Overview

DuckDB-Wasm's (dynamic) extension loading is modeled after regular DuckDB's extension loading, with a few relevant differences due to the difference in platform. A growing subset of core, community, and external extensions is supported.

This page explains the extension format, how INSTALL and LOAD behave, autoloading, which extensions are available, signing, and how extensions are fetched and served.

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, this custom section is required to be the last one, but this requirement may be 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 performs no durable installation, because there is no cross-session storage; it only records where an extension will be fetched from (for example, with INSTALL ⟨name⟩ FROM community), deferring the actual fetch to LOAD. 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.

Available Extensions

A growing subset of extensions is supported for DuckDB-Wasm, spanning core extensions, community extensions, and external extensions. Most are fetched on the fly when they are autoloaded or explicitly loaded, rather than being bundled into the DuckDB-Wasm binary. For example:

-- Explicitly load a core extension
LOAD icu;

-- Load spatial for geospatial support
INSTALL spatial;
LOAD spatial;

-- Install a community extension, then load it
INSTALL h3 FROM community;
LOAD h3;

-- Install an external extension from a repository
INSTALL sqlite_scanner FROM 'https://extensions.duckdb.org';
LOAD sqlite_scanner;

The core extensions commonly used with DuckDB-Wasm include:

WebAssembly is essentially an additional platform, and there may be platform-specific limitations that prevent some extensions from matching their native capabilities or that make them behave differently. Relevant differences for DuckDB-hosted extensions are documented below.

HTTPFS

The native HTTPFS extension is not compiled into DuckDB-Wasm, because HTTPS capabilities have to go through an additional layer, the browser, which adds both differences and some restrictions relative to native. Instead, DuckDB-Wasm ships a separate JavaScript implementation of the same functionality. Running LOAD httpfs; opts into this re-implemented HTTP stack, which is interchangeable with the native extension for most purposes but does not support all use cases, as it must follow the security rules imposed by the browser.

In particular:

• Requests are always upgraded to HTTPS.
• Because of the browser's CORS policy, any request for data must target a site that allows (using CORS headers) the site hosting the DuckDB-Wasm instance to access that data.

The MDN website is a great resource for more information regarding CORS. If a query over a remote file fails with a network error, see Troubleshoot.

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 --compressed 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 for the extensions must be CORS enabled for a browser to allow the connection; see Troubleshoot if a load fails.

Tooling

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

Further Reading

• Deploy — serving and mirroring extensions from a custom endpoint.
• Core Extensions — the core extensions available across all DuckDB clients.
• Community Extensions — third-party extensions installable with INSTALL … FROM community.
• Import Data — using the Parquet, JSON, and Wasm-flavored httpfs extensions to read files.
• Troubleshoot — CORS errors when loading extensions from a custom repository.