Troubleshooting and Optimization

What this domain actually tests

Debugging, tracing, and performance tuning — the smallest domain on the exam and the most practical. If you have ever operated a serverless application at 3am, most of this is already yours.

X-Ray answers every "trace a request across multiple services" question. You need the daemon (or the managed integration), segments and subsegments, and one distinction the exam tests specifically:

Annotations are indexed and searchable. Metadata is not.

If a question wants to filter or search traces by some value, that value must be an annotation. Metadata is along for the ride but cannot be queried. It is a small fact and it is worth a mark.

CloudWatch covers custom metrics, the embedded metric format, metric filters (which turn a log pattern into a metric you can alarm on), and alarms themselves.

The traps

The API Gateway error codes. Learn these three; they turn vague troubleshooting questions into instant answers:

• 502 Bad Gateway — your Lambda returned a malformed response. The response must contain statusCode and body. This is the most common one and it is almost always a code bug, not a config one.
• 504 Gateway Timeout — the integration timed out.
• 429 Too Many Requests — throttling, from a usage plan or an account concurrency limit.

Reserved versus provisioned concurrency. Two settings with confusingly similar names doing genuinely opposite things, and the exam knows it:

• Provisioned concurrency keeps execution environments warm. It eliminates cold starts. Use it when latency matters.
• Reserved concurrency caps how much concurrency a function may consume, so it cannot starve the rest of your account — and, as a side effect, it throttles that function.

If a question is about latency and cold starts, it is provisioned. If it is about one function consuming all available concurrency and breaking everything else, it is reserved.

Memory is the performance lever, and it also controls CPU. Raising a Lambda's memory raises its CPU allocation proportionally, so a compute-bound function can become faster and cheaper at higher memory, because it finishes sooner. This is counter-intuitive and therefore reliably tested.

DynamoDB throttling points at either under-provisioned capacity or a hot partition. Read the key design in the stem before assuming it is a capacity problem.

How to study it

Break things on purpose. Return a raw string from a Lambda behind API Gateway and watch the 502. Set a one-second timeout on something slow and watch the 504. Hammer an endpoint until you get a 429.

Ten minutes of deliberately causing each error teaches you more than any amount of reading, because on exam day the question gives you the symptom and expects you to name the cause — and you will simply recognise it.

Then write down the reserved-versus-provisioned distinction in your own words. If you cannot explain which one prevents cold starts without hesitating, you do not have it yet.