feat(flags): Add ETag support for local evaluation caching #91
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Add HTTP ETag caching for feature flag definitions to reduce bandwidth when polling for flag updates. When flags haven't changed, the server returns 304 Not Modified and cached flags are preserved.
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Pull request overview
This PR implements HTTP ETag-based caching for feature flag definitions in the PostHog PHP library, reducing bandwidth consumption during flag polling. When flags haven't changed, the server returns a 304 Not Modified response, allowing the client to reuse cached flag data instead of re-transmitting the full payload.
Key Changes
- Added ETag extraction and storage from HTTP responses with If-None-Match header support for conditional requests
- Enhanced HttpResponse class to include ETag metadata and 304 status detection
- Updated MockedHttpClient test infrastructure to support ETag scenarios and response queuing
Reviewed changes
Copilot reviewed 11 out of 11 changed files in this pull request and generated no comments.
Show a summary per file
| File | Description |
|---|---|
| lib/HttpResponse.php | Added ETag property, getter method, and isNotModified() helper for 304 detection |
| lib/HttpClient.php | Implemented ETag extraction from response headers, 304 status handling, and token masking utility |
| lib/Client.php | Added ETag caching state, If-None-Match header injection, and 304 response handling in loadFlags() |
| lib/PostHog.php | Added getClient() method to expose Client instance for accessing ETag functionality |
| test/EtagSupportTest.php | Comprehensive test suite covering ETag storage, If-None-Match headers, 304 handling, and edge cases |
| test/HttpClientTest.php | New tests for token masking functionality in URLs |
| test/MockedHttpClient.php | Enhanced mock to support ETag responses and sequential response queuing for testing |
| test/PostHogTest.php | Updated test expectations to include includeEtag request option |
| test/FeatureFlagTest.php | Updated test expectations to include includeEtag request option |
| test/FeatureFlagLocalEvaluationTest.php | Updated test expectations to include includeEtag request option |
| example.php | Added interactive ETag polling example demonstrating bandwidth optimization |
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dustinbyrne
approved these changes
Dec 4, 2025
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Add HTTP ETag caching for feature flag definitions to reduce bandwidth when polling for flag updates. When flags haven't changed, the server returns 304 Not Modified and cached flags are preserved.
Based on the work we did for PostHog/posthog-js#2678