AWS Limits Review

Goal

Catch AWS service limit/quota violations in infrastructure code before they cause production issues.

When to use

Reviewing Terraform, CloudFormation, CDK, or Pulumi code
Code that provisions or configures AWS resources
PR reviews involving AWS infrastructure changes
Before deploying infrastructure changes to production

When not to use

Non-AWS cloud providers (future skills: cloudflare-limits, gcp-limits)
Application code without infrastructure components
AWS SDK usage for read-only operations

Inputs

Infrastructure code files (.tf, .json, .yaml, .yml, .ts, .py)
Context about expected scale/traffic patterns (if available)

Outputs

List of potential limit violations with severity
Links to official AWS documentation
Recommended mitigations

Default workflow

Identify AWS services in the code:
- Scan for resource types (e.g., aws_lambda_function, AWS::Lambda::Function)
- Note service patterns in SDK calls
Check against known limits in REFERENCE.md:
- Hard limits (cannot be increased)
- Soft limits at default values
- Limits that commonly cause production issues
Flag violations with context:
- Quote the specific code
- Explain the limit and why it matters
- Link to AWS documentation
- Suggest mitigation if applicable
Prioritize by severity:
- Critical: Hard limits that will fail immediately
- High: Default limits likely to be hit at moderate scale
- Medium: Limits that may cause issues at high scale
- Low: Informational, worth noting for awareness

Key patterns to watch

Lambda

Functions with >15 min expected runtime (hard limit: 15 min)
Synchronous payloads >6MB
VPC-connected functions without sufficient ENI capacity
High concurrency without reserved/provisioned concurrency

API Gateway

Integrations expecting >29s response time (default timeout)
Missing throttling configuration for public APIs
WebSocket connections without connection management

S3

Single-prefix designs with high throughput expectations
Missing retry logic for 503 responses

DynamoDB

Items approaching 400KB
Single-table designs without GSI capacity planning
On-demand tables without understanding burst limits

Step Functions

Long-running workflows approaching 25,000 event limit
Map states without concurrency limits
Large payloads (>256KB state data)

Load Balancing

Multiple load balancers sharing target groups (limit: 5 LBs per target group)
Target groups with many targets without health check tuning
Weighted routing with many target groups per action

Cognito

Auth flows without rate limit handling (default: 120 RPS)
Bulk user imports without throttling

Validation checklist

All identified AWS services checked against REFERENCE.md
Hard limits flagged as Critical
Soft limits noted with current defaults
Links to AWS docs included for each finding
Mitigations suggested where applicable

Output format

## AWS Limits Review

### Critical
- **[Service]: [Limit name]**
  - File: `path/to/file.tf:42`
  - Issue: [Description]
  - Limit: [Value] ([hard/soft])
  - Docs: [AWS link]
  - Mitigation: [Suggestion]

### High
...

### Medium
...

### Low / Informational
...

### No Issues Found
- [List of services reviewed with no concerns]

Examples

Example 1: Lambda timeout

Input: Terraform defining Lambda with Step Functions integration

resource "aws_lambda_function" "processor" {
  timeout = 900  # 15 minutes
}

Output:

High: Lambda timeout at maximum

File: lambda.tf:12

Issue: Lambda timeout set to maximum (900s). Any operation exceeding this will fail.

Limit: 900 seconds (hard)

Docs: https://docs.aws.amazon.com/lambda/latest/dg/gettingstarted-limits.html

Mitigation: For longer processes, use Step Functions with Lambda iterations or switch to Fargate.

Example 2: Multiple LBs per target group

Input: Terraform with shared target groups

resource "aws_lb_target_group" "shared" {
  name = "shared-targets"
}

resource "aws_lb_listener" "lb1" {
  default_action {
    target_group_arn = aws_lb_target_group.shared.arn
  }
}
# ... repeated for 6 load balancers

Output:

Critical: Target group shared across too many load balancers

File: alb.tf:15-45

Issue: Target group "shared-targets" referenced by 6 load balancers. Maximum is 5.

Limit: 5 load balancers per target group (hard)

Docs: https://docs.aws.amazon.com/elasticloadbalancing/latest/application/load-balancer-limits.html

Mitigation: Create separate target groups for additional load balancers.

aws-limits

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