PostgreSQL is an open-source object-relational database management system that is widely used in the industry. It provides various replication mechanisms to ensure high availability and data consistency across multiple nodes. One such mechanism is logical replication slots, which was introduced in PostgreSQL 9.4.
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Logical replication slots are a feature that helps to keep track of data changes and ensures that the changes are replicated to the subscriber nodes without any data loss or corruption. In this article, we will explore what logical replication slots are, how they work, and their benefits.
What Are Replication Slots
Before we dive into logical replication slots, let’s first understand what replication slots are. Replication slots are objects that represent a physical point in the WAL (Write-Ahead Log) stream of the primary node.
They help to keep track of the last transaction that was sent to the replica node(s). When a replica connects to the primary node, it requests a slot and starts streaming WAL from that point onwards.
Replication slots can be either physical or logical. Physical replication slots keep track of all changes made to the database, while logical replication slots only track changes made to specific tables or databases.
What Are Logical Replication Slots
Logical replication slots are a type of replication slot introduced in PostgreSQL 9. They allow for selective replication of data changes based on specific criteria such as tables or rows within tables. These criteria are defined by using publications and subscriptions.
Publications define what data should be sent from the primary node to the subscriber nodes. They can include one or more tables along with any constraints or filters needed for selective replication. Subscriptions define which nodes should receive this data and how it should be consumed.
How Do Logical Replication Slots Work
When a publication is created, a corresponding logical slot is automatically created on the primary node. This slot is used to keep track of the last transaction that was sent to the subscribers for that publication. Subscribers can then connect to the primary node and request a replication stream for that publication.
When a subscriber connects, it requests a logical slot from the primary node. The primary node then streams WAL records to the subscriber based on the criteria defined in the publication. As changes are made to the selected tables on the primary node, they are streamed to all subscribers subscribed to that publication.
Benefits of Using Logical Replication Slots
Logical replication slots offer several benefits over physical replication slots, including:
- Selective Replication: Logical replication slots allow for selective replication of data changes based on specific criteria such as tables or rows within tables. This helps reduce network traffic and disk usage by only replicating relevant data.
- High Availability: Logical replication slots help ensure high availability by providing a way to keep track of data changes and replicate them to subscriber nodes without any data loss or corruption.
- Data Consistency: Logical replication slots help maintain data consistency across multiple nodes by ensuring that all nodes have access to the same set of data changes.
In conclusion, logical replication slots are an important feature in PostgreSQL that helps ensure high availability and data consistency across multiple nodes. They allow for selective replication of data changes based on specific criteria such as tables or rows within tables, making it easier to manage large datasets. If you’re working with PostgreSQL, consider using logical replication slots as part of your replication strategy.
