How to Implement Logical Replication In PostgreSQL?

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Logical replication in PostgreSQL allows you to replicate selected tables or parts of tables from one PostgreSQL database to another. It provides a flexible and powerful way to create replication setups for various use cases.

To implement logical replication in PostgreSQL, you need to follow these steps:

  1. Set up the publisher database: The database from which the data will be replicated is called the publisher. You need to enable logical replication on the publisher database by configuring the wal_level parameter in the PostgreSQL configuration file to logical. Restart the PostgreSQL server for the changes to take effect.
  2. Create a publication: A publication defines which tables or parts of tables will be replicated. Use the CREATE PUBLICATION command to create a publication and specify the tables or parts of tables using the FOR TABLE clause.
  3. Set up the subscriber database: The database that will receive the replicated data is called the subscriber. You need to enable logical replication on the subscriber database by configuring the wal_level parameter to logical and max_replication_slots to a value that defines the maximum number of replication slots. Restart the PostgreSQL server.
  4. Create a replication slot: A replication slot is a connection between the publisher and subscriber databases. Use the pg_create_logical_replication_slot function to create a replication slot on the publisher database. This function returns the slot name and the initial LSN (Log Sequence Number) position.
  5. Create a subscription: A subscription defines the connection between the publisher and subscriber databases. Use the CREATE SUBSCRIPTION command on the subscriber database to create a subscription. Specify the publisher's connection details, the publication name, and the replication slot name.
  6. Monitor replication: You can check the status of replication by querying the pg_stat_replication view on the publisher and subscriber databases. This view provides information about the replication connection, lag, and other useful details.
  7. Handling conflict resolution: In case of conflicts between updates on the publisher and subscriber databases, you can define conflict resolution rules. PostgreSQL provides options like DO NOTHING, PRESERVE UPDATE, or user-defined conflict resolution functions, allowing you to customize the behavior.

By following these steps, you can successfully implement logical replication in PostgreSQL. It enables you to replicate data efficiently and selectively between databases, providing flexibility and control over your replication setup.

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How to handle data consistency in logical replication?

To handle data consistency in logical replication, you can follow these steps:

  1. Ensure consistency at the source database: Before starting the replication process, make sure the source database is in a consistent state. This can be achieved by performing data integrity checks, resolving any conflicts or inconsistencies, and ensuring that no incomplete transactions are present.
  2. Synchronize data before replication: Before setting up logical replication, synchronize the data between the source and target databases. This can be done by taking a base backup of the source database and restoring it on the target database.
  3. Monitor changes and capture updates: Use the logical replication mechanism to capture all the changes happening on the source database. This can be done using a suitable replication method, such as using logical decoding plugins or replication slots.
  4. Apply changes on the target database: Once the updates are captured, apply them to the target database in the same order as they were applied on the source database. This ensures that the data remains consistent between the source and target databases.
  5. Handle conflicts and data transformations: During the replication process, there may be conflicts or data transformations that need to be resolved. These can be handled by using replication filters, transformation scripts, or custom logic to ensure the data consistency is maintained.
  6. Monitor and verify replication status: Continuously monitor the replication process and verify that the data is being replicated correctly. This can be done by comparing the data between the source and target databases and checking for any discrepancies.
  7. Handle replication failures: If any replication failures occur, such as network issues or system failures, promptly investigate and resolve them to ensure data consistency is maintained. This may involve resynchronizing the databases or applying missing updates.
  8. Regularly test and validate replication: Periodically perform tests and validations to ensure the logical replication is working as expected. This can involve running tests on the target database, comparing it with the source database, and verifying that the data matches.

By following these steps, you can ensure data consistency in logical replication and maintain synchronization between the source and target databases.

How to install PostgreSQL on Linux?

To install PostgreSQL on Linux, you can follow these steps:

  1. Update the package index on your system: For Debian-based systems (Ubuntu, Mint, etc.), run: sudo apt update For Red Hat-based systems (CentOS, Fedora, etc.), run: sudo yum update
  2. Install PostgreSQL using the package manager: For Debian-based systems, run: sudo apt install postgresql For Red Hat-based systems, run: sudo yum install postgresql-server postgresql-contrib This command will install the PostgreSQL server and some additional modules.
  3. Start and enable the PostgreSQL service: For systemd-based systems, run: sudo systemctl start postgresql sudo systemctl enable postgresql For SysVinit-based systems, run: sudo service postgresql start sudo chkconfig postgresql on
  4. Verify the installation by connecting to the default PostgreSQL database: sudo -u postgres psql This will open the PostgreSQL command prompt.
  5. Set a password for the default PostgreSQL user (postgres): \password postgres Follow the prompt to set your desired password.
  6. Exit the PostgreSQL prompt: \q

PostgreSQL should now be successfully installed on your Linux system. You can start using it by connecting to databases using tools like psql or PgAdmin.

What are the limitations of logical replication in PostgreSQL?

There are several limitations of logical replication in PostgreSQL:

  1. Replication slots: Logical replication requires the use of replication slots, which means that a limited number of slots are available. Once all slots are occupied, new replication streams cannot be established until a slot becomes available.
  2. Transactional boundaries: Logical replication works on a per-transaction basis, meaning that changes made within the same transaction are replicated together. This can result in delays if a transaction is long-running or includes a large volume of changes.
  3. DDL replication: While logical replication can replicate most DML (Data Manipulation Language) statements, such as inserts, updates, and deletes, it does not replicate DDL (Data Definition Language) statements, such as table creations and alterations. This means that schema changes need to be manually implemented on the replica.
  4. Resource consumption: Logical replication can consume significant resources, especially when replicating a large number of changes or replicating to multiple replicas. This can result in higher CPU and disk usage on the replica database.
  5. Replication lag: Due to the inherent nature of logical replication, there can be a certain amount of replication lag between the primary and replica databases. This can impact real-time data availability and may require additional monitoring and management.
  6. Conflicts and data integrity: Logical replication does not handle conflicts automatically. If conflicting changes are made on both the primary and replica databases, manual intervention is required to resolve the conflicts and ensure data integrity.
  7. Replication setup complexity: Setting up logical replication requires additional configuration and setup compared to physical replication. It involves configuring replication slots, setting up publication and subscription, and managing replication identities.

It is important to consider these limitations when implementing logical replication in PostgreSQL and assess whether they align with the specific requirements and constraints of the application.

What is an upstream database in logical replication?

In logical replication, an upstream database refers to the source database from which changes or updates are replicated to one or more downstream databases. It is the database that initiates the replication process and sends the changes to be replicated to the downstream databases.

The upstream database is responsible for capturing the changes made to the data, packaging them into logical replication messages or events, and sending them to the downstream databases. It can also be considered as the "publisher" or "source" database in the replication process.

The downstream databases, also known as subscriber or replica databases, receive and apply the changes sent by the upstream database to keep their data in sync with the source database.

Overall, the upstream database is an essential component in logical replication, as it drives the replication process by broadcasting the changes made to the data to the downstream databases.

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