Скачать или смотреть How to Manage Transaction-wise Global Variables in PostgreSQL

Discover how to effectively handle transaction-wise variables in PostgreSQL for smoother function interactions. Learn about shared memory and synchronization techniques.
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How to Manage Transaction-wise Global Variables in PostgreSQL

Managing global variables in PostgreSQL can be quite challenging due to its architecture of multiprocess handling. This guide will dive into a common scenario where you want to ensure that certain transactions can communicate through shared data. Specifically, we will address the question of whether it's feasible to create a transaction-wise global variable in PostgreSQL.

The Problem at Hand

Imagine you have two functions:

Function F1: This function writes data into a buffer, which is subsequently saved to external files when certain conditions are met, specifically when its execution context is released.

Function F2: This function depends on the data that F1 writes to the external files.

The crux of the issue lies in the fact that F2 needs to confirm whether the buffer from F1 is empty or not before proceeding. If there’s data in F1's buffer, F2 should write it out manually; otherwise, it can proceed without doing so. The challenge arises since both functions operate independently, yet F2 relies on the state of F1's buffer in the same transaction.

Solution Overview

Unfortunately, due to PostgreSQL's design, one backend process cannot see the global variables of another. However, there are two primary methods you can use to manage shared data between transactions:

Using Shared Memory Segments

Utilizing the LISTEN / NOTIFY Functionality

Let's break down these solutions.

1. Using Shared Memory Segments

You can create a shared memory segment to store transaction-wise data that multiple backend processes can access. This approach involves a few steps:

Create a _PG_init function: This function initializes your shared memory segment within your PostgreSQL extension.

Use Existing Extensions as a Reference: Looking at examples like pg_stat_statements, which utilizes shared memory, can provide insights on how to implement this.

Adjust Your Configuration:

Ensure your library is included in the shared_preload_libraries configuration of the PostgreSQL server. This step is crucial for making your extension available across different backend processes.

Although this method provides a clear means of data sharing, it does require a deeper understanding of PostgreSQL internals and memory management.

2. Utilizing the LISTEN / NOTIFY Functionality

A simpler and more straightforward alternative is to use PostgreSQL's built-in LISTEN and NOTIFY functionalities. This allows different backend processes to communicate with each other without the need for complex memory management.

How It Works:

LISTEN: A process can listen for notifications on a specific channel.

NOTIFY: A process can send a notification on that channel, which can alert any listeners that a change or important event has occurred.

Implementation Steps:

Modify Function F1 to send a NOTIFY command after writing to the buffer.

In Function F2, use LISTEN to wait for notifications from F1 before proceeding with its logic.

This method provides a more elegant and efficient means of ensuring F2 can check the status of F1's buffer without resorting to shared memory.

Conclusion

While managing transaction-wise global variables in PostgreSQL presents its challenges, understanding the architecture's limitations opens pathways to viable solutions. Utilizing either shared memory segments or the LISTEN/NOTIFY system can help synchronize actions between functions while allowing them to remain independent.

Whether you choose the more complex path of shared memory or the straightforward approach with notification systems, the key is to tailor your solution to fit the needs of your application effectively. With these strat