|Limits on the source database
- The source database must have PRIMARY KEY or UNIQUE constraints and all fields must
be unique. Otherwise, the destination database may contain duplicate data records.
- The following requirements for data logs must be met:
- The data logging feature must be enabled.
- Data logs are retained for at least 7 days during initial full data synchronization.
You can wait until initial full data synchronization is complete, and then clear the
data logs generated in the source database after the DTS task is run.
Note To ensure data security, the DTS server stores only 50 GB of data logs or the data
logs for the last 24 hours. If the limit is exceeded, the DTS server automatically
clears the cached logs.
Warning If you clear the data logs of the source database during initial full data synchronization,
the data synchronization task may fail. For example, initial full data synchronization
takes more than 24 hours due to the large data volume in the source database and abnormal
writing in the destination database. In this case, if the data logs of the source
database are cleared during initial full data synchronization, DTS cannot obtain the
data logs generated 24 hours ago. Therefore, the data synchronization task may fail.
- Limits on operations:
To ensure that the data synchronization task runs as expected, you can perform primary/secondary
switchover only on an ApsaraDB RDS for PostgreSQL instance V11. In this case, you
must set the
. For more information, see Use the failover slot feature for logical subscriptions
Warning If you perform primary/secondary switchover on a self-managed PostgreSQL database or an ApsaraDB RDS for PostgreSQL instance of other versions, the data synchronization
- Requirements for the objects to be synchronized:
- Only tables can be selected as the objects to be synchronized.
- DTS does not synchronize the following types of data: BIT, VARBIT, GEOMETRY, ARRAY,
UUID, TSQUERY, TSVECTOR, and TXID_SNAPSHOT.
- A single data synchronization task can synchronize data from only one database. To
synchronize data from multiple databases, you must create a data synchronization task
for each database.
- During data synchronization, if you select a schema as the object to be synchronized,
take note of the following limits: If you create a table in the schema or run the
RENAME command to rename the table, you must run the
ALTER TABLE schema.table REPLICA IDENTITY FULL; command before you write data to the table. This ensures data consistency.
Note Replace the
table in the preceding sample command with the actual schema name and table name.
- To ensure that the latency of data synchronization is accurate, DTS adds a heartbeat
table to the source database. The name of the heartbeat table is dts_postgres_heartbeat.
- During data synchronization, DTS creates a replication slot for the source database.
The replication slot is prefixed with
dts_sync_. DTS automatically clears historical replication slots every 90 minutes to reduce
Note If the data synchronization task is released or fails, DTS automatically clears the
replication slot. If a primary/secondary switchover is performed on the source ApsaraDB
RDS for PostgreSQL instance, you must log on to the secondary database to clear the
- Before you synchronize data, evaluate the impact of data synchronization on the performance
of the source and destination databases. We recommend that you synchronize data during
off-peak hours. During initial full data synchronization, DTS uses read and write
resources of the source and destination databases. This may increase the loads of
the database servers.
- During initial full data synchronization, concurrent INSERT operations cause fragmentation
in the tables of the destination database. After initial full data synchronization
is complete, the tablespace of the destination database is larger than that of the
- We recommend that you do not use gh-ost or pt-online-schema-change to perform data
definition language (DDL) operations on source tables during data synchronization.
Otherwise, data synchronization may fail.
- If you use only DTS to write data to the destination database, you can use Data Management
(DMS) to perform online DDL operations on source tables during data synchronization.
For more information, see Change schemas without locking tables.
Warning If you use tools other than DTS to write data to the destination database, we recommend
that you do not use DMS to perform online DDL operations. Otherwise, data loss may
occur in the destination database.