SQLite WASM: Troubleshooting Multiple Database Attachments Without OPFS

Issue Overview: SQLite WASM Fails to Attach Non-Main Databases

When working with SQLite in a WebAssembly (WASM) environment, particularly without the Origin Private File System (OPFS), users often encounter issues when attempting to attach or create databases that are not named main. The core problem manifests in two distinct ways:

1. Database Creation and Deserialization Failures: When creating or deserializing a database that is not named main, the sqlite3_deserialize() function fails, resulting in an error code SQLITE_ERROR. This occurs even when the database file is correctly loaded into memory and the deserialization parameters appear to be valid. The error message not an error is misleading and does not provide actionable insights.

2. Data Missing or Incorrect Schema References: Even if the deserialization step is bypassed or ignored, queries against the non-main database fail to return expected results. For example, querying sqlite_master for tables returns an empty result set, or attempting to reference the database by its name (e.g., foobar.sqlite_master) results in a no such table error. This indicates that the database schema is not being correctly recognized or attached.

Additionally, attempts to attach an existing database file to a main database connection result in an empty database being created, rather than the expected attachment of the existing data. This behavior persists regardless of the Virtual File System (VFS) used, whether it is unix-none or memdb.

Possible Causes: Misaligned Expectations and WASM Limitations

The root causes of these issues stem from a combination of misaligned expectations regarding SQLite’s WASM implementation and inherent limitations of the environment. Below are the key factors contributing to the problem:

1. Schema Name Mismatch in Deserialization: The sqlite3_deserialize() function requires the schema name to match an existing schema in the database connection. In the provided example, the schema name foobar does not exist in the mainDatabase connection, leading to a failure. The function explicitly disconnects from the specified schema before attempting to deserialize, which means the schema must already be present. This behavior is consistent with the SQLite C API but is not immediately intuitive in the WASM context.

2. WASM Environment Constraints: SQLite’s WASM implementation is primarily designed for browser environments, where file system access is limited or non-existent. Without OPFS, the WASM runtime relies on in-memory file systems or custom VFS implementations. These environments may not fully support traditional file-based operations, such as attaching external database files. The unix-none VFS, for example, does not provide persistent storage, which complicates the attachment of existing databases.

3. Incorrect Use of sqlite3_deserialize(): The sqlite3_deserialize() function is intended to load a serialized database into an existing schema. However, the example code attempts to use it to create a new schema (foobar), which is not supported. This misuse leads to the observed errors and data inconsistencies.

4. Lack of Node.js Support: The SQLite WASM implementation is optimized for browser use and does not officially support Node.js. While the provided example uses Node.js for testing, the behavior of SQLite WASM in this environment may differ from its behavior in a browser. This discrepancy can lead to unexpected issues, such as the inability to attach databases or incorrect VFS behavior.

5. VFS Configuration and Limitations: The choice of VFS (unix-none vs. memdb) affects how databases are handled. The memdb VFS is designed for in-memory databases and may not support file-based operations like attaching external databases. Conversely, the unix-none VFS lacks persistent storage, making it unsuitable for scenarios requiring file system access.

Troubleshooting Steps, Solutions & Fixes: Resolving Database Attachment Issues

To address the issues outlined above, follow these detailed troubleshooting steps and implement the recommended solutions:

1. Correct Schema Name Usage in Deserialization

The primary issue with sqlite3_deserialize() is the mismatch between the schema name and the existing database connection. To resolve this:

• Ensure Schema Exists Before Deserialization: Before calling sqlite3_deserialize(), verify that the schema name matches an existing schema in the database connection. For example, if the database is named foobar, ensure that the schema foobar is already present.

• Use main for Deserialization: If the goal is to load a database into memory, use the main schema for deserialization. This avoids the need to create or attach additional schemas and ensures compatibility with the sqlite3_deserialize() function.

Example:

const mainRc = sqlite3.capi.sqlite3_deserialize(
 mainDatabase.pointer!,
 "main", // Use "main" instead of "foobar"
 mainMemory,
 main.byteLength,
 main.byteLength,
 sqlite3.capi.SQLITE_DESERIALIZE_FREEONCLOSE |
  sqlite3.capi.SQLITE_DESERIALIZE_RESIZEABLE,
);

2. Leverage Browser-Specific Features

Since SQLite WASM is optimized for browser environments, consider using browser-specific features to achieve the desired functionality:

• Use OPFS for Persistent Storage: If persistent storage is required, enable OPFS in the browser environment. This allows SQLite to access a private file system, making it possible to attach and manage multiple databases.

• Avoid Node.js for Testing: Test the SQLite WASM implementation in a browser environment to ensure compatibility and avoid Node.js-specific issues. Use tools like Webpack or Vite to bundle and run the code in a browser.

3. Implement Custom VFS for File-Based Operations

If OPFS is not available and file-based operations are necessary, consider implementing a custom VFS:

• Create a Custom VFS: Develop a custom VFS that supports the required file system operations, such as reading and writing database files. This VFS can be tailored to the specific needs of the application.

• Integrate with Existing Libraries: Use libraries like sql.js or emscripten to integrate custom VFS implementations with SQLite WASM. These libraries provide additional functionality for managing file systems in a WASM environment.

4. Use Alternative Approaches for Database Attachment

If attaching databases is a critical requirement, explore alternative approaches:

• Serialize and Deserialize Data Manually: Instead of attaching databases, serialize the data from the source database and deserialize it into the target database. This approach avoids the limitations of the ATTACH DATABASE command in a WASM environment.

• Use In-Memory Databases: If persistent storage is not required, use in-memory databases for all operations. This simplifies the management of multiple databases and avoids issues related to file system access.

5. Report Issues and Contribute to the Community

If the issues persist, consider reporting them to the SQLite community or contributing to the development of the WASM implementation:

• Submit Bug Reports: File detailed bug reports with the SQLite team, including reproducible examples and environment details. This helps the team identify and address issues more effectively.

• Contribute Code: If you have the expertise, contribute code or patches to improve the SQLite WASM implementation. This could include enhancements to the VFS, deserialization logic, or support for additional environments.

By following these steps and implementing the recommended solutions, you can overcome the challenges of attaching multiple databases in SQLite WASM without OPFS. The key is to align your approach with the limitations and capabilities of the WASM environment while leveraging available tools and features to achieve your goals.