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This topic provides information about how to configure certain characteristics of Oracle database objects that you will be replicating with SharePlex.
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.
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.
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.
An alternative to a key definition is to create or use a unique index based on one or more columns that establish uniqueness.
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)|
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.
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.
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:
SQL> update table X set a=a+1; commit
The new values become the following, and they are replicated to the target system:
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 Configure replication to maintain multiple peer databases.
The correct use of indexes is important in a replication environment. An index maintains the integrity of the target data.
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.
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:
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.
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:
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.
DML changes resulting from a trigger firing on the source system enter the redo log and, therefore, are replicated and posted 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|
|Reporting, data sharing, other basic one-way replication||
For important information about how to use the trigger scripts, see the SharePlex Reference Guide.
Integrity constraints have a effect on replication. Follow these guidelines to ensure that they are handled.
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.
SharePlex allows ON DELETE CASCADE constraints to remain enabled on the target tables, but this feature must be enabled. 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
Set the following SharePlex parameters:
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.
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 Understand the concept of synchronization.
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.
SharePlex does not support peer-to-peer replication of identical sequences. For more information, see Configure replication to maintain multiple peer databases.
To configure sequences for replication
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.
SharePlex uses the ALTER SEQUENCE command to update sequences in a target database as follows:
Change the increment value to:
ALTER the sequence(s) again by setting the following values:
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.