As of [sept 2022, Fastly's Compute@Edge used Spidermonkey.wasm](https://news.ycombinator.com/item?id=32916019). > These are good questions! Here's some answers from the corner of the world I > know best as a Wasmtime contributor at Fastly: > > 1. Spidermonkey.wasm is the basis of Fastly's JavaScript on Compute@Edge > support. We have found it to be faster than QuickJS. The source code is here: > https://github.com/fastly/js-compute-runtime. > > 2. Fastly Compute@Edge is built on wasmtime. You can develop web services for > it in Rust, JS, and Go: https://developer.fastly.com/learning/compute/ > > 3. Fastly's multi-tenant platform is closed source, but our single-tenant > local development platform, which also uses wasmtime under the hood as well, > is open source: https://github.com/fastly/viceroy. It isn't a big leap to > make viceroy multi-tenant: Wasmtime provides everything you need, and all > Viceroy would have to do is dispatch on e.g. HTTP host header to the correct > tenant. Our multi-tenant platform is closed source because it is very > specialized for use on Fastly's edge, not because the multi-tenant aspect is > special. Fastly's https://github.com/fastly/spidermonkey-wasi-embedding