  # doctrine dbal connection name
      table: oro_message_queue             # table name where messages will be stored
      pid_file_dir: /tmp/oro-message-queue # RedeliverOrphanMessagesExtension stores consumer pid files here
      consumer_process_pattern: ':consume' # used by RedeliverOrphanMessagesExtension to check the working or non-working consumers
                                           # (see limitations section for more details)
      polling_interval: 1000               # consumer polling interval in milliseconds
                                           # (see limitations section for more details)

Limitations

As RDBMS are not designed to work as message queue, the implementation has several limitations.

• There is no way to use event-driven model and listen for new inserts into the DB. We use polling model to ask the DB if it has new messages. We run such queries once per second by default, and it means that every consumer receives only one message per second. Use polling_interval option to change this value, but low interval values may cause DB load.

• When the consumer receives a message, it updates the DB record with a unique identifier, so any other consumer cannot receive this message. After the job is done and the message is acknowledged, the consumer removes this record from the DB. This is the best case scenario, however, exceptions may occur. For instance, if a fatal error happens, the message consumer process may finish with blocking message still remaining in the DB. For such cases, the RedeliverOrphanMessagesExtension is used. It periodically searches for the messages which are consumed but not acknowledged, and then redelivers them.

AMQP Transport (RabbitMQ)

RabbitMQ Broker

The RabbitMQ broker comes with Enterprise Editions of Oro applications.

RabbitMQ is one of the most popular Message Queue brokers that supports many features and messaging protocols.

The integration with RabbitMQ in Oro applications is implemented based on the AMQP protocol and supports most of its features that are actively used in Oro applications, including:

• Multiple Queues

• Separate Consumer pools for different queues

• Routing of messages from Exchange to the different queues based on Message Topic, Message Headers, etc.

The main drawback of the RabbitMQ broker is that it can be relatively complicated to set up and configure, as opposed to the DBAL broker.

AMQP (RabbitMQ) Transport

RabbitMQ provides better and faster messages delivery as opposed to DBAL. It is recommended to use RabbitMQ, if possible.

Options

The application reads config settings (a user named guest with the default password of guest, granted full access to the / virtual host) from the ORO_MQ_DSN environment variable. The format is as follow amqp://guest:guest@localhost:5672. The default value for ORO_MQ_DSN environment variable is set in the config/config.yml file:

config/config.yml

 oro_message_queue:
     client: ~
 parameters:
     message_queue_transport_dsn: '%env(ORO_MQ_DSN)%'
     env(ORO_MQ_DSN): 'dbal:'

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See Also

• Message Queue Architecture Guide