This project builds LevelDb as a Wasm library to give sites direct access to a performant device-local key/value store.

Emscripten is used to compile C++ to Web Assembly.

The data is stored via the site's Origin Private File System.

Status

The project should be considered a POC or example of how to build a Wasm library.

• A limited set of LevelDb operations are thus far provided (e.g. no WriteOptions).
• Few performance optimizations have been pursued.
• Test coverage is minimal.

Building

Set up CMake

$ sudo apt install cmake

Get and build the code

$ git clone --recurse-submodules git@github.com:a-sully/leveldb.git
$ cd leveldb

Set up the Emscripten SDK

$ cd third_party/emsdk
$ ./emsdk install latest
$ ./emsdk activate latest
$ source ./emsdk_env.sh

Set up a CMake build directory

$ cd ../..
$ mkdir -p build && cd build
$ emcmake cmake -DCMAKE_BUILD_TYPE=Release ..

Build

$ cmake --build . --target leveldbwasmdemo

Tip: set CMAKE_BUILD_PARALLEL_LEVEL or use -jN to build faster.

Run

Setting up a local web server

Using threads in WASM requires SharedArrayBuffer, which is only enabled when a page is cross-origin isolated, which requires adding the following headers to all HTTP responses:

Cross-Origin-Embedder-Policy: require-corp
Cross-Origin-Opener-Policy: same-origin

The usual local development servers such as the one built into Python don’t send these headers. One built with Rust however, called sfz provides a --coi option to enable the necessary headers.

Demo

Build the leveldbwasmdemo target and direct your browser to demo.html on your local server, e.g. http://localhost:5000/demo.html.

Benchmark

Build the leveldbwasmbench target and direct your browser to benchmark.html on your local server, e.g. http://localhost:5000/benchmark.html.

This compares the speed of LevelDB via Wasm to IndexedDB (wrapped with IDB-Keyval) for some simple key/value store operations, mainly put, get and delete. DevTools should be able to show additional metrics such as local storage space used (in Chromium this is found under the Application tab).

Preliminary results indicate a 25-35% speed-up for fetching data, with similar write speed and storage footprint.

Develop

The public-facing API is published in ./wasm/js-api/. Some code for interfacing between ES and C++ resides in ./wasm/idl/. For usage examples, refer to the demo and benchmark apps

Authors

• Reilly Grant (reillyg@google.com)
• Nathan Memmot (memmott@google.com)
• Evan Stade (estade@google.com)
• Austin Sullivan (asully@google.com)

References

• LevelDb
• Emscripten
• WebIDL Binder
• WasmFS
• C/C++ debugging extension
• [https://developer.chrome.com/articles/origin-private-file-system/](Origin Private File System)
• OPFS explorer extension
• sfz
• IDB-Keyval
• Previous porting effort