This topic provides information about how to configure certain characteristics of Oracle database objects that you will be replicating with SharePlex.

Ensure row uniqueness

SharePlex must have a way to ensure that the row it is changing on the target is the correct one that matches the source row. This is accomplished by ensuring a one-to-one relationship through the use of keys and indexes.

The role of keys

SharePlex works fastest when there is a primary key or a unique key on all source and target tables being replicated, especially large tables and tables containing LONG columns. When choosing a key to use, SharePlex uses the best available key column in the following order of priority.

  • Primary key
  • Unique key with the fewest columns, where at least one of the columns is NOT NULL

  • Unique key with the fewest columns.

For best performance, it is recommended that you enable primary and unique key supplemental logging.

If a table does not have a primary or unique key, or if Oracle logs the wrong unique key for SharePlex, you can specify columns for SharePlex to use as a key when you create the configuration file. This is known as a key definition and is specified in the configuration file. For more information, see Define a Unique Key in the SharePlex Administration Guide.

An alternative to a key definition is to create or use a unique index based on one or more columns that establish uniqueness.

Ensure that the correct key is logged

When primary and unique key supplemental logging is enabled and a table does not have a primary key, Oracle has to decide what type of unique key to log. If the table has multiple unique keys, Oracle determines the best key to use and logs those column values for every UPDATE. If a table does not have any type of key, Oracle logs all columns that are not a LONG or a LOB.

SharePlex must also identify a key to use to replicate data. Like Oracle, SharePlex chooses a key in this order:

  • primary key if there is one
  • the best (or only) unique key if there is one
  • all the columns

If a table being replicated by SharePlex does not have a primary key, but does have multiple unique keys, it is possible that the key columns that Oracle logs are not those that SharePlex requires.

Tables without a key or unique index

If SharePlex cannot detect a key or unique index on a table, it constructs a key using all of the columns except LONGs and LOBs. This key is maintained internally and is not created on the table itself.

This is not a desirable option, because the resultant WHERE clause causes Oracle to perform a full table scan on the target table to find the row, and that significantly slows replication. Additionally, row uniqueness cannot be enforced.

For example, if there is a possibility that non-LONG columns in different rows could contain identical values, but the LONG columns could have different values, the table can be out of synchronization without being detected by you or SharePlex. The following example illustrates the problem. The rows in the table are identical except for the LONG columns, and there is no primary or unique key.

COLUMN A COLUMN B COLUMN C (LONG)
10 20 100
10 20 200
10 20 300

Suppose a user on the source system changes Column A to 15 in the first row. When constructing the SQL statement to apply the change to the target table, SharePlex constructs a key using Columns A and B (UPDATE tablename SET Column A to 15 WHERE Column A = 10 and Column B = 20) to locate the row to change. There are three rows that meet this criteria, so SharePlex could post the change to the wrong row.

Keys with nulls

If a key permits NULLs, SharePlex cannot ensure the uniqueness of a row for UPDATEs and DELETES, so there is a possibility it could change the wrong row on the target system. To control how SharePlex addresses keys that permit NULLs, set the SP_SYS_IN_SYNC parameter. See the SharePlex Reference Guide for more information.

Changes to key values

SharePlex handles changes to the value of a key column without any special settings. However, if sequences are used for keys and if there is a possibility that those values will be updated, create the sequences so that the updates cannot result in a duplication of keys on the target system. Otherwise, if a new value is used to apply an operation, and that value already exists as a key in another row in the target table, SharePlex will return unique-key constraint violations and out-of-sync errors. This type of error can happen when you update values using an “x +n” formula, where n is an incremental increase. It is possible for one of the “x +n” values to equal an existing value.

The following is an example, where the values in the key column are increased by 1:

Key_Col

1

4

5

7

SQL> update table X set a=a+1; commit

The new values become the following, and they are replicated to the target system:

Key_Col

2

5

6

8

SharePlex performs the update in the order that the operations enter the redo log:

update x set a=2 where a=1; (Succeeds)

update x set a=5 where a=4; (Fails because a value of 5 already exists.)

update x set a=6 where a=5; (Succeeds)

update x set a=8 where a=7; (Succeeds)

The pre-image value that Post uses for the target sequence is the same as the increased value replicated from the source. Oracle rejects the operation as a unique constraint violation. Another example would be a transaction that updates A to B and then updates B to C.

Important! If you plan to use peer-to-peer replication, there are additional requirements for keys. For more information, see the Configure Replication to Maintain Multiple Peer Databases section in the SharePlex Administration Guide.

Indexes

The correct use of indexes is important in a replication environment. An index maintains the integrity of the target data.

  • When replicating a source table that has a unique index, the target table also should have a unique index.
  • All large tables should have a unique index on the target system. If there is no unique index, Oracle scans the entire table in order to locate the rows to be changed by Post.
  • Some applications do not use primary key constraints, and therefore a unique index is not created by default. Often, however, there will be an index that was created, but not named, as a unique index (not using CREATE UNIQUE INDEX command), although it was created on one or more columns populated with unique values, such as a person’s name and employee identification number. If no unique index exists for a table, it is recommended that you create one or specify a user-defined key when you create the configuration file. For more information, see the Define a Unique Key section in the SharePlex Administration Guide.
  • Once you have identified or created a unique index, you can ensure that Oracle uses it by employing the hints feature of SharePlex. For more information, see the Use Oracle Index Hints section in the SharePlex Administration Guide.

  • If tables have foreign keys, make sure the appropriate columns are indexed so that modifications to the foreign keys do not cause full table scans.

  • Keep your indexes up-to-date, or it can slow the Post process. Rebuild the ones that are fragmented.

If there are too many indexes on a target table, Oracle must update them all as rows are added and deleted. This slows the entire system, including replication. Consider limiting the number of indexes to the ones that have the most utility.

For applications that perform mostly one type of DML, consider the following:

  • INSERTs: Use just a few indexes, to limit their maintenance.
  • UPDATEs: Use an index on a column that does not change after an INSERT statement.
  • DELETEs: Remove as many indexes as you can.

If you run large batch jobs that do millions of SQL operations, remove unnecessary indexes before the batch job, then rebuild them at the end. This makes SharePlex run faster, and you will have more organized indexes afterward.

Bitmap indexes

For performance purposes, avoid the use of bitmap indexes while the Post process is applying the data. These indexes can adversely affect the performance of the Post process.

If you must use bitmap indexes on target tables, weigh their benefit for queries against the their impact on the transactions applied by Post:

  • When Oracle adds, updates or deletes a bitmap entry, it effectively locks all the rows associated with the bitmap segment.
  • A bitmap segment can contain references to hundreds of rows. As a result, changes made by different Post sessions (there is a Post session for every session on the source system) can block each other if their work updates bitmap entries in the same bitmap segment.
  • To proceed, Post must detect and resolve the blocking, which delays posting significantly if the number of locks is high.
  • In general, frequent inserts by multiple concurrent sessions into a table with bitmap indexes will incur lock conflicts, but random update and delete activity on such a table will not. SharePlex follows the Oracle recommendation to have a bitmap index on a more static table.

Note: Replicating bitmap indexes is not recommended. Every time you change a table with a bitmap index, the index is rebuilt. This associated cost (Oracle time and resources) to rebuild is added to your SQL UPDATE statement.

Prevent triggers from firing on the target

DML changes resulting from a trigger firing on the source system enter the redo log and are replicated to the target database by SharePlex. Consequently, if the same trigger fires on the target system and initiates the same DML changes (which already were made through replication) there will be out-of-sync errors.

For example, if an INSERT to TableA on the source system triggers an INSERT into TableB, SharePlex replicates both INSERTs to the target system. The Post process applies the first INSERT to TableA on the target system, triggering an INSERT into TableB. Thus, when Post attempts to post the replicated INSERT to TableB, there will be a unique-key violation. The row already exists because the trigger fired for TableA.

Triggers can be handled as follows, depending on your replication strategy:

Replication strategy How to handle triggers on the target

High availability

and

Peer-to-peer
  1. Enable triggers for users other than SharePlex, either in preparation for failover or because transactions are performed on multiple source systems.
  2. Disable triggers for the SharePlex user by running the sp_add_trigger.sql script. This script puts a WHEN clause into each trigger’s procedural statement that tells it to ignore operations posted by the SharePlex user.
Reporting, data sharing, other basic one-way replication
  • Either disable triggers entirely on the target system or run the sp_add_trigger.sql script to ignore operations posted by the SharePlex user.

    • Triggers on objects not in the replication configuration can remain active.

    For important information about how to use the trigger scripts, see theSharePlex Reference Guide.

    Configure integrity constraints

    Integrity constraints have a effect on replication. Follow these guidelines to ensure that they are handled.

    Foreign key constraints

    Foreign key constraints must be disabled on the target tables. SharePlex replicates the results of the source foreign key constraints. Tables with foreign keys to one another must all be included in the replication configuration for accurate replication of the source foreign key results. All tables with referential constraints must exist in the target database. If you leave one or more out, the referential integrity could become corrupted.

    Note: If constraints are DEFERRED on the target tables, the Post transaction can fail on the constraint validation. To work around this problem, enable the SP_OPO_DISABLE_OBJNUM parameter to allow the transaction to succeed. The underlying target table still remains out-of-sync until it is resynchronized.

    ON DELETE CASCADE constraints

    SharePlex provides a feature that allows ON DELETE CASCADE constraints to remain enabled on the target tables, but it must be enabled explicitly through a parameter setting. Post detects the ON DELETE CASCADE dependencies and suppresses the posting of any replicated cascaded deletes to the child tables.

    If you do not enable this support through SharePlex you must disable these constraints manually on the target. Otherwise SharePlex replicates both the primary delete and the cascaded deletes, which results in conflicts and errors when the deletes cascade on the target.

    To enable ON DELETE CASCADE support:

    1. Enable the logging of primary keys, unique index columns, and foreign key columns on the source.

    2. Set the following SharePlex parameters:

      • SP_OPO_DEPENDENCY_CHECK parameter to 2
      • SP_OCT_REDUCED_KEY parameter to 0
      • SP_OPO_REDUCED_KEY parameter to 0, 1 or 2

    NOTE: In peer-to-peer replication, SP_OPO_REDUCED_KEY must be set to 0.

    Check constraints

    Disable check constraints on the target system. They add unnecessary overhead. These checks are redundant in a well maintained, synchronized replication environment because they are satisfied on the source system. For high-availability purposes, you can build a script that re-enables the constraints as part of the failover procedure.

    Prevent access to target objects

    In all scenarios except peer-to-peer replication, the SharePlex database user must be the only user allowed to perform DML or DDL on the target objects. If DML or DDL changes are made to target objects by other individuals, jobs or applications, the target data may no longer reflect the state of the data on the source system. For more information, see the Understand the Concept of Synchronization section in the SharePlex Administration Guide.

    Configure sequences

    SharePlex replicates changes to Oracle sequences that are made in an ALTER SEQUENCE and DROP SEQUENCE commands and during DML transactions. It may not be necessary to replicate sequences in certain replication strategies.

    • High availability: Yes

      The way that SharePlex replicates sequences allows users to begin using the failover database immediately without worrying about incrementing the sequences or reusing one.

    • Reporting, data sharing, other basic one-way replication: No

      If sequences are unnecessary on the target system, do not replicate them. It can slow down replication. Even if a sequence is used to generate keys in a source table, the sequence values are part of the key columns when the replicated rows are inserted on the target system. The sequence itself does not have to be replicated.

    • Peer-to-peer: No

      SharePlex does not support peer-to-peer replication of identical sequences. For more information, see the Configure Replication to Maintain Multiple Peer Databases section in the SharePlex Administration Guide.

    To configure sequences for replication:

    1. To replicate sequences, the supplemental logging of primary and unique keys must be enabled at the database level, or you must enable supplemental logging for primary keys on the sys.seq$ table.

    2. Use caching and set the cache to increments of at least 20. When sequences are cached, SharePlex can replicate the values as a group. When sequences are not cached, SharePlex must go to disk every time a value is obtained from the sequence, which slows replication of more critical data.

    3. To ensure the uniqueness of sequences on the target system, the start value of the target sequence must be larger than the start value of the source sequence. Use the following formula to determine the target START_WITH value:

      source_current_value+ (source_INCREMENT_BY_value x source_CACHE_value) =target_START_WITH_value

      Important! (source_INCREMENT_BY_value x source_CACHE_value) should not exceed 2 GB, or replication of sequences will fail.

    1. Specify sequences in the configuration by owner and name, as you would a table.
    1. Because changes to sequences are DDL commands, the Post process pauses all posting until the sequence update is finished. For that reason, it is recommended that you process sequences through a separate post queue from the tables, especially if the sequences are not cached. For more information, see the Configure SharePlex to Replicate Data section in the .SharePlex Administration Guide.

    SharePlex uses the ALTER SEQUENCE command to update sequences in a target database as follows:

    • Change the increment value to:

      source_INCREMENT_BY_valuexsource_CACHE_value

    • Set to NOCACHE.
    • UPDATE the sequence(s).

    • ALTER the sequence(s) again by setting the following values:

      Increment_value=source_INCREMENT_BY_value

      Cache_value=source_CACHE_value

    SharePlex treats ALTER SEQUENCE operations like a simple SELECT (UPDATE) to a sequence because the redo log records make no distinctions between the two operations.