port ffe feature flagging sdk to php

[leoromanovsky]

Motivation

Add FFE support to dd-trace-php. PHP applications can evaluate feature flags delivered via Remote Config using the same datadog-ffe Rust engine used by Ruby and Python.

Changes

• Rust FFI layer (components-rs/ffe.rs): C-callable bridge to datadog-ffe::rules_based — config store, evaluate, result accessors
• C extension (ext/ddtrace.c): ffe_evaluate, ffe_has_config, ffe_config_changed, ffe_load_config internal functions that marshal PHP arrays to FfeAttribute structs
• Remote Config (components-rs/remote_config.rs): Register FfeFlags product + FfeFlagConfigurationRules capability; handle add/remove of FFE configs
• PHP Provider (src/DDTrace/FeatureFlags/Provider.php): Singleton that checks RC config state, calls native evaluate, parses JSON results, reports exposures
• Exposure pipeline: LRU dedup cache (65K entries) + batched writer to /evp_proxy/v2/api/v2/exposures (1000 event buffer cap)
• OpenFeature adapter (src/DDTrace/OpenFeature/DataDogProvider.php): Implements AbstractProvider for the open-feature/sdk composer package
• Build: Add datadog-ffe to RUST_FILES in Makefile for PECL packaging
• Tests: LRU cache unit tests, exposure cache unit tests, 220 evaluation correctness tests from JSON fixtures
• Config: DD_EXPERIMENTAL_FLAGGING_PROVIDER_ENABLED gating via X-macro in ext/configuration.h

Decisions

Evaluation in Rust, not PHP. All flag evaluation (UFC parsing, targeting rules, shard hashing, allocation resolution) happens in libdatadog's datadog-ffe crate via FFI. PHP only handles orchestration (config lifecycle, exposure dedup, HTTP transport). This matches Ruby and Python — no language re-implements evaluation logic.

Global config behind Mutex<FfeState>. The Rust FFE config is stored in a lazy_static global with a Mutex. PHP is single-threaded per process, so RwLock would be unnecessary complexity. The changed flag and config are bundled in one struct to avoid torn reads.

Reuses existing RC pipeline. FFE configs flow through the same sidecar → ddog_process_remote_configs() path as APM Tracing and Live Debugger. No new polling mechanism.

Structured attributes, not JSON blobs. The C extension converts PHP arrays into FfeAttribute structs (typed: string/number/bool) before calling Rust, avoiding JSON encode/decode overhead on the hot path.

Exposure dedup uses length-prefixed composite keys. Key = len(flag):flag:subject, value = len(variant):variant:allocation. Avoids collision from delimiters appearing in flag/subject strings.

ExposureWriter caps at 1000 events per request. Matches Ruby and Python. Flush via register_shutdown_function.

Cargo.lock: minimal additions only. Started from master's lockfile and ran cargo check — only 73 new crates added (datadog-ffe transitive deps), 2 unavoidable minor bumps (once_cell 1.20→1.21 required by pyo3, os_info 3.9→3.14 required by crashtracker). No edition2024 crates, no gratuitous upgrades.

Test Results

Unit tests (local, no extension): 243 tests, 75 assertions, 1 skipped
Unit tests (Docker with extension): 243 tests, 735 assertions — all pass

System tests (FEATURE_FLAGGING_AND_EXPERIMENTATION, apache-mod-8.0):

13 tests, 0 failures, 0 errors, 1 xfail
12 passed, 1 skipped (exposure cache cross-request dedup — PHP shared-nothing arch)

Cross-language dogfood parity (6 flags, 6 SDKs):

ALL MATCH: boolean, float, integer, JSON, string flags across Go, Java, Node, PHP, Python, Ruby

Companion PRs

• libdatadog: DataDog/libdatadog#1532
• system-tests: DataDog/system-tests#6243

LOC (excluding Cargo.lock + JSON fixtures)

Category	Lines
Rust FFI	323
C extension	126
PHP source	829
PHP tests	559
Build/wiring	8
Total	1,845

[datadog-datadog-prod-us1]

⚠️ Tests

✨ Fix all issues with Cursor

⚠️ Warnings

🧪 1029 Tests failed

    ❄️ Known flaky: Endpoints are sent() from com.datadog.appsec.php.integration.Laravel8xTests (Fix with Cursor)

❄️ Known flaky: Endpoints are sent() from com.datadog.appsec.php.integration.Symfony62Tests (Datadog) (Fix with Cursor)

Assertion failed: 

assert endpoints.size() == 6
       |         |      |
       |         1      false
       [com.datadog.appsec.php.TelemetryHelpers$Endpoint@4b8a0d03]

Assertion failed: 

assert endpoints.size() == 6
...

❄️ Known flaky: telemetry data is received() from com.datadog.appsec.php.integration.TelemetryTests (Datadog) (Fix with Cursor)

Assertion failed: 

assert wafInit.points[0][1] == 1.0
       |       |     |  |   |
       |       |     |  5.0 false
       |       |     [1770903346, 5.0]
       |       [[1770903346, 5.0]]
       com.datadog.appsec.php.TelemetryHelpers$Metric@396f2724

Assertion failed: 
...

View all

ℹ️ Info

❄️ No new flaky tests detected

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: 6bdc4f9 | Docs | Datadog PR Page | Was this helpful? Give us feedback!}

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 62.11%. Comparing base (b9986f9) to head (6bdc4f9).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3630      +/-   ##
==========================================
- Coverage   62.21%   62.11%   -0.11%     
==========================================
  Files         141      141              
  Lines       13387    13387              
  Branches     1753     1753              
==========================================
- Hits         8329     8315      -14     
- Misses       4260     4273      +13     
- Partials      798      799       +1     

see 3 files with indirect coverage changes

---

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[pr-commenter]

Benchmarks [ tracer ]

Benchmark execution time: 2026-02-12 14:24:20

Comparing candidate commit 6bdc4f9 in PR branch feature/ffe-feature-flagging with baseline commit b9986f9 in branch master.

Found 4 performance improvements and 40 performance regressions! Performance is the same for 147 metrics, 3 unstable metrics.

scenario:ComposerTelemetryBench/benchTelemetryParsing

• 🟥 mem_peak [+153.224KB; +153.224KB] or [+3.835%; +3.835%]
• 🟩 execution_time [-2.210µs; -1.190µs] or [-17.404%; -9.368%]

scenario:ContextPropagationBench/benchExtractHeaders128Bit

• 🟥 mem_peak [+153.224KB; +153.224KB] or [+3.834%; +3.834%]

scenario:ContextPropagationBench/benchExtractHeaders128Bit-opcache

• 🟩 execution_time [-819.063ns; -780.937ns] or [-49.540%; -47.234%]

scenario:ContextPropagationBench/benchExtractHeaders64Bit

• 🟥 mem_peak [+153.224KB; +153.224KB] or [+3.834%; +3.834%]

scenario:ContextPropagationBench/benchExtractTraceContext128Bit

• 🟥 mem_peak [+153.224KB; +153.224KB] or [+3.834%; +3.834%]

scenario:ContextPropagationBench/benchExtractTraceContext128Bit-opcache

• 🟩 execution_time [-898.754ns; -747.246ns] or [-33.573%; -27.914%]

scenario:ContextPropagationBench/benchExtractTraceContext64Bit

• 🟥 mem_peak [+153.224KB; +153.224KB] or [+3.834%; +3.834%]

scenario:ContextPropagationBench/benchInject128Bit

• 🟥 mem_peak [+153.224KB; +153.224KB] or [+3.834%; +3.834%]

scenario:ContextPropagationBench/benchInject64Bit

• 🟥 mem_peak [+153.224KB; +153.224KB] or [+3.834%; +3.834%]

scenario:EmptyFileBench/benchEmptyFileBaseline

• 🟥 mem_peak [+153.232KB; +153.232KB] or [+2.961%; +2.961%]

scenario:EmptyFileBench/benchEmptyFileDdprof

• 🟥 mem_peak [+153.127KB; +155.631KB] or [+2.955%; +3.003%]

scenario:EmptyFileBench/benchEmptyFileOverhead

• 🟥 mem_peak [+153.232KB; +153.232KB] or [+2.961%; +2.961%]

scenario:HookBench/benchHookOverheadInstallHookOnFunction

• 🟥 mem_peak [+153.224KB; +153.224KB] or [+3.834%; +3.834%]

scenario:HookBench/benchHookOverheadInstallHookOnMethod

• 🟥 mem_peak [+153.224KB; +153.224KB] or [+3.834%; +3.834%]

scenario:HookBench/benchHookOverheadTraceFunction

• 🟥 mem_peak [+153.310KB; +153.313KB] or [+3.451%; +3.451%]

scenario:HookBench/benchHookOverheadTraceMethod

• 🟥 mem_peak [+153.310KB; +153.315KB] or [+3.402%; +3.402%]

scenario:HookBench/benchWithoutHook

• 🟥 mem_peak [+153.224KB; +153.224KB] or [+3.835%; +3.835%]

scenario:LaravelBench/benchLaravelBaseline

• 🟥 mem_peak [+153.264KB; +153.264KB] or [+2.960%; +2.960%]

scenario:LaravelBench/benchLaravelDdprof

• 🟥 mem_peak [+152.601KB; +154.582KB] or [+2.944%; +2.982%]

scenario:LaravelBench/benchLaravelOverhead

• 🟥 mem_peak [+153.264KB; +153.264KB] or [+2.960%; +2.960%]

scenario:MessagePackSerializationBench/benchMessagePackSerialization

• 🟥 mem_peak [+153.312KB; +153.312KB] or [+3.604%; +3.604%]

scenario:PDOBench/benchPDOBaseline

• 🟥 mem_peak [+153.312KB; +153.312KB] or [+3.801%; +3.801%]

scenario:PHPRedisBench/benchRedisBaseline

• 🟥 mem_peak [+153.224KB; +153.224KB] or [+3.834%; +3.834%]

scenario:SamplingRuleMatchingBench/benchGlobMatching1

• 🟥 mem_peak [+153.224KB; +153.224KB] or [+3.835%; +3.835%]

scenario:SamplingRuleMatchingBench/benchGlobMatching2

• 🟥 mem_peak [+153.224KB; +153.224KB] or [+3.835%; +3.835%]

scenario:SamplingRuleMatchingBench/benchGlobMatching3

• 🟥 mem_peak [+153.224KB; +153.224KB] or [+3.835%; +3.835%]

scenario:SamplingRuleMatchingBench/benchGlobMatching4

• 🟥 mem_peak [+153.224KB; +153.224KB] or [+3.835%; +3.835%]

scenario:SamplingRuleMatchingBench/benchRegexMatching1

• 🟥 execution_time [+105.882ns; +145.118ns] or [+9.222%; +12.640%]
• 🟥 mem_peak [+153.224KB; +153.224KB] or [+3.834%; +3.834%]

scenario:SamplingRuleMatchingBench/benchRegexMatching2

• 🟥 execution_time [+92.399ns; +148.201ns] or [+7.994%; +12.822%]
• 🟥 mem_peak [+153.224KB; +153.224KB] or [+3.834%; +3.834%]

scenario:SamplingRuleMatchingBench/benchRegexMatching3

• 🟥 execution_time [+83.650ns; +141.550ns] or [+7.139%; +12.081%]
• 🟥 mem_peak [+153.224KB; +153.224KB] or [+3.834%; +3.834%]

scenario:SamplingRuleMatchingBench/benchRegexMatching4

• 🟥 execution_time [+124.256ns; +177.544ns] or [+10.873%; +15.536%]
• 🟥 mem_peak [+153.224KB; +153.224KB] or [+3.834%; +3.834%]

scenario:SpanBench/benchDatadogAPI

• 🟥 mem_peak [+153.224KB; +153.224KB] or [+3.835%; +3.835%]

scenario:SymfonyBench/benchSymfonyBaseline

• 🟥 mem_peak [+153.264KB; +153.264KB] or [+2.960%; +2.960%]

scenario:SymfonyBench/benchSymfonyDdprof

• 🟥 mem_peak [+153.081KB; +155.085KB] or [+2.953%; +2.992%]

scenario:SymfonyBench/benchSymfonyOverhead

• 🟥 mem_peak [+153.264KB; +153.264KB] or [+2.960%; +2.960%]

scenario:TraceAnnotationsBench/benchTraceAnnotationOverhead

• 🟥 mem_peak [+153.312KB; +153.312KB] or [+3.396%; +3.396%]

scenario:TraceFlushBench/benchFlushTrace

• 🟥 mem_peak [+153.312KB; +153.313KB] or [+3.760%; +3.760%]

scenario:TraceSerializationBench/benchSerializeTrace

• 🟥 mem_peak [+153.312KB; +153.312KB] or [+3.674%; +3.674%]
• 🟩 execution_time [-25.458µs; -13.942µs] or [-5.858%; -3.208%]