Feat/streaming

[FredrikOseberg]

Summary

This PR introduces experimental streaming mode support to the
Unleash Go SDK, enabling real-time feature flag updates through
Server-Sent Events (SSE) instead of traditional polling. The
implementation provides instant updates when feature flags change
on the server.

Key Features

• Real-time updates: SSE-based streaming eliminates polling delay
  for instant feature flag changes
• Automatic fallback: Seamlessly falls back to polling mode if
  streaming is unavailable
• Delta processing: Efficient processing of incremental changes
  using hydration and delta events
• Backward compatibility: Existing polling-based clients continue
  to work without changes
• Robust error handling: Proper reconnection logic and error
  recovery

Architecture Changes

New Components

• Streaming Client (streaming_client.go): Handles SSE connections
  and event processing
• Streaming Processor (streaming_processor.go): Processes delta
  events and manages feature state
• Delta API (api/delta.go): Defines event types for streaming
  updates including:
  • HydrationEvent: Full state initialization on connection
  • FeatureUpdatedEvent: Individual feature changes
  • FeatureRemovedEvent: Feature deletions
  • SegmentUpdatedEvent: Segment modifications
  • SegmentRemovedEvent: Segment deletions

Key Implementation Details

• Uses LaunchDarkly's eventsource library for reliable SSE
  connections
• Implements proper delta processing with hydration for initial
  state
• Maintains separate streaming and polling code paths for clean
  separation
• Clean architecture: Streaming client uses only delta
  processing, eliminating redundant full-response logic

Configuration

Enable streaming mode by setting the experimental streaming
configuration:

unleash.Initialize(
unleash.WithUrl("https://your-unleash-instance.com/api/"),
unleash.WithAppName("my-app"),
unleash.WithExperimental(map[string]interface{}{
"type": "streaming",
}),
)

Testing

• Comprehensive unit tests for streaming components
• Integration tests with mock SSE server
• Delta processing tests covering all event types
• Backward compatibility tests ensuring polling mode continues to
  work

Files Changed

• Core streaming implementation: 6 new files (~1,200 lines)
• Delta API support: New event types and parsing logic
• Configuration updates: Experimental streaming mode support
• Documentation: Updated README with streaming mode usage
• Dependencies: Added github.com/launchdarkly/eventsource for SSE
  support

[FredrikOseberg]

listen to update event

[kwasniew]

leftover

[kwasniew]

weird formatting

[kwasniew]

this is weird that repository created a streaming client and passes itself as argument

[kwasniew]

can it depend on some inverted dependency being the operation that will be called. because now we have a cyclical dependency where repository creates delta processor and processor calls repository creating a cycle.

[kwasniew]

some claude analysis told me it may lead to race conditions with isEnabled calls but haven't dug deeper