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Troubleshooting Replication Problems

This appendix contains troubleshooting guidelines for managing a replication environment. This appendix contains the following topics:

• Diagnosing Problems with Database Links
• Diagnosing Problems with Master Sites
• Diagnosing Problems with the Deferred Transaction Queue
• Diagnosing Problems with Materialized Views

Diagnosing Problems with Database Links

If you think a database link is not functioning properly, then you can drop and re-create it using Oracle Enterprise Manager, SQL*Plus, or another tool.

• Make sure that the database link name is the same as the global name of the target database.
• Make sure that the scheduled interval is what you want.
• Make sure that the scheduled interval is not shorter than the required execution time.

If you used a connection qualifier in a database link to a given site, then the other sites that link to that site must have the same connection qualifier. For example, suppose you create a database link as follows:

CREATE DATABASE LINK dbs1.world@myethernet CONNECT TO repadmin
  IDENTIFIED BY secret USING 'connect_string_myethernet'

All the sites, whether masters or materialized views, associated with dbs1.world@myethernet must include myethernet as the connection qualifier.

Diagnosing Problems with Master Sites

Problems can arise in a multimaster replication system. The following sections discuss some problems and ways to solve them:

• Replicated Objects Not Created at New Master Site
• DDL Changes Not Propagated to Master Site
• DML Changes Not Asynchronously Propagated to Other Sites
• DML Cannot be Applied to Replicated Table
• Bulk Updates and Constraint Violations
• Recreating a Replicated Object
• Unable to Generate Replication Support for a Table
• Problems with Replicated Procedures or Triggers
• Problems With ON DELETE CASCADE and Integrity Constraints

Replicated Objects Not Created at New Master Site

If you add a new master site to a master group, and the appropriate objects are not created at the new site, then try the following:

• Ensure that the necessary private database links exist between the new master site and the existing master sites. If you used the Replication Management tool's Setup Wizard to setup your sites, then you should not have any problems. You must have links both to the new site from each existing site, and from the new site to each existing site.
• Make sure that the administrative requests at all sites have completed successfully. If requests have not been executed yet, then you can manually execute pending administrative requests to complete the operation immediately.

DDL Changes Not Propagated to Master Site

If you create a new master group object or alter the definition of a master group object at the master definition site and the modification is not propagated to a master site, then first ensure that the administrative requests at all sites have completed successfully. If requests are pending execution, then you can manually execute them to complete the operation immediately.

When you execute DDL statements through the replication API, Oracle executes the statements on behalf of the user who submits the DDL. When a DDL statement applies to an object in a schema other than the submitter's schema, the submitter needs appropriate privileges to execute the statement. In addition, the statement must explicitly name the schema. For example, assume that you supply the following as the ddl_text parameter to the DBMS_REPCAT.CREATE_MASTER_ REPOBJECT procedure:

CREATE TABLE oe.new_employees AS SELECT * FROM hr.employees WHERE ...;

Because each table name contains a schema name, this statement works whether the replication administrator is oe, hr, or another user, as long as the administrator has the required privileges.

DML Changes Not Asynchronously Propagated to Other Sites

If you make an update to your data at a master site, and that change is not asynchronously propagated to the other sites in your replication environment, then try the following:

• Use the Replication Management tool in Oracle Enterprise Manager to check whether the corresponding deferred transaction has been pushed to the destination. If not, then you can also check to see how much longer it will be before the scheduled link pushes the queue to the destination site. If you do not want to wait for the next scheduled push across a link, then you can execute deferred transaction manually.
• If a scheduled link's interval has passed and corresponding deferred transactions have not been pushed, then check the corresponding job for the link.
• Even after propagating a deferred transaction to a destination, it may not execute because of an error. Check the DEFERROR data dictionary view at the destination site for errors.

  See Also: Oracle9i Replication Management API Reference for information about modifying tables without replicating the modifications, which may be necessary when you need to manually synchronize the data in replicated tables

DML Cannot be Applied to Replicated Table

If you receive the deferred_rpc_quiesce exception when you attempt to modify a replicated table, then the master group to which your replicated object belongs is quiescing or quiesced. To proceed, your replication administrator must resume replication activity for the master group.

Bulk Updates and Constraint Violations

A single update statement applied to a replicated table can update zero or more rows. The update statement causes zero or more update requests to be queued for deferred execution, one for each row updated. This distinction is important when constraints are involved, because Oracle effectively performs constraint checking at the end of each statement. While a bulk update may not violate a uniqueness constraint, for example, some equivalent sequence of individual updates may violate uniqueness.

If the ordering of updates is important, then update one row at a time in an appropriate order. This lets you define the order of update requests in the deferred transactions queue.

Recreating a Replicated Object

If you add an object such as a package, procedure, or view to a master group, then the status of the object must be valid. If the status of an object is invalid, then recompile the object or drop and re-create the object before adding it to a master group. Check the DBA_REPOBJECT data dictionary view for the status of replication objects.

Unable to Generate Replication Support for a Table

When you generate replication support for a table, Oracle activates an internal trigger at the local site. EXECUTE privileges for most of the packages involved with replication, such as DBMS_REPCAT and DBMS_DEFER, need to be granted to replication administrators and users that own replicated objects. The Replication Management tool's Setup Wizard and the DBMS_REPCAT_ADMIN package both perform the grants needed by the replication administrators for many typical replication scenarios. When the owner of a replicated object is not a replication administrator, however, you must explicitly grant EXECUTE privilege on DBMS_DEFER to the object owner.

Problems with Replicated Procedures or Triggers

If you discover an unexpected unresolved conflict, and you were mixing procedural and row-level replication on a table, then carefully review the procedure to ensure that the replicated procedure did not cause the conflict. Complete the following checks:

• Ensure that ordering conflicts between procedural and row-level updates are not possible.
• Check if the replicated procedure locks the table in EXCLUSIVE mode before performing updates or uses some other mechanism of avoiding conflicts with row-level updates.
• Check that row-level replication is disabled at the start of the replicated procedure and re-enabled at the end.
• Ensure that row-level replication is re-enabled even if exceptions occur when the procedure executes.
• Check to be sure that the replicated procedure executed at all master sites.

You should perform similar checks on any replicated triggers that you have defined on replicated tables.

Problems With ON DELETE CASCADE and Integrity Constraints

ON DELETE CASCADE is not supported for replicated tables with a configuration similar to the following example. This example assumes there are three tables named A, B, and C.

• Table B contains a record dependent on a record in Table A because of a foreign key constraint.
• Table C contains a record dependent on a record in Table B because of a foreign key constraint.
• Table C also contains a record dependent on a record in Table A because of a foreign key constraint.

If you use ON DELETE CASCADE in such a configuration, then the deletes may not be propagated in the correct order when you delete a record. In these cases, the following error is returned:

ORA-02292: integrity constraint (HR.DEPT_MGR_FK) violated - child record found

If you encounter this error, then create triggers that are aware of replication to perform this functionality instead of using ON DELETE CASCADE.

Diagnosing Problems with the Deferred Transaction Queue

If deferred transactions at a site are not being pushed to their destinations, then the following sections explain some possible causes for the problem:

• Check Jobs for Scheduled Links
• Distributed Transaction Problems with Synchronous Replication
• Incomplete Database Link Specifications
• Incorrect Replication Catalog Views

Check Jobs for Scheduled Links

When you create a scheduled link, Oracle adds a corresponding job to the site's job queue. If you have scheduled a link to push deferred transactions at a periodic interval, and you encounter a problem, then you should first be certain that you are not experiencing a problem with the job queue.

Distributed Transaction Problems with Synchronous Replication

When you use synchronous replication, Oracle uses a distributed transaction to ensure that the transaction has been properly committed at the remote site. Distributed transactions use two-phase commit. Asynchronous replication does not use two-phase commit.

Incomplete Database Link Specifications

If you notice that transactions are not being pushed to a given remote site, then you may have a problem with how you have specified the destination for the transaction. When you create a scheduled link, you must provide the full database link name.

Incorrect Replication Catalog Views

Having the wrong view definitions can lead to erroneous deferred transaction behavior. The DEFCALLDEST and DEFTRANDEST views are defined differently in catdefer.sql and catrepc.sql. The definitions in catrepc.sql should be used whenever replication is used. If catdefer.sql is ever (re)loaded, then ensure that the view definitions in catrepc.sql are subsequently loaded.

Diagnosing Problems with Materialized Views

There are a number of problems that might happen with materialized view sites in a replication system. The following sections discuss some problems and ways to troubleshoot them:

• Problems Creating Replicated Objects at Materialized View Site
• Refresh Problems
• Advanced Troubleshooting of Refresh Problems

Problems Creating Replicated Objects at Materialized View Site

If you unsuccessfully attempt to create a new object at a materialized view site, then try the following:

• For an updatable materialized view, check that the associated master table or master materialized view has a materialized view log.
• Make sure that you have the necessary privileges to create the object. For a materialized view, you need SELECT privilege on the master table or master materialized view and its materialized view log. See "Assign Privileges" for more information.
• If you are trying to add an existing materialized view to a materialized view group, then try re-creating the materialized view when you add it to the group.
• If you are trying to create a fast refresh primary key or subquery materialized view, then make sure that the materialized view log on the master table or master materialized view logs primary keys.
• If you are trying to create a fast refresh rowid materialized view, then make sure that the materialized view log on the master table logs rowids.
• Check if the materialized view log has the required columns added for subquery materialized views. See "Logging Columns in the Materialized View Log" for information.
• Check if the materialized view log exists for all tables that are involved in a fast refresh materialized view. If the materialized view contains a subquery, then each table referenced in the subquery should have a materialized view log.

Problems Performing Offline Instantiation of a Deployment Template

If you receive and error stating that Oracle is unable to initialize the extent in the temporary tablespace when you try to instantiate a deployment template offline, then you may need to adjust the datafile for the temporary database so that it auto extends.

For example, issue the following statement to adjust the datafile:

ALTER DATABASE TEMPFILE '/u02/oracle/rbdb1/temp.dbf' 
    AUTOEXTEND ON
    NEXT 10M;

After you have made this adjustment, instantiate the deployment template offline at the materialized view site.

Refresh Problems

The following sections explain several common materialized view refresh problems.

Common Problems

Several common factors can prevent the automatic refresh of a group of materialized views:

• The lack of a job queue process at the materialized view database
• An intervening network or server failure
• An intervening server shutdown

When a materialized view refresh group is experiencing problems, ensure that none of the preceding situations is preventing Oracle from completing group refreshes.

Automatic Refresh Retries

When Oracle fails to refresh a group automatically, the group remains due for its refresh to complete. Oracle will retry an automatic refresh of a group with the following behavior:

• Oracle retries the group refresh first one minute later, then two minutes later, four minutes later, and so on, with the retry interval doubling with each failed attempt to refresh the group.
• Oracle does not allow the retry interval to exceed the refresh interval itself.
• Oracle retries the automatic refresh up to sixteen times.

If after 16 attempts to refresh a refresh group Oracle continues to encounter errors, then Oracle considers the group broken. The General page of the Refresh Group property sheet in Schema Manager indicates when a refresh group is broken. You can also query the BROKEN column of the USER_REFRESH and USER_REFRESH_CHILDREN data dictionary views to see the current status of a refresh group.

The errors causing Oracle to consider a materialized view refresh group broken are recorded in a trace file. After you correct the problems preventing a refresh group from refreshing successfully, you must refresh the group manually. Oracle then resets the broken flag so that automatic refreshes can happen again.

Fast Refresh Errors at New Materialized View Sites

In some cases, a materialized view log for a master table or master materialized view might be purged during the creation of a materialized view at a new materialized view site. When this happens, you may encounter the following errors:

ORA-12004 REFRESH FAST cannot be used for materialized view materialized_view_
name
ORA-12034 materialized view log on materialized_view_name younger than last 
refresh

Materialized Views Continually Refreshing

If you encounter a situation where Oracle continually refreshes a group of materialized views, then check the group's refresh interval. Oracle evaluates a group's automatic refresh interval before starting the refresh. If a group's refresh interval is less than the amount of time it takes to refresh all materialized views in the group, then Oracle continually starts a group refresh each time the job queue process checks the queue of outstanding jobs.

Materialized View Logs Growing Too Large

If a materialized view log at a master site or master materialized view site is growing too large, then check to see whether a network or site failure has prevented the master site or master materialized view site from becoming aware that a materialized view has been dropped. You may need to purge part of the materialized view log or unregister the unused materialized view site.

Advanced Troubleshooting of Refresh Problems

If you have a problem refreshing a materialized view, then try the following:

• Check the NEXT_DATE value in the DBA_REFRESH_CHILDREN view to determine if the refresh has been scheduled.
• If the refresh interval has passed, then check the DBA_REFRESH view for the associated job number for the materialized view refresh and then diagnose the problem with job queues.
• Check if there are job queue process running. Check the JOB_QUEUE_PROCESSES initialization parameter, query the DBA_JOBS_RUNNING view, and use your operating system to check if the job queue processes are still running.
• You may also encounter an error if you attempt to define a master detail relationship between two materialized views. You should define master detail relationships only on the master tables by using declarative referential integrity constraints. The related materialized views should then be placed in the same refresh group to preserve this relationship. However, you can define deferred (or deferrable) constraints on materialized views.
• If there are any outstanding conflicts recorded at the master site or master materialized view site for the materialized views, then you can only refresh the materialized views by setting the parameter REFRESH_AFTER_ERRORS to true. This parameter can be set when you create or alter a materialized view refresh group. There is a corresponding parameter for the Replication Management tool's property sheets.
• Materialized views in the same refresh groups have their rows updated in a single transaction. Such a transaction can be very large, requiring either a large rollback segment at the materialized view site, with the rollback segment specified to be used during refresh, or more frequent refreshes to reduce the transaction size.
• If Oracle error ORA-12004 occurs, then the master site or master materialized view site may have run out of rollback segments when trying to maintain the materialized view log, or the materialized view log may be out of date. For example, the materialized view log may have been purged or re-created.
• Complete refreshes of a single materialized view internally use the TRUNCATE feature to increase speed and reduce rollback segment requirements. However, until the materialized view refresh is complete, users may temporarily see no data in the materialized view. Refreshes of multiple materialized views (for example, refresh groups) do not use the TRUNCATE feature.
• Reorganization of the master table (for example, to reclaim system resources) should TRUNCATE the master table to force rowid materialized views to do complete refreshes. Otherwise, the materialized views have incorrect references to master table rowids. You use the BEGIN_TABLE_REORGANIZATION and END_TABLE_REORGANIZATION procedures in the DBMS_MVIEW package to reorganize a master table. See the Oracle9i Replication Management API Reference for more information.
• If while refreshing you see an ORA-942 (table or view does not exist), then check your database links and make sure you still have the required privileges on the master table or master materialized view and the materialized view log.
• If a fast refresh was succeeding but then fails, then check whether:
  • The materialized view log was truncated, purged, or dropped
  • You still have the required privileges on the materialized view log
• If a force refresh takes an inordinately long time, then check if the materialized view log used by the refresh has been dropped.
• If the materialized view was created with BUILD DEFERRED, and its first fast refresh fails, then make sure a previous complete refresh was done successfully before checking for other problems.

  See Also: Oracle9i Replication Management API Reference for information about managing materialized view logs