AWS Developer - Associate Interview Preparation Guide

Reserved Concurrency sets a hard ceiling on the maximum number of concurrent executions a specific Lambda function can consume, protecting downstream databases from overload. Provisioned Concurrency, by contrast, pre-initializes a specified number of execution environments so that functions respond instantly without cold start latency. While reserved concurrency restricts scaling, provisioned concurrency guarantees readiness and incurs additional hourly infrastructure costs for the reserved execution slots.

Global Secondary Indexes can be created at any time, have partition keys and sort keys independent of the base table, and scale their provisioned read and write capacity units separately. Local Secondary Indexes, however, must be created at the exact time the table is created, share the provisioned throughput pool of the base table, and share the same partition key. For almost all modern applications, GSIs are preferred due to their flexibility and independent scaling properties.

API Gateway HTTP APIs are designed for building APIs with minimal features, offering lower latency and significantly lower cost (up to 70% cheaper than REST APIs). They natively support OIDC/OAuth2 JWT authorizers and CORS configuration. REST APIs offer advanced features such as request validation, AWS WAF integration, API keys, request transformations, usage plans, and private integrations. Use HTTP APIs for modern serverless microservices requiring high performance and lower cost, and REST APIs for enterprise integrations requiring robust edge protection and request mapping.

If an incoming request hits an existing warm execution environment (context reuse), the contents of the /tmp directory persist from the previous invocation. However, developers should never rely on this data persisting across requests for critical state, because AWS can spin down execution environments at any time, and subsequent cold start invocations will provision a completely fresh and empty /tmp directory.

To prevent poison pill messages from repeatedly failing and blocking queue throughput, developers should configure a Dead Letter Queue (DLQ) on the SQS queue or specify an OnFailure destination on the Lambda event source mapping. Additionally, setting an appropriate maximum retry attempt count and ensuring that queue visibility timeouts exceed the maximum Lambda execution duration prevents premature message redelivery.

The principle of least privilege dictates that a user, role, or service should only be granted the minimum permissions necessary to perform its specific tasks. In serverless applications, this means avoiding wildcard permissions (Action: '*', Resource: '*') on Lambda execution roles. Instead, developers must scope permissions down to exact action verbs (e.g., 'dynamodb:Query', 's3:GetObject') and specific resource ARNs matching individual tables or bucket prefixes.

Relational normalization splits data across multiple tables with foreign key relationships, relying on SQL JOIN operations to assemble entity graphs at query time. DynamoDB single-table design stores multiple distinct entities and relationships within a single table using composite primary keys and attribute overloading. This eliminates expensive cross-table joins, enabling predictable single-digit millisecond response times at massive scale, though it requires upfront knowledge of all query access patterns.

A cold start occurs when an incoming request requires AWS to provision a new execution environment, download the deployment package, initialize the runtime, and execute global initialization code. Cold starts are mitigated by keeping deployment packages small, utilizing lightweight runtimes, initializing database connection pools outside the handler, and configuring Provisioned Concurrency for latency-critical APIs.

When a Lambda function is configured to run inside a VPC, AWS must attach Elastic Network Interfaces (ENIs) to the function's execution environment. Historically, this ENI creation process added significant network initialization latency during cold starts. Although modern AWS microvm architectures have substantially optimized VPC attachment times, improper subnet IP address exhaustion or NAT Gateway bottlenecks can still impact networking performance.

AWS SAM is an extension of AWS CloudFormation designed specifically for serverless applications, using concise YAML or JSON syntax to define functions, APIs, and tables. AWS CDK allows developers to define cloud infrastructure using familiar general-purpose programming languages like TypeScript, Python, and Java, generating CloudFormation templates under the hood. CDK provides powerful object-oriented abstractions, loops, and conditional logic that simplify complex multi-service architectures compared to raw YAML.