Fixed-Interval Jitter - Source Monitor

Objective

Wire the fixed-interval scheduling path to use the existing jitter_percent configuration instead of scheduling at exact intervals. Currently, apply_adaptive_interval! line 31-33 sets next_fetch_at = Time.current + fixed_minutes.minutes with zero jitter, causing thundering herd effects when many sources share the same interval. REQ-FT-04.

Context

@ lib/source_monitor/fetching/feed_fetcher/adaptive_interval.rb -- line 31-33: fixed-interval path has no jitter; line 53-67: adjusted_interval_with_jitter and jitter_offset already exist for adaptive path
@ lib/source_monitor/configuration/fetching_settings.rb -- jitter_percent already configurable (default 0.1), REQ-FT-08 already satisfied
@ test/lib/source_monitor/fetching/feed_fetcher_adaptive_interval_test.rb -- existing tests use jitter: ->(_) { 0 } to zero out jitter for deterministic assertions

Tasks

06-02-T1: Wire fixed-interval path to use jitter

Files: lib/source_monitor/fetching/feed_fetcher/adaptive_interval.rb

In the else branch of apply_adaptive_interval! (fixed-interval path, line 31-33), change from:

ruby

fixed_minutes = [source.fetch_interval_minutes.to_i, 1].max
attributes[:next_fetch_at] = Time.current + fixed_minutes.minutes

to:

ruby

fixed_minutes = [source.fetch_interval_minutes.to_i, 1].max
fixed_seconds = fixed_minutes * 60.0
attributes[:next_fetch_at] = Time.current + adjusted_interval_with_jitter(fixed_seconds)

This reuses the existing adjusted_interval_with_jitter method which already calls jitter_offset (which respects jitter_proc injection and jitter_percent_value from config). No new methods needed.

Acceptance: The fixed-interval path calls adjusted_interval_with_jitter instead of adding exact minutes.

06-02-T2: Write tests for fixed-interval jitter

Files: test/lib/source_monitor/fetching/feed_fetcher_adaptive_interval_test.rb

Add tests: (1) "fixed-interval sources get jitter when jitter_percent is non-zero" -- create a source with adaptive_fetching_enabled=false and fetch_interval_minutes=60, fetch with default jitter (no jitter proc override), verify next_fetch_at is NOT exactly Time.current + 60.minutes (has some offset within ±10%). (2) "fixed-interval jitter respects jitter_proc override" -- use jitter: ->(interval) { interval * 0.05 }, verify next_fetch_at equals Time.current + (6060 + 6060*0.05) seconds. (3) "fixed-interval with zero jitter_percent has no jitter" -- configure config.fetching.jitter_percent = 0, verify next_fetch_at is exactly Time.current + 60.minutes.

Note: Existing tests that use jitter: ->(_) { 0 } will continue to pass unchanged since they explicitly zero out jitter.

Acceptance: All new and existing adaptive interval tests pass. bin/rails test test/lib/source_monitor/fetching/feed_fetcher_adaptive_interval_test.rb exits 0.

Verification

bash

bin/rails test test/lib/source_monitor/fetching/feed_fetcher_adaptive_interval_test.rb
bin/rubocop lib/source_monitor/fetching/feed_fetcher/adaptive_interval.rb

Success Criteria

Fixed-interval sources get ±10% jitter on next_fetch_at by default
Jitter uses existing config jitter_percent (default 0.1)
jitter_proc injection still overrides for test determinism
Zero jitter_percent produces exact interval (no randomness)
All existing adaptive interval tests pass unchanged
RuboCop zero offenses