How detect changes of an api-CodePudding

I have a api that returns if the building is open and how much peapole are in there. Now i want my Discord Bot to send a message when the Building opens. How do i do that?

if the api recives a request the response is looks this :

state: {
open: false/true
}

CodePudding user response：

It may be helpful to clear out the terminology: the "API" in this context is the endpoints exposed by the server and their request/response schemas (you can think of it as the fields you send and receive back). Now, this doesn't change in your case: it's the same endpoint, and the same fields. What changes is the value.

Now, you are probably doing a HTTP request to a given URL, where the server is. And in HTTP world, we say that we are requesting a resource. The resource behind https://stackoverflow.com is the homepage of this website. The resource behind the endpoint you are calling is a building's state. This resource changes overtime, it may open or closed at any time, people going in and out. But the API doesn't change in this case.

Let's reword your question, so it can be clearer: How can a client know when a HTTP resource changes? If your server only exposes this endpoint to know the state of the building, the answer is a sad "it can't". Let's say that I close the building, the server knows it somehow and now the building's state is {"open": false}. But the server doesn't have any mechanism to say to your client that the state changed, the server just waits for the client to ask what the state is, and returns. Allowing a server to send data to your client without the client requesting first adds some complexity to your architecture, and although there's a bonus (the client will know of state changes as soon as possible), in your case, it may not be necessary.

One alternative is long polling, in long polling your client makes a request to the server and the server doesn't respond immediately, it... waits. Waits for an update, like a change in building's state. When an update happens, then it sends a response. The client, in turn, requests again! And waits for the server to send an update... In practice, the client will keep up with the server state. The mentioned article for long polling gives a good example: https://javascript.info/long-polling#regular-polling

The one caveat is that the server must also support long polling. If the server just returns whatever the resource's state is, then the client will keep receiving the same state over and over. Another valid solution is instead of waiting for updates, the client keeps requesting the server for every few seconds. You may miss some updates! But in some cases, it's fine to lose track of a few updates.

Ok, enough theory. What about your case? If you want to know if a door is open or closed, but don't care to know when it happens, you can just request the server every five seconds or so:

In some pseudo javascript code, and very inspired by the long polling article mentioned before:

async function subscribe() {
  let buildingState = null
  while (true) {
    const response = await fetch("/subscribe")
    if (response.status != 200) {
      // An error - let's show it
      showMessage(response.statusText)
    } 

    // Get and show the message
    const message = await response.json()
    // a function that returns true/false if the state is different
    if (stateChanged(message, buildingState)) {
      updateDiscordBot(message)
      buildingState = message
    }
    // wait five seconds and repeat
    await new Promise(resolve => setTimeout(resolve, 5000));
  } 
}
subscribe();

This works fine. But if you want for your client to immediately know when a door is opened or closed, this is not enough: opening and closing a door is a quite fast action, and even if you request the server every second, you may lose updates. In this case, the server needs to change. Either by implementing long polling, or something like websockets... you didn't mention that the server is in your control, but if it doesn't, it may be helpful to talk with who maintains it to work out a solution.

CodePudding user response：

If you only have pull access to that API then the only way to detect state change is to periodically send a request, store response, and trigger your bot on stored response change.