Jeff’s Note #
Unlike generic exam dumps, ADH analyzes this scenario through the lens of a Real-World Lead Developer.
For DVA-C02 candidates, the confusion often lies in how to effectively cache POST requests, since they are typically considered non-idempotent and thus not cacheable by default. In production, this is about knowing exactly which AWS services support caching for which HTTP methods and how to integrate them for optimal resource usage. Let’s drill down.
The Certification Drill (Simulated Question) #
Scenario #
Skyline Tech, a startup specializing in real-time data analytics, built their backend API on AWS. Their API leverages Amazon CloudFront as the CDN, Amazon API Gateway to expose REST endpoints, and AWS Lambda for the business logic layer. The API experiences a steady flow of requests — at least four per second. The development team notices that many clients repeatedly issue the same POST request with identical payload to fetch frequently used data. To optimize backend resource consumption and reduce latency, the team wants to implement caching for these POST requests.
The Requirement: #
Identify the best architectural approach to cache POST requests in this API setup to improve efficiency without compromising on correctness or scalability.
The Options #
- A) Configure CloudFront caching policies to cache POST request responses, modifying the cache key behavior to use default headers.
- B) Enable and override the API Gateway cache for the stage and specifically configure caching on the POST method.
- C) Modify the Lambda function to persist the latest response in the /tmp directory and check this cache before processing.
- D) Store the latest request/response pair in AWS Systems Manager Parameter Store and update Lambda code to retrieve cached data from there.
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Correct Answer #
B) Enable and override the API Gateway cache for the stage and specifically configure caching on the POST method.
Quick Insight: The Developer Imperative #
- API Gateway supports caching at the stage level and allows configuring caching for POST methods by overriding the default “no-cache” behavior.
- CloudFront does not cache POST requests by default as POSTs are considered non-idempotent; caching them requires extensive customization which can introduce complexity or inconsistency.
- Lambda’s /tmp directory is ephemeral to the container lifecycle and not reliable as a caching layer for distributed requests.
- Using Systems Manager Parameter Store for caching introduces latency and is not designed as a cache.
Content Locked: The Expert Analysis #
You’ve identified the answer. But do you know the implementation details that separate a Junior from a Senior?
The Expert’s Analysis #
Correct Answer #
Option B
The Winning Logic #
API Gateway offers a built-in caching mechanism that can cache responses at the stage level and be explicitly enabled for specific HTTP methods, including POST. While GET requests are cached by default, POST requests require method cache override because POSTs generally modify server state and aren’t cache-friendly by default.
- This approach uses fully managed caching that reduces Lambda function invocations and backend hits, improving latency and cost efficiency.
- It preserves API security, request/response parameter handling, and integrates smoothly into deployment pipelines.
- API Gateway caching uses an in-memory cache per region, optimized for low latency.
The Trap (Distractor Analysis): #
-
Why not A?
CloudFront is primarily designed to cache GET/HEAD requests. It can be configured to forward and cache POST responses, but that is complex, uncommon, and usually not recommended for dynamically generated content behind API Gateway and Lambda. Fine-grained cache key manipulation for POST is not straightforward. -
Why not C?
The Lambda /tmp storage is ephemeral and isolated per container instance. It cannot reliably share cached data across concurrent/parallel executions or across multiple Lambda containers, so it is unsuitable as a distributed cache. -
Why not D?
Parameter Store is for configuration and secrets retrieval, not low-latency cache. Using Parameter Store introduces API call overhead and latency, which defeats the purpose of caching for performance.
The Technical Blueprint #
# Enable API Gateway stage cache with POST method override using AWS CLI
aws apigateway update-stage \
--rest-api-id <api-id> \
--stage-name <stage-name> \
--patch-operations op=replace,path=/cacheClusterEnabled,value=true
# Override method caching for POST
aws apigateway update-method \
--rest-api-id <api-id> \
--resource-id <resource-id> \
--http-method POST \
--patch-operations op=replace,path=/methodSettings/*/caching/enabled,value=true
The Comparative Analysis #
| Option | API Complexity | Performance Impact | Use Case |
|---|---|---|---|
| A | High — CloudFront caching POST is complex | Medium — Possible but risky | Not suited for dynamic API POST requests |
| B | Moderate — API Gateway cache config | High — Reduces Lambda invocations | Ideal for API Gateway backed REST APIs |
| C | Low — custom Lambda code required | Low — local cache only, no sharing | Unreliable and limited scope caching |
| D | Medium — Integration complexity | Low — SSM latency impacts | Not designed for caching, best for config data |
Real-World Application (Practitioner Insight) #
Exam Rule #
“For API caching of POST requests behind API Gateway, always enable and override caching at the API Gateway stage and method level.”
Real World #
“In production, teams sometimes employ dedicated caching layers like Redis or Elasticache for complex POST request caching scenarios, especially when cache invalidation logic is critical.”
(CTA) Stop Guessing, Start Mastering #
Disclaimer
This is a study note based on simulated scenarios for the AWS DVA-C02 exam.