All about Logical Replication in Postgres.

What is Logical Replication

Logical replication in PostgreSQL is a feature that allows you to replicate data from one database to another, in a flexible and customizable way. Unlike physical replication, which replicates the entire database and its structure, logical replication allows you to specify which tables and columns you want to replicate, and how you want the data to be replicated. This means that you can replicate only the data that is relevant to your application, and you can apply custom transformations or filters to the replicated data to meet your specific needs.

Logical replication is implemented using a publish-subscribe model, where one database is designated as the publisher and another database is designated as the subscriber. The publisher sends the changes that occur on its tables to the subscriber, and the subscriber applies those changes to its own database. This allows the subscriber to maintain an up-to-date copy of the data on the publisher, and it enables you to use the replicated data for various purposes, such as reporting, analysis, or backup.

Logical replication is an important feature in PostgreSQL because it allows you to replicate data between different databases, even if they are running on different platforms or versions. This makes it a powerful tool for data integration, migration, and disaster recovery scenarios. It is also a useful alternative to physical replication when you need more flexibility and control over how the data is replicated.

What would you use Logical Replication for in Postgres?

There are many use cases for logical replication in PostgreSQL, some of which include:

  • Data integration: Logical replication can be used to replicate data from different sources, such as different databases or different tables within the same database, and integrate it into a single destination. This can be useful for applications that need to combine data from multiple sources and provide a unified view of the data.
  • Data migration: Logical replication can be used to migrate data from an old database to a new database, without interrupting the operation of the application. This can be useful when you want to upgrade or change your database, or when you want to move your data to a different platform or environment.
  • Disaster recovery: Logical replication can be used to maintain a standby database that can be used as a failover in case the primary database becomes unavailable. This can be useful for ensuring high availability and minimizing downtime in case of disasters or other unexpected events.
  • Reporting and analysis: Logical replication can be used to replicate data from the production database to a reporting or analytics database, where it can be queried and analyzed without affecting the performance of the production database. This can be useful for applications that need to generate reports or perform complex analysis on the data.
  • Backup and archiving: Logical replication can be used to replicate data from the production database to a backup or archiving database, where it can be stored for long-term retention or for compliance purposes. This can be useful for ensuring that the data is preserved and can be recovered in case of data loss or corruption.

These are just some examples of how logical replication can be used in PostgreSQL. There are many other potential uses for this feature, and it can be a powerful tool for managing and manipulating data in a variety of scenarios.

How do you set up Logical Replication in Postgres

To set up logical replication in PostgreSQL, you need to perform the following steps:

  1. Set up the publisher server and the subscriber server. The publisher server is the server that will send the changes to the subscriber server, while the subscriber server will receive and apply the changes to its own database.
  2. On the publisher server, create a new publication using the CREATE PUBLICATION statement. This publication will define which tables and columns will be replicated from the publisher server to the subscriber server. For example, to create a publication named my_publication that replicates the employees table, you can use the following statement:
  1. On the subscriber server, create a new subscription using the CREATE SUBSCRIPTION statement. This subscription will define the connection details for the publisher server and the publication that will be used for replication. For example, to create a subscription named my_subscription that uses the my_publication publication on the publisher server, you can use the following statement:

Replace publisher_server_hostname, replication_user, and replication_password with the actual hostname, username, and password of the publisher server and the user that will be used for replication.

  1. Once the subscription is created, the subscriber server will connect to the publisher server and start receiving the changes. The changes will be automatically applied to the local database on the subscriber server, and the data on the subscriber server will be kept in sync with the data on the publisher server.

Note that these are the basic steps for setting up logical replication in PostgreSQL. There are many other factors to consider and configure, such as security, performance, and monitoring, to ensure that your replication setup is reliable and efficient. It is recommended to consult the PostgreSQL documentation and other resources to learn more about the details and best practices for setting up logical replication in PostgreSQL.

How can you monitor logical replication in postgres

To monitor logical replication in PostgreSQL, you can use the following methods:

  1. Use the pg_stat_subscription view to get information about the status and performance of the replication process. This view contains several columns that provide information about the replication, such as the name of the subscription, the status of the replication, the lag in replication, and the number of rows replicated. You can use this view to monitor the overall health of the replication process, and to detect and troubleshoot any issues that may arise.
  2. Use the pg_replication_slots view to get information about the replication slots that are used by the subscriptions. This view contains several columns that provide information about the slots, such as the name of the slot, the status of the slot, the number of bytes received, and the number of bytes sent. You can use this view to monitor the utilization and efficiency of the replication slots, and to ensure that there are enough slots available for the subscriptions.
  3. Use the pg_stat_activity view to get information about the connections that are used by the subscriptions. This view contains several columns that provide information about the connections, such as the process ID of the connection, the user that is connected, the database that is connected to, and the state of the connection. You can use this view to monitor the connections that are used by the subscriptions, and to detect and troubleshoot any problems with the connections.
  4. Use the pg_stat_replication view to get information about the replication slots that are used by the subscriptions. This view contains several columns that provide information about the slots, such as the name of the slot, the PID of the process that is using the slot, the user that is using the slot, and the state of the slot. You can use this view to monitor the slots that are used by the subscriptions, and to detect and troubleshoot any issues with the slots.

How do you monitor logical replication with the pg_stat_subscription view?

To monitor logical replication using the pg_stat_subscription view in PostgreSQL, you can use a query like the following:

This query will return the following information for each subscription:

  • subid: The ID of the subscription.
  • subname: The name of the subscription.
  • pid: The process ID of the process that is responsible for the replication.
  • relid: The ID of the table that is being replicated.
  • sent_lsn: The LSN (log sequence number) of the last transaction that was sent by the publisher.
  • write_lsn: The LSN of the last transaction that was written to the write-ahead log (WAL) on the subscriber.
  • flush_lsn: The LSN of the last transaction that was flushed to disk on the subscriber.
  • replay_lsn: The LSN of the last transaction that was replayed on the subscriber.
  • sync_priority: The priority of the subscription, which determines the order in which the transactions are replayed on the subscriber.
  • sync_state: The state of the subscription, which can be one of sync, async, or potential.

This information can be used to monitor the status and performance of the replication process, and to detect and troubleshoot any issues that may arise. For example, you can use the sent_lsn and write_lsn columns to check the lag in replication, and you can use the sync_state column to check the state of the replication. You can also use this information to monitor the overall health of the replication process, and to ensure that it is working as expected.

How do you monitor logical replication with the pg_replication_slots view?

To monitor logical replication using the pg_replication_slots view in PostgreSQL, you can use a query like the following:

This query will return the following information for each replication slot:

  • slot_name: The name of the replication slot.
  • plugin: The plugin that is used by the replication slot.
  • slot_type: The type of the replication slot, which can be physical or logical.
  • active: Whether the replication slot is active or inactive.
  • xmin: The transaction ID of the oldest transaction that is still needed by the slot.
  • catalog_xmin: The transaction ID of the oldest transaction that is still needed by the slot for catalog operations.
  • restart_lsn: The LSN of the last transaction that was replayed by the slot.
  • confirmed_flush_lsn: The LSN of the last transaction that was confirmed as flushed by the slot.
  • slot_bytes: The number of bytes that are currently used by the slot.

This information can be used to monitor the utilization and efficiency of the replication slots, and to ensure that there are enough slots available for the subscriptions. For example, you can use the active column to check the number of active slots, and you can use the slot_bytes column to check the amount of data that is stored in the slots. You can also use this information to monitor the overall health of the replication process, and to detect and troubleshoot any issues that may arise with the slots.

How do you monitor logical replication with the pg_stat_activity view?

To monitor logical replication using the pg_stat_activity view in PostgreSQL, you can use a query like the following:

This query will return the following information for each connection that is used by the replication process:

  • pid: The process ID of the connection.
  • datname: The name of the database that the connection is connected to.
  • application_name: The name of the application that is using the connection.
  • state: The state of the connection, which can be one of active, idle, or idle in transaction.
  • query: The current query that is being executed on the connection.

This information can be used to monitor the connections that are used by the subscriptions, and to detect and troubleshoot any problems with the connections. For example, you can use the state column to check the activity on the connections, and you can use the query column to see the queries that are being executed on the connections. You can also use this information to monitor the overall health of the replication process, and to ensure that the connections are working as expected.

How do you monitor logical replication with the pg_stat_replication view?

To monitor logical replication using the pg_stat_replication view in PostgreSQL, you can use a query like the following:

This query will return the following information for each replication slot that is used by the subscriptions:

  • client_addr: The IP address of the client that is connected to the slot.
  • client_hostname: The hostname of the client that is connected to the slot.
  • client_port: The port number that is used by the client to connect to the slot.
  • backend_start: The timestamp of when the client connected to the slot.
  • backend_xmin: The transaction ID of the oldest transaction that is still needed by the client.
  • state: The state of the slot, which can be one of streaming, catching up, or stalled.
  • sent_lsn: The LSN of the last transaction that was sent by the publisher.
  • write_lsn: The LSN of the last transaction that was written to the write-ahead log (WAL) on the subscriber.
  • flush_lsn: The LSN of the last transaction that was flushed to disk on the subscriber.
  • replay_lsn: The LSN of the last transaction that was replayed on the subscriber.
  • sync_priority: The priority of the subscription, which determines the order in which the transactions are replayed on the subscriber.
  • sync_state: The state of the subscription, which can be one of sync, async, or potential.

This information can be used to monitor the slots that are used by the subscriptions, and to detect and troubleshoot any issues with the slots. For example, you can use the state column to check the status of the slots, and you can use the sent_lsn, write_lsn, flush_lsn, and replay_lsn columns to check the lag in replication. You can also use this information to monitor the overall health of the replication process, and to ensure that the slots are working as expected.

In Postgres how do you fix it when logical replication slots become inactive?

If logical replication slots become inactive in PostgreSQL, you can try the following steps to fix the issue:

  1. Use the SELECT pg_drop_replication_slot(slot_name) statement to drop the inactive slots. This will free up the resources that are used by the slots, and it will allow you to create new slots for the subscriptions.
  2. Use the SELECT pg_create_logical_replication_slot(slot_name, plugin) statement to create new slots for the subscriptions. This will ensure that there are enough slots available for the subscriptions, and it will allow the replication process to continue without interruption.
  3. Use the SELECT pg_logical_slot_get_changes(slot_name, count, start_lsn, forward, skip_empty_xacts) statement to retrieve the changes that were missed during the inactive period. This will allow you to catch up with the changes that were not replicated, and it will help you to restore the consistency of the data on the subscriber.
  4. Use the SELECT pg_logical_slot_peek_changes(slot_name, forward, start_lsn, until_lsn, count) statement to check the status of the changes that are being replicated. This will allow you to monitor the progress of the replication, and it will help you to detect any issues that may arise with the changes.

These are some of the steps that you can try to fix the issue of inactive logical replication slots in PostgreSQL. It is recommended to consult the PostgreSQL documentation and other resources to learn more about the details and best practices for managing and troubleshooting logical replication slots in PostgreSQL.

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