feat(core): Scale expression isolate pool to 0 after inactivity

[ivov]

https://linear.app/n8n/issue/CAT-2781/

• I have seen this code, I have run this code, and I take responsibility for this code.

[codecov]

Bundle Report

Changes will increase total bundle size by 42 bytes (0.0%) ⬆️. This is within the configured threshold ✅

Detailed changes

Bundle name	Size	Change
editor-ui-esm	45.75MB	42 bytes (0.0%) ⬆️

Affected Assets, Files, and Routes:

view changes for bundle: editor-ui-esm

Assets Changed:

Asset Name	Size Change	Total Size	Change (%)
assets/constants-*.js	42 bytes	3.14MB	0.0%

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[cubic-dev-ai]

No issues found across 9 files

Architecture diagram

sequenceDiagram
    participant CLI as CLI / BaseCommand
    participant Pool as IsolatePool
    participant Bridge as RuntimeBridge (V8 Isolate)
    participant Obs as Observability / Metrics

    Note over CLI,Obs: Engine Initialization
    CLI->>Pool: initialize()
    Pool->>Bridge: NEW: createBridge() x N
    Pool->>Obs: reportPoolSize(N)
    Pool->>Pool: NEW: startIdleTimer(idleTimeoutMs)

    Note over CLI,Obs: Normal Request Flow (Activity)
    CLI->>Pool: acquire()
    Pool->>Pool: CHANGED: startIdleTimer() (Reset)
    Pool-->>CLI: return bridge
    Pool->>Bridge: execute expression
    CLI->>Pool: release(bridge)
    Pool->>Bridge: dispose()
    Pool->>Bridge: replenish()

    Note over CLI,Obs: NEW: Scale-to-Zero Flow (Inactivity)
    Pool->>Pool: idleTimer expires
    Pool->>Pool: scaleToZero()
    Pool->>Pool: waitForReplenishment() (Safety check)
    
    alt isIdle is still true
        Pool->>Bridge: dispose() all warm bridges
        Pool->>Obs: NEW: counter("scaled_to_zero")
        Pool->>Obs: gauge("pool.size", 0)
    end

    Note over CLI,Obs: NEW: Wake-up Flow (Recovery)
    CLI->>Pool: acquire()
    
    alt wasIdle
        Pool->>Pool: isIdle = false
        Pool->>Obs: NEW: counter("scaled_up")
        Pool->>Pool: startIdleTimer()
        Pool->>Pool: replenish() (Scale back to N)
    end

    Pool-->>CLI: throw PoolExhaustedError (While warming)

    loop replenishment
        Pool->>Bridge: createBridge()
        Bridge-->>Pool: ready
        Pool->>Obs: gauge("pool.size", count++)
    end

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[codspeed-hq]

Merging this PR will degrade performance by 33.02%

❌ 1 regressed benchmark
✅ 31 untouched benchmarks
⏩ 20 skipped benchmarks¹

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
❌	vm micro: Script Compilation - cache hit (repeated expression)	357.7 µs	534 µs	-33.02%

---

_{Comparing cat-2781-scale-isolate-pool-down-to-0-after-inactivity (6788235) with master (39189c3)}

Footnotes

1. 20 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[github-actions]

Performance Comparison

Comparing current → latest master → 14-day baseline

Memory consumption baseline with starter plan resources

Metric	Current	Latest Master	Baseline (avg)	vs Master	vs Baseline	Status
memory-heap-used-baseline	114.95 MB	114.05 MB	113.86 MB (σ 0.84)	+0.8%	+1.0%	⚠️
memory-rss-baseline	291.23 MB	287.98 MB	284.98 MB (σ 42.51)	+1.1%	+2.2%	✅

docker-stats

Metric	Current	Latest Master	Baseline (avg)	vs Master	vs Baseline	Status
docker-image-size-n8n	1269.76 MB	1269.76 MB	1269.76 MB (σ 0.00)	+0.0%	+0.0%	—
docker-image-size-runners	393.00 MB	393.00 MB	391.63 MB (σ 11.06)	+0.0%	+0.3%	✅

Idle baseline with Instance AI module loaded

Metric	Current	Latest Master	Baseline (avg)	vs Master	vs Baseline	Status
instance-ai-heap-used-baseline	186.76 MB	186.52 MB	186.34 MB (σ 0.24)	+0.1%	+0.2%	⚠️
instance-ai-rss-baseline	385.54 MB	388.20 MB	372.63 MB (σ 22.95)	-0.7%	+3.5%	✅

How to read this table

• Current: This PR's value (or latest master if PR perf tests haven't run)
• Latest Master: Most recent nightly master measurement
• Baseline: Rolling 14-day average from master
• vs Master: PR impact (current vs latest master)
• vs Baseline: Drift from baseline (current vs rolling avg)
• Status: ✅ within 1σ | ⚠️ 1-2σ | 🔴 >2σ regression