What is the execution mechanism of Koa's onion model and its practical application scenarios

Koa's Onion Model is its most core design feature, implementing middleware execution flow control through async/await. In the onion model, middleware executes in registration order, forming a layered structure similar to an onion.

Execution flow:

1. Request enters from outer middleware, passing inward layer by layer
2. Each middleware executes "pre-logic" before await next()
3. Execute await next() to enter the next layer of middleware
4. After reaching the innermost layer, start returning outward layer by layer
5. Each middleware executes "post-logic" after await next()
6. Final response returns to client from outermost middleware

Code example:

javascript
const Koa = require('koa');
const app = new Koa();

app.use(async (ctx, next) => {
  console.log('1 - pre');
  await next();
  console.log('1 - post');
});

app.use(async (ctx, next) => {
  console.log('2 - pre');
  await next();
  console.log('2 - post');
});

app.use(async (ctx) => {
  console.log('3 - core');
  ctx.body = 'Hello Koa';
});

// Execution order: 1-pre -> 2-pre -> 3-core -> 2-post -> 1-post

Advantages of onion model:

1. Clear execution order: Pre and post logic separated, clear code structure
2. Flexible control: Each middleware can decide whether to continue downstream
3. Unified error handling: Can catch all middleware errors through try-catch
4. Middleware reuse: Can reuse middleware logic at different positions
5. Request/response handling: Convenient to execute different logic when request enters and response returns

Practical application scenarios:

• Logging: Record request and response info in pre/post logic
• Error handling: Unified capture and handle errors in outer middleware
• Authentication: Verify user identity in pre-logic
• Response time statistics: Calculate total request processing time
• Response formatting: Unified response format handling in post-logic