Middleware patterns in Deno - Performance & Optimization

Performance: Middleware patterns

MEDIUM IMPACT

Middleware patterns affect request processing speed and server response time by adding layers of code that run before the final handler.

Adding multiple middleware functions to handle HTTP requests

Deno

app.use(async (ctx, next) => { await Promise.all([someHeavyTask(), anotherHeavyTask()]); await next(); });
app.use(async (ctx) => { ctx.response.body = 'Done'; });

Runs independent tasks in parallel, reducing total wait time before response.

📈 Performance GainReduces blocking time by running tasks concurrently, improving INP and lowering CPU spikes.

Adding multiple middleware functions to handle HTTP requests

Deno

app.use(async (ctx, next) => { await someHeavyTask(); await next(); });
app.use(async (ctx, next) => { await anotherHeavyTask(); await next(); });
app.use(async (ctx) => { ctx.response.body = 'Done'; });

Each middleware runs heavy tasks sequentially, increasing total response time and CPU load.

📉 Performance CostBlocks response for each middleware task, increasing INP and CPU usage linearly with number of middlewares.

Performance Comparison

Pattern	Middleware Count	Sequential Tasks	Parallel Tasks	Verdict
Sequential heavy middleware	3	3	0	[X] Bad
Parallelized middleware tasks	2	2	2	[OK] Good

Rendering Pipeline

Middleware runs during the server's request handling phase before sending the response. Each middleware adds processing time before the browser receives data.

→Request Processing

→Response Generation

⚠️ BottleneckSequential middleware execution causing increased server response time

Core Web Vital Affected

LCP

Middleware patterns affect request processing speed and server response time by adding layers of code that run before the final handler.

Optimization Tips

1Avoid heavy synchronous tasks in middleware to prevent blocking.

2Run independent middleware tasks in parallel to reduce total processing time.

3Keep middleware count minimal and focused to reduce overhead.

Performance Quiz - 3 Questions

What is the main performance drawback of using many sequential middleware functions?

A They improve browser rendering speed.

B They reduce the size of the response payload.

C They increase server response time by adding sequential processing delays.

D They decrease CPU usage on the server.

DevTools: Network

How to check: Open DevTools, go to Network tab, reload the page, and check the Time column for server response delays.

What to look for: Look for long waiting (TTFB) times indicating slow server processing due to middleware.

Practice

1. What is the main purpose of middleware in a Deno web application?

easy

A. To compile TypeScript code into JavaScript

B. To store data permanently in a database

C. To run code step-by-step for each incoming request

D. To create user interface components

Solution

1. Step 1: Understand middleware role

   Middleware processes each request in order, allowing code to run before sending a response.

2. Step 2: Compare options

   Only To run code step-by-step for each incoming request describes this step-by-step request handling. Other options describe unrelated tasks.

3. Final Answer:

   To run code step-by-step for each incoming request -> Option C

4. Quick Check:

   Middleware = step-by-step request code [OK]

Hint: Middleware runs code for each request in order [OK]

Common Mistakes:

• Confusing middleware with database operations
• Thinking middleware builds UI components
• Assuming middleware compiles code

2. Which of the following is the correct way to call the next middleware in Deno?

easy

A. next.await()

B. next()

C. call next()

D. await next()

Solution

1. Step 1: Recall async middleware pattern

   Middleware functions are async and must await next() to continue the chain.

2. Step 2: Check syntax correctness

   Only 'await next()' correctly waits for the next middleware to finish before continuing.

3. Final Answer:

   await next() -> Option D

4. Quick Check:

   Use await next() to proceed [OK]

Hint: Always use await before next() in async middleware [OK]

Common Mistakes:

• Calling next() without await causing unexpected behavior
• Using incorrect syntax like call next()
• Trying to use next.await() which is invalid

3. Given this middleware code snippet in Deno:

async function logger(ctx, next) {
  console.log('Before');
  await next();
  console.log('After');
}

async function handler(ctx) {
  ctx.response.body = 'Hello';
}

// Middleware chain: logger -> handler

What will be printed to the console when a request is handled?

medium

A. Before\nAfter

B. After\nBefore

C. Before

D. After

Solution

1. Step 1: Analyze middleware flow

   Logger prints 'Before', then awaits next middleware (handler), then prints 'After'.

2. Step 2: Understand order of prints

   Since handler runs between the two console.logs, output order is 'Before' then 'After'.

3. Final Answer:

   Before\nAfter -> Option A

4. Quick Check:

   Middleware logs before and after next() [OK]

Hint: Middleware logs before and after await next() [OK]

Common Mistakes:

• Assuming 'After' prints before 'Before'
• Thinking only 'Before' or 'After' prints
• Ignoring await causing wrong order

4. Identify the error in this Deno middleware code:

async function auth(ctx, next) {
  if (!ctx.request.headers.get('Authorization')) {
    ctx.response.status = 401;
    ctx.response.body = 'Unauthorized';
  }
  await next();
}

medium

A. Missing await before ctx.request.headers.get

B. Should return after setting 401 to stop middleware chain

C. ctx.response.body must be a JSON object

D. next() should not be awaited

Solution

1. Step 1: Understand middleware flow control

   If authorization header is missing, response is set to 401, but code still calls await next(), continuing the chain.

2. Step 2: Identify fix to stop chain

   To prevent further middleware running, function should return immediately after setting 401 response.

3. Final Answer:

   Should return after setting 401 to stop middleware chain -> Option B

4. Quick Check:

   Return early to stop middleware chain [OK]

Hint: Return after response to stop middleware chain [OK]

Common Mistakes:

• Not returning causes next middleware to run anyway
• Thinking await is missing on header get
• Believing response body must be JSON always

5. You want to create a middleware in Deno that measures how long each request takes and adds a custom header X-Response-Time with the duration in milliseconds. Which of the following middleware implementations correctly achieves this?

hard

A. async function responseTime(ctx, next) {
const start = performance.now();
await next();
const ms = performance.now() - start;
ctx.response.headers.set('X-Response-Time', ms.toFixed(2));
}

B. async function responseTime(ctx, next) {
const start = Date.now();
const ms = Date.now() - start;
ctx.response.headers.set('X-Response-Time', `${ms}ms`);
await next();
}

C. async function responseTime(ctx, next) {
const start = Date.now();
next();
const ms = Date.now() - start;
ctx.response.headers.set('X-Response-Time', `${ms}ms`);
}

D. async function responseTime(ctx, next) {
await next();
const ms = Date.now();
ctx.response.headers.set('X-Response-Time', ms);
}

Solution

1. Step 1: Measure time before and after next()

   Start timer before await next(), then calculate duration after it finishes.

2. Step 2: Use precise timer and set header correctly

   performance.now() gives better precision than Date.now(). Setting header with ms.toFixed(2) formats nicely.

3. Final Answer:

   Middleware using performance.now(), await next(), and setting header after next() -> Option A

4. Quick Check:

   Use performance.now() and await next() then set header [OK]

Hint: Start timer before await next(), set header after [OK]

Common Mistakes:

• Calculating time before await next() runs
• Using Date.now() which is less precise
• Setting header before next() completes