Synchronize data dictionary FAQ - Data Management - Alibaba Cloud Documentation Center

This topic provides answers to common questions about refreshing or synchronizing the data dictionary (metadata) in Data Management (DMS), covering general issues, Redis-specific topics, and Oracle-specific topics.

General

Why are databases or tables missing in DMS even though they exist in my database?

DMS does not always read database and table information from the database in real time.

Secure hosting instances: DMS collects and stores instance metadata. A full metadata collection is triggered when an instance is first registered, and a scheduled full collection runs daily at 18:00.
Non-secure hosting instances: DMS persists only database-level information. Other metadata is loaded in real time after you log on to the instance.
DDL changes executed through DMS trigger an incremental metadata collection. The updated schema is typically available in DMS within about 5 seconds after the operation completes, though the actual time may vary depending on database load and collection task status.
Changes made through the database console or other clients may require a manual refresh or data dictionary synchronization.

For more information, see Database metadata collection, display, and search.

How do I manually refresh database and table information?

You can use either of the following methods:

Method 1: In the instance list on the left side of the DMS homepage, right-click the target instance and select Refresh/Synchronize Data Dictionary. Confirm the operation and wait for the collection to complete.
Method 2: In the top navigation bar, choose Data Assets > Instance Management. In the instance list, find the target instance and choose More > Refresh/Synchronize Data Dictionary.

If the changes were made through another client, you can also try logging on to the instance again to reload the metadata.

For more information, see Synchronize the data dictionary of a database instance and When and why DMS reloads database metadata.

What should I do if databases are still missing after synchronizing the data dictionary?

We recommend that you check the following items in order:

1. Check the permissions of the database account used by DMS

DMS can only collect metadata for databases that the configured database account has permission to access. This issue may occur with ApsaraDB RDS for MySQL, ApsaraDB RDS for PostgreSQL, ApsaraDB RDS for SQL Server, PolarDB for MySQL, PolarDB for PostgreSQL, as well as third-party or self-managed databases.

Verify the following:

The database account and password configured in DMS are valid.
The account has access permissions on the target database.
After a new database is created, the database has been authorized for the account used by DMS.
To display all databases under an instance, the account must have access permissions on all target databases.
For PostgreSQL-based databases, also check permissions on the target database, schema, and related objects.

Note

For ApsaraDB RDS for MySQL, ApsaraDB RDS for PostgreSQL, ApsaraDB RDS for SQL Server, PolarDB for MySQL, and PolarDB for PostgreSQL, a newly created database may not be automatically authorized for the account used by DMS. In this case, the database is not displayed in DMS even after you synchronize the data dictionary. You must first grant access permissions on the new database to the account used by DMS, and then synchronize the data dictionary again.

The authorization methods vary by database engine. For more information, see the following topics:

2. Check the instance connection status

Verify the following:

The database instance is running properly.
The database address, port, account, and password are correct.
The DMS IP addresses have been added to the database whitelist.
Security groups, firewalls, and network links allow access from DMS.
If SSL/TLS is enabled on the database, the corresponding SSL connection method is also configured in DMS.

For more information, see Troubleshoot database logon and connection issues and Add DMS IP addresses to the instance whitelist.

3. Check whether system database filter rules are blocking the database

The Security collaboration control mode hides certain system databases by default. Security rules can also define custom filters for databases or schemas. When a database is filtered, DMS does not collect its metadata, and the database cannot be searched or displayed.

For more information, see System databases blocked by DMS by default and Customize system database filters.

4. Check the metadata access permissions of the current DMS user

Even if the database account has access permissions, the current DMS user may not have the required permissions. When metadata access control is enabled, users without instance or database permissions cannot view the corresponding resources.

If administrators can see the database but regular users cannot, check the following items first:

Whether user-level, database-level, or instance-level metadata access control is enabled.
Whether the user has query, export, or change permissions on the target database.
Whether a DBA or administrator has assigned the required permissions to the user.

For more information, see Configure metadata access control.

Why does data dictionary synchronization take a long time to complete?

The time required for metadata synchronization depends on the number of databases and tables in the instance. If the instance contains a large number of tables, the synchronization task may take several minutes or longer.

The current system imposes a maximum timeout of 120 seconds for metadata collection SQL statements. If a single database contains an extremely large number of tables (for example, on the order of 100,000 or more), the collection SQL may time out, preventing metadata synchronization from completing. As a result, DMS cannot display the table list for that database. This is a known system limitation. The 100,000-table figure is an approximate reference and not a strict threshold. Whether a timeout occurs also depends on database performance, load, and network conditions.

Recommendations:

Avoid triggering synchronization repeatedly in a short period of time.
Wait for the current synchronization task to complete, and then check the results.
Verify that the database connection is stable.
Verify that the database account permissions have not changed.
For non-secure hosting instances, try logging on to the instance again.

For more information, see Synchronize the data dictionary of a database instance.

Why can only some users not see a database?

This is typically a DMS user permission issue rather than a metadata collection issue.

Check the following:

Whether user-level, database-level, or instance-level metadata access control is enabled.
Whether the user has query, export, or change permissions on the target database.
Whether a DBA or administrator has assigned instance or database permissions to the user.

If administrators can see the database but regular users cannot, prioritize checking this area.

Why are system databases or certain schemas not visible?

The Security collaboration control mode hides certain system databases by default. For example:

MySQL: mysql, information_schema, performance_schema, and others.
SQL Server: master, model, msdb, tempdb, and others.
PostgreSQL: Primarily filters system schemas such as information_schema and pg_catalog.

In addition, for instances in the Security collaboration control mode, you can define custom filters by navigating to Security Rules > Others > Metadata Synchronization > Filter System Databases. You can filter system databases or sensitive business databases by exact name or regular expression. For PostgreSQL-based databases, the filter applies to schemas. After you modify the filter rules, you must synchronize the data dictionary again, or wait for the daily automatic metadata synchronization at 18:00 for the changes to take effect.

For more information, see System databases blocked by DMS by default for each database type and Customize system database filters in DMS.

Why are schemas not visible after the database is locked or in an abnormal state?

When DMS refreshes or synchronizes the data dictionary, it connects to the target database and runs metadata collection SQL statements. If the ApsaraDB RDS instance or the target database is locked or unavailable, DMS cannot retrieve schema and other metadata. Repeatedly synchronizing the data dictionary typically does not resolve this issue.

We recommend the following steps:

Check the running status of the instance and target database in the corresponding cloud database console.
If the database is locked, follow the instructions in the cloud database console to unlock it and restore normal access.
In DMS, test the connection to confirm that the database is accessible.
Run Refresh/Synchronize Data Dictionary again and wait for the collection to complete before refreshing the object list.

For more information, see Troubleshoot database logon and connection issues.

Redis

Why can I only see DB0 after registering a Redis cluster over the Internet?

According to DMS documentation, after a third-party or self-managed Redis cluster is registered in DMS, only DB0 is displayed.

This is a known product limitation. Repeatedly refreshing or synchronizing the data dictionary does not resolve this issue.

Recommendations:

For Alibaba Cloud Tair or Redis instances, register the instance using the Alibaba Cloud database source.
For third-party or self-managed Redis clusters, confirm the DMS display capabilities in advance.

For more information, see Register a third-party or self-managed database.

How does DMS identify the Redis cluster mode?

The ability to identify Redis cluster architectures depends on the instance source and registration method.

For Alibaba Cloud Tair or Redis cluster instances, we recommend registering the instance using the Alibaba Cloud database source.
Redis clusters registered over the Internet or from third-party or self-managed sources have limited capabilities.
As stated in the documentation, third-party and self-managed Redis clusters currently support only DB0.

Oracle

Why are schemas, tables, or columns still missing after synchronizing the data dictionary for an Oracle instance?

When DMS collects Oracle metadata, it queries Oracle system tables or system views using the database account configured during instance registration. If the account lacks query permissions on the relevant objects, metadata such as schemas, tables, columns, indexes, constraints, and comments may be collected incompletely, and they remain invisible in DMS even after you synchronize the data dictionary.

Make sure the database account used by DMS has at least query permissions on the following system tables or views:

V$INSTANCE
SYS.DBA_USERS
SYS.DBA_TABLES
SYS.DBA_SEGMENTS
SYS.DBA_TAB_COMMENTS
SYS.DBA_TAB_COLUMNS
SYS.DBA_COL_COMMENTS
SYS.DBA_SYNONYMS
SYS.DBA_OBJECTS
SYS.DBA_INDEXES
SYS.DBA_CONSTRAINTS
SYS.DBA_IND_COLUMNS

Note

V$INSTANCE is typically a synonym for a dynamic performance view. When granting permissions, you may need the Oracle DBA to grant query permissions on the underlying object SYS.V_$INSTANCE. The specific requirements depend on the Oracle version and permission configuration.

After granting the required permissions, run Refresh/Synchronize Data Dictionary again.