http spec：等待创建“文件”

Question

Is there a http related spec which helps me to implement "wait for creation of file"?

Here is what should be implemented:

1. Server called "sink" connects to server "middle" and waits until a file on this server gets created
2. Server "creator" creates a file and sends it to server "middle"
3. As soon as the whole file is available at server "middle" server "sink" should start the download.

I know that I could use websockets and custom methods to implement this.

But before going this way I would like to know if there is a http related spec which does cover this use case.

Answer 1

Short answer

Yes, the HTTP specification itself ( RFC7230 ) already covers your use case and there is also a known mechanism called HTTP Long Polling described in RFC6202 where a server waits for something for many seconds before returning a response.

Long answer

HTTP is a flexible protocol, so much so that the HTTP specification itself ( RFC7230 ) already covers your use case. In Section 1 (emphasis mine):

HTTP is a generic interface protocol for information systems. It is designed to hide the details of how a service is implemented by presenting a uniform interface to clients that is independent of the types of resources provided . Likewise, servers do not need to be aware of each client's purpose: an HTTP request can be considered in isolation rather than being associated with a specific type of client or a predetermined sequence of application steps. The result is a protocol that can be used effectively in many different contexts and for which implementations can evolve independently over time.

The specification allows the server to connect to other server and wait for something ("hide the details of how a service is implemented") and that something can be a file or anything else ("independent of the types of resources provided").

You should notice that the specification does not define the internal workings of the server. It does not define what the server must do in order to provide the response, as explained also in Section 1 (emphasis mine):

One consequence of this flexibility is that the protocol cannot be defined in terms of what occurs behind the interface .

In your use case, server "sink" is actually a client because it connects to server "middle" expecting to receive a file. The fact that server "middle" is waiting for another server to do something is irrelevant because the specification does not define what should occur behind the interface.

However, if you are looking for a similar mechanism where the server also waits for something, one example would be HTTP Long Polling described in RFC6202 (emphasis mine):

HTTP Long Polling: The server attempts to "hold open" ( not immediately reply to ) each HTTP request, responding only when there are events to deliver.

In HTTP Long Polling, after receiving a client's request, the server does not reply immediately ("not immediately reply to"). It simply waits for an event to occur. How long the server can wait is not even defined. This mechanism works within the confines of how HTTP works and therefore complies with HTTP specification.