🧠 Core Idea
Lambda runs code without servers, but functions aren't directly accessible over the internet. If clients — websites, mobile apps, external users — need to reach your Lambda functions, something has to expose them as web APIs.
That's Amazon API Gateway: the front door to your serverless app, receiving client requests and forwarding them to backend services like Lambda.
🌐 What is API Gateway?
A fully managed service for creating, publishing, securing, and monitoring APIs. AWS handles the infrastructure — you focus on the application logic.
🤔 Why Do We Need API Gateway?
Picture a serverless app built with Lambda and DynamoDB. Lambda can process requests and read/write data — but users on the internet can't invoke a Lambda function directly.
Something needs to expose that function as a web endpoint. API Gateway does exactly that: it accepts requests from external clients and forwards them to the right Lambda function.
🏗️ How It Works
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<div class="af-inner">
<div class="af-controls">
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Play
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Reset
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<!-- Client (Supericons: lucide:monitor) -->
<g class="af-node" id="nodeClient" transform="translate(55,55) scale(2.9) translate(-12,-12)" fill="none" stroke="var(--client-color)" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round">
<rect width="20" height="14" x="2" y="3" rx="2"/>
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<line x1="12" x2="12" y1="17" y2="21"/>
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<text x="55" y="147" text-anchor="middle" class="af-label">Client</text>
<!-- connector 1: Client <-> API Gateway -->
<line class="af-connector" id="lineC" x1="185" y1="60" x2="345" y2="60" marker-end="url(#afArrowR)" marker-start="url(#afArrowL)"/>
<text x="265" y="40" class="af-arrow-label">REST API</text>
<!-- API Gateway (Supericons: lucide:door-open) -->
<g class="af-node" id="nodeGW">
<rect x="345" y="15" width="90" height="90" fill="var(--gw-color)"/>
<g transform="translate(367,37) scale(1.9)" fill="none" stroke="#ffffff" stroke-width="1.8" stroke-linecap="round" stroke-linejoin="round">
<path d="M11 20H2"/>
<path d="M11 4.562v16.157a1 1 0 0 0 1.242.97L19 20V5.562a2 2 0 0 0-1.515-1.94l-4-1A2 2 0 0 0 11 4.561z"/>
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<path d="M14 12h.01"/>
<path d="M22 20h-3"/>
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<text x="390" y="147" class="af-label">API Gateway</text>
<!-- connector 2: API Gateway <-> Lambda -->
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<text x="572" y="40" class="af-arrow-label">PROXY REQUESTS</text>
<!-- Lambda (lambda glyph in circle) -->
<g class="af-node" id="nodeLambda">
<circle cx="755" cy="60" r="48" fill="none" stroke="var(--lambda-color)" stroke-width="3"/>
<text x="755" y="82" text-anchor="middle" font-size="58" font-family="Georgia, serif" fill="var(--lambda-color)">λ</text>
</g>
<text x="755" y="147" class="af-label">Lambda</text>
<!-- connector 3: Lambda <-> DynamoDB -->
<line class="af-connector" id="lineL" x1="805" y1="60" x2="1090" y2="60" marker-end="url(#afArrowR)" marker-start="url(#afArrowL)"/>
<text x="947" y="40" class="af-arrow-label">CRUD</text>
<!-- DynamoDB (Supericons: lucide:database) -->
<g class="af-node" id="nodeDDB">
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<path d="M3 5V19A9 3 0 0 0 21 19V5"/>
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var resetBtn = wrapper.querySelector('#afReset');
var packet = wrapper.querySelector('#afPacket');
var lineC = wrapper.querySelector('#lineC');
var lineG = wrapper.querySelector('#lineG');
var lineL = wrapper.querySelector('#lineL');
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var nodeGW = wrapper.querySelector('#nodeGW');
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var nodeDDB = wrapper.querySelector('#nodeDDB');
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playBtn.disabled = true;
var d = 1150;
var t = 0;
// request: client -> gateway -> lambda -> dynamodb
nodeClient.classList.add('is-pulsing');
packet.classList.add('is-visible');
timers.push(setTimeout(function () {
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lineC.classList.add('is-lit-fwd');
packet.setAttribute('cx', 345);
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timers.push(setTimeout(function () {
lineC.classList.remove('is-lit-fwd');
nodeGW.classList.add('is-pulsing');
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nodeGW.classList.remove('is-pulsing');
lineG.classList.add('is-lit-fwd');
packet.setAttribute('cx', 705);
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nodeLambda.classList.add('is-pulsing');
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nodeDDB.classList.add('is-pulsing');
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// response: dynamodb -> lambda -> gateway -> client
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packet.setAttribute('cx', 440);
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lineG.classList.remove('is-lit-back');
nodeGW.classList.add('is-pulsing');
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nodeGW.classList.remove('is-pulsing');
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</div>Client sends an HTTP request
API Gateway receives it
Forwards it to a Lambda function
Lambda processes the request, reads/writes DynamoDB if needed
Response flows back through API Gateway to the client
Together, API Gateway (requests) + Lambda (logic) + DynamoDB (data) form a fully serverless app — no servers to provision or manage.
API Gateway is serverless itself: no infrastructure to manage, automatic scaling, built-in high availability. Traffic spikes are handled automatically.
REST APIs — standard HTTP methods (GET, POST, PUT, DELETE); used by websites, mobile/web apps, third-party integrations
WebSocket APIs — connections stay open for real-time communication; used for live chat, gaming, dashboards, notifications, streaming
Authentication — only authorized users/apps can invoke the API
API keys — identify clients, control access, track usage
Throttling — limits requests per period to prevent abuse and control costs
Monitoring — tracks traffic, request volume, performance, and errors
Whether you get a few requests a day or thousands per second, API Gateway scales automatically — no manual intervention needed.
🌟 Why Use API Gateway?
Fully managed, serverless
Automatic scaling
Easy to create and secure APIs
Built-in auth, API keys, throttling, monitoring
Native Lambda integration
🎓 Exam Tips
API Gateway = entry point for serverless APIs
Lambda functions aren't exposed directly to the internet — API Gateway is the front door
Commonly paired with Lambda and DynamoDB
Supports both REST and WebSocket APIs
Built-in: authentication, API keys, throttling, monitoring, auto-scaling
"Expose a Lambda function as an HTTP API" → Amazon API Gateway
🧠 Key Takeaways
API Gateway is a fully managed service for building and securing APIs
It lets external clients reach Lambda functions securely
Serverless and auto-scaling by design
Supports REST and WebSocket APIs
Built-in authentication, API keys, throttling, and monitoring
Common architecture: Client → API Gateway → Lambda → DynamoDB