Client-side SDK local evaluation

[alexcardell]

Problem

Evaluation of flags and context is done server-side, via network request. This is not always appropriate for hot path code, adds latency, etc.

Running evaluations locally would be more performant, more easily testable.

Ideal Solution

• Allow clients to run all evaluation locally, this should be opt in to start
• Allow clients to operate on a locally cached flag context, and use that for evaluations -- this is to make side effects explicit
• Allow clients to receive flag context updates via streaming -- Offer Server Sent Events (SSE) as an alternative to polling for client SDKs #2122
• Preferably this would be an SDK specific solution rather than embedding binaries to do the evaluation part

Search

• I searched for other open and closed issues before opening this

Additional Context

The benefit of server side evaluation is always using the latest flag state. In case Flipt can't be reached, but a flag has been disabled, local evaluations are an interesting case, especially with caching

[markphelps]

@alexcardell yup we have something in the works. Im not sure it will solve all the 'ideal solution' bullet points out the gate, but will definitely make evaluations much much faster by occurring client side.

Will give you a ping once we have something that is releasable to try.

May I ask which language(s) would you want the clients for?

[markphelps]

FYI @alexcardell we just released a first pass at client side evaluation: https://www.flipt.io/blog/new-client-side-evaluation

Next week I'm going to focus on packaging up the languages we support and pushing to the various registries, then will add additional languages.

Any language that you would like to see prioritized?

[markphelps]

Closing as we now have official Client Side Eval SDKs. If there are any languages you would like us to support that we dont already, please open an issue in the client SDK monorepo: https://github.com/flipt-io/flipt-client-sdks