I am building an application that must react when the timestamp for a certain Firestore document becomes older than the current time. Is there a way to setup this type of query listener as a Cloud Function, or otherwise achieve the desired goal of reacting to a document when its timestamp crosses the current time?
From what I can tell reading the Firestore and Cloud Function documentation, query listeners may not be possible to setup as Cloud Functions. Furthermore, this is not just a regular query listener - the query criteria (time) is dynamic, so it isn't the typical query structure ("is A < 5") but a dynamic one ("is T < now" where "now" is changing every moment).
If it's true this is not possible as a query listener, I'd certainly appreciate any suggestions on how to achieve this goal through another means. One idea I had was to create a Cloud Function that triggers every 60 seconds and runs the queries based on the time at that moment, but this would not allow constant listening (and 60 seconds is unfortunately too long for our usage). Thank you so much in advance
CodePudding user response：
Firestore queries can only filter on literal values that are explicitly stored in the documents they return. There's no way to perform a calculation in a query, so any time you need a now in the query - that timestamp will be calculated at the moment the query is created.
There are two common ways to implement the time-to-live type functionality that you describe:
Set up a process that periodically runs (e.g. a time-based Cloud Function), and every time the process runs perform a query to determine what documents have expired.
As a variant of this, you could start a permanent listener for updates each time the Cloud Function triggers and keep that active for slightly less than the interval until the next trigger.
Create a Cloud Task for each document that expires/triggers when the document needs to be processed. While this may seem more complex, it actually ends up being simpler due to the fact that your callbacks now trigger on individual documents.
Also see: Is there any TTL (Time To Live ) for Documents in Firebase Firestore, which includes a link to Doug's excellent article on How to schedule a Cloud Function to run in the future with Cloud Tasks (to build a Firestore document TTL).