Replication slots in PostgreSQL are a mechanism for streaming physical and logical replication to keep the standby servers current with changes from the primary server. Replication slots help manage the data flow between primary and standby servers. Physical replication slots and logical replication slots are the two primary categories of replication slots in PostgreSQL.
1. Physical Replication Slots
Streaming replication, which involves replicating the physical changes made to the database on the primary server to the standby servers, uses physical replication slots. High availability configurations frequently use physical replication.
Physical replication slots can be configured with different options, such as:
– `max_wal_size`: The maximum amount of WAL (Write-Ahead Logging) data that can be retained for a replication slot.
– `min_wal_size`: The minimum amount of WAL data that must be retained for a replication slot.
– `max_slot_wal_keep_size`: The maximum amount of WAL data to retain specifically for replication slots.
2. Logical Replication Slots
Logical replication slots are used for logical replication, where changes are captured in a more application-friendly format rather than the raw physical changes. This allows for more flexible replication scenarios, such as selective table replication or transformation of data during replication.
Logical replication slots can also be configured with different options, such as:
– `plugin`: The name of the logical replication plugin to use with the slot.
– `plugin_opts`: Options specific to the chosen logical replication plugin.
– `create_slot`: Boolean flag indicating whether the replication slot should be created during server startup.
In addition to these main types, there is another concept called “replication slot types” in PostgreSQL, which refers to how replication slots are treated in terms of retention and cleanup. Replication slot types include:
1. Physical Slots
Physical replication slots are used for streaming replication and are associated with a specific WAL segment. They are typically retained until the standby acknowledges receipt of the corresponding data.
2. Logical Slots
Logical replication slots are used for logical replication and are associated with a specific point in the WAL. They are retained until the logical replication consumer acknowledges receipt of the corresponding data.
It’s important to note that the specific options and behaviors related to replication slots might vary depending on the version of PostgreSQL you are using. Always refer to the PostgreSQL documentation for the most up-to-date and accurate information.
Use cases for Physical replication Slots
Database management systems like PostgreSQL have physical replication slots, which give administrators more power and flexibility over replication operations. They are especially helpful in high-availability configurations and scenarios involving database replication. Following are a few frequent uses for physical replication slots:
1. Continuous Archiving and Backup: To ensure continuous archiving and backup of the transaction logs, physical replication slots can be used. This is crucial for backup and point-in-time recovery purposes. You can make sure that the database server keeps the necessary WAL (Write-Ahead Log) segments until they have been successfully replicated to the standby server by keeping a replication slot open.
2. Streaming Replication: In streaming replication configurations, physical replication slots are essential. The replication slot aids in controlling the flow of WAL segments when a standby server is replicating changes from the primary server. The replication slot can store the WAL segments until the standby catches up if the standby is running behind, preventing the primary server from prematurely removing the logs.
3. High Availability and Failover: In high-availability configurations, physical replication slots guarantee that a standby server can seamlessly replace a primary server in the event of a failure. The standby server is always up to date with the WAL segments thanks to the maintenance of a replication slot, which speeds up and improves the reliability of the failover process.
4. Load Balancing and Read Scalability: Replication slots can be used in configurations where multiple standby servers are used to distribute read traffic, improving read scalability. By allocating a separate replication slot to each standby, it is possible to make sure that they all receive the proper updates from the primary server.
5. Backup Integrity: It’s crucial to make sure that backups are consistent and current when performing backups. To preserve data integrity, replication slots make sure that the backup procedure doesn’t begin until all required WAL segments have been archived and replicated.
6. Delayed Standbys: In some circumstances, it’s necessary to have a standby server that purposefully lags behind the primary. This may be helpful for testing, analytical queries, or having a backup server that keeps track of data changes up to a specific time. To accomplish this, replication slots can be configured to delay replication.
7. Data Migration and Upgrades: Before proceeding with a data migration or database upgrade, you might want to make a copy of the current database for testing purposes. Replication slots ensure that the replica and the production database stay in sync.
8. Disaster Recovery: Having a well-maintained replication slot on a standby server can significantly speed up the recovery process and lower data loss in the event of a catastrophic event that affects the primary database server.
Keep in mind that depending on database management system being used. The replication setup requirements, the implementation and configuration of physical replication slots, may differ.
Use cases for Logical replication Slots
PostgreSQL has a feature called logical replication slots. In a logical replication setup, these slots are used to manage and control the flow of changes (replication data) from the source database to the replica database. In contrast to physical replication, which duplicates entire block-level data files, logical replication duplicates specific database changes (rows, statements). Examples of logical replication slots in use are as follows:
1. Database Migration: To ensure a seamless switch from the old database to the new one, replication slots can be used during database migrations. You can record all changes made during the migration process and apply them to the new database once it is ready by setting up a replication slot on the old database. This minimizes downtime and ensures data consistency.
2. Reporting and analytics: Data can be replicated from a production database to a reporting or analytics database using replication slots. This enables you to remove reporting queries that require a lot of resources from the production database, preventing them from slowing down the main application.
3. High Availability and Failover: To achieve high availability and failover scenarios, replication slots can be useful. You can make sure that standby databases are constantly updated with changes from the primary database by using replication slots. This configuration makes sure that in the event of a primary database failure, the standby databases are prepared to take over.
4. Data Warehousing: To replicate data from a transactional database to a data warehouse for analysis, use logical replication slots. This makes it possible to separate operational from analytical workloads, enhancing the efficiency of both systems.
5. Selective Replication: You can set up selective replication using replication slots, which allows you to choose to replicate only particular tables or schemas as opposed to replicating the entire database. When you want to replicate only pertinent data to the replica database, this is helpful.
6. Data Sharing and Distribution: Sharing scenarios, in which various pieces of a sizable dataset are dispersed across several databases, can make use of replication slots. The synchronization of data changes between the shared databases is managed with the aid of replication slots.
7. Testing and development: To create development or testing environments that are current with the production data, replication slots can be used. This enables testing and development teams to use real-world data scenarios without affecting the live database.
8. Point-in-Time Recovery and Rollback: Point-in-time recovery and reverting to a previous state can both be accomplished with the help of replication slots. Utilizing replication slots allows you to recover the database to a previous state by replaying changes made up to a certain point in time.
9. Data Incorporation: Data integration between various systems can also be made easier by replication slots. For instance, you can use logical replication to combine data from various applications or microservices that make use of different.
PostgreSQL’s logical replication slots provide a versatile and effective method for controlling the flow of updates between databases, enabling a range of use cases from high availability and failover to data warehousing and reporting.
Step by Step guide on creating replication slots
Creating replication slots is a crucial aspect of setting up logical replication in PostgreSQL. Replication slots allow a standby server to connect to the primary server and receive updates in real-time. Here’s a step-by-step guide on creating replication slots in PostgreSQL:
1. Install PostgreSQL: Ensure that PostgreSQL is installed on both the primary and standby servers. You can download and install PostgreSQL from the official website or package manager for your operating system.
2. Configure Primary Server: Edit the `postgresql.conf` file on the primary server to enable logical replication:
“`
wal_level = logical
max_replication_slots = <desired_number_of_slots>
max_wal_senders = <desired_number_of_senders>
“`
3. Restart PostgreSQL: After making changes to the configuration file, restart the PostgreSQL service on the primary server to apply the changes.
4. Create Replication Slot: Connect to the primary server using `psql` or any PostgreSQL client, and execute the following SQL command to create a replication slot:
“`sql
SELECT * FROM pg_create_logical_replication_slot(‘<slot_name>’, ‘pgoutput’);
“`
Replace `<slot_name>` with a name for your replication slot.
The second argument `’pgoutput’` specifies the replication plugin. PostgreSQL provides several replication plugins; `’pgoutput’` is commonly used for logical replication.
5. Retrieve Replication Slot Details: After successfully creating the replication slot, PostgreSQL will return a result set containing the slot’s details, including its `slot_name` and `consistent_point`.
6. Configure Standby Server: Edit the `recovery.conf` file on the standby server to configure it to connect and replicate from the primary server. This file might be named `recovery.conf` or `postgresql.auto.conf` depending on your PostgreSQL version.
“`
primary_conninfo = ‘host=<primary_host> port=<primary_port> user=<replication_user> password=<password>’
primary_slot_name = ‘<slot_name>’
restore_command = ‘pg_waldump %f %p’
standby_mode = ‘on’
“`
Replace `<primary_host>`, `<primary_port>`, `<replication_user>`, `<password>`, and `<slot_name>` with appropriate values.
7. Start Standby Server: Start the PostgreSQL service on the standby server. It will connect to the primary server, use the specified replication slot, and begin replicating changes.
8. Monitor Replication: You can monitor the replication progress by checking the logs on both the primary and standby servers. Additionally, you can use PostgreSQL’s system views to get insights into the replication status.
Remember to ensure proper network connectivity and security settings between the primary and standby servers for successful replication.
