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To achieve zero-downtime schema migrations under high write traffic, implement the Expand/Contract (Parallel Run) pattern combined with AWS Database Migration Service (DMS) and pt-online-schema-change or gh-ost:

• Step 1: Create a Read Replica and perform schema alterations on the replica or use gh-ost triggers on master without table locks.
• Step 2: Use AWS DMS with Change Data Capture (CDC) to stream live delta changes from source to target tables continuously.
• Step 3: Update application code to dual-write or dynamically switch database endpoints using Route 53 CNAME / RDS Proxy.

Reserved Concurrency guarantees a maximum upper limit of concurrent executions for a specific Lambda function to prevent a single function from exhausting your AWS account’s total regional concurrency pool (default 1,000).

Provisioned Concurrency keeps a designated number of execution environments pre-initialized and warm, completely eliminating cold start latency for latency-sensitive microservices and API Gateway endpoints.

Cross-account access should strictly avoid long-lived IAM user access keys. Instead, use IAM Roles with STS AssumeRole:

• Define an IAM Role in Account B (Resource Account) with an explicit Trust Policy naming Account A’s IAM Principal.
• Enforce an ExternalId condition check in the trust policy to prevent the confused deputy problem.
• Account A users call aws sts assume-role to receive temporary 15-minute to 12-hour credentials.