|Oracle® Streams Advanced Queuing User's Guide and Reference
Part Number B10785-01
This chapter introduces Oracle Messaging Gateway (MGW) features and functionality.
This chapter contains these topics:
Messaging Gateway enables communication between applications based on non-Oracle messaging systems and Oracle Streams AQ.
Oracle Streams AQ provides propagation between two Oracle Streams AQ queues to enable e-business (HTTP through IDAP). Messaging Gateway extends this to applications based on non-Oracle messaging systems.
Because Messaging Gateway is integrated with Oracle Streams AQ and Oracle Database, it offers reliable message delivery. Messaging Gateway guarantees that messages are delivered once and only once between Oracle Streams AQ and non-Oracle messaging systems that support persistence. The PL/SQL interface provides an easy-to-learn administrative API, especially for developers already proficient in using Oracle Streams AQ.
This release of Messaging Gateway supports the integration of Oracle Streams AQ with applications based on WebSphere MQ 5.3, TIB/Rendezvous 6.9 and TIB/Rendezvous 7.2.
Messaging Gateway provides the following features:
Extends Oracle Streams AQ message propagation
Messaging Gateway propagates messages between Oracle Streams AQ and non-Oracle messaging systems. Messages sent by Oracle Streams AQ applications can be received by non-Oracle messaging system applications. Conversely, messages published by non-Oracle messaging system applications can be consumed by Oracle Streams AQ applications.
Support for Java Message Service (JMS) messaging systems
Messaging Gateway propagates messages between Oracle Java Message Service (Oracle JMS) and WebSphere MQ Java Message Service (WebSphere MQ JMS).
Native message format support
Messaging Gateway supports the native message formats of messaging systems. Oracle Streams AQ messages can have
RAW or any Oracle object type payload. WebSphere MQ messages can be text or byte messages. TIB/Rendezvous messages can be any TIB/Rendezvous wire format datatype except the nested datatype
MSG and those with unsigned integers.
Messaging Gateway facilitates message conversion between Oracle Streams AQ messages and non-Oracle messaging system messages. Messages are converted through either automatic routines provided by Messaging Gateway or customized message transformation functions that you provide.
Note:Messaging Gateway does not support message propagation between JMS and non-JMS messaging systems.
Integration with Oracle Database
Messaging Gateway is managed through a PL/SQL interface similar to that of Oracle Streams AQ. Configuration information is stored in Oracle Database tables. Message propagation is carried out by an external process of the Oracle Database server.
Guaranteed message delivery
If the messaging systems at the propagation source and propagation destination both support transactions, then Messaging Gateway guarantees that persistent messages are propagated exactly once. If messages are not persistent or transactions are not supported by the messaging systems at the propagation source or propagation destination, then at-most-once propagation is guaranteed.
Messaging Gateway supports client authentication of Oracle Database and non-Oracle messaging systems.
Messaging Gateway has two main components:
Administration Package DBMS_MGWADM
Messaging Gateway Agent
Figure 18-1 shows how these components work together with Oracle Database and non-Oracle messaging systems.
Figure 18-1 Messaging Gateway Architecture
The Messaging Gateway administration package
DBMS_MGWADM provides an interface for managing the Messaging Gateway agent, creating messaging system links, registering non-Oracle queues, and setting up propagation.
Users call the procedures in the package to make configuration changes regardless of whether the Messaging Gateway agent is running. If the Messaging Gateway agent is running, then the procedures in the package send notifications for configuration changes to the agent. The agent dynamically alters its configuration for most configuration changes, although some changes require that the agent be shut down and restarted before they take effect. All the procedures in the package are serialized to guarantee that the Messaging Gateway agent receives and processes notifications in the same order as they are made.
See Also:PL/SQL Packages and Types Reference for more information on
The Messaging Gateway agent runs as an external process of the Oracle Database server and processes propagation jobs. It is started and shut down by calling the
SHUTDOWN procedures in
The Messaging Gateway agent contains a multithreaded propagation engine and a set of drivers for messaging systems. The propagation engine fairly schedules propagation jobs and processes propagation jobs concurrently. The polling thread in the agent periodically polls the source queues of enabled propagation jobs and wakes up worker threads to process propagation jobs if messages are available. The drivers for non-Oracle messaging systems run as clients of the messaging systems for all messaging operations.
As an Oracle Database feature, Messaging Gateway provides a mechanism of message propagation between Oracle Streams AQ and non-Oracle messaging systems. Oracle Streams AQ is involved in every propagation job as either propagation source or propagation destination.
Messaging Gateway is managed through the PL/SQL administration package
DBMS_MGWADM. All configuration information and execution state information of Messaging Gateway are stored in Oracle Database and can be accessed through database views.
The Messaging Gateway agent runs as an external procedure of the Oracle Database server. Therefore, it runs only when its associated database server is running.
The Messaging Gateway agent connects to non-Oracle messaging systems through messaging system links. Messaging system links are communication channels between the Messaging Gateway agent and non-Oracle messaging systems. Users can use the administration package
DBMS_MGWADM to configure multiple links to the same or different non-Oracle messaging systems.
Queues in non-Oracle messaging systems, such as WebSphere MQ queues, TIB/Rendezvous subjects, and WebSphere MQ JMS destinations (queues and topics) can all serve as propagation sources and destinations for Messaging Gateway. They are referred to as foreign queues. All foreign queues involved in message propagation as source queues, destination queues, or exception queues must be registered through the administration package. The registration of a foreign queue does not create the physical queue in a non-Oracle messaging system, but merely records information about the queue, such as the messaging system link to access it, its native name, and its domain (queue or topic). The physical queue must be created through the administration interface of the non-Oracle messaging system.
See Also:"Registering a Non-Oracle Queue"
To propagate messages, propagation jobs must be created. A propagation job consists of a propagation subscriber and a propagation schedule. A propagation subscriber defines the source queue and destination queue of a propagation job. A propagation schedule controls when the propagation job is processed.
If the propagation source is a queue (point-to-point), then the Messaging Gateway agent moves all messages in the queue to the destination. If the propagation source is a topic (publish/subscribe), then the Messaging Gateway agent creates a subscription on the propagation source topic. The agent moves all messages that are published to the topic after the subscription is created.
A propagation job is processed when its schedule is enabled. Disabling a propagation schedule stops propagation job processing, but does not stop message subscription.
When the Messaging Gateway agent processes a propagation job, it dequeues messages from the source queue and enqueues the messages to the destination queue. As each message is propagated, it is converted from its native format in the source messaging system to its native format in the destination messaging system. Messaging Gateway provides automatic message conversions between simple and commonly used message formats. You can customize message conversions by providing your own message transformation functions.
When the Messaging Gateway agent fails to convert a message from the source format to the destination format, the agent moves the message from the source queue to an exception queue, if the exception queue exists, and continues to process the propagation job.
If the Messaging Gateway agent runs into failures when processing a propagation job, it retries up to sixteen times in an exponential backoff scheme (from two seconds up to thirty minutes) before it stops retrying.
To guarantee reliable message delivery, Messaging Gateway requires logging queues in messaging systems that support transactions and persistent messages. The Messaging Gateway agent uses the logging queues to store the processing states of propagation jobs so that it can restore propagation processing from failures.