PostgreSQL version 9.5.2 uses physical replication slots to manage write ahead log (WAL) retention on the source DB instance. For each cross-Region read replica instance, Amazon RDS creates and associates a physical replication slot.
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| Name | Type | References | Description |
|---|---|---|---|
slot_name | name | A unique, cluster-wide identifier for the replication slot | |
plugin | name | The base name of the shared object containing the output plugin this logical slot is using, or null for physical slots. | |
slot_type | text | The slot type - physical or logical | |
datoid | oid | | The OID of the database this slot is associated with, or null. Only logical slots have an associated database. |
database | text | | The name of the database this slot is associated with, or null. Only logical slots have an associated database. |
temporary | boolean | True if this is a temporary replication slot. Temporary slots are not saved to disk and are automatically dropped on error or when the session has finished. | |
active | boolean | True if this slot is currently actively being used | |
active_pid | integer | The process ID of the session using this slot if the slot is currently actively being used. NULL if inactive. | |
xmin | xid | The oldest transaction that this slot needs the database to retain. VACUUM cannot remove tuples deleted by any later transaction. | |
catalog_xmin | xid | The oldest transaction affecting the system catalogs that this slot needs the database to retain. VACUUM cannot remove catalog tuples deleted by any later transaction. | |
restart_lsn | pg_lsn | The address (LSN) of oldest WAL which still might be required by the consumer of this slot and thus won't be automatically removed during checkpoints. NULL if the LSN of this slot has never been reserved. | |
confirmed_flush_lsn | pg_lsn | The address (LSN) up to which the logical slot's consumer has confirmed receiving data. Data older than this is not available anymore. NULL for physical slots. |
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- Before you can use logical decoding, you must set wallevel to logical and maxreplicationslots to at least 1. 1 Create Replication slot and check pgreplicationslots as per below snapshot:: In the above slot information you can see one slot is active and the newly created slot is not active yet.
- Using a replication slot requires Postgres's replication privileges. At this time, the replication privilege is only available for the server's admin user.
- Replication Slots Replication in PostgreSQL works by processing WAL files. In order for this to work, all WAL files have to be transiently available until they are processed. Therefore a mechanism is needed to tell the main WAL management not to recycle or remove files.
- Learn how to monitor native PostgreSQL replication using the system views and functions PostgreSQL provides. Specific queries are also provided. This tutorial assumes you have already setup 1) streaming replication between a primary and a replica PostgreSQL database cluster, and 2) are using replication slots.
