Lambda VS ECS

Aspect	AWS Lambda	Amazon ECS
Architecture	Serverless, event-driven functions	Container-based microservices
Execution Limits	15-minute maximum	No time limits
Scaling	Automatic, per-function scaling	Automatic container scaling with ECS capacity providers
Cost Model	Pay per request and compute time	Pay for EC2/Fargate resources used
State Management	Stateless by default	Stateful or stateless possible
Cold Starts	Yes, can impact latency	Minimal with warm container pools
Deployment	Function versioning, complex coordination	Standard container deployment practices
Testing	Complex integration testing	Traditional container testing approaches
Code Reuse	Requires layers or code duplication	Native code sharing within containers
Resource Limits	Memory: 10GB max, tmp storage: 10GB	Based on underlying compute resources
Monitoring	CloudWatch integration	CloudWatch + container-level metrics
Network Integration	VPC integration possible but adds latency	Native VPC networking
Development Experience	Function-oriented development	Traditional application development
Maintenance Overhead	Low for individual functions, higher for complex apps	Medium, managed container orchestration
Best For	Event processing, small discrete tasks	Complex applications, long-running services

Monolitic refactoring

While Lambda is serverless and highly scalable, it has key limitations for this scenario:

1. Significant code refactoring would be needed to convert the monolithic application to Lambda functions

2. Lambda has execution time limits (15 minutes max) which may not suit all application components

3. Lambda cold starts could impact performance

4. Lambda isn't ideal for keeping "as much existing code as possible" as stated in requirements

5. Breaking down and managing a large monolith in Lambda requires complex coordination between functions

ECS (option D) better supports gradual migration while maintaining existing code structure.

Lambda challeges

Managing a monolithic application in Lambda functions presents several coordination challenges:

1. Function dependencies - Lambda functions need to communicate with each other, requiring careful API design and management

2. Shared code/resources - Breaking down shared components requires duplicating code or creating complex layers

3. State management - Coordinating state across multiple functions is complex compared to a container-based approach

4. Deployment complexity - Need to manage and version multiple function deployments in sync

5. Testing challenges - Integration testing across multiple functions is more difficult than testing containerized services

ECS simplifies these challenges by keeping related components together in containers while still enabling modular development.

Lambda Deployment complexigy

Deploying multiple Lambda functions creates several synchronization challenges:

1. Version Control - Each function needs separate versioning and tracking of dependencies

2. Rollbacks - Must coordinate rollbacks across multiple interconnected functions

3. Staged Deployments - Canary or blue-green deployments become complex across multiple functions

4. API Versioning - API Gateway versions must align with function versions

5. Cross-Function Dependencies - Changes in one function may require updates in dependent functions

6. Infrastructure Changes - Managing IAM roles, permissions, and resources across multiple functions

7. Deployment Order - Functions must be deployed in correct sequence to maintain application stability

With ECS, deployment is simpler as containers package related functionality together and use standard container orchestration practices.

Lambda Testing complexity

Testing Lambda functions across multiple functions presents unique challenges:

1. Limited local testing - Lambda's cloud-native nature makes local testing more complex than containers

2. Dependency mocking - Each function interaction needs careful mocking for proper testing

3. End-to-end testing complexity - Testing complete workflows requires orchestrating multiple function executions

4. Environmental variables - Managing different configs across test environments is more complex

5. Debugging difficulty - Tracing issues across function calls is harder than in a containerized environment

With ECS containers, testing remains closer to traditional application testing patterns, making it easier for teams to maintain quality.