ConcurrentModification
IAM returns `ConcurrentModification` when overlapping write operations target the same IAM resource at the same time, producing a control-plane conflict. IAM returns HTTP 409 for this error.
Last reviewed: March 29, 2026|Editorial standard: source-backed technical guidance
What Does Concurrent Modification Mean?
IAM rejected the write due to concurrent mutations on the same entity, so control-plane changes must be serialized and retried with jittered backoff.
Common Causes
- -Multiple automation processes update the same IAM role/user/policy concurrently.
- -Retry loops replay IAM write calls before previous mutation has settled.
- -CI/CD jobs and drift-remediation jobs overlap on identical IAM targets.
- -No locking or sequencing exists for IAM control-plane writes.
How to Fix Concurrent Modification
- 1Add jittered retry with bounded attempts for concurrent-modification failures.
- 2Serialize writes per IAM entity using distributed locks or single-writer orchestration.
- 3Re-read latest IAM state before applying retry mutation.
- 4Pause overlapping jobs that target the same IAM objects.
Step-by-Step Diagnosis for Concurrent Modification
- 1Correlate CloudTrail event timeline for overlapping IAM write operations.
- 2Identify all pipelines/services mutating the same IAM entity concurrently.
- 3Inspect retry policies for aggressive immediate replay patterns.
- 4Diff desired-state commits to find conflicting concurrent changes.
Per-Entity Write Contention Analysis
- -Identify all writers mutating the same IAM entity (example: deploy pipeline and drift remediator both patch role policy simultaneously).
- -Correlate overlapping API calls using CloudTrail request timestamps (example: two UpdateRole calls overlap within seconds).
Mutation Serialization and Retry Discipline
- -Apply bounded exponential backoff with jitter for IAM writes (example: fixed-delay retries keep colliding with in-flight mutation).
- -Enforce single-writer lock per IAM entity scope (example: distributed lock keyed by role ARN).
How to Verify the Fix
- -Confirm previously conflicting IAM operation succeeds after sequencing controls.
- -Validate conflict rate drops across repeated deployment cycles.
- -Ensure IAM state converges deterministically under concurrent workload tests.
How to Prevent Recurrence
- -Adopt single-writer model for each IAM entity domain.
- -Implement lock-aware orchestration for IAM mutations in CI/CD and remediation jobs.
- -Alert on concurrent modification spikes and auto-throttle conflicting workflows.
Pro Tip
- -store per-entity change tokens in orchestration state so subsequent IAM updates must verify they are operating on latest committed revision.
Decision Support
Compare Guide
409 Conflict vs 412 Precondition Failed: When to Use Each
Choose 412 when If-Match or If-Unmodified-Since checks fail; choose 409 for state conflicts without failed precondition headers during concurrent updates.
Playbook
Conflict and Concurrency Playbook (409 / 412 / OptimisticLock)
Use this playbook to separate true write conflicts from stale precondition failures, then apply safe re-fetch, optimistic-lock, and retry choices.
Official References
Provider Context
This guidance is specific to AWS services. Always validate implementation details against official provider documentation before deploying to production.