Reactor信号基础问题：subscribe时request(unbounded)流向疑问

Hey there! Awesome question—this is one of those RxJS nuances that trips up a lot of folks when they start digging into backpressure and subscription flows. Let’s break this down clearly.

My Original Question #

When subscribing, the subscribe operation requests an unbounded number of elements. Why does the request(unbounded) event seem to travel downward through the pipeline instead of upward?

My Uncertain Initial Answer #

I initially thought the unbounded request first travels upward to the take operator, and then take passes it downward... but I’m not sure if that’s the right way to think about it.

Clarifying the Flow (The Correct Breakdown) #

First, let’s set straight the two key directions in RxJS pipelines:

• Subscription flow: Moves upward from your observer to the source observable (think: connecting the dots from end to start).
• Request & data flow: Requests for data move upward (observer → operators → source), while emitted data moves downward (source → operators → observer).

Wait, so your core question’s premise might have a tiny mix-up: request(unbounded) doesn’t travel downward—it travels upward. Let’s use a concrete example with take to see exactly how this plays out:

Suppose we have this code:

interval(100).pipe(take(5)).subscribe();

Here’s the step-by-step flow:

1. When you call subscribe(), a default observer is attached to the take operator’s observable.
2. The take operator then subscribes to its source (the interval observable)—this is the upward subscription flow.
3. The default observer automatically sends an unbounded request to the take operator’s subscription. This request travels upward to take.
4. take receives the unbounded request, but since it’s only allowed to pass 5 elements downstream, it modifies the request: it sends a request for 5 elements upward to the interval source.
5. The interval source starts emitting elements, which travel downward through take to the observer.
6. Once 5 elements are emitted, take completes the subscription—this completion signal travels upward to interval, telling it to stop emitting.

So Where Might the Confusion Come From? #

If you’ve seen logs that make it look like request(unbounded) is moving downward, it’s probably because some operators log requests as they propagate them upstream. For example, a tap operator might log "received request for unbounded" when it gets the request from downstream, then pass it upward to its source. That log could make it seem like the request is moving down, but it’s actually just the operator acknowledging the request before sending it up.

Your initial thought was partially right—take does process the unbounded request—but the direction was reversed: the request reaches take from below (the observer), and take sends a modified request upward to the source.

内容的提问来源于stack exchange，提问作者Stav Alfi