Design an error model

Design an error model

One consistent error contract for a service (and ideally the whole fleet): typed exceptions → a global filter → one response shape, so clients parse errors the same way and logs are uniform. Examples NestJS/TS, neutral @org scope; the base class is AppException. principle → ▸ Example → ▸ Other stacks. This is the content of the shared infra-exception package (see structure-a-shared-backend-lib); it pairs with write-cross-cutting-code (the filter is a cross-cutting primitive) and write-service-code §7 (logging).

Core principle

Never let raw framework/vendor errors reach the client, and never hand-shape error JSON in handlers. Throw a typed exception that carries its status + a stable body; one global filter turns every throw — typed, validation, or unexpected — into the same response shape with a trace id. Handlers just throw new NotFoundError(...); they never build an error response.

1. A typed exception hierarchy

• One base exception extends the framework's HTTP exception and carries a structured payload { statusCode, message, data? }, plus a prepareResponse(traceId) that produces the wire body. One subclass per HTTP status fixes the code so call sites read intent, not numbers.

  export class AppException extends HttpException {
    constructor(private readonly info: { statusCode: HttpStatus; message: string; data?: unknown }) {
      super(info.message, info.statusCode);
    }
    prepareResponse(traceId?: string) { return { ...this.info, traceId }; }   // the ONE wire shape
  }
  export class BadRequestError    extends AppException { constructor(m: string, d?: unknown) { super({ statusCode: 400, message: m, data: d }); } }
  export class UnauthorizedError  extends AppException { constructor(m = 'Unauthorized')      { super({ statusCode: 401, message: m }); } }
  export class NotFoundError      extends AppException { constructor(m = 'Not found')         { super({ statusCode: 404, message: m }); } }
  export class InternalServerError extends AppException { constructor(m = 'Internal error', d?: unknown) { super({ statusCode: 500, message: m, data: d }); } }
  

• Call sites throw the typed subclass, never a bare framework error and never a hand-built JSON body: if (!listing) throw new NotFoundError('Listing not found'); ▸ Other stacks: a base AppError + subclasses (Go error types + errors.As, Python exception hierarchy, a sealed error enum). Principle: typed errors carrying status + a stable payload, thrown by call sites.

2. Flatten validation errors into the same shape

• Map validator failures to the same body via a static factory — flatten nested field errors into a keyed { "field": { rule: msg }, "parent.child": { … } } object so clients can show per-field messages. Wire it into the global ValidationPipe's exceptionFactory so every DTO validation failure produces your BadRequestError automatically.

  static fromValidationErrors(errors: ValidationError[]): BadRequestError {
    const data = {}; const walk = (errs, out, parent) => errs.forEach((e) => {
      const key = parent ? `${parent}.${e.property}` : e.property;
      if (e.constraints) out[key] = e.constraints; else if (e.children?.length) walk(e.children, out, key);
    });
    walk(errors, data); return new BadRequestError('Validation failed', data);
  }
  // bootstrap: new ValidationPipe({ whitelist: true, transform: true, exceptionFactory: BadRequestError.fromValidationErrors })
  

▸ Other stacks: same idea — convert the validation library's error list into your standard error body in one adapter, wired once at the request boundary.

3. A global filter → one body, trace id, right severity

• One catch-all filter maps everything to the wire body: a typed exception → its prepareResponse; a framework not-found → your NotFoundError; anything unexpected → a generic 500 that hides internals (no stack/SQL to the client — expose a verbose body only behind an explicit config flag, never a NODE_ENV branch). Attach a trace id (from an incoming header or a generated correlation id) so a client report maps to your logs.
• Log by severity: 4xx → warn, 5xx → error (with the cause + ids). Wire the filter once globally, not per controller.

  @Catch()
  export class GlobalExceptionFilter implements ExceptionFilter {
    catch(ex: unknown, host: ArgumentsHost) {
      const res = host.switchToHttp().getResponse(); const req = host.switchToHttp().getRequest();
      const app = ex instanceof AppException ? ex
        : ex instanceof NotFoundException ? new NotFoundError()
        : new InternalServerError(undefined, this.verbose ? (ex as Error).stack : undefined);
      this.logBySeverity(req, app, ex);                       // warn 4xx / error 5xx
      res.status(app.getStatus()).json(app.prepareResponse(req.header('x-trace-id')));
    }
  }
  

▸ Other stacks: a global error middleware / handler that maps any error to the standard body, logs by severity, and never leaks internals on 5xx.

4. An RPC/microservice filter twin

• A service-to-service call needs the same discipline: an exception filter on the message handler maps a throw to the failed envelope { success: false, message, statusCode, data } (the success twin of the RPC envelope in integrate-internal-services §1), so callers get a consistent failure to classify (retryable vs terminal) — never a raw stack over the wire. ▸ Other stacks: gRPC status + error details; map domain errors to transport-level error codes once.

5. Log-and-swallow for fire-and-forget handlers

• An in-process event handler / background callback must not crash the bus if it throws. A @CatchException() method decorator wraps the method: run it, and on throw log + return (optionally a custom handler) instead of propagating — the in-process twin of the SQS "don't throw in a consumer" rule (write-service-code §6). Use it on fire-and-forget side effects, not on request handlers (those should surface the error to the global filter).

  @EventsHandler(ListingPublishedEvent)
  class NotifyOnPublish {
    @CatchException()                       // log + swallow; one failing handler won't break siblings
    async handle(e: ListingPublishedEvent) { await this.mailer.send(/* … */); }
  }
  

▸ Other stacks: a wrapper/aspect that logs and absorbs in async event handlers; classify which call sites swallow (side effects) vs surface (request path).

Verification

• Call sites throw typed subclasses; no hand-built error JSON, no raw framework/vendor errors to clients.
• DTO validation failures produce the same body via the ValidationPipe exceptionFactory.
• A single global filter maps every throw to one shape, attaches a trace id, logs 4xx=warn/5xx=error, and hides internals on 5xx (verbose only behind a config flag, not NODE_ENV).
• Service-to-service failures use the failed envelope, not a raw stack.
• Fire-and-forget handlers log + swallow; request handlers surface to the filter.

Related

• structure-a-shared-backend-lib — the infra-exception package this skill describes.
• write-cross-cutting-code — filters/decorators as cross-cutting primitives; the @CatchException mechanics.
• integrate-internal-services §1 (RPC envelope), §3 (consumer ack-vs-DLQ) · write-service-code §6/§7/§9.