In Database systems that use logical replication, replication slots are especially important for database replication. The logical replication mechanism in PostgreSQL’s database system supports replication slots.
Logical database replication depends on replication slots, which are essential. They manage replication lag, prevent replicas from becoming overloaded, guarantee dependable data delivery from the primary to replicas, and support point-in-time recovery. Replication slots help to keep the overall stability and integrity of the replication process stable and synchronized by ensuring a controlled and coordinated flow of changes.
Here’s how replication slots work and their role in database replication
1. Capturing and Retention of changes: Between the primary database (source) and the replica database (target), replication slots serve as a buffer. They serve as essential named markers or bookmarks that monitor the replication process. Prior to being sent to the replica, the primary database writes changes (INSERTs, UPDATEs, and DELETEs) to the replication slots.
2. Guaranteed Retention: Replication slots ensure that data changes required for replication are kept in the transaction logs of the primary database (also known as WAL logs or write-ahead logs) up until the point at which all subscribed replicas have successfully received and processed them. This makes sure that even if replicas become temporarily disconnected, they won’t miss any changes.
3. Preventing Omissions due to Overload: To keep the replica databases from becoming overloaded, replication slots are also important. The replication slot aids in regulating the rate of change delivery from the primary, if a replica experiences delays in processing changes due to load or network issues. This keeps the replica from being overloaded and enables it to catch up without running the risk of data loss.
4. Point-in-Time Recovery: For situations involving point-in-time recovery, replication slots are also useful. Recovering to a specific timestamp is possible when a replica uses a replication slot to pinpoint a particular location in the replication stream. When you need to restore the replica to a particular consistent state, this is especially helpful.
5. Option of Manual or Automatic Slot management in replication slots: Based on the connectivity and lag of the replica, PostgreSQL automatically creates and deletes replication slots. This lessens the need for extra data to be retained in the main database.
6. Lag Monitoring: Replication slots can be used to track the replication lag, or the interval between changes made in the primary and application of changes made in the replica. System administrators may find this information helpful in ensuring the functionality and health of the replication setup.
Benefits of using replication Slots for different replication scenarios
In the streaming replication of PostgreSQL, replication slots are especially helpful. In various replication scenarios, replication slots offer several advantages that improve data consistency, availability, performance, and management flexibility. These are crucial components of database replication systems like PostgreSQL that work to maintain the accuracy of the data and the dependability of the system. The advantages of using replication slots for various replication scenarios are listed in detail below.
1. Preventing Data Loss: Replication slots aid in data loss prevention. These make sure that the WAL (Write-Ahead Logs) needed by standby servers are not deleted by the primary database. WAL segments will remain in the primary database until all standby servers confirm that all the segments have been received and applied. This guarantees that backup servers are current and capable of data recovery even during outages.
2. Throttling and Resource Management: You can regulate the speed of data replication to backup servers using replication slots. You can restrict the number of replication slots that are available. to control the amount of data that is streamed to each standby. This avoids potential performance issues brought on by overtaxing the network or the standby servers.
3. Selective replication: In selective replication particular data changes are replicated to particular standbys. This can be implemented using replication slots. You can replicate only a portion of the data for particular use cases, such as reporting or analytics, this can be helpful.
4. Load Balancing: For e.g., Read traffic can be split among several standby servers using replication slots. You can distribute the load from read queries, lessening the load on the primary database, and enhancing performance, by setting up multiple standbys and using replication slots.
5. High Availability: Replication slots improve your database’s high availability by ensuring that backup servers are constantly prepared to replace the primary database in the event of a failure. The risk of data loss is decreased, and downtime is minimized thanks to the constant updating of the backup servers.
7. Delayed Replication: Delayed replication uses replication slots to implement standby servers that purposefully lag the primary by a predetermined amount of time. This can be helpful in situations where you need to preserve a window of time during which unintentional data changes can be undone before being applied to the standby.
8. Switchover and Failover: During scenarios involving database switchover and failover, replication slots are essential. Replication slots make sure that the new primary can carry on streaming changes from where the old primary left off. This minimizes data loss and downtime when a standby is promoted to become the new primary.