Server middleware in GraphQL

Introduction

Server middleware helps manage requests before they reach your GraphQL server. It can check, change, or log data to keep things safe and organized.

When you want to check if a user is logged in before they get data.

When you need to log every request for debugging or tracking.

When you want to add extra information to requests, like user roles.

When you want to limit how many requests a user can make in a time.

When you want to handle errors or modify responses before sending them.

Syntax

GraphQL

app.use((req, res, next) => {
  // Your middleware code here
  next();
});

app.use adds middleware to your server.

next() moves to the next middleware or the main handler.

Examples

This middleware logs every request before continuing.

GraphQL

app.use((req, res, next) => {
  console.log('Request received');
  next();
});

This middleware checks if the user sent an authorization header. If not, it stops the request.

GraphQL

app.use((req, res, next) => {
  if (!req.headers.authorization) {
    res.status(401).send('Unauthorized');
  } else {
    next();
  }
});

Sample Program

This example sets up a GraphQL server with middleware that logs every request URL before handling it.

GraphQL

const express = require('express');
const { graphqlHTTP } = require('express-graphql');
const { buildSchema } = require('graphql');

const app = express();

// Middleware to log requests
app.use((req, res, next) => {
  console.log(`Received request for ${req.url}`);
  next();
});

// Simple GraphQL schema
const schema = buildSchema(`
  type Query {
    hello: String
  }
`);

// Root resolver
const root = {
  hello: () => 'Hello world!'
};

// GraphQL endpoint
app.use('/graphql', graphqlHTTP({
  schema: schema,
  rootValue: root,
  graphiql: true
}));

app.listen(4000, () => {
  console.log('Server running on http://localhost:4000/graphql');
});

Important Notes

Middleware runs in the order you add it, so order matters.

Always call next() unless you want to stop the request.

Middleware can modify the request or response objects to add useful info.

Summary

Server middleware runs before your GraphQL code to manage requests.

Use middleware to check, log, or change requests safely.

Remember to call next() to continue processing.

Practice

1. What is the main purpose of server middleware in a GraphQL server?

easy

A. To run code before GraphQL resolvers to manage requests

B. To define the GraphQL schema types

C. To execute database queries directly

D. To format the GraphQL query results

Solution

1. Step 1: Understand middleware role

   Middleware runs before the main GraphQL code to handle requests.

2. Step 2: Identify middleware tasks

   It can check, log, or modify requests before resolvers run.

3. Final Answer:

   To run code before GraphQL resolvers to manage requests -> Option A

4. Quick Check:

   Middleware manages requests before resolvers [OK]

Hint: Middleware runs before resolvers to manage requests [OK]

Common Mistakes:

• Confusing middleware with schema definition
• Thinking middleware executes database queries
• Assuming middleware formats results

2. Which of the following is the correct way to call the next middleware function in a GraphQL server middleware?

easy

A. next();

B. continue();

C. proceed();

D. runNext();

Solution

1. Step 1: Recall middleware function signature

   Middleware functions receive a next callback to continue processing.

2. Step 2: Identify correct call syntax

   The standard call to continue is next(); exactly.

3. Final Answer:

   next(); -> Option A

4. Quick Check:

   Use next() to continue middleware chain [OK]

Hint: Call next() to continue middleware processing [OK]

Common Mistakes:

• Using incorrect function names like continue()
• Forgetting to call next() causing request hang
• Calling next without parentheses

3. Given this middleware code snippet, what will be logged when a request is made?

function logMiddleware(req, res, next) {
  console.log('Request received');
  next();
  console.log('Request processed');
}

medium

A. Request processed\nRequest received

B. Request received\nRequest processed

C. Only Request received

D. Only Request processed

Solution

1. Step 1: Analyze middleware execution order

   First, 'Request received' logs before calling next().

2. Step 2: Understand next() behavior

   After next() completes, 'Request processed' logs.

3. Final Answer:

   Request received\nRequest processed -> Option B

4. Quick Check:

   Logs before and after next() run in order [OK]

Hint: Logs before and after next() run in order [OK]

Common Mistakes:

• Assuming next() blocks logging after it
• Thinking logs reverse order
• Ignoring that next() allows later code to run

4. Identify the error in this middleware code:

function authMiddleware(req, res, next) {
  if (!req.user) {
    res.status(401).send('Unauthorized');
  }
  next();
}

medium

A. req.user check should be after next()

B. Missing call to next() inside if block

C. res.status should be res.sendStatus

D. next() is called even after sending response

Solution

1. Step 1: Check flow after sending response

   If !req.user, response is sent but next() still runs.

2. Step 2: Understand middleware flow control

   Calling next() after sending response can cause errors or duplicate handling.

3. Final Answer:

   next() is called even after sending response -> Option D

4. Quick Check:

   Stop calling next() after response sent [OK]

Hint: Don't call next() after sending response [OK]

Common Mistakes:

• Calling next() unconditionally after response
• Not returning after sending response
• Misplacing user check after next()

5. You want to add middleware that logs the request time and then modifies the request object to add a requestId before GraphQL resolvers run. Which middleware code correctly achieves this?

hard

A. function middleware(req, res, next) {
req.requestTime = Date.now();
next();
res.requestId = Math.random().toString(36).slice(2);
}

B. function middleware(req, res) {
req.requestTime = Date.now();
next();
req.requestId = Math.random().toString(36).slice(2);
}

C. function middleware(req, res, next) {
req.requestTime = Date.now();
req.requestId = Math.random().toString(36).slice(2);
next();
}

D. function middleware(req, res, next) {
next();
req.requestTime = Date.now();
req.requestId = Math.random().toString(36).slice(2);
}

Solution

1. Step 1: Ensure middleware modifies request before next()

   Properties must be set on req before calling next() to be available in resolvers.

2. Step 2: Check correct use of next() and property assignment

   function middleware(req, res, next) { req.requestTime = Date.now(); req.requestId = Math.random().toString(36).slice(2); next(); } sets both properties before next(), ensuring proper order and correct object.

3. Final Answer:

   function middleware(req, res, next) { req.requestTime = Date.now(); req.requestId = Math.random().toString(36).slice(2); next(); } -> Option C

4. Quick Check:

   Set req properties before next() call [OK]

Hint: Set req properties before next() to pass data [OK]

Common Mistakes:

• Calling next() before setting properties
• Modifying res instead of req for request data
• Missing next() parameter or call