2 Oracle Streams Information Capture

Implicit Capture

With implicit capture, data definition language (DDL) and data manipulation language (DML) changes are captured automatically either by a capture process or by synchronous capture. A specific type of message called logical change record (LCR) describes these database changes. Both a capture process and synchronous capture can filter database changes with user-defined rules. Therefore, only changes to specified objects are captured.

The following topics describe capture processes and synchronous captures:

• Capture Processes

• Synchronous Captures

Explicit Capture

With explicit capture, applications generate messages and enqueue them. These messages can be formatted as LCRs, or they can be formatted into different types of messages for consumption by other applications. Messages can also be enqueued explicitly by an apply process or by an apply handler for an apply process.

Explicit capture is useful in the following situations:

• When applications generate messages that must be processed by other applications.

• When you have a heterogeneous replication environment in which an apply process in an Oracle database applies changes that originated at a non-Oracle database. In this case, an application captures LCRs based on the changes at the non-Oracle database, and these LCRs are processed by an apply process at an Oracle database.

Logical Change Records (LCRs)

An LCR is a message with a specific format that describes a database change. There are two types of LCRs: row LCRs and DDL LCRs. A capture process, a synchronous capture, or an application can capture LCRs.

You can capture the following types of LCRs with Oracle Streams:

• A captured LCR is an LCR that is captured implicitly by a capture process and enqueued into the buffered queue portion of an ANYDATA queue.

• A persistent LCR is an LCR that is enqueued into the persistent queue portion of an ANYDATA queue. A persistent LCR can be enqueued in one of the following ways:

  • Captured implicitly by a synchronous capture and enqueued

  • Constructed explicitly by an application and enqueued

  • Dequeued by an apply process and enqueued by the same apply process using the SET_ENQUEUE_DESTINATION procedure in the DBMS_APPLY_ADM package

  The only difference between the persistent LCRs captured in these three ways is that persistent LCRs captured by a synchronous capture have more attributes than those constructed by an application or enqueued by an apply process.

• A buffered LCR is and LCR that is constructed explicitly by an application and enqueued into the buffered queue portion of an ANYDATA queue.

The following sections contain information about LCRs:

• Row LCRs

• DDL LCRs

• Extra Information in LCRs

User Messages

Messages that do not contain LCRs are called user messages. User messages can be of any type (except an LCR type). User messages can be created by an application and consumed by an application. For example, a business application might create a user message for each order, and these messages might be processed by another application.

You can capture the following types of user messages with Oracle Streams:

• A persistent user message is a non-LCR message of a user-defined type that is enqueued into a persistent queue. A persistent user message can be enqueued in one of the following ways:

  • Created explicitly by an application and enqueued

  • Dequeued by an apply process and enqueued by the same apply process using the SET_ENQUEUE_DESTINATION procedure in the DBMS_APPLY_ADM package

  A persistent user message can be enqueued into the persistent queue portion of an ANYDATA queue or a typed queue.

• A buffered user message is a non-LCR message of a user-defined type that is created explicitly by an application and enqueued into a buffered queue. A buffered user message can be enqueued into the buffered queue portion of an ANYDATA queue or a typed queue.

Introduction to Capture Processes

Every Oracle database has a set of two or more redo log files. The redo log files for a database are collectively known as the database redo log. The primary function of the redo log is to record all of the changes made to the database.

Redo logs are used to guarantee recoverability in the event of human error or media failure. A capture process is an optional Oracle background process that scans the database redo log to capture data manipulation language (DML) and data definition language (DDL) changes made to database objects. When a capture process is configured to capture changes from a redo log, the database where the changes were generated is called the source database for the capture process.

When a capture process captures a database change, it converts it into a specific message format called a logical change record (LCR). After capturing an LCR, a capture process enqueues a message containing the LCR into a queue. A capture process is always associated with a single ANYDATA queue, and it enqueues messages into this queue only. For improved performance, captured LCRs always are stored in a buffered queue, which is System Global Area (SGA) memory associated with a queue. You can create multiple queues and associate a different capture process with each queue.

Captured LCRs can be sent to queues in the same database or other databases by propagations. Captured LCRs can also be dequeued by apply processes. In some situations, an optimization enables capture processes to send LCRs to apply processes more efficiently. This optimization is called combined capture and apply.

A capture process can run on its source database or on a remote database. When a capture process runs on its source database, the capture process is a local capture process. When a capture process runs on a remote database, the capture process is a downstream capture process, and the remote database is called the downstream database.

Figure 2-1 shows a capture process capturing LCRs.

Figure 2-1 Capture Process

Description of "Figure 2-1 Capture Process"

Capture Process Rules

A capture process either captures or discards changes based on rules that you define. 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 or negative rule set for the capture process.

If a rule evaluates to TRUE for a change, and the rule is in the positive rule set for a capture process, then the capture process captures the change. If a rule evaluates to TRUE for a change, and the rule is in the negative rule set for a capture process, then the capture process discards the change. If a capture process has both a positive and a negative rule set, then the negative rule set is always evaluated first.

You can specify capture process rules at the following levels:

• A table rule captures 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 captures or discards either row changes resulting from DML changes or DDL changes to the database objects in a particular schema.

• A global rule captures or discards either all row changes resulting from DML changes or all DDL changes in the database.

Data Types Captured by Capture Processes

When capturing the row changes resulting from DML changes made to tables, a capture process can capture changes made to columns of the following data types:

• VARCHAR2

• NVARCHAR2

• FLOAT

• NUMBER

• LONG

• DATE

• BINARY_FLOAT

• BINARY_DOUBLE

• TIMESTAMP

• TIMESTAMP WITH TIME ZONE

• TIMESTAMP WITH LOCAL TIME ZONE

• INTERVAL YEAR TO MONTH

• INTERVAL DAY TO SECOND

• RAW

• LONG RAW

• CHAR

• NCHAR

• UROWID

• CLOB with BASICFILE or SECUREFILE storage

• NCLOB with BASICFILE or SECUREFILE storage

• BLOB with BASICFILE or SECUREFILE storage

• XMLType stored as CLOB

Types of DML Changes Captured by Capture Processes

When you specify that DML changes made to certain tables should be captured, a capture process captures the following types of DML changes made to these tables:

• INSERT

• UPDATE

• DELETE

• MERGE

• Piecewise operations

A capture process converts each MERGE change into an INSERT or UPDATE change. MERGE is not a valid command type in a row LCR.

Supplemental Logging in an Oracle Streams Environment

Supplemental logging places additional column data into a redo log whenever an operation is performed. A capture process captures this additional information and places it in LCRs. Supplemental logging is always configured at a source database, regardless of location of the capture process that captures changes to the source database.

Typically, supplemental logging is required in Oracle Streams replication environments. In these environments, an apply process needs the additional information in the LCRs to properly apply changes that are replicated from a source database to a destination database. However, supplemental logging can also be required in environments where changes are not applied to database objects directly by an apply process. In such environments, an apply handler can process the changes without applying them to the database objects, and the supplemental information might be needed by the apply handlers.

Local Capture and Downstream Capture

You can configure a capture process to run locally on a source database or remotely on a downstream database. A single database can have one or more capture processes that capture local changes and other capture processes that capture changes from a remote source database. That is, you can configure a single database to perform both local capture and downstream capture.

Downstream Capture

Downstream capture means that a capture process runs on a database other than the source database. The following types of downstream capture configurations are possible: real-time downstream capture and archived-log downstream capture. The downstream_real_time_mine capture process parameter controls whether a downstream capture process performs real-time downstream capture or archived-log downstream capture. A real-time downstream capture process and one or more archived-log downstream capture processes can coexist at a downstream database.

Note:

• References to "downstream capture processes" in this document apply to both real-time downstream capture processes and archived-log downstream capture processes. This document distinguishes between the two types of downstream capture processes when necessary.

• A downstream capture process only can capture changes from a single source database. However, multiple downstream capture processes at a single downstream database can capture changes from a single source database or multiple source databases.

• To configure downstream capture, the source database must be an Oracle Database 10g Release 1 or later database.

Real-Time Downstream Capture

A real-time downstream capture configuration works in the following way:

• Redo transport services use the log writer process (LGWR) at the source database to send redo data to the downstream database either synchronously or asynchronously. At the same time, the LGWR records redo data in the online redo log at the source database.

• A remote file server process (RFS) at the downstream database receives the redo data over the network and stores the redo data in the standby redo log.

• A log switch at the source database causes a log switch at the downstream database, and the ARCHn process at the downstream database archives the current standby redo log file.

• The real-time downstream capture process captures changes from the standby redo log whenever possible and from the archived standby redo log files whenever necessary. A capture process can capture changes in the archived standby redo log files if it falls behind. When it catches up, it resumes capturing changes from the standby redo log.

Figure 2-2 Real-Time Downstream Capture

Description of "Figure 2-2 Real-Time Downstream Capture"

The advantage of real-time downstream capture over archived-log downstream capture is that real-time downstream capture reduces the amount of time required to capture changes made at the source database. The time is reduced because the real-time downstream capture process does not need to wait for the redo log file to be archived before it can capture data from it.

Note:

You can configure more than one real-time downstream capture process that captures changes from the same source database, but you cannot configure real-time downstream capture for multiple source databases at one downstream database.

Archived-Log Downstream Capture

An archived-log downstream capture configuration means that archived redo log files from the source database are copied to the downstream database, and the capture process captures changes in these archived redo log files. You can copy the archived redo log files to the downstream database using redo transport services, the DBMS_FILE_TRANSFER package, file transfer protocol (FTP), or some other mechanism.

Figure 2-3 Archived-Log Downstream Capture

Description of "Figure 2-3 Archived-Log Downstream Capture"

The advantage of archived-log downstream capture over real-time downstream capture is that archived-log downstream capture allows downstream capture processes from multiple source databases at a downstream database. You can copy redo log files from multiple source databases to a single downstream database and configure multiple archived-log downstream capture processes to capture changes in these redo log files.

See Also:

Oracle Data Guard Concepts and Administration for more information about redo transport services

The Downstream Database Performs Most Change Capture Actions

If you configure either real-time or archived-log downstream capture, then the following actions are performed at the downstream database:

• The first time a downstream capture process is started at the downstream database, Oracle Database uses data dictionary information in the redo data from the source database to create a LogMiner data dictionary at the downstream database. The DBMS_CAPTURE_ADM.BUILD procedure is run at the source database to extract the source data dictionary information to the redo log at the source database. Next, the redo data is copied to the downstream database from the source database. Additional downstream capture processes for the same source database can use this existing LogMiner data dictionary, or they can create new LogMiner data dictionaries. Also, a real-time downstream capture process can share a LogMiner data dictionary with one or more archived-log downstream capture processes.

• A capture process scans the redo data from the source database for changes using LogMiner.

• The rules engine evaluates changes based on the rules in one or more of the capture process rule sets.

• The capture process enqueues changes that satisfy the rules in its rule sets into a local ANYDATA queue. The capture process formats the changes as LCRs.

• If the captured LCRs are shared with one or more other databases, then one or more propagations propagate these LCRs from the downstream database to the other databases.

In a downstream capture configuration, the following actions are performed at the source database:

• The DBMS_CAPTURE_ADM.BUILD procedure is run at the source database to extract the data dictionary to the redo log.

• Supplemental logging at the source database places additional information that might be needed for apply in the redo log.

• If database objects at the source database must be instantiated at other databases in the environment, then the objects must be prepared for instantiation, and a mechanism such as an Export utility must be used to make a copy of the database objects.

In addition, the redo data must be copied from the computer system running the source database to the computer system running the downstream database. In a real-time downstream capture configuration, redo transport services use LGWR to send redo data to the downstream database. Typically, in an archived-log downstream capture configuration, redo transport services copy the archived redo log files to the downstream database.

See Also:

Chapter 5, "How Rules Are Used in Oracle Streams" for more information about rule sets for Oracle Streams clients and for information about how messages satisfy rule sets

Advantages of Downstream Capture

The following are the advantages of using downstream capture:

• Capturing changes uses fewer resources at the source database because the downstream database performs most of the required work.

• If you plan to capture changes originating at multiple source databases, then capture process administration can be simplified by running multiple archived-log downstream capture processes with different source databases at one downstream database. That is, one downstream database can act as the central location for change capture from multiple sources. In such a configuration, one real-time downstream capture process can run at the downstream database in addition to the archived-log downstream capture processes.

• Copying redo data to one or more downstream databases provides improved protection against data loss. For example, redo log files at the downstream database can be used for recovery of the source database in some situations.

• The ability to configure at one or more downstream databases multiple capture processes that capture changes from a single source database provides more flexibility and can improve scalability.

Optional Database Link From the Downstream Database to the Source Database

When you create or alter a downstream capture process, you optionally can specify the use of a database link from the downstream database to the source database. This database link must have the same name as the global name of the source database. Such a database link simplifies the creation and administration of a downstream capture process. You specify that a downstream capture process uses a database link by setting the use_database_link parameter to TRUE when you run the CREATE_CAPTURE or ALTER_CAPTURE procedure on the downstream capture process. The name of the database link must match the global name of the source database.

When a downstream capture process uses a database link to the source database, the capture process connects to the source database to perform the following administrative actions automatically:

• In certain situations, runs the DBMS_CAPTURE_ADM.BUILD procedure at the source database to extract the data dictionary at the source database to the redo log when a capture process is created.

• Prepares source database objects for instantiation.

• Obtains the first SCN for the downstream capture process if the first system change number (SCN) is not specified during capture process creation. The first SCN is needed to create a capture process.

If a downstream capture process does not use a database link, then you must perform these actions manually.

Note:

During the creation of a downstream capture process, if the first_scn parameter is set to NULL in the CREATE_CAPTURE procedure, then the use_database_link parameter must be set to TRUE. Otherwise, an error is raised.

See Also:

Oracle Streams Replication Administrator's Guide for information about when the DBMS_CAPTURE_ADM.BUILD procedure is run automatically during capture process creation if the downstream capture process uses a database link

Operational Requirements for Downstream Capture

The following are operational requirements for using downstream capture:

• The source database must be running at least Oracle Database 10g and the downstream capture database must be running the same release of Oracle Database as the source database or later.

• The downstream database must be running Oracle Database 10g Release 2 or later to configure real-time downstream capture. In this case, the source database must be running Oracle Database 10g Release 1 or later.

• The operating system on the source and downstream capture sites must be the same, but the operating system release does not need to be the same. In addition, the downstream sites can use a different directory structure than the source site.

• The hardware architecture on the source and downstream capture sites must be the same. For example, a downstream capture configuration with a source database on a 32-bit Sun system must have a downstream database that is configured on a 32-bit Sun system. Other hardware elements, such as the number of CPUs, memory size, and storage configuration, can be different between the source and downstream sites.

SCN Values Related to a Capture Process

This section describes system change number (SCN) values that are important for a capture process. You can query the DBA_CAPTURE data dictionary view to display these values for one or more capture processes.

• Captured SCN and Applied SCN

• First SCN and Start SCN

COLUMN FIRST_CHANGE# HEADING 'First SCN' FORMAT 999999999
COLUMN NAME HEADING 'Log File Name' FORMAT A50

SELECT DISTINCT FIRST_CHANGE#, NAME FROM V$ARCHIVED_LOG
  WHERE DICTIONARY_BEGIN = 'YES';

Oracle Streams Capture Processes and RESTRICTED SESSION

When you enable restricted session during system startup by issuing a STARTUP RESTRICT statement, capture processes do not start, even if they were running when the database shut down. When restricted session is disabled with an ALTER SYSTEM statement, each capture process that was running when the database shut down is started.

When restricted session is enabled in a running database by the SQL statement ALTER SYSTEM ENABLE RESTRICTED SESSION clause, it does not affect any running capture processes. These capture processes continue to run and capture changes. If a stopped capture process is started in a restricted session, then the capture process does not actually start until the restricted session is disabled.

Capture Process Components

A capture process is an optional Oracle background process whose process name is CPnn, where nn can include letters and numbers. A capture process captures changes from the redo log by using the infrastructure of LogMiner. Oracle Streams configures LogMiner automatically. The underlying LogMiner process name is MSnn, where nn can include letters and numbers. You can create, alter, start, stop, and drop a capture process, and you can define capture process rules that control which changes a capture process captures.

A capture process consists of the following components:

• One reader server that reads the redo log and divides the redo log into regions.

• One or more preparer servers that scan the regions defined by the reader server in parallel and perform prefiltering of changes found in the redo log. Prefiltering involves sending partial information about changes, such as schema and object name for a change, to the rules engine for evaluation, and receiving the results of the evaluation. You can control the number of preparer servers using the parallelism capture process parameter.

• One builder server that merges redo records from the preparer servers. These redo records either evaluated to TRUE during partial evaluation or partial evaluation was inconclusive for them. The builder server preserves the system change number (SCN) order of these redo records and passes the merged redo records to the capture process.

• The capture process (CPnn) performs the following actions for each change when it receives merged redo records from the builder server:

  • Formats the change into an LCR

  • If the partial evaluation performed by a preparer server was inconclusive for the change in the LCR, then sends the LCR to the rules engine for full evaluation

  • Receives the results of the full evaluation of the LCR if it was performed

  • Discards the LCR if it satisfies the rules in the negative rule set for the capture process or if it does not satisfy the rules in the positive rule set

  • Enqueues the LCR into the queue associated with the capture process if the LCR satisfies the rules in the positive rule set for the capture process

Each reader server, preparer server, and builder server is a process.

Capture User

Changes are captured in the security domain of the capture user for a capture process. The capture user captures all changes that satisfy the capture process rule sets. In addition, the capture user runs all custom rule-based transformations specified by the rules in these rule sets. The capture user must have the necessary privileges to perform these actions, including EXECUTE privilege on the rule sets used by the capture process, EXECUTE privilege on all custom rule-based transformation functions specified for rules in the positive rule set, and privileges to enqueue messages into the capture process queue. A capture process can be associated with only one user, but one user can be associated with many capture processes.

Capture Process States

The state of a capture process describes what the capture process is doing currently. You can view the state of a capture process by querying the STATE column in the V$STREAMS_CAPTURE dynamic performance view. The following capture process states are possible:

• INITIALIZING - Starting up.

• WAITING FOR DICTIONARY REDO - Waiting for redo log files containing the dictionary build related to the first SCN to be added to the capture process session. A capture process cannot begin to scan the redo log files until all of the log files containing the dictionary build have been added.

• DICTIONARY INITIALIZATION - Processing a dictionary build.

• MINING (PROCESSED SCN = scn_value) - Mining a dictionary build at the SCN scn_value.

• LOADING (step X of Y) - Processing information from a dictionary build and currently at step X in a process that involves Y steps, where X and Y are numbers.

• CAPTURING CHANGES - Scanning the redo log for changes that satisfy the capture process rule sets.

• WAITING FOR REDO - Waiting for new redo log files to be added to the capture process session. The capture process has finished processing all of the redo log files added to its session. This state is possible if there is no activity at a source database. For a downstream capture process, this state is possible if the capture process is waiting for new log files to be added to its session.

• EVALUATING RULE - Evaluating a change against a capture process rule set.

• CREATING LCR - Converting a change into a logical change record (LCR).

• ENQUEUING MESSAGE - Enqueuing an LCR that satisfies the capture process rule sets into the capture process queue.

• PAUSED FOR FLOW CONTROL - Unable to enqueue LCRs either because of low memory or because propagations and apply processes are consuming messages slower than the capture process is creating them. This state indicates flow control that is used to reduce spilling of captured LCRs when propagation or apply has fallen behind or is unavailable.

• WAITING FOR A SUBSCRIBER TO BE ADDED - Waiting for a subscriber to the capture process's queue to be added. A subscriber can be a propagation or an apply process.

• WAITING FOR THE BUFFERED QUEUE TO SHRINK - Waiting for the buffered queue to change to a smaller size. The buffered queue shrinks when there is a memory limitation or when an administrator reduces its size.

• WAITING FOR n SUBSCRIBER(S) INITIALIZING - Waiting for apply processes that receive LCRs from the capture process to start, where n is the number of apply processes.

• SUSPENDED FOR AUTO SPLIT/MERGE - Waiting for a merge operation to complete.

• SHUTTING DOWN - Stopping.

• ABORTING - Aborting.

Capture Process Parameters

After creation, a capture process is disabled so that you can set the capture process parameters for your environment before starting it for the first time. Capture process parameters control the way a capture process operates. For example, the parallelism capture process parameter controls the number of preparer servers used by a capture process, and the time_limit capture process parameter specifies the amount of time a capture process runs before it is shut down automatically. You set capture process parameters using the DBMS_CAPTURE_ADM.SET_PARAMETER procedure.

Persistent Capture Process Status Upon Database Restart

A capture process maintains a persistent status when the database running the capture process is shut down and restarted. For example, if a capture process is enabled when the database is shut down, then the capture process automatically starts when the database is restarted. Similarly, if a capture process is disabled or aborted when a database is shut down, then the capture process is not started and retains the disabled or aborted status when the database is restarted.

Introduction to Synchronous Capture

Synchronous capture is an optional Oracle Streams client that captures data manipulation language (DML) changes made to tables. Synchronous capture uses an internal mechanism to capture DML changes to specified tables. When synchronous capture is configured to capture changes to tables, the database that contains these tables is called the source database.

When a DML change it made to a table, it can result in changes to one or more rows in the table. Synchronous capture captures each row change and converts it into a specific message format called a row logical change record (row LCR). After capturing a row LCR, synchronous capture enqueues a message containing the row LCR into a queue. Row LCRs created by synchronous capture always contain values for all the columns in a row, even if some of the columns where not modified by the change.

Figure 2-4 shows a synchronous capture capturing LCRs.

Figure 2-4 Synchronous Capture

Description of "Figure 2-4 Synchronous Capture"

Synchronous Capture and Queues

Synchronous capture is always associated with a single ANYDATA queue, and it enqueues messages into this queue only. The queue used by synchronous capture must be a commit-time queue. Commit-time queues ensure that messages are grouped into transactions, and that transactions groups are in commit system change number (CSCN) order. Synchronous capture always enqueues row LCRs into the persistent queue. The persistent queue is the portion of a queue that only stores messages on hard disk in a queue table, not in memory. You can create multiple queues and associate a different synchronous capture with each queue.

Although synchronous capture must enqueue messages into a commit-time queue, messages captured by synchronous capture can be propagated to queues that are not commit-time queues. Therefore, any intermediate queues that store messages captured by synchronous capture do not need to be commit-time queue. Also, apply processes that apply messages captured by synchronous capture can use queues that are not commit-time queues.

Synchronous Capture Rules

Synchronous capture either captures or discards changes based on rules that you define. 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. If a rule evaluates to TRUE for a change, and the rule is in the positive rule set for synchronous capture, then synchronous capture captures the change. Synchronous capture does not use negative rule sets.

You can specify synchronous capture rules at the table level. A table rule captures or discards row changes resulting from DML changes to a particular table. Subset rules are table rules that include a subset of the row changes to a particular table. Synchronous capture does not use schema or global rules.

All synchronous capture rules must be created with one of the following procedures in the DBMS_STREAMS_ADM package:

• ADD_TABLE_RULES

• ADD_SUBSET_RULES

Synchronous capture does not capture changes based on the following types of rules:

• Rules added to the synchronous capture rules set by any procedure other than ADD_TABLE_RULES or ADD_SUBSET_RULES in the DBMS_STREAMS_ADM package.

• Rules created by the DBMS_RULE_ADM package.

If these types of rules are in a synchronous capture rule set, then synchronous capture ignores these rules.

A synchronous capture can use a rule set created by the CREATE_RULE_SET procedure in the DBMS_RULE_ADM package, but you must add rules to the rule set with the ADD_TABLE_RULES or ADD_SUBSET_RULES procedure.

If the specified synchronous capture does not exist when you run the ADD_TABLE_RULES or ADD_SUBSET_RULES procedure, then the procedure creates it automatically. You can also use the CREATE_SYNC_CAPTURE procedure in the DBMS_CAPTURE_ADM package to create a synchronous capture.

Data Types Captured by Synchronous Capture

When capturing the row changes resulting from DML changes made to tables, synchronous capture can capture changes made to columns of the following data types:

• VARCHAR2

• NVARCHAR2

• NUMBER

• FLOAT

• DATE

• BINARY_FLOAT

• BINARY_DOUBLE

• TIMESTAMP

• TIMESTAMP WITH TIME ZONE

• TIMESTAMP WITH LOCAL TIME ZONE

• INTERVAL YEAR TO MONTH

• INTERVAL DAY TO SECOND

• RAW

• CHAR

• NCHAR

• UROWID

Types of DML Changes Captured by Synchronous Capture

When you specify that DML changes made to specific tables should be captured, synchronous capture captures the following types of DML changes made to these tables:

• INSERT

• UPDATE

• DELETE

• MERGE

Synchronous capture converts each MERGE change into an INSERT or UPDATE change. MERGE is not a valid command type in a row LCR.

Capture User for Synchronous Capture

Changes are captured in the security domain of the capture user for a synchronous capture. The capture user captures all changes that satisfy the synchronous capture rule set. In addition, the capture user runs all custom rule-based transformations specified by the rules in these rule sets. The capture user must have the necessary privileges to perform these actions, including EXECUTE privilege on the rule set used by synchronous capture, EXECUTE privilege on all custom rule-based transformation functions specified for rules in the rule set, and privileges to enqueue messages into the synchronous capture queue. A synchronous capture can be associated with only one user, but one user can be associated with many synchronous captures.

Multiple Synchronous Captures in a Single Database

Oracle recommends that each ANYDATA queue used by a synchronous capture, propagation, or apply process have messages from at most one synchronous capture from a particular source database. Therefore, use a separate queue for each synchronous capture that captures changes originating at a particular source database, and ensure that each queue has its own queue table. Also, messages from two or more synchronous captures in the same source database should not be propagated to the same destination queue.

Types of Messages That Can Be Enqueued Explicitly

Applications can create and enqueue different types of messages for various purposes in an Oracle Streams environment. These messages can be messages of a user-defined type called user messages, or they can be LCRs.

This section contains these topics:

• User Messages

• Logical Change Records (LCRs)

Enqueue Features

The enqueue features available with Oracle Streams Advanced Queuing include the following:

• Enqueue into a buffered queue or a persistent queue

• Ordering of messages by priority enqueue time, or commit time

• Array enqueue of messages

• Correlation identifiers

• Message grouping

• Sender identification

• Time specification and scheduling