Pipeline Internals — Series 3

Preview — 3 of 10 questions

How are the pipes applied across these two arguments?

javascript

@Post(':id')
update(
  @Param('id', ParseIntPipe) id: number,
  @Body(new ValidationPipe({ whitelist: true })) dto: UpdateCatDto,
) {}

AAll pipes from all arguments are collected into one chain applied to the whole request

BThe arguments are processed in parallel, so a failure in one may not prevent the other's pipes from running

CPer argument: each parameter's applicable pipes — global, controller, method, then its own list — are reduced in order over that argument's extracted value, and the arguments are processed in sequence, so the first pipe to throw aborts binding and the handler is never invoked

DOnly the first argument's pipes run; the rest are applied lazily on access

What structure produces the onion behaviour?

javascript

Global → Controller → Route → handler

AThe chain is folded from the innermost outward, each interceptor receiving a CallHandler whose handle() invokes the next layer — and because that invocation is deferred until subscription, nothing executes until the outermost stream is subscribed to

BEach interceptor is invoked in turn and its return value discarded, with the handler called separately at the end

CThe interceptors are pushed onto a stack and popped after the handler completes

DEach interceptor spawns a child execution context, and the results are merged

Why does this filter return a value rather than write a response?

javascript

@Catch(RpcException)
export class RpcFilter implements ExceptionFilter {
  catch(exception: RpcException, host: ArgumentsHost) {
    return throwError(() => exception.getError());
  }
}

ABecause RPC filters are executed twice, once to return and once to write

BBecause ArgumentsHost is read-only for non-HTTP transports

CBecause microservice filters must be registered globally

DBecause a non-HTTP context has no response object to write to: the transport's exception context expects the filter to produce the error value it will send back over the wire, so the return value is the response

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