Why production Deno needs careful architecture - See It in Action

Why production Deno needs careful architecture

📖 Scenario: You are building a simple Deno server to handle user requests in a production environment. You want to understand why careful architecture is important to keep the server reliable, maintainable, and secure.

🎯 Goal: Build a basic Deno server with clear separation of concerns and configuration to demonstrate the need for careful architecture in production.

📋 What You'll Learn

Create a data structure to hold user information

Add a configuration variable for maximum allowed users

Implement core logic to filter users based on the configuration

Complete the server setup with proper routing and response

💡 Why This Matters

🌍 Real World

In real production Deno servers, clear architecture helps keep code maintainable, secure, and scalable as the app grows.

💼 Career

Understanding how to structure Deno projects with data, config, and server logic is key for backend developers working with modern JavaScript runtimes.

1

DATA SETUP: Create user data

Create a constant called users that is an array of objects with these exact entries: { id: 1, name: "Alice" }, { id: 2, name: "Bob" }, and { id: 3, name: "Charlie" }.

Deno

# Your code here

Hint

Use const users = [ ... ] with objects inside the array.

2

CONFIGURATION: Set maximum users allowed

Create a constant called maxUsers and set it to the number 2.

Deno

const users = [
  { id: 1, name: "Alice" },
  { id: 2, name: "Bob" },
  { id: 3, name: "Charlie" }
];

# Your code here

Hint

Use const maxUsers = 2; to set the limit.

3

CORE LOGIC: Filter users based on maxUsers

Create a constant called activeUsers that contains the first maxUsers entries from the users array using the slice method.

Deno

const users = [
  { id: 1, name: "Alice" },
  { id: 2, name: "Bob" },
  { id: 3, name: "Charlie" }
];

const maxUsers = 2;

# Your code here

Hint

Use slice(0, maxUsers) to get the first users.

4

COMPLETION: Setup Deno server to respond with active users

Import serve from "https://deno.land/std@0.177.0/http/server.ts". Then call serve with an async function that returns a new Response with JSON.stringify(activeUsers) and header {"Content-Type": "application/json"}.

Deno

const users = [
  { id: 1, name: "Alice" },
  { id: 2, name: "Bob" },
  { id: 3, name: "Charlie" }
];

const maxUsers = 2;

const activeUsers = users.slice(0, maxUsers);

# Your code here

Hint

Use serve with an async function returning a JSON response of activeUsers.

Practice

1. Why is it important to plan permissions carefully when deploying a Deno app to production?

easy

A. Because Deno restricts access to files, network, and environment by default for security.

B. Because Deno automatically grants all permissions to any script.

C. Because permissions do not affect Deno apps in production.

D. Because permissions only matter during development, not in production.

Solution

1. Step 1: Understand Deno's default security model

   Deno runs scripts with no permissions by default to protect your system.

2. Step 2: Recognize the need for explicit permission granting in production

   In production, you must carefully grant only needed permissions to avoid security risks.

3. Final Answer:

   Because Deno restricts access to files, network, and environment by default for security. -> Option A

4. Quick Check:

   Default security = careful permission planning [OK]

Hint: Remember: Deno locks down permissions by default [OK]

Common Mistakes:

• Assuming Deno grants all permissions automatically
• Ignoring permissions in production
• Thinking permissions only matter during development

2. Which of the following is the correct way to run a Deno script with network access permission?

easy

A. deno run --allownetwork script.ts

B. deno run --net-allow script.ts

C. deno run --enable-network script.ts

D. deno run --allow-net script.ts

Solution

1. Step 1: Recall Deno's permission flags syntax

   Deno uses flags like --allow-net to grant network access.

2. Step 2: Identify the correct flag from options

   Only --allow-net matches Deno's official permission flag syntax.

3. Final Answer:

   deno run --allow-net script.ts -> Option D

4. Quick Check:

   Correct flag syntax = --allow-net [OK]

Hint: Look for exact permission flag syntax with dashes [OK]

Common Mistakes:

• Using incorrect flag names like --net-allow
• Combining words without dashes
• Assuming permission flags are optional

3. Consider this Deno code snippet:

try {
  const data = await Deno.readTextFile('config.json');
  console.log('Config loaded:', data);
} catch (error) {
  console.error('Failed to load config:', error.message);
}

What will happen if the file config.json does not exist and the script is run with deno run --allow-read script.ts?

medium

A. The catch block will run and print an error message.

B. The script will crash with an uncaught error.

C. The script will print 'Config loaded:' with empty data.

D. The script will silently ignore the error and continue.

Solution

1. Step 1: Understand try-catch behavior in async file reading

   If the file is missing, Deno.readTextFile throws an error caught by catch.

2. Step 2: Analyze the catch block's effect

   The catch block logs 'Failed to load config:' plus the error message.

3. Final Answer:

   The catch block will run and print an error message. -> Option A

4. Quick Check:

   Missing file triggers catch block [OK]

Hint: Missing file triggers catch block error handling [OK]

Common Mistakes:

• Expecting the script to crash without catch
• Assuming empty string is returned for missing file
• Thinking error is ignored silently

4. You have this Deno script:

const data = await Deno.readTextFile('data.txt');
console.log(data);

When running deno run script.ts without any flags, what error will you get?

medium

A. SyntaxError: Unexpected token

B. PermissionDenied: read access to "data.txt" is not allowed

C. ReferenceError: Deno is not defined

D. No error, script runs fine

Solution

1. Step 1: Recall Deno's default permission restrictions

   Deno denies file read access unless --allow-read is given.

2. Step 2: Identify the error from missing permission

   Running without --allow-read causes a PermissionDenied error.

3. Final Answer:

   PermissionDenied: read access to "data.txt" is not allowed -> Option B

4. Quick Check:

   Missing --allow-read causes PermissionDenied [OK]

Hint: No --allow-read means PermissionDenied error [OK]

Common Mistakes:

• Expecting syntax or reference errors
• Assuming script runs without permissions
• Confusing permission errors with runtime errors

5. You want to build a Deno app that reads a config file, fetches data from an API, and writes logs to a file. Which architecture approach best ensures security and maintainability in production?

hard

A. Grant all permissions globally and write all code in one file.

B. Run the app without permissions and rely on user input for data.

C. Use modular code, grant only needed permissions per module, and handle errors properly.

D. Avoid error handling to keep code simple and grant all permissions at runtime.

Solution

1. Step 1: Consider security by limiting permissions

   Grant only the permissions each module needs to reduce risk.

2. Step 2: Organize code modularly and handle errors

   Modular code improves maintainability; error handling improves reliability.

3. Final Answer:

   Use modular code, grant only needed permissions per module, and handle errors properly. -> Option C

4. Quick Check:

   Modular + limited permissions + error handling = best practice [OK]

Hint: Modular code + least permissions + error handling wins [OK]

Common Mistakes:

• Granting all permissions globally
• Ignoring error handling
• Putting all code in one file