Health checks and monitoring in Deno - Mini Project: Build & Apply

Health Checks and Monitoring with Deno

📖 Scenario: You are working on a simple web service that needs to report its health status. This helps system administrators and monitoring tools know if the service is running well or if it has problems.We will build a small Deno server that responds to health check requests.

🎯 Goal: Create a Deno server that responds to /health requests with a JSON status. The server should also have a configurable threshold to simulate a health check condition.

📋 What You'll Learn

Create a Deno HTTP server

Add a configuration variable for health status threshold

Implement a health check endpoint at /health

Return JSON response with status and uptime

Print the server start message

💡 Why This Matters

🌍 Real World

Health checks are used by monitoring systems to know if a service is working well. This helps keep websites and apps reliable.

💼 Career

Understanding health checks and monitoring is important for DevOps roles to maintain system uptime and troubleshoot issues quickly.

1

Create a basic Deno HTTP server

Create a Deno HTTP server using serve from std/http/server.ts. The server should listen on port 8000 and respond with "Server is running" to any request.

Deno

import { serve } from "https://deno.land/std@0.203.0/http/server.ts"

// Your code here

Hint

Use serve function and return a new Response with the text "Server is running".

2

Add a health status threshold variable

Add a variable called healthThreshold and set it to 5. This will simulate a threshold for health checks.

Deno

import { serve } from "https://deno.land/std@0.203.0/http/server.ts"

const healthThreshold = 5

// Your code here

Hint

Declare const healthThreshold = 5 before starting the server.

3

Implement the /health endpoint with health check logic

Modify the server handler to check if the request URL is /health. If yes, respond with JSON containing status set to "healthy" if healthThreshold is greater than 0, otherwise "unhealthy". Also include uptime in seconds since server start. For other URLs, respond with "Server is running".

Deno

import { serve } from "https://deno.land/std@0.203.0/http/server.ts"

const healthThreshold = 5

const startTime = Date.now()

serve((req) => {
  // Your code here
}, { port: 8000 })

Hint

Use new URL(req.url).pathname to check the path. Calculate uptime by subtracting startTime from current time. Return JSON with status and uptime.

4

Print server start message

Add a console.log statement before starting the server to print "Server started on http://localhost:8000".

Deno

import { serve } from "https://deno.land/std@0.203.0/http/server.ts"

const healthThreshold = 5

const startTime = Date.now()

// Your code here

serve((req) => {
  if (new URL(req.url).pathname === "/health") {
    const uptime = Math.floor((Date.now() - startTime) / 1000)
    const status = healthThreshold > 0 ? "healthy" : "unhealthy"
    const body = JSON.stringify({ status, uptime })
    return new Response(body, { headers: { "Content-Type": "application/json" } })
  }
  return new Response("Server is running")
}, { port: 8000 })

Hint

Use console.log("Server started on http://localhost:8000") before calling serve.

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