Lambda is throttled at the account limit

Symptom

Troubleshooting scenario: Lambda Throttles rises, messages wait, and direct callers may see TooManyRequestsException. Historical context: docs/40-aws-job-pipeline.md records this account’s concurrency quota as 10 on 2026-10-15 and a refused reservation. That is not a general AWS default or a current quota reading.

Read the error first

Lambda refused an invocation for lack of concurrency: simultaneous executions. Under Reach / Allowed / Password, this is capacity; the request reached Lambda, and a password or IAM allow cannot add capacity. An IAM AccessDenied with missing allow or explicit deny is a different failure. Check whether the limit is account-wide or function-specific.

Decide

flowchart TB
  A["Lambda throttles"] --> B{"Function reservation exhausted?"}
  B -->|"yes"| C["Compare reservation with all its triggers"]
  B -->|"no"| D{"Regional shared quota exhausted?"}
  D -->|"yes"| E["Check other functions and quota"]
  D -->|"no"| F["Inspect scaling-rate or other throttle reason"]
  C --> G["Align capacity, consumers and downstream limits"]
  E --> G

Likely causes

Ranked for rising throttles while shared concurrency is full.

1. Other functions consume the Region’s available shared concurrency.
2. A function reservation is too small for its combined triggers, or zero disables execution.
3. A burst reaches a scaling-rate limit before total concurrency is full; read the throttle reason.

How to check

Console

Lambda → Account settings: current Regional quota. CloudWatch: compare account concurrency with each function’s ConcurrentExecutions and Throttles. Check Configuration → Concurrency and each SQS trigger’s maximum concurrency; correlate queue age and receive failures.

CLI

Use the stack’s Region, us-east-1; these read limits and trigger settings.

export AWS_PROFILE=ticketlite
aws lambda get-account-settings --region us-east-1 --query '{Limit:AccountLimit.ConcurrentExecutions,Unreserved:AccountLimit.UnreservedConcurrentExecutions}'
aws lambda get-function-concurrency --function-name minitickets-dev-worker --region us-east-1
aws lambda list-event-source-mappings --function-name minitickets-dev-worker --region us-east-1

CloudTrail

Search PutFunctionConcurrency, DeleteFunctionConcurrency and UpdateEventSourceMapping for configuration changes. These cannot show simultaneous usage; use CloudWatch metrics. Ordinary Event history does not contain every invocation.

Fix

Coordinate quota and allocations before increasing consumers. Current mappings cap email at 5 and tickets at 2 concurrent batches; these reserve no capacity. The committed config sets no worker reservation. After any approved quota increase, review SES/DynamoDB limits too.

Prevent

Monitor throttles, shared concurrency and queue age. Keep mapping caps within available capacity; repeated receives during throttling can move healthy messages toward the DLQ.

infra/src/jobs/queue-trigger.ts → createQueueTrigger; Inspect the email mapping →.

Say it in 30 seconds

Say it like this (20-30 seconds)

I compare function throttles with the Region’s current concurrency quota and other functions’ usage. Our email and ticket queue mappings cap parallel batches at five and two, but reserve no capacity, so another function can still consume the shared pool. I check reservations and all triggers before changing limits. The account historically had a quota of ten; I read today’s value and coordinate quota, consumer caps and downstream sending limits.

Advanced: Three concurrency settings, three jobs

The Regional account quota limits the pool. Reserved concurrency allocates and caps a function’s share; a mapping’s maximum concurrency caps that queue’s use without reserving capacity. Provisioned concurrency initializes environments and costs money; it does not replace the account quota. The historical quota of ten came from a read-only account-settings check in docs/40-aws-job-pipeline.md, not a witnessed production throttling incident. No current account query was performed for this page. AWS documents account concurrency metrics and reservation versus provisioned concurrency. Do not generalize an old account-specific reservation refusal into AWS’s current universal minimum; read the present quota and service response. infra/src/jobs/worker-lambda.ts → createWorkerLambda; infra/src/tickets/lambda.ts → createTicketLambda.