Deliver invoked snapshots directly

[SandroMaglione]

Summary

• deliver invoked child active snapshots directly from the child runtime
• remove the replay PubSub, subscription, watcher fiber, and watcher teardown retained by snapshot-mapped invokes
• keep the observer internal and lazy while preserving stale-session checks, ordered delivery, and callback isolation
• update the runtime memory profile to describe the direct observation path

The previous stream-based implementation retained approximately 16 KiB above an otherwise identical parent-child family. Four order-balanced local process pairs reduced snapshot-observed family heap by 28.5%, from 57.5 KiB to 41.1 KiB, with unrelated throughput remaining neutral within local variability.

Changeset

• Added or updated for a library or package-metadata change
• Not required because this PR does not change src/ or package.json

Validation

• pnpm check
• Relevant example checks, when examples changed (not applicable)
• Reviewed the automated type-performance report, when the public TypeScript API or inference changed; local budgets are unchanged and no public API changed
• Reviewed the automated runtime-performance report, when runtime behavior changed

[github-actions]

Type performance

Measured with TypeScript 6.0.3 and skipLibCheck=true.

Scenario	Base	PR	Difference
Effect only	55	55	0 (0.0%)
Import effect-machine	55	55	0 (0.0%)
Machine.defineStates (3 states)	2,965	2,965	0 (0.0%)
Machine.make (3 states, 2 events)	8,715	8,715	0 (0.0%)
machine.handle (3 states, 2 transitions)	23,492	23,492	0 (0.0%)
machine.handle (depth 8)	117,075	117,075	0 (0.0%)
machine.handle (depth 12)	129,713	129,713	0 (0.0%)
machine.handle (depth 16)	145,055	145,055	0 (0.0%)
machine.handle (depth 24)	183,851	183,851	0 (0.0%)
machine.handle (wide depth 16)	214,189	214,189	0 (0.0%)
machine.handle (parallel/history/choice)	132,667	132,667	0 (0.0%)
machine.handle (4 successive calls)	128,674	128,674	0 (0.0%)
machine exact input/output/error/services	110,584	110,584	0 (0.0%)
execution adapter readiness	127,854	127,854	0 (0.0%)

Marginal instantiations are measured against the matching setup without that API call:

Scenario	Base	PR	Difference
Import effect-machine	0	0	0
Machine.defineStates (3 states)	2,910	2,910	0 (0.0%)
Machine.make (3 states, 2 events)	5,742	5,742	0 (0.0%)
machine.handle (3 states, 2 transitions)	14,777	14,777	0 (0.0%)
machine.handle (depth 8)	109,601	109,601	0 (0.0%)
machine.handle (depth 12)	121,039	121,039	0 (0.0%)
machine.handle (depth 16)	135,181	135,181	0 (0.0%)
machine.handle (depth 24)	171,577	171,577	0 (0.0%)
machine.handle (wide depth 16)	202,224	202,224	0 (0.0%)
machine.handle (parallel/history/choice)	114,331	114,331	0 (0.0%)
machine.handle (4 successive calls)	113,703	113,703	0 (0.0%)
machine exact input/output/error/services	100,670	100,670	0 (0.0%)
execution adapter readiness	102,092	102,092	0 (0.0%)

Check times (informational)

Scenario	Base	PR
Effect only	0.02s	0.02s
Import effect-machine	0.02s	0.02s
Machine.defineStates (3 states)	0.07s	0.07s
Machine.make (3 states, 2 events)	0.11s	0.11s
machine.handle (3 states, 2 transitions)	0.16s	0.18s
machine.handle (depth 8)	0.37s	0.38s
machine.handle (depth 12)	0.41s	0.41s
machine.handle (depth 16)	0.44s	0.43s
machine.handle (depth 24)	0.49s	0.50s
machine.handle (wide depth 16)	0.54s	0.54s
machine.handle (parallel/history/choice)	0.46s	0.45s
machine.handle (4 successive calls)	0.43s	0.41s
machine exact input/output/error/services	0.37s	0.38s
execution adapter readiness	0.42s	0.42s

Type instantiations are the comparison metric. Check time varies with runner load and is informational only.

[github-actions]

Runtime performance

Median of 5 independent benchmark processes on AMD EPYC 9V74 80-Core Processor with Node v24.18.0.

Pull request baseline

Scenario	Effect Machine	XState 5	XState 6 alpha
Plan counter transitions	108,818 transitions/s	140,861 transitions/s	116,979 transitions/s
Drain burst with terminal fence	66,852 increments/s	392,497 increments/s	107,768 increments/s
Drain burst with a change observer	55,728 increments/s	388,780 increments/s	108,557 increments/s
Lookup and send to one child	61,447 increments/s	388,731 increments/s	105,697 increments/s
Start and stop a machine	16,847 machines/s	183,891 machines/s	74,738 machines/s
Start and stop a parent with one child	5,417 families/s	72,886 families/s	30,704 families/s

Memory profile	Effect Machine	XState 5	XState 6 alpha	Effect runtime primitives
Idle machine	17.8 KiB	3.8 KiB	6.1 KiB	—
Raw managed process	13.1 KiB	—	—	—
Two independent idle machines	34.9 KiB	7.3 KiB	11.8 KiB	—
Idle parent with one child	41.0 KiB	5.5 KiB	8.8 KiB	—
Parent with observed child registry	52.4 KiB	—	—	—
Parent with observed invoked child snapshots	41.2 KiB	—	—	—
Suspended Effect fiber	—	—	—	0.7 KiB
Effect queue with waiting fiber	—	—	—	2.8 KiB
Effect mailbox actor shell	—	—	—	3.2 KiB

Effect Machine change from base

Metric	Base	Base variability	PR	PR variability	Difference
Plan counter transitions	109,912 transitions/s	0.4% MAD	108,818 transitions/s	0.9% MAD	-1.0%
Drain burst with terminal fence	67,064 increments/s	0.8% MAD	66,852 increments/s	0.8% MAD	-0.3%
Drain burst with a change observer	55,648 increments/s	0.4% MAD	55,728 increments/s	0.2% MAD	+0.1%
Lookup and send to one child	61,506 increments/s	0.5% MAD	61,447 increments/s	0.3% MAD	-0.1%
Start and stop a machine	16,961 machines/s	1.7% MAD	16,847 machines/s	1.3% MAD	-0.7%
Start and stop a parent with one child	5,390 families/s	0.6% MAD	5,417 families/s	1.5% MAD	+0.5%
Idle machine heap per unit	17.8 KiB	0.0% MAD	17.8 KiB	0.0% MAD	+0.1%
Raw managed process heap per unit	13.1 KiB	0.0% MAD	13.1 KiB	0.0% MAD	+0.1%
Two independent idle machines heap per unit	34.9 KiB	0.0% MAD	34.9 KiB	0.0% MAD	+0.0%
Idle parent with one child heap per unit	41.6 KiB	0.0% MAD	41.0 KiB	0.0% MAD	-1.5%
Parent with observed child registry heap per unit	53.0 KiB	0.0% MAD	52.4 KiB	0.0% MAD	-1.2%
Parent with observed invoked child snapshots heap per unit	57.5 KiB	0.0% MAD	41.2 KiB	0.0% MAD	-28.3%

Versions and interpretation

• Effect Machine: 0.3.0
• XState 5: 5.32.5
• XState 6 alpha: 6.0.0-alpha.27
• Effect runtime primitives: 4.0.0-beta.105

Higher throughput is better; lower heap is better. Variability is the median absolute deviation across independent processes, relative to their median. Runtime measurements on shared GitHub-hosted hardware remain informational, so small differences should be confirmed across multiple workflow runs.