3
Streams Staging and Propagation

This chapter explains the concepts relating to staging events in a queue and propagating events from one queue to another.

This chapter contains these topics:

• Introduction to Event Staging and Propagation
• Captured and User-Enqueued Events
• Event Propagation Between Queues
• Messaging Clients
• SYS.AnyData Queues and User Messages
• SYS.AnyData Queues and Oracle Real Application Clusters
• Streams Staging and Propagation Architecture

  See Also: Chapter 10, "Managing Staging and Propagation"

Introduction to Event Staging and Propagation

Streams uses queues of type SYS.AnyData to stage events. In Streams, there are two types of events that can be encapsulated into a SYS.AnyData event and staged in a SYS.AnyData queue: logical change records (LCRs) and user messages. An LCR is an object that contains information about a change to a database object, while a user message is a message of a user-defined type created by users or applications. Both types of events can be used for information sharing within a single database or between databases.

Staged events can be consumed or propagated, or both. Staged events can be consumed by an apply process, by a messaging client, or by a user application. A running apply process implicitly dequeues events, but messaging clients and user applications explicitly dequeue events. Even after an event is consumed, it may remain in the queue if you also have configured Streams to propagate the event to one or more other queues or if message retention is specified for user-enqueued messages. The queues to which events are propagated may reside in the same database or in different databases than the queue from which the events are propagated. In either case, the queue from which the events are propagated is called the source queue, and the queue that receives the events is called the destination queue. There can be a one-to-many, many-to-one, or many-to-many relationship between source and destination queues. Figure 3-1 shows propagation from a source queue to a destination queue.

Figure 3-1 Propagation from a Source Queue to a Destination Queue

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You can create, alter, and drop a propagation, and you can define propagation rules that control which events are propagated. The user who owns the source queue is the user who propagates events, and this user must have the necessary privileges to propagate events.

These privileges include the following:

• Execute privilege on the rule sets used by the propagation
• Execute privilege on all rule-based transformation functions used in the rule sets
• Enqueue privilege on the destination queue if the destination queue is in the same database

If the propagation propagates events to a destination queue in a remote database, then the owner of the source queue must be able to use the propagation's database link, and the user to which the database link connects at the remote database must have enqueue privilege on the destination queue.

Captured and User-Enqueued Events

Events can be enqueued in two ways:

• A capture process enqueues captured changes in the form of events containing LCRs. An event containing an LCR that was originally captured and enqueued by a capture process is called a captured event.
• A user application enqueues user messages encapsulated in events of type SYS.AnyData. These user messages can contain LCRs or any other type of message. Any user message that was explicitly enqueued by a user or an application is called a user-enqueued event. Events that were enqueued by a user procedure called from an apply process are also user-enqueued events.

So, each captured event contains an LCR, but a user-enqueued event may or may not contain an LCR. Propagating a captured event or a user-enqueued event enqueues the event into the destination queue.

Events can be dequeued in two ways:

• An apply process dequeues either captured or user-enqueued events. If the event contains an LCR, then the apply process either can apply it directly or call a user-specified procedure for processing. If the event does not contain an LCR, then the apply process can invoke a user-specified procedure called a message handler to process it. In addition, captured LCRs that are dequeued by an apply process and then enqueued using the SET_ENQUEUE_DESTINATION procedure in the DBMS_APPLY_ADM package are user-enqueued events.
• A user application explicitly dequeues user-enqueued events and processes them. The user application may or may not use a Streams messaging client. Captured events cannot be dequeued by a user application. Captured events must be dequeued by an apply process. However, if a user procedure called by an apply process explicitly enqueues an event, then the event is a user-enqueued event and can be explicitly dequeued, even if the event was originally a captured event.

The dequeued events may have originated at the same database where they are dequeued, or they may have originated at a different database.

Event Propagation Between Queues

You can use Streams to configure event propagation between two queues, which may reside in different databases. Streams uses job queues to propagate events.

A propagation is always between a source queue and a destination queue. Although propagation is always between two queues, a single queue may participate in many propagations. That is, a single source queue may propagate events to multiple destination queues, and a single destination queue may receive events from multiple source queues. However, only one propagation is allowed between a particular source queue and a particular destination queue. Also, a single queue may be a destination queue for some propagations and a source queue for other propagations.

A propagation may propagate all of the events in a source queue to a destination queue, or a propagation may propagate only a subset of the events. Also, a single propagation can propagate both captured and user-enqueued events. You can use rules to control which events in the source queue are propagated to the destination queue and which events are discarded.

Depending on how you set up your Streams environment, changes could be sent back to the site where they originated. You need to ensure that your environment is configured to avoid cycling a change in an endless loop. You can use Streams tags to avoid such a change cycling loop.

Propagation Rules

A propagation either propagates or discards events based on rules that you define. For LCR events, each rule specifies the database objects and types of changes for which the rule evaluates to TRUE. You can place these rules in a positive rule set for the propagation or a negative rule set for the propagation.

If a rule evaluates to TRUE for an event, and the rule is in the positive rule set for a propagation, then the propagation propagates the change. If a rule evaluates to TRUE for an event, and the rule is in the negative rule set for a propagation, then the propagation discards the change. If a propagation has both a positive and a negative rule set, then the negative rule set is always evaluated first.

You can specify propagation rules for LCR events at the following levels:

• A table rule propagates or discards either row changes resulting from DML changes or DDL changes to a particular table. Subset rules are table rules that include a subset of the row changes to a particular table.
• A schema rule propagates or discards either row changes resulting from DML changes or DDL changes to the database objects in a particular schema.
• A global rule propagates or discards either all row changes resulting from DML changes or all DDL changes in the source queue.

For non-LCR events, you can create your own rules to control propagation.

A queue subscriber that specifies a condition causes the system to generate a rule. The rule sets for all subscribers to a queue are combined into a single system-generated rule set to make subscription more efficient.

Ensured Event Delivery

A user-enqueued event is propagated successfully to a destination queue when the enqueue into the destination queue is committed. A captured event is propagated successfully to a destination queue when both of the following actions are completed:

• The event is processed by all relevant apply processes associated with the destination queue.
• The event is propagated successfully from the destination queue to all of its relevant destination queues.

When an event is successfully propagated between two SYS.AnyData queues, the destination queue acknowledges successful propagation of the event. If the source queue is configured to propagate an event to multiple destination queues, then the event remains in the source queue until each destination queue has sent confirmation of event propagation to the source queue. When each destination queue acknowledges successful propagation of the event, and all local consumers in the source queue database have consumed the event, the source queue can drop the event.

This confirmation system ensures that events are always propagated from the source queue to the destination queue, but, in some configurations, the source queue can grow larger than an optimal size. When a source queue grows, it uses more SGA memory and may use more disk space.

There are two common reasons for source-queue growth:

• If an event cannot be propagated to a specified destination queue for some reason (such as a network problem), then the event will remain in the source queue until the destination queue becomes available. This situation could cause the source queue to grow large. So, you should monitor your queues regularly to detect problems early.
• Suppose a source queue is propagating captured events to multiple destination queues, and one or more destination databases acknowledge successful propagation of events much more slowly than the other queues. In this case, the source queue can grow because the slower destination databases create a backlog of events that have already been acknowledged by the faster destination databases. In an environment such as this, consider creating more than one capture process to capture changes at the source database. In this case, you can use one source queue for the slower destination databases and another source queue for the faster destination databases.

  See Also: Chapter 2, "Streams Capture Process" "Monitoring a SYS.AnyData Queue and Messaging"

Directed Networks

A directed network is one in which propagated events may pass through one or more intermediate databases before arriving at a destination database. An event may or may not be processed by an apply process at an intermediate database. Using Streams, you can choose which events are propagated to each destination database, and you can specify the route that events will traverse on their way to a destination database. Figure 3-2 shows an example of a directed networks environment.

Figure 3-2 Example Directed Networks Environment

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The advantage of using a directed network is that a source database does not need to have a physical network connection with a destination database. So, if you want events to propagate from one database to another, but there is no direct network connection between the computers running these databases, then you can still propagate the events without reconfiguring your network, as long as one or more intermediate databases connect the source database to the destination database.

If you use directed networks, and an intermediate site goes down for an extended period of time or is removed, then you may need to reconfigure the network and the Streams environment.

Queue Forwarding and Apply Forwarding

An intermediate database in a directed network may propagate events using queue forwarding or apply forwarding. Queue forwarding means that the events being forwarded at an intermediate database are the events received by the intermediate database. The source database for an event is the database where the event originated.

Apply forwarding means that the events being forwarded at an intermediate database are first processed by an apply process. These events are then recaptured by a capture process at the intermediate database and forwarded. When you use apply forwarding, the intermediate database becomes the new source database for the events because the events are recaptured from the redo log generated there.

Consider the following differences between queue forwarding and apply forwarding when you plan your Streams environment:

• With queue forwarding, an event is propagated through the directed network without being changed, assuming there are no capture or propagation transformations. With apply forwarding, events are applied and recaptured at intermediate databases and may be changed by conflict resolution, apply handlers, or apply transformations.
• With queue forwarding, a destination database must have a separate apply process to apply events from each source database. With apply forwarding, fewer apply processes may be required at a destination database because recapturing of events at intermediate databases may result in fewer source databases when changes reach a destination database.
• With queue forwarding, one or more intermediate databases are in place between a source database and a destination database. With apply forwarding, because events are recaptured at intermediate databases, the source database for an event can be the same as the intermediate database connected directly with the destination database.

A single Streams environment may use a combination of queue forwarding and apply forwarding.

Advantages of Queue Forwarding

Queue forwarding has the following advantages compared to apply forwarding:

• Performance may be improved because an event is captured only once.
• Less time may be required to propagate an event from the database where the event originated to the destination database, because the events are not applied and recaptured at one or more intermediate databases. In other words, latency may be lower with queue forwarding.
• The origin of an event can be determined easily by running the GET_SOURCE_DATABASE_NAME member procedure on the LCR contained in the event. If you use apply forwarding, then determining the origin of an event requires the use of Streams tags and apply handlers.
• Parallel apply may scale better and provide more throughput when separate apply processes are used because there are fewer dependencies, and because there are multiple apply coordinators and apply reader processes to perform the work.
• If one intermediate database goes down, then you can reroute the queues and reset the start SCN at the capture site to reconfigure end-to-end capture, propagation, and apply.

  If you use apply forwarding, then substantially more work may be required to reconfigure end-to-end capture, propagation, and apply of events, because the destination database(s) downstream from the unavailable intermediate database were using the SCN information of this intermediate database. Without this SCN information, the destination databases cannot apply the changes properly.

Advantages of Apply Forwarding

Apply forwarding has the following advantages compared to queue forwarding:

• A Streams environment may be easier to configure because each database can apply changes only from databases directly connected to it, rather than from multiple remote source databases.
• In a large Streams environment where intermediate databases apply changes, the environment may be easier to monitor and manage because fewer apply processes may be required. An intermediate database that applies changes must have one apply process for each source database from which it receives changes. In an apply forwarding environment, the source databases of an intermediate database are only the databases to which it is directly connected. In a queue forwarding environment, the source databases of an intermediate database are all of the other source databases in the environment, whether they are directly connected to the intermediate database or not.

  See Also: Chapter 4, "Streams Apply Process" Oracle Streams Replication Administrator's Guide for an example of an environment that uses queue forwarding and for an example of an environment that uses apply forwarding

Binary File Propagation

You can propagate a binary file between databases by using Streams. To do so, you put one or more BFILE attributes in a message payload and then propagate the message to a remote queue. Each BFILE referenced in the payload is transferred to the remote database after the message is propagated, but before the message propagation is committed. The directory object and filename of each propagated BFILE are preserved, but you can map the directory object to different directories on the source and destination databases. The message payload can be a BFILE wrapped in a SYS.AnyData payload, or the message payload can be one or more BFILE attributes of an object wrapped in a SYS.AnyData payload.

The following are not supported in a message payload:

• One or more BFILE attributes in a varray
• A user-defined type object with a SYS.AnyData attribute that contains one or more BFILE attributes

Propagating a BFILE in Streams has the same restrictions as the procedure DBMS_FILE_TRANSFER.PUT_FILE.

Messaging Clients

A messaging client consumes user-enqueued events when it is invoked by an application or a user. You use rules to specify which user-enqueued events in the queue are dequeued by a messaging client. These user-enqueued events may be user-enqueued LCRs or user-enqueued messages.

You can create a messaging client by specifying dequeue for the streams_type parameter when you run one of the following procedures in the DBMS_STREAMS_ADM package:

• ADD_MESSAGE_RULE
• ADD_TABLE_RULES
• ADD_SUBSET_RULES
• ADD_SCHEMA_RULES
• ADD_GLOBAL_RULES

When you create a messaging client, you specify the name of the messaging client and the SYS.AnyData queue from which the messaging client dequeues messages. These procedures also can add rules to the positive or negative rule set of a messaging client. You specify the message type for each rule, and a single messaging client can dequeue messages of different types.

The user who creates a messaging client is granted the privileges to dequeue from the queue using the messaging client. This user is the messaging client user. The messaging client user can dequeue messages that satisfy the messaging client rule sets. A messaging client can be associated with only one user, but one user may be associated with many messaging clients.

Figure 3-3 shows a messaging client dequeuing user-enqueued events.

Figure 3-3 Messaging Client

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SYS.AnyData Queues and User Messages

Streams enables messaging with queues of type SYS.AnyData. These queues can stage user messages whose payloads are of SYS.AnyData type. A SYS.AnyData payload can be a wrapper for payloads of different datatypes.

By using SYS.AnyData wrappers for message payloads, publishing applications can enqueue messages of different types into a single queue, and subscribing applications can dequeue these messages, either explicitly using a messaging client or an application, or implicitly using an apply process. If the subscribing application is remote, then the messages can be propagated to the remote site, and the subscribing application can dequeue the messages from a local queue in the remote database. Alternatively, a remote subscribing application can dequeue messages directly from the source queue using a variety of standard protocols, such as PL/SQL and OCI.

Streams includes the features of Advanced Queuing (AQ), which supports all the standard features of message queuing systems, including multiconsumer queues, publish and subscribe, content-based routing, internet propagation, transformations, and gateways to other messaging subsystems.

You can wrap almost any type of payload in a SYS.AnyData payload. To do this, you use the Convertdata_type static functions of the SYS.AnyData type, where data_type is the type of object to wrap. These functions take the object as input and return a SYS.AnyData object.

You cannot enqueue SYS.AnyData payloads that contain payloads of the following datatypes into a SYS.AnyData queue:

• CLOB
• NCLOB
• BLOB

In addition, you cannot enqueue SYS.AnyData payloads that contain payloads of object types with attributes of these datatypes into a SYS.AnyData queue, nor object types that use type evolution or type inheritance.

SYS.AnyData Queues and Oracle Real Application Clusters

You can configure a SYS.AnyData queue to stage and propagate captured and user-enqueued events in a Real Application Clusters (RAC) environment. In a RAC environment, only the owner instance may have a buffer for a queue, but different instances may have buffers for different queues. A buffered queue is System Global Area (SGA) memory associated with a SYS.AnyData queue that contains only captured events. Buffered queues are discussed in more detail later in this chapter.

A SYS.AnyData queue that contains only user-enqueued events behaves the same as a typed queue in a RAC environment. However, if a SYS.AnyData queue contains or will contain captured events in a RAC environment, then each propagation that propagates captured events to a RAC destination database must use an instance-specific database link that refers to the owner instance of the destination queue. If the propagation connects to any other instance, then the propagation will raise an error.

Each capture process and apply process is started on the owner instance for its SYS.AnyData queue, even if the start procedure is run on a different instance. If the owner instance for a queue table containing a destination queue becomes unavailable, then queue ownership is transferred automatically to another instance in the cluster. If this happens, then database links from remote source queues must be reconfigured manually to connect to the instance that owns the destination queue.

Streams processes and jobs support primary instance and secondary instance specifications for queue tables. If you use these specifications, then the secondary instance assumes ownership of a queue table when the primary instance becomes unavailable, and ownership is transferred back to the primary instance when it becomes available again. If both the primary and secondary instance for a queue table containing a destination queue become unavailable, then queue ownership is transferred automatically to another instance in the cluster. In this case, if the primary or secondary instance becomes available again, then ownership is transferred back to one of them accordingly. You can set primary and secondary instance specifications using the ALTER_QUEUE_TABLE procedure in the DBMS_AQADM package.

The DBA_QUEUE_TABLES data dictionary view contains information about the owner instance for a queue table. A queue table may contain multiple queues. In this case, each queue in a queue table has the same owner instance as the queue table.

Streams Staging and Propagation Architecture

In general, SYS.AnyData queues use the same infrastructure as typed queues. However, unlike typed queues, which stage all events in a queue table, SYS.AnyData queues have a buffered queue to stage captured events in shared memory. This section describes buffered queues, propagation jobs, and secure queues, and how they are used in Streams. In addition, this section discusses how transactional queues handle captured and user-enqueued events, as well as the need for a Streams data dictionary at databases that propagate captured events.

This section contains the following topics:

• Buffered Queues
• Propagation Jobs
• Secure Queues
• Transactional and Nontransactional Queues
• Streams Data Dictionary for Propagations

  See Also: Oracle Streams Advanced Queuing User's Guide and Reference for more information about AQ infrastructure PL/SQL Packages and Types Reference for more information about the DBMS_JOB package

Buffered Queues

A buffered queue includes the following storage areas:

• System Global Area (SGA) memory associated with a SYS.AnyData queue that contains only captured events
• Part of a queue table for a SYS.AnyData queue that stores captured events that have spilled from memory

A buffered queue enables Oracle to optimize captured events by buffering captured events in the SGA instead of always storing them in a queue table. This buffering of captured events happens in any database where captured events are staged in a SYS.AnyData queue. Such a database may be a source database, an intermediate database, or a destination database. Captured events are always stored in a buffered queue, but user-enqueued LCR events and user-enqueued non-LCR events are always stored in queue tables, not in buffered queues. Also, when a transaction is moved to the error queue, all events in the transaction are stored in a queue table, not in a buffered queue.

Buffered queues improve performance, but some of the information in a buffered queue may be lost if the instance containing the buffered queue shuts down normally or abnormally. Streams automatically recovers from these cases, assuming full database recovery is performed on the instance.

In a single database, you can specify that Streams memory be allocated from a new pool in the SGA called the Streams pool. To configure the Streams pool, specify the size of the pool in bytes using the STREAMS_POOL_SIZE initialization parameter. If the size of the Streams pool is greater than zero, then any SGA memory used by Streams is allocated from the Streams pool. If the size of the Streams pool is zero, then the memory used by Streams is allocated from the shared pool and may use up to 10% of the shared pool.

Captured events in a buffered queue may spill from memory into the queue table if they have been staged in the buffered queue for a period of time without being dequeued, or if there is not enough space in memory to hold all of the captured events. Captured events that spill from memory are stored in the appropriate AQ$_queue_table_name_p table, where queue_table_name is the name of the queue table for the queue.

Propagation Jobs

A Streams propagation is configured internally using the DBMS_JOBS package. Therefore, a propagation job is the mechanism that propagates events from a source queue to a destination queue. Like other jobs configured using the DBMS_JOBS package, propagation jobs have an owner, and they use job queue processes (Jnnn) as needed to execute jobs.

A propagation job may be used by more than one propagation. All destination queues at a database receive events from a single source queue through a single propagation job. By using a single propagation job for multiple destination queues, Streams ensures that an event is sent to a destination database only once, even if the same message is received by multiple destination queues in the same database. Communication resources are conserved because messages are not sent more than once to the same database.

Propagation Scheduling and Streams Propagations

A propagation schedule specifies how often a propagation job propagates events from a source queue to a destination queue. Therefore, all propagations that use a propagation job have the same propagation schedule. A default propagation schedule is established for the new propagation job when you create the propagation job using one of the following procedures:

• The ADD_GLOBAL_PROPAGATION_RULES procedure in the DBMS_STREAMS_ADM package
• The ADD_SCHEMA_PROPAGATION_RULES procedure in the DBMS_STREAMS_ADM package
• The ADD_TABLE_PROPAGATION_RULES procedure in the DBMS_STREAMS_ADM package
• The ADD_SUBSET_PROPAGATION_RULE procedure in the DBMS_STREAMS_ADM package
• The CREATE_PROPAGATION procedure in the DBMS_PROPAGATION_ADM package

The default schedule has the following properties:

• The start time is SYSDATE().
• The duration is NULL, which means infinite.
• The next time is NULL, which means that propagation restarts as soon as it finishes the current duration.
• The latency is three seconds, which is the wait time after a queue becomes empty to resubmit the propagation job. Therefore, the latency is the maximum wait, in seconds, in the propagation window for a message to be propagated after it is enqueued.

If you want to alter the default schedule for a propagation job, then use the ALTER_PROPAGATION_SCHEDULE procedure in the DBMS_AQADM package.

Propagation Jobs and RESTRICTED SESSION

When the restricted session is enabled during system startup by issuing a STARTUP RESTRICT statement, propagation jobs with enabled propagation schedules do not propagate events. When the restricted session is disabled, each propagation schedule that is enabled and ready to run will run when there is an available job queue process.

When the restricted session is enabled in a running database by the SQL statement ALTER SYSTEM with the ENABLE RESTRICTED SESSION clause, any running propagation job continues to run to completion. However, any new propagation job submitted for a propagation schedule is not started. Therefore, propagation for an enabled schedule may eventually come to a halt.

Secure Queues

Secure queues are queues for which AQ agents must be associated explicitly with one or more database users who can perform queue operations, such as enqueue and dequeue. The owner of a secure queue can perform all queue operations on the queue, but other users cannot perform queue operations on a secure queue, unless they are configured as secure queue users. In Streams, secure queues can be used to ensure that only the appropriate users and Streams clients enqueue events into a queue and dequeue events from a queue.

Secure Queues and the SET_UP_QUEUE Procedure

All SYS.AnyData queues created using the SET_UP_QUEUE procedure in the DBMS_STREAMS_ADM package are secure queues. When you use the SET_UP_QUEUE procedure to create a queue, any user specified by the queue_user parameter is configured as a secure queue user of the queue automatically, if possible. The queue user is also granted ENQUEUE and DEQUEUE privileges on the queue. To enqueue events into and dequeue events from a queue, a queue user must also have EXECUTE privilege on the DBMS_STREAMS_MESSAGING package or the DBMS_AQ package. The SET_UP_QUEUE procedure does not grant either of these privileges. Also, an event cannot be enqueued into a queue unless a subscriber who can dequeue the event is configured.

To configure a queue user as a secure queue user, the SET_UP_QUEUE procedure creates an AQ agent with the same name as the user name, if one does not already exist. The user must use this agent to perform queue operations on the queue. If an agent with this name already exists and is associated with the queue user only, then the existing agent is used. SET_UP_QUEUE then runs the ENABLE_DB_ACCESS procedure in the DBMS_AQADM package, specifying the agent and the user.

If you use the SET_UP_QUEUE procedure in the DBMS_STREAMS_ADM package to create a secure queue, and you want a user who is not the queue owner and who was not specified by the queue_user parameter to perform operations on the queue, then you can configure the user as a secure queue user of the queue manually. Alternatively, you can run the SET_UP_QUEUE procedure again and specify a different queue_user for the queue. In this case, SET_UP_QUEUE will skip queue creation, but it will configure the user specified by queue_user as a secure queue user of the queue.

If you create a SYS.AnyData queue using the DBMS_AQADM package, then you use the secure parameter when you run the CREATE_QUEUE_TABLE procedure to specify whether the queue is secure or not. The queue is secure if you specify true for the secure parameter when you run this procedure. When you use the DBMS_AQADM package to create a secure queue, and you want to allow users to perform queue operations on the secure queue, then you must configure these secure queue users manually.

Secure Queues and Streams Clients

When you create a capture process or an apply process, an AQ agent of the secure queue associated with the Streams process is configured automatically, and the user who runs the Streams process is specified as a secure queue user for this queue automatically. Therefore, a capture process is configured to enqueue into its secure queue automatically, and an apply process is configured to dequeue from its secure queue automatically. In either case, the AQ agent has the same name as the Streams client.

For a capture process, the user specified as the capture_user is the user who runs the capture process. For an apply process, the user specified as the apply_user is the user who runs the apply process. If no capture_user or apply_user is specified, then the user who invokes the procedure that creates the Streams process is the user who runs the Streams process.

Also, if you change the capture_user for a capture process or the apply_user for an apply process, then the specified capture_user or apply_user is configured as a secure queue user of the queue used by the Streams process. However, the old capture user or apply user remains configured as a secure queue user of the queue. To remove the old user, run the DISABLE_DB_ACCESS procedure in the DBMS_AQADM package, specifying the old user and the relevant AQ agent. You may also want to drop the agent if it is no longer needed. You can view the AQ agents and their associated users by querying the DBA_AQ_AGENT_PRIVS data dictionary view.

When you create a messaging client, an AQ agent of the secure queue with the same name as the messaging client is associated with the user who runs the procedure that creates the messaging client. This messaging client user is specified as a secure queue user for this queue automatically. Therefore, this user can use the messaging client to dequeue messages from the queue.

A capture process, an apply process, or a messaging client can be associated with only one user. However, one user may be associated with multiple Streams clients, including multiple capture processes, apply processes, and messaging clients. For example, an apply process cannot have both hr and oe as apply users, but hr may be the apply user for multiple apply processes.

If you drop a capture process, apply process, or messaging client, then the users who were configured as secure queue users for these Streams clients remain secure queue users of the queue. To remove these users as secure queue users, run the DISABLE_DB_ACCESS procedure in the DBMS_AQADM package for each user. You may also want to drop the agent if it is no longer needed.

Transactional and Nontransactional Queues

A transactional queue is one in which user-enqueued events can be grouped into a set that are applied as one transaction. That is, an apply process performs a COMMIT after it applies all the user-enqueued events in a group. The SET_UP_QUEUE procedure in the DBMS_STREAMS_ADM package always creates a transactional queue.

A nontransactional queue is one in which each user-enqueued event is its own transaction. That is, an apply process performs a COMMIT after each user-enqueued event it applies. In either case, the user-enqueued events may or may not contain user-created LCRs.

The difference between transactional and nontransactional queues is important only for user-enqueued events. An apply process always applies captured events in transactions that preserve the transactions executed at the source database. Table 3-1 shows apply process behavior for each type of event and each type of queue.

Table 3-1 Apply Process Behavior for Transactional and Nontransactional Queues

Event Type	Transactional Queue	Nontransactional Queue
Captured Events	Apply process preserves the original transaction	Apply process preserves the original transaction
User-Enqueued Events	Apply a user-specified group of user-enqueued events as one transaction	Apply each user-enqueued event in its own transaction

Streams Data Dictionary for Propagations

When a database object is prepared for instantiation at a source database, a Streams data dictionary is populated automatically at the database where changes to the object are captured by a capture process. The Streams data dictionary is a multiversioned copy of some of the information in the primary data dictionary at a source database. The Streams data dictionary maps object numbers, object version information, and internal column numbers from the source database into table names, column names, and column datatypes. This mapping keeps each captured event as small as possible because the event can store numbers rather than names internally.

The mapping information in the Streams data dictionary at the source database is needed to evaluate rules at any database that propagates the captured events from the source database. To make this mapping information available to a propagation, Oracle automatically populates a multiversioned Streams data dictionary at each database that has a Streams propagation. Oracle automatically sends internal messages that contain relevant information from the Streams data dictionary at the source database to all other databases that receive captured events from the source database.

The Streams data dictionary information contained in these internal messages in a queue may or may not be propagated by a propagation. Which Streams data dictionary information to propagate depends on the rule sets for the propagation. When a propagation encounters Streams data dictionary information for a table, the propagation rule sets are evaluated with partial information that includes the source database name, table name, and table owner. If the partial rule evaluation of these rule sets determines that there may be relevant LCRs for the given table from the specified database, then the Streams data dictionary information for the table is propagated.

When Streams data dictionary information is propagated to a destination queue, it is incorporated into the Streams data dictionary at the database that contains the destination queue, in addition to being enqueued into the destination queue. Therefore, a propagation reading the destination queue in a directed networks configuration can forward LCRs immediately without waiting for the Streams data dictionary to be populated. In this way, the Streams data dictionary for a source database always reflects the correct state of the relevant database objects for the LCRs relating to these database objects.