The difference between synchronous and asynchronous middleware

The Concept of Synchronous and Asynchronous Middleware

Middleware is a core mechanism in the Koa2 framework, allowing a series of operations to be executed between requests and responses. Synchronous middleware executes in the order of the code, while asynchronous middleware may involve non-blocking operations and require special handling. Understanding the differences between these two types of middleware is crucial for building efficient applications.

Characteristics of Synchronous Middleware

Synchronous middleware is the simplest form, where each middleware function executes sequentially until encountering a next() call. Koa2 pauses the current middleware, executes subsequent middleware, and then returns to the current middleware to continue execution.

app.use(async (ctx, next) => {  
  console.log('Middleware 1 starts');  
  await next();  
  console.log('Middleware 1 ends');  
});  

app.use(async (ctx, next) => {  
  console.log('Middleware 2 starts');  
  ctx.body = 'Hello World';  
  console.log('Middleware 2 ends');  
});  

The output order will be:

Middleware 1 starts  
Middleware 2 starts  
Middleware 2 ends  
Middleware 1 ends  

Handling Asynchronous Middleware

When middleware contains asynchronous operations, async/await or returning a Promise must be used. Koa2's onion model requires proper handling of asynchronous flows; otherwise, the middleware execution order may become chaotic.

app.use(async (ctx, next) => {  
  const start = Date.now();  
  await next();  
  const ms = Date.now() - start;  
  console.log(`Request duration: ${ms}ms`);  
});  

app.use(async (ctx) => {  
  await new Promise(resolve => setTimeout(resolve, 1000));  
  ctx.body = 'Delayed response';  
});  

Differences in Error Handling

Errors in synchronous middleware can be caught directly with try-catch, while asynchronous middleware requires special handling:

// Synchronous error  
app.use((ctx, next) => {  
  try {  
    throw new Error('Synchronous error');  
  } catch (err) {  
    ctx.status = 500;  
    ctx.body = err.message;  
  }  
});  

// Asynchronous error  
app.use(async (ctx, next) => {  
  try {  
    await someAsyncOperation();  
  } catch (err) {  
    ctx.status = 500;  
    ctx.body = 'Async operation failed';  
  }  
});  

Performance Impact

Synchronous middleware blocks the event loop, while asynchronous middleware better leverages Node.js's non-blocking nature. Consider this file-reading example:

// Synchronous version (not recommended)  
app.use((ctx) => {  
  const data = fs.readFileSync('largefile.txt');  
  ctx.body = data;  
});  

// Asynchronous version (recommended)  
app.use(async (ctx) => {  
  const data = await fs.promises.readFile('largefile.txt');  
  ctx.body = data;  
});  

Combined Usage Scenarios

In real-world applications, the two types of middleware are often mixed. For example, authentication middleware might be synchronous, while database operation middleware is asynchronous:

app.use((ctx, next) => {  
  if (!ctx.headers.authorization) {  
    ctx.throw(401, 'Unauthorized');  
  }  
  return next();  
});  

app.use(async (ctx) => {  
  const user = await User.findOne({ token: ctx.headers.authorization });  
  ctx.body = user;  
});  

Pitfalls in Middleware Execution Order

Asynchronous operations can lead to unexpected execution sequences. The following example demonstrates a common issue:

app.use(async (ctx, next) => {  
  console.log('A starts');  
  await next();  
  console.log('A ends');  
});  

app.use(async (ctx, next) => {  
  console.log('B starts');  
  setTimeout(() => {  
    console.log('Timer callback');  
  }, 0);  
  await next();  
  console.log('B ends');  
});  

app.use((ctx) => {  
  console.log('C');  
  ctx.body = 'Done';  
});  

The output might be:

A starts  
B starts  
C  
B ends  
A ends  
Timer callback  

Best Practice Recommendations

1. Prefer declaring middleware with async functions.
2. All asynchronous operations should be awaited or return a Promise.
3. Avoid synchronous I/O operations in middleware.
4. For complex asynchronous flows, consider using koa-compose.
5. Error handling should be placed in the outermost middleware.

// Using koa-compose to combine middleware  
const compose = require('koa-compose');  

const middleware1 = async (ctx, next) => {  
  // ...  
  await next();  
};  

const middleware2 = async (ctx, next) => {  
  // ...  
  await next();  
};  

app.use(compose([middleware1, middleware2]));