Running a scheduler for each user in Firebase Cloud Functions?

Question

I wish to run a scheduler which runs according to each user in my cloud functions and not universal scheduler which runs at the same time for all users.

For example,

Answer 1

Cloud Functions alone won't be able to schedule function executions indepently for each user. You can only schedule a single function on a schedule that you provide in the function configuration. Obviously, it won't scale for you to deploy a new function for each user.

You have two choices:

1. Deploy a scheduled function that runs as frequently as possible (every 1 minute). Have it query a database to figure out which items of work need to be run per-user, and arrange for them to execute. Based on the volume of work, you might want to offload those to other function invocations via pubsub.

2. On a per-user basis, use Cloud Tasks to schedule the first item of work that should be done for the user (which could be implemented to invoke an HTTP trigger). The payload of the task should describe the work to be done (minimally it should contain the id of the user it represents). When that task completes, the HTTP trigger will have to figure out when the next periodic invocation should be, and schedule it as such. You will effectively always have one task in a queue "pending" execution. If somehow your function fails to establish the next task, then the periodic nature will break, and you will have to somehow have to figure out how to start it up again.

Option 1 is easier to implement, IMO, but you lose the granularity of timing if the invocation needs to happen precisely on the exact second of the minute. You will also need to maintain a database or some other store to query to figure out how to tell the per-minute function should do more work for any given user.

Option 2 is much harder to implement correctly, requires the use of a whole new product with a new API, but you will be able to schedule tasks with much finer granularity.