Persistent WAL/SHM Files in SQLite: Troubleshooting and Solutions

BySQLite.work Team Hours Last Updated on
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Understanding the Role of WAL and SHM Files in SQLite

SQLite’s Write-Ahead Logging (WAL) mode is a popular feature that enhances concurrency by allowing readers and writers to operate simultaneously without blocking each other. When a database is in WAL mode, two additional files are created: the -wal (Write-Ahead Log) file and the -shm (Shared Memory) file. The -wal file contains changes that have been committed but not yet written to the main database file, while the -shm file is used for coordinating access to the -wal file among multiple processes.

In a typical setup, these files are temporary and are automatically deleted when the last connection to the database is closed. However, this behavior can cause issues in scenarios where a read-only process needs to continue accessing the database after the writer process has closed its connection and deleted the -wal and -shm files. This leads to the core issue: How can we ensure that the -wal and -shm files persist even after the writer process has closed, allowing read-only processes to continue functioning?

Why WAL and SHM Files Are Deleted and When It Becomes Problematic

The deletion of -wal and -shm files is a default behavior in SQLite, designed to clean up temporary files and maintain a tidy filesystem. When the last connection to a database in WAL mode is closed, SQLite assumes that no further operations will be performed on the database, and it deletes the -wal and -shm files. This behavior is generally efficient and avoids leaving behind unnecessary files.

However, this default behavior becomes problematic in specific use cases. For example, consider a scenario where a writer process is responsible for updating the database, while multiple read-only processes are continuously querying the database. If the writer process closes its connection, the -wal and -shm files are deleted, causing the read-only processes to fail with errors such as sqlite3.OperationalError: unable to open database file. This is because the read-only processes rely on the -wal file to access the latest committed changes.

The issue is exacerbated when the read-only processes do not have write permissions to the directory containing the database files. In such cases, they cannot recreate the -wal and -shm files, leading to a complete breakdown of the read operations. This raises the question: Is there a way to make the -wal and -shm files persistent, ensuring that read-only processes can continue to function even after the writer process has closed its connection?

Solutions for Making WAL and SHM Files Persistent

1. Using the SQLITE_FCNTL_PERSIST_WAL File Control

SQLite provides a file control option called SQLITE_FCNTL_PERSIST_WAL that can be used to prevent the automatic deletion of the -wal and -shm files. When this option is enabled, the -wal file is not deleted when the last connection to the database is closed. Instead, it remains on disk, allowing read-only processes to continue accessing the database.

To enable this option, you need to invoke the sqlite3_file_control function with the SQLITE_FCNTL_PERSIST_WAL flag. Here is an example of how to do this in Python using the sqlite3 module:

import sqlite3

# Open the database connection
conn = sqlite3.connect('example.db')

# Enable persistent WAL mode
conn.execute('PRAGMA journal_mode=WAL')
conn.execute('PRAGMA wal_autocheckpoint=0')
conn.execute('PRAGMA locking_mode=EXCLUSIVE')
conn.execute('PRAGMA synchronous=NORMAL')
conn.execute('PRAGMA journal_size_limit=0')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(RESTART)')
conn.execute('PRAGMA wal_checkpoint(TRUNCATE)')
conn.execute('PRAGMA wal_checkpoint(PASSIVE)')
conn.execute('PRAGMA wal_checkpoint(FULL)')
conn.execute('PRAGMA wal_checkpoint(R

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