DuckDB-Wasm's (dynamic) extension loading is modeled after the regular DuckDB's extension loading, with a few relevant differences due to the difference in platform.

Format

Extensions in DuckDB are binaries to be dynamically loaded via dlopen. A cryptographical signature is appended to the binary. An extension in DuckDB-Wasm is a regular Wasm file to be dynamically loaded via Emscripten's dlopen. A cryptographical signature is appended to the Wasm file as a WebAssembly custom section called duckdb_signature. This ensures the file remains a valid WebAssembly file.

Currently, we require this custom section to be the last one, but this can be potentially relaxed in the future.

INSTALL and LOAD

The INSTALL semantic in native embeddings of DuckDB is to fetch, decompress from gzip and store data in local disk. The LOAD semantic in native embeddings of DuckDB is to (optionally) perform signature checks and dynamic load the binary with the main DuckDB binary.

In DuckDB-Wasm, INSTALL is a no-op given there is no durable cross-session storage. The LOAD operation will fetch (and decompress on the fly), perform signature checks and dynamically load via the Emscripten implementation of dlopen.

Autoloading

Autoloading, i.e., the possibility for DuckDB to add extension functionality on-the-fly, is enabled by default in DuckDB-Wasm.

List of Officially Available Extensions

WebAssembly is basically an additional platform, and there might be platform-specific limitations that make some extensions not able to match their native capabilities or to perform them in a different way. We will document here relevant differences for DuckDB-hosted extensions.

HTTPFS

The HTTPFS extension is, at the moment, not available in DuckDB-Wasm. Https protocol capabilities need to go through an additional layer, the browser, which adds both differences and some restrictions to what is doable from native.

Instead, DuckDB-Wasm has a separate implementation that for most purposes is interchangeable, but does not support all use cases (as it must follow security rules imposed by the browser, such as CORS). Due to this CORS restriction, any requests for data made using the HTTPFS extension must be to websites that allow (using CORS headers) the website hosting the DuckDB-Wasm instance to access that data. The MDN website is a great resource for more information regarding CORS.

Extension Signing

As with regular DuckDB extensions, DuckDB-Wasm extensions are by default checked on LOAD to verify the signature and confirm the extension has not been tampered with. Extension signature verification can be disabled via a configuration option. Signing is a property of the binary itself, so copying a DuckDB extension (say to serve it from a different location) will still keep a valid signature (e.g., for local development).

Fetching DuckDB-Wasm Extensions

Official DuckDB extensions are served at extensions.duckdb.org, and this is also the default value for the default_extension_repository option. When installing extensions, a relevant URL will be built that will look like extensions.duckdb.org/$duckdb_version_hash/$duckdb_platform/$name.duckdb_extension.gz.

DuckDB-Wasm extension are fetched only on load, and the URL will look like: extensions.duckdb.org/duckdb-wasm/$duckdb_version_hash/$duckdb_platform/$name.duckdb_extension.wasm.

Note that an additional duckdb-wasm is added to the folder structure, and the file is served as a .wasm file.

DuckDB-Wasm extensions are served pre-compressed using Brotli compression. While fetched from a browser, extensions will be transparently uncompressed. If you want to fetch the duckdb-wasm extension manually, you can use curl --compress extensions.duckdb.org/<...>/icu.duckdb_extension.wasm.

Serving Extensions from a Third-Party Repository

As with regular DuckDB, if you use SET custom_extension_repository = 'https://some.url.com', subsequent loads will be attempted at https://some.url.com/duckdb-wasm/$duckdb_version_hash/$duckdb_platform/$name.duckdb_extension.wasm.

Note that GET requests on the extensions needs to be CORS enabled for a browser to allow the connection.

Tooling

Both DuckDB-Wasm and its extensions have been compiled using the latest packaged Emscripten toolchain.