Health checks and monitoring in Deno - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a simple HTTP server that responds with 'OK' for health checks.

Deno

import { serve } from "https://deno.land/std@0.177.0/http/server.ts";

serve((_req) => new Response("OK"), { port: [1] });

Drag options to blanks, or click blank then click option'

A3000

B5000

C80

D8080

Attempts:

3 left

💡 Hint

Common Mistakes

Using port 80 without permissions

Choosing a port number outside the typical range

The server listens on port 3000, a common port for development servers.

2fill in blank

medium

Complete the code to check the health endpoint path '/health'.

Deno

import { serve } from "https://deno.land/std@0.177.0/http/server.ts";

serve((req) => {
  if (req.url.endsWith([1])) {
    return new Response("OK");
  }
  return new Response("Not Found", { status: 404 });
}, { port: 3000 });

Drag options to blanks, or click blank then click option'

A"/health"

B"/check"

C"/status"

D"/ping"

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect endpoint paths

Missing the leading slash in the path

The health check endpoint is commonly '/health' to indicate service status.

3fill in blank

hard

Fix the error in the code to correctly parse the request URL for health check.

Deno

serve((req) => {
  const url = new URL(req.[1]);
  if (url.pathname === "/health") {
    return new Response("OK");
  }
  return new Response("Not Found", { status: 404 });
}, { port: 3000 });

Drag options to blanks, or click blank then click option'

AurlString

Bpath

Curl

DurlRaw

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'path' instead of 'url'

Trying to access a non-existent property

The Request object has a 'url' property containing the full URL string.

4fill in blank

hard

Fill the blank to create a JSON response with status and uptime.

Deno

serve(() => {
  const uptime = performance.[1]();
  return new Response(JSON.stringify({ status: "OK", uptime: uptime }), {
    headers: { "Content-Type": "application/json" }
  });
}, { port: 3000 });

Drag options to blanks, or click blank then click option'

A/

B*

Cseconds

Dnow

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect Deno API

Using division instead of multiplication

performance.now() returns uptime in milliseconds since the Deno process started.

5fill in blank

hard

Fill both blanks to create a health check that returns status, uptime, and memory usage in JSON.

Deno

serve(() => {
  const metrics = performance.[1]();
  const memory = Deno.[2]();
  return new Response(JSON.stringify({
    status: "OK",
    uptime: metrics,
    memory: memory.rss
  }), {
    headers: { "Content-Type": "application/json" }
  });
}, { port: 3000 });

Drag options to blanks, or click blank then click option'

Anow

BmemoryUsage

C*

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up API names

Using division instead of multiplication for uptime

performance.now() provides uptime in ms, Deno.memoryUsage() provides memory info, and memory.rss is resident set size in bytes.

Practice

1. What is the main purpose of a health check endpoint in a Deno application?

easy

A. To verify if the application is running and responsive

B. To deploy the application to production

C. To increase the application's memory usage

D. To compile the Deno code into machine code

Solution

1. Step 1: Understand health check concept

   A health check endpoint is designed to confirm that the app is alive and responding to requests.

2. Step 2: Identify the correct purpose

   Options B, C, and D describe unrelated tasks like deployment, resource usage, or compilation, which are not health checks.

3. Final Answer:

   To verify if the application is running and responsive -> Option A

4. Quick Check:

   Health check = Verify app responsiveness [OK]

Hint: Health checks confirm app is alive and responding [OK]

Common Mistakes:

• Confusing health checks with deployment steps
• Thinking health checks improve performance directly
• Mixing health checks with code compilation

2. Which of the following is the correct way to create a simple health check HTTP endpoint in Deno?

easy

A. import { serve } from "https://deno.land/std/http/server.ts";
serve(req => console.log(req), { port: 8000 });

B. import { listenAndServe } from "https://deno.land/std/http/server.ts";
listenAndServe(8000, () => console.log("OK"));

C. import { serve } from "https://deno.land/std/http/server.ts";
serve(() => console.log("OK"), { port: 8000 });

D. import { serve } from "https://deno.land/std/http/server.ts";
serve(req => new Response("OK"), { port: 8000 });

Solution

1. Step 1: Check correct import and usage

   import { serve } from "https://deno.land/std/http/server.ts"; serve(req => new Response("OK"), { port: 8000 }); correctly imports 'serve' and returns a Response with "OK" for each request, which is a valid health check endpoint.

2. Step 2: Identify incorrect usage in other options

   import { listenAndServe } from "https://deno.land/std/http/server.ts"; listenAndServe(8000, () => console.log("OK")); uses deprecated 'listenAndServe' and does not respond to requests properly. Options A and C use console.log instead of returning a Response, so they won't respond correctly.

3. Final Answer:

   import { serve } from "https://deno.land/std/http/server.ts"; serve(req => new Response("OK"), { port: 8000 }); -> Option D

4. Quick Check:

   serve + Response("OK") = Correct health endpoint [OK]

Hint: Use serve with a Response object to reply to requests [OK]

Common Mistakes:

• Using console.log instead of returning Response
• Using deprecated functions like listenAndServe
• Not returning a response to HTTP requests

3. Given this Deno code snippet for a health check endpoint, what will be the output when accessed via HTTP?

import { serve } from "https://deno.land/std/http/server.ts";
serve(req => {
  if (new URL(req.url).pathname === "/health") {
    return new Response("Healthy", { status: 200 });
  } else {
    return new Response("Not Found", { status: 404 });
  }
}, { port: 8080 });

medium

A. Response with body 'Healthy' and status 404 when URL is '/health'

B. Response with body 'Healthy' and status 200 when URL is '/health'

C. Response with body 'Not Found' and status 200 for all URLs

D. Server crashes due to syntax error

Solution

1. Step 1: Analyze request URL condition

   The code checks if the request URL is exactly '/health'. If yes, it returns a 200 status with "Healthy".

2. Step 2: Understand else branch

   If the URL is anything else, it returns a 404 status with "Not Found".

3. Final Answer:

   Response with body 'Healthy' and status 200 when URL is '/health' -> Option B

4. Quick Check:

   URL '/health' = 200 Healthy [OK]

Hint: Check URL condition to predict response status and body [OK]

Common Mistakes:

• Confusing status codes for different URLs
• Assuming server crashes without syntax errors
• Mixing response body and status code

4. You wrote this Deno health check server code but it does not respond to requests properly. What is the error?

import { serve } from "https://deno.land/std/http/server.ts";
serve(req => {
  console.log("Received request");
});

medium

A. The serve function requires a port number as a string

B. The import statement is incorrect

C. The handler does not return a Response object

D. console.log is not allowed inside serve

Solution

1. Step 1: Check handler function

   The handler function only logs to console but does not return any Response, so HTTP requests get no valid reply.

2. Step 2: Verify other parts

   The import is correct, serve accepts an options object for port, and console.log is allowed. The main issue is missing Response return.

3. Final Answer:

   The handler does not return a Response object -> Option C

4. Quick Check:

   Handler must return Response [OK]

Hint: Always return a Response object in serve handler [OK]

Common Mistakes:

• Forgetting to return Response in handler
• Thinking console.log sends HTTP response
• Misunderstanding serve function parameters

5. You want to monitor your Deno app's health and expose metrics like uptime and request count. Which approach best combines health checks and monitoring in one server?

hard

A. Create two endpoints: '/health' returns 200 OK; '/metrics' returns JSON with uptime and request count

B. Use only '/health' endpoint returning plain text 'OK' without metrics

C. Create a single endpoint that returns HTML with uptime but no status code

D. Run two separate Deno servers, one for health and one for metrics, on different ports

Solution

1. Step 1: Understand combined health and monitoring needs

   Health checks confirm app is alive; monitoring provides metrics like uptime and request count.

2. Step 2: Evaluate options for best practice

   Create two endpoints: '/health' returns 200 OK; '/metrics' returns JSON with uptime and request count cleanly separates concerns with '/health' for status and '/metrics' for detailed data, both accessible on one server.

3. Step 3: Reject less effective options

   Use only '/health' endpoint returning plain text 'OK' without metrics lacks metrics, C lacks proper status code, D adds complexity by running two servers unnecessarily.

4. Final Answer:

   Create two endpoints: '/health' returns 200 OK; '/metrics' returns JSON with uptime and request count -> Option A

5. Quick Check:

   Separate endpoints for health and metrics = Best practice [OK]

Hint: Use separate endpoints for health and metrics on same server [OK]

Common Mistakes:

• Mixing metrics and health in one endpoint without status code
• Ignoring metrics in health checks
• Running multiple servers unnecessarily