Alternative Approaches to Liquibase Locking (Use with Caution)

2024-06-23

Liquibase uses a locking mechanism to prevent concurrent updates to a database schema. This means only one Liquibase instance can modify the database schema at a time. This avoids conflicts that could arise if multiple developers or servers tried to make changes simultaneously.

DATABASECHANGELOGLOCK Table

Liquibase manages these locks through a special table in the target database called DATABASECHANGELOGLOCK. This table has a single row and a LOCKED column. When a Liquibase update starts, it sets LOCKED to 1 to indicate the schema is being modified. Once the update finishes, Liquibase releases the lock by setting LOCKED back to 0.

Reasons for Liquibase Locks

There are a couple of reasons why you might encounter a Liquibase lock:

  1. Another Liquibase Instance is Running: If another Liquibase process is already updating the database schema, you'll encounter a lock error. This could be due to another developer running Liquibase on the same database, or an automated deployment process.

  2. Unexpected Termination: If a Liquibase update is interrupted (due to a crash, server restart, etc.), the lock might not be released properly. This can leave the database schema locked even though no update is actually happening.

If you encounter a Liquibase lock, here are a couple of ways to resolve it:

  • Wait for the Update to Finish: If another Liquibase instance holds the lock, the easiest solution might be to wait for it to complete its update.

  • Release the Lock Manually: If the lock seems stuck, you can manually release it using the liquibase release-locks command. This removes the lock from the DATABASECHANGELOGLOCK table. However, use this with caution as it could disrupt an ongoing update.




Checking for Lock Status (Shell Script):

This basic shell script uses the liquibase list-locks command to check if the database is locked:

#!/bin/bash

locked=$(liquibase --url jdbc:oracle:thin:@//localhost:1521/XEPDB1 list-locks)

if [[ $locked == *"locked: true"* ]]; then
  echo "Database is locked by Liquibase."
else
  echo "Database is not locked."
fi

Releasing Lock Manually (Java):

This Java code snippet demonstrates using the Liquibase API to release the lock:

import liquibase.Liquibase;
import liquibase.database.Database;
import liquibase.database.jvm.JdbcConnection;
import liquibase.exception.LiquibaseException;

public class ReleaseLiquibaseLock {

  public static void main(String[] args) throws LiquibaseException {
    String url = "jdbc:oracle:thin:@//localhost:1521/XEPDB1";
    String username = "your_username";
    String password = "your_password";

    Database database = new liquibase.database.oracle.OracleDatabase(((JdbcConnection) Liquibase.connect(url, username, password).getConnection()).getUnderlyingConnection());
    Liquibase liquibase = new Liquibase("changelog.xml", new XmlReader(), database);
    liquibase.releaseLocks();

    System.out.println("Liquibase lock released (if any).");
  }
}



External Coordination:

  • Deployment Script: If you have a controlled deployment process, you can implement logic within the deployment script to ensure only one instance of Liquibase runs at a time. This could involve using a distributed locking mechanism like Zookeeper or a semaphore within your deployment platform.

  • Manual Coordination: For smaller deployments, you can simply rely on manual coordination between developers to avoid running Liquibase updates concurrently. This can be error-prone and requires discipline, but might be suitable for low-traffic environments.

Database-level Locking:

Some databases offer built-in locking mechanisms that you could potentially leverage. However, this approach requires careful implementation and might not be portable across different database platforms. It's also important to ensure your database locks don't conflict with Liquibase's internal locking mechanism.

Liquibase No-ChangelogLock Extension (Last Resort):

Liquibase offers an extension called "no-changelog-lock" that disables the built-in locking entirely. This should be used with extreme caution as it removes the protection against concurrent updates and could lead to data corruption if multiple instances modify the schema simultaneously. This is generally not recommended for production environments.

Choosing the Right Method:

The best alternative method depends on your specific needs and risk tolerance. Here's a quick breakdown:

  • External Coordination: - Good for controlled deployments with low risk of accidental conflicts.
  • Database-level Locking: - Potentially complex to implement and might not be portable. Use with caution.
  • Liquibase No-ChangelogLock: - Only for very specific scenarios where the risk of concurrent updates is minimal. Not recommended for production.

database oracle liquibase


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