Node.js server uses async model in one single thread, which means at any time, only one request (connId) is under execution by Node (except... you have multiple node.js instance, but let's keep the scenario simple and ignore this case).

When one request is processed (running its handler code), it may start an async task such as read a file, and continue execution. The request itself's handler code would be executed without waiting for async task, and when this handler code is finished running, from Node.js point of view, the request handling itself is done -- the handling of async task's result is another thing in another time, node does not care about the progress of it.

Thus, in order to return remaining time of all requests -- I guess this is the remaining time of other request's async task, because remaining time of other request's handler code execution does not make any sense, there must be some place to store the information of all requests, including:

• request's connId and startTime (the time when request is received).
• request's timeout value, which is passed as parameter in URL.
• request's estimated remaining time, this information is mission specific and must be retrieved from other async task related services (you can pull time by time using setInterval or make other services push the latest remaining time). Node.js doesn't know the remaining time information of any async task.

In this way, you can track all running requests and their remaining time. Before one request is returned, you can check the above "some place" to calculate all requests' remaining time. This "some place" could be global variable, memory database such as Redis, or even a plain database such as MySQL.

Please note: the calculated remaining time would not be accurate, as the read&calculation itself would cost time and introduce error.