# Atlas Stream Processing Lambda external functions and error semantics > Source-anchored research for Atlas Stream Processing Lambda external functions and error semantics. Run status: COMPLETED. Parent: [MongoDB Atlas Stream Processing](https://llms-explorer.com/tree/mongodb-atlas-stream-processing/) · 4 facets · 17 facts · page: https://llms-explorer.com/tree/atlas-stream-processing-lambda-external-functions-and-error-semantics/ ## How it works - Datadog distinguishes invocation errors from function errors and explains that Lambda Errors counts function failures, excluding invocation errors. Monitor invocation failures separately when diagnosing ASP-to-Lambda execution. (low confidence) — [source](https://www.datadoghq.com/blog/key-metrics-for-monitoring-aws-lambda/) ## Problems, failure modes and limitations - onError defaults to dlq; ignore drops the affected document; fail terminates the processor. (medium confidence) (verified as of 2026-10-14) — [source](https://www.mongodb.com/docs/atlas/atlas-stream-processing/sp-agg-externalFunction.md) - Payload JSON conversion failures or invalid final BSON objects go to the ASP DLQ regardless of onError. Invalid operator expressions instead fail the processor. (low confidence) — [source](https://www.mongodb.com/docs/atlas/atlas-stream-processing/sp-agg-externalFunction.md) - After asynchronous acceptance, Lambda owns its event queue. Its documented default is two additional attempts for function errors, including timeouts; duplicates can occur even without an error. (medium confidence) (verified as of 2026-10-27) — [source](https://docs.aws.amazon.com/lambda/latest/api/API_Invoke.html) - Configure Lambda-side failure capture for asynchronous execution: a Lambda DLQ captures discarded events, and an on-failure destination captures failed-invocation records. ASP onError does not catch those later function failures. (high confidence) — [source](https://www.mongodb.com/docs/atlas/atlas-stream-processing/sp-agg-externalFunction.md) - Lambda asynchronous delivery can duplicate events even without function errors. Make external side effects idempotent; stream processing completion cannot establish exactly-once Lambda side effects. The latter is an inference from the documented invocation boundary. (medium confidence) — [source](https://www.mongodb.com/docs/atlas/atlas-stream-processing/sp-agg-externalFunction.md) ## Comparisons and alternatives - ASP onError covers invocation API errors in both modes and Lambda function errors only in synchronous mode. AWS async invocation returns acceptance rather than function results. (high confidence) — [source](https://www.mongodb.com/docs/atlas/atlas-stream-processing/sp-agg-externalFunction.md) - ASP retries potentially transient request failures; its reference specifies neither retry count nor backoff. This differs from Lambda-managed asynchronous execution retries. (medium confidence) — [source](https://www.mongodb.com/docs/atlas/atlas-stream-processing/sp-agg-externalFunction.md) - The payload subpipeline supports $project, $addFields, $replaceRoot and $set. Lambda responses must be buffered rather than streamed. (low confidence) (verified as of 2026-10-14) — [source](https://www.mongodb.com/docs/atlas/atlas-stream-processing/sp-agg-externalFunction.md) ## Facts and statements - $externalFunction invokes AWS Lambda through a registered connection. Synchronous execution returns enrichment results; asynchronous execution continues without awaiting function completion. (high confidence) — [source](https://www.mongodb.com/docs/atlas/atlas-stream-processing/sp-agg-externalFunction.md) - execution defaults to sync. as is required for synchronous nonterminal stages, forbidden for async, and unset for a zero-byte response. (low confidence) (verified as of 2026-10-14) — [source](https://www.mongodb.com/docs/atlas/atlas-stream-processing/sp-agg-externalFunction.md) - Lambda must share the ASP AWS region. IAM needs lambda:InvokeFunction and lambda:GetFunctionConfiguration; ASP reads the configured timeout to wait through execution. Only buffered responses are supported. (low confidence) (verified as of 2026-10-14) — [source](https://www.mongodb.com/docs/atlas/atlas-stream-processing/sp-agg-externalFunction.md) - Lambda Invoke HTTP success does not imply function success: synchronous function failures use FunctionError and the response payload. An accepted asynchronous invocation returns HTTP 202. (low confidence) — [source](https://docs.aws.amazon.com/lambda/latest/api/API_Invoke.html) - Datadog’s monitoring guidance recommends watching Lambda Errors and dead-letter delivery errors and correlating invocation metrics with logs. This supplies Lambda-side diagnostic guidance, not evidence of an ASP incident. (low confidence) — [source](https://www.datadoghq.com/blog/key-metrics-for-monitoring-aws-lambda/) - MongoDB’s changelog dates $externalFunction support to 26 March 2025, while its introductory blog is dated 3 July 2025; the blog date should not be substituted for the documented release date. (medium confidence) — [source](https://www.mongodb.com/docs/atlas/atlas-stream-processing/changelog.md) - The changelog records externalFunction parallelism on 5 August 2026 and Lambda Private Link on 19 August 2026. The Private Link guide requires its endpoint in the processors’ region. (medium confidence) — [source](https://www.mongodb.com/docs/atlas/atlas-stream-processing/changelog.md) - For asynchronous function errors, Lambda defaults to two additional attempts, waiting one minute then two minutes. These defaults are configurable; they are not an ASP retry policy. (high confidence) (verified as of 2026-10-27) — [source](https://docs.aws.amazon.com/lambda/latest/dg/invocation-async-error-handling.html)