Amazon Web Services services
5 / 24
Amazon Web Services

Serverless — AWS Lambda

Run code in response to events without provisioning or managing any servers.

Lambda runs a function when an event arrives. There is no server to size, patch or keep running.

A motion-sensor light. It draws zero power in the dark and switches on instantly the moment someone walks by, for exactly as long as needed — you never pay to leave it burning all night.

Key Concepts

1
    S3 object created      -> resize the image
    API Gateway request    -> handle the HTTP call
    SQS message            -> process the job
    EventBridge schedule   -> nightly report
    DynamoDB Stream        -> react to a row change
2
You choose memory; CPU follows it. 1,792 MB is roughly one full vCPU. So a CPU-bound function is often cheaper at higher memory, because it finishes proportionally faster — a counter-intuitive point worth knowing.
3
Billing is per millisecond of execution, with nothing charged while idle. That is the real difference from a server sitting at 5% utilisation all night.
4
Cold starts are the limitation to be honest about.
    cold   new execution environment: download, init runtime,
           run your init code -- tens to hundreds of ms
    warm   environment reused, handler only
5
Keep the init light, reuse SDK clients across invocations, and use provisioned concurrency when a latency-sensitive path cannot tolerate it.
6
Concurrency is the scaling and the blast radius.
    account limit    1,000 concurrent executions by default
    reserved         guarantee capacity for a critical function AND
                     cap it so it cannot starve the others
    provisioned      pre-warmed environments, no cold start, charged
7
Reserved concurrency as a cap is the under-appreciated use. Without it, one runaway function can consume the whole account limit and throttle everything else.
8
The hard limits.
    15 minutes maximum execution
    10 GB memory, 10 GB ephemeral /tmp
    6 MB synchronous payload, 256 KB asynchronous
9
Functions are stateless. Anything that must persist goes to S3, DynamoDB or ElastiCache. The /tmp directory survives only while the environment is reused, which makes it a cache and never a store.
/tmp
10
What the interviewer is probing.1. "What causes a cold start and how do you reduce it?" Probing: whether you can be specific. Stalls: "The first call is slow." Moves up: a new execution environment must download the code and initialise the runtime; keep init light, reuse clients across invocations, and use provisioned concurrency for latency-sensitive paths.
11
2. "Would raising memory ever make a function cheaper?" Probing: the CPU relationship, which is counter-intuitive. Stalls: "No, memory costs more." Moves up: yes for CPU-bound work — CPU scales with memory, so it finishes proportionally faster and the millisecond bill can fall.
12
3. "What does reserved concurrency do?" Probing: both of its uses. Stalls: "It guarantees capacity." Moves up: it guarantees capacity and caps it, so one runaway function cannot consume the account limit and throttle everything else.
13
4. "What are the hard limits you design around?" Probing: the boundaries. Stalls: "There are none." Moves up: 15 minutes, 10GB memory and /tmp, 6MB synchronous payload — so long work goes asynchronous and large payloads go via S3.
Use cases:lambdaserverlessevent-drivenapi-gateway