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fix(lambda): return partial CreateFleet instances instead of discarding them #5131

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fix(lambda): return partial CreateFleet instances instead of discarding them #5131

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b8c45d8
fix(lambda): return partial CreateFleet instances instead of discardi…
vegardx May 26, 2026
e8cdc4e
refactor: address review feedback — mechanism-agnostic log, stronger …
vegardx May 26, 2026
d4f3be6
Merge branch 'main' into fix/partial-createfleet-return-instances
edersonbrilhante Jun 19, 2026
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16 changes: 14 additions & 2 deletions lambdas/functions/control-plane/src/aws/runners.test.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -513,7 +513,7 @@ describe('create runner with errors', () => {

await expect(createRunner(createRunnerConfig(defaultRunnerConfig))).rejects.toMatchObject({
name: 'ScaleError',
failedInstanceCount: 2,
failedInstanceCount: 1, // numberOfRunners when zero instances created
});

@vegardx vegardx May 26, 2026

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Agreed. Fixed in e8cdc4e — test now uses numberOfRunners: 3 so the assertion actually validates the value.

expect(mockEC2Client).toHaveReceivedCommandWith(
CreateFleetCommand,
Expand Down Expand Up @@ -543,6 +543,16 @@ describe('create runner with errors', () => {
);
});

it('returns partial instances on recognized scale error instead of throwing', async () => {
createFleetMockWithErrors(['UnfulfillableCapacity'], ['i-partial']);

await expect(createRunner(createRunnerConfig(defaultRunnerConfig))).resolves.toEqual(['i-partial']);
expect(mockEC2Client).toHaveReceivedCommandWith(
CreateFleetCommand,
expectedCreateFleetRequest(defaultExpectedFleetRequestValues),
);

@vegardx vegardx May 26, 2026

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Agreed. Fixed in e8cdc4e — test now requests 3 runners and gets 1 instance back, making it a genuine partial-success scenario.

});

it('test error by create fleet call is thrown.', async () => {
mockEC2Client.on(CreateFleetCommand).rejects(new Error('Some error'));

Expand Down Expand Up @@ -688,12 +698,14 @@ describe('create runner with errors fail over to OnDemand', () => {
// fallback to on demand for UnfulfillableCapacity but InsufficientInstanceCapacity is thrown
createFleetMockWithWithOnDemandFallback(['UnfulfillableCapacity'], instancesIds);

// Partial success: 1 instance created, unrecognized error for the rest.
// Returns partial instances instead of throwing to prevent orphans.
await expect(
createRunner({
...createRunnerConfig(defaultRunnerConfig),
numberOfRunners: 2,
}),
).rejects.toBeInstanceOf(Error);
).resolves.toEqual(['i-123']);

expect(mockEC2Client).toHaveReceivedCommandTimes(CreateFleetCommand, 1);

Expand Down
21 changes: 19 additions & 2 deletions lambdas/functions/control-plane/src/aws/runners.ts
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Expand Up @@ -201,9 +201,26 @@ async function processFleetResult(

const failedCount = countScaleErrors(errors, scaleErrors);
if (failedCount > 0) {
logger.warn('Create fleet failed, ScaleError will be thrown to trigger retry for ephemeral runners.');
if (instances.length > 0) {
logger.warn(
`Partial fleet success: ${instances.length}/${runnerParameters.numberOfRunners} instances created. ` +
`Returning partial results; caller will retry the shortfall via SQS.`,
{ data: fleet.Errors },

@vegardx vegardx May 26, 2026

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Good point. Fixed in e8cdc4e — removed SQS reference, reworded to mechanism-agnostic, and added created/requested to log metadata.

);
return instances;
}
Comment on lines +223 to +230

@vegardx vegardx May 26, 2026

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The caller (scaleUp in scale-up.ts) already handles instances.length < numberOfRunners — it tracks how many instances were successfully created vs. how many SQS messages it received, and marks the shortfall as batchItemFailures. This is the existing contract; we're not changing it.

Introducing a structured return type would be a larger refactor that touches every callsite of createRunner (scale-up, pool, on-demand fallback recursion) for no behavioral benefit — the length comparison already provides the signal.

logger.warn('Create fleet failed with zero instances, ScaleError will be thrown to trigger retry.');
logger.debug('Create fleet failed.', { data: fleet.Errors });
throw new ScaleError(failedCount);
throw new ScaleError(runnerParameters.numberOfRunners);
}

if (instances.length > 0) {
logger.warn(
`Partial fleet success: ${instances.length}/${runnerParameters.numberOfRunners} instances created. ` +
`Error not recognized as scaling error; returning partial results.`,
{ data: fleet.Errors },
);
return instances;
}

logger.warn('Create fleet failed, error not recognized as scaling error.', { data: fleet.Errors });
Expand Down