typescript 42 lines · 5 steps

Recording HTTP metrics with a NestJS interceptor

A NestJS interceptor times every HTTP request and increments Prometheus counters when the response finishes.

Explained by highlit
NestJS Intermediate interceptor observability prometheus rxjs middleware
1import {
2 CallHandler,
3 ExecutionContext,
4 Injectable,
5 NestInterceptor,
6} from '@nestjs/common';
7import { HttpArgumentsHost } from '@nestjs/common/interfaces';
8import { Observable } from 'rxjs';
9import { finalize } from 'rxjs/operators';
10import { InjectMetric } from '@willsoto/nestjs-prometheus';
11import { Counter, Histogram } from 'prom-client';
12import { Request, Response } from 'express';
13 
14@Injectable()
15export class MetricsInterceptor implements NestInterceptor {
16 constructor(
17 @InjectMetric('http_request_duration_seconds')
18 private readonly duration: Histogram<string>,
19 @InjectMetric('http_requests_total')
20 private readonly total: Counter<string>,
21 ) {}
22 
23 intercept(context: ExecutionContext, next: CallHandler): Observable<unknown> {
24 if (context.getType() !== 'http') {
25 return next.handle();
26 }
27 
28 const http: HttpArgumentsHost = context.switchToHttp();
29 const req = http.getRequest<Request>();
30 const res = http.getResponse<Response>();
31 const route = req.route?.path ?? req.path;
32 const stopTimer = this.duration.startTimer({ method: req.method, route });
33 
34 return next.handle().pipe(
35 finalize(() => {
36 const status = String(res.statusCode);
37 stopTimer({ status });
38 this.total.inc({ method: req.method, route, status });
39 }),
40 );
41 }
42}
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Interceptors wrap the request lifecycle, letting you run code before and after the handler responds.
  2. 2The rxjs finalize operator fires on both success and error, making it ideal for cleanup like stopping a timer.
  3. 3Checking context.getType() keeps HTTP-only logic from breaking on non-HTTP transports.

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