(feature) relayproxy -> implement OpenFeature ADR0008 "Server-Sent Events (SSE) for bulk evaluation changes"

[vi-verrone]

Requirements

Current behaviour
The relayproxy seems to not include the eventStreams field into the OFREPBulkEvaluationSuccess response model when using OFREP endpoint ofrep/v1/evaluate/flags.

Expected behaviour
The relayproxy should implement OpenFeature ADR0008 to let OFREP providers use the eventStreams field and eventually read for flag changes, without the needs of polling.

Proposals and Considerations
The relayproxy already expose am SSE endpoint /stream/v1/sse/flag/change that can be included in BulkEvaluationSuccess responses, for example:
{ "flags": [...], "eventStreams": [{ "type": "sse", "url": "${RESOLVED_SSE_HOST}/stream/v1/sse/flag/change", "inactivityDelaySec": ${RESOLVED_INACTIVITY_DELAY} } ] }

RESOLVED_SSE_HOST may be:

• resolved automatically from the incoming request, by using an internal context property in the payload like gofeatureflag.sseEndpoint or some request header like x-gofeatureflag-sse-endpoint
• resolved from a mapping configuration in the config file. For example a new sse.endpoint or eventStream.endpoint property.

The same may be applied to RESOLVED_INACTIVITY_DELAY.

NOTE: The feature may be initially considered opt-in through a flag in the config file like sse.ofrepEnabled or something else that fits better. This will let unchanged config files to make relayproxy to behave as-is.

Eventual work to be done

• Update the codebase of relayproxy
• Update documentation (configuration, usage, use cases, etc.)
  • Probably a dedicated section for AWS Lambda streaming responses + eventual use case when integrating with AWS API Gateway streaming responses

[thomaspoignant]

@vi-verrone thanks for opening this issue.
The SSE has been done to be compatible with OFREP ADR but until now we haven't returned the eventStreams field.

It should be straight forward to implement the change, but as you said we must check if it is a breaking change to add it directly or if we need an optin mode for that.

[vi-verrone]

@thomaspoignant if you agree I can draft a PR with the possible implementation, let me know.

[thomaspoignant]

Yes feel free to open a PR for this.

[github-actions]

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