[RUM-15525] Add sessionSampleRate configuration option

[cdn34dd]

Motivation

Adds support for client-side session sampling in the Electron SDK. Without this, every session is always collected. Implements sessionSampleRate (0–100) analogous to the other SDKs.

Tracked in RUM-15525.

Changes

New sessionSampleRate configuration option (src/config.ts)

• Optional in InitConfiguration, required (defaults to 100) in the resolved Configuration.
• Validated the same way as telemetrySampleRate — invalid values log an error and the validator returns undefined, which aborts init() (no configuration is built, init() returns false).

Deterministic sampler (src/tools/Sampler.ts)

• isSessionSampled uses the Knuth-factor consistent-sampling method, aligned with the Datadog cross-language spec (and the browser-core implementation): (uuid_node * knuthFactor) % 2^64 compared against the rate, using BigInt to avoid 64-bit overflow.
• Same session ID always produces the same decision — intentional, to enable future replay correlation when sessionReplaySampleRate is added.
• Fractional rates (e.g. 0.1) are supported.
• If the session ID doesn't match the expected UUID format, the session is dropped (not sampled).
• Also adds correctedChildSampleRate(parentRate, childRate) ported from browser-core, in preparation for the upcoming Session Replay PR. It is currently unused (no caller yet) but lands here since the SR PR follows immediately.

Session sampling propagation (src/domain/session/SessionManager.ts)

• isSampled is computed once at session creation via isSessionSampled(id, sessionSampleRate).
• A sampled session is recorded in the session history; a non-sampled session is simply never added. No on-disk format change is required, so existing session-history files remain fully compatible — no migration needed.

Hook behavior for non-sampled sessions (src/domain/session/SessionContext.ts)

• Because a non-sampled session is absent from the history, all three hooks miss on it: RUM and span events return DISCARDED, and telemetry returns SKIPPED (telemetry still sends, without session attribution). SessionContext also exposes getActiveSessionId() (see below); remaining changes are a cosmetic local rename.

Internal context (src/index.ts, src/domain/session/SessionManager.ts, src/domain/session/SessionContext.ts)

• getInternalContext() now returns undefined for any session that isn't currently tracked — including non-sampled (and expired) sessions — instead of always returning the active session's id. This prevents leaking a session_id (used for cross-product correlation) for a session that produces no RUM.
• Backed by a single source of truth: SessionContext.getActiveSessionId() resolves the session from the same history the RUM/span/telemetry hooks consult, so internal context stays consistent with what those hooks attribute (a non-sampled session is absent; an expired one is closed).

Test instructions

Checklist

• Tested locally (playground)
• Added unit tests for this change.
• Added e2e/integration tests for this change.
• Updated related documentation.

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[Copilot]

Pull request overview

Adds client-side session sampling to the Electron SDK via a new sessionSampleRate config option and deterministic sampling based on the session UUID, with propagation into session lifecycle handling and unit tests.

Changes:

• Added sessionSampleRate to configuration and validation logic.
• Introduced a deterministic sampler utility (isSessionSampled) and unit tests for sampling behavior.
• Wired session sampling into session creation so non-sampled sessions are discarded from RUM attribution.

Architectural Flow

buildConfiguration() resolves sessionSampleRate → init() passes resolved config into SessionManager.start() → SessionManager.createNewSession() generates a UUID and makes a deterministic sampling decision → session history (SessionContext/DiskValueHistory) is updated to drive hook enrichment (RUM/telemetry/spans) based on history lookups.

Score

2.6 / 5.0

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 9 comments.

Show a summary per file

File	Description
src/tools/Sampler.ts	Adds deterministic session sampling + helper rate math.
src/tools/Sampler.spec.ts	Adds unit tests for sampler determinism and boundary conditions.
src/tools/DiskValueHistory.ts	Minor typing/cleanup in disk history restore logic.
src/mocks.specUtil.ts	Updates test configuration helper with sessionSampleRate.
src/index.ts	Passes resolved configuration into SessionManager.start().
src/domain/session/SessionManager.ts	Computes sampling decision on session creation and gates session history updates.
src/domain/session/SessionManager.spec.ts	Updates SessionManager tests and adds sampling-related scenarios.
src/domain/session/SessionContext.ts	Minor refactor of hook code (variable naming).
src/config.ts	Adds sessionSampleRate and updates sample-rate validation/build logic.
src/config.spec.ts	Adds config validation tests for sessionSampleRate and updates telemetry sample-rate invalid cases.

[bcaudan]

It could be nice to document the new option in the README.md

[Copilot]

Pull request overview

Copilot reviewed 11 out of 11 changed files in this pull request and generated 3 comments.