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CDC Control Task

The CDC Control task is used to control the life cycle of change data capture (CDC) packages. It handles CDC package synchronization with the initial load package, the management of Log Sequence Number (LSN) ranges that are processed in a run of a CDC package. In addition, the CDC Control task deals with error scenarios and recovery.

The CDC Control task maintains the state of the CDC package in an SSIS package variable and it can also persist it in a database table so that the state is maintained across package activations and between multiple packages that together perform a common CDC process (for example, one task may be responsible for the initial loading and the other for the trickle-feed updates).

The CDC Control task supports two groups of operations. One group handles the synchronization of initial load and change processing, and the other manages the change-processing range of LSNs for a run of a CDC package and keeps track of what was processed successfully.

The following operations handle the synchronization of initial load and change processing:

Operation

Description

ResetCdcState

This operation is used to reset the persistent CDC state associated with the current CDC context. After this operation is run, the current maximum LSN from the LSN-timestamp sys.fn_cdc_get_max_lsn table becomes the start of the range for the next processing range. This operation requires a connection to the source database.

MarkInitialLoadStart

This operation is used at the beginning of an initial-load package to record the current LSN in the source database before the initial-load package starts reading the source tables. This requires a connection to the source database to call sys.fn_cdc_get_max_lsn.

If you select MarkInitialLoadStart when working on SQL Server 2014 CDC (that is, not Oracle) the user specified in the connection manager must be either db_owner or sysadmin.

MarkInitialLoadEnd

This operation is used at the end of an initial-load package to record the current LSN in the source database after the initial-load package finished reading the source tables. This LSN is determined by recording the current time when this operation occurred and then querying the cdc.lsn_time_mapping table in the CDC database looking for a change that occurred after that time.

If you select MarkInitialLoadEnd when working on SQL Server 2014 CDC (that is , not Oracle) the user specified in the connection manager must be either db_owner or sysadmin.

MarkCdcStart

This operation is used when then the initial load is made from a snapshot database. In this case, the change processing should start immediately after the snapshot LSN. You can specify the name of the snapshot database to use and the CDC Control task queries SQL Server 2014 for the snapshot LSN. You also have the option to directly specify the snapshot LSN.

If you select MarkCdcStart when working on SQL Server 2014 CDC (that is , not Oracle) the user specified in the connection manager must be either db_owner or sysadmin.

The following operations are used to manage the processing range:

Operation

Description

GetProcessingRange

This operation is used before invoking the data flow that uses the CDC Source data flow. It establishes a range of LSNs that the CDC Source data flow reads when invoked. The range is stored in an SSIS package variable that is used by the CDC Source during data-flow processing.

For more information about the states that are stored, see Define a State Variable.

MarkProcessedRange

: This operation is executed after each CDC run (after the CDC data flow is completed successfully) to record the last LSN that was fully processed in the CDC run. The next time GetProcessingRange is executed, this position is the start of the processing range.

The CDC Control task maintains a persistent state between activations. The information stored in the CDC state is used to determine and maintain the processing range for the CDC package and for detecting error conditions. The persistent state is stored as a string. For more information, see Define a State Variable.

The CDC Control task supports two types of state persistency

  • Manual State Persistency: In this case, the CDC Control task manages the state stored in a package variable but the package developer must read the variable from a persistent store before calling the CDC Control and then write it back to that persistent store after the CDC Control is last called and the CDC run completes.

  • Automatic State Persistency: The CDC state is stored in a table in a database. The state is stored under a name provided in the StateName property in a table named in the Table to Use for Storing State property, which is located in a selected connection manager for storing the state. The default is the source connection manager but the common practice is for it to be the target connection manager. The CDC Control task updates the state value in the state table and this is committed as part of the ambient transaction.

The CDC Control task may report an error when:

  • It fails to read the persistent CDC state or when updating the persistent state fails.

  • It fails to read the current LSN information from the source database.

  • The CDC state read is not consistent.

In all of these cases, the CDC Control task reports an error that can be handled in the standard way SSIS handles control-flow errors.

The CDC Control task may also report a warning when the Get Processing Range operation is invoked directly after another Get Processing Range operation without Mark Processed Range being called. This is an indication that the previous run failed or that another CDC package may be running using the same CDC state name.

You can set properties through SSIS Designer or programmatically.

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