how to handle syncing topics across multiple devices in firebase

Question

I have a system where one user can be logged into multiple devices. The use case is as follows:

Suppose the user A subscribes to a weather topic from device A. This user will now get regular push messages from this topic.

Now assume that the same user is logged in to a device B. This device also needs to get subscribed to the weather topic as there is no gurantee that the user will just use device A.

Same use case can be applied in reverse case of unsubscription as well as more than two devices.

What is the best possible way to solve such a problem.

The current approaches which i have thought of are:

1) Make a node under each user id inside firebase db called subscriptions. This node will have subscribed data in the form of

Topidname :boolean issubscribed

This approach has a listener attached to this location in a background service. Each device with the same user id will always listen to the same location thus solving the issue in most cases. It can't gurantee integrity due to the normal service lifecycle reasons.

A service is used because if the listener is kept only for the lifetime of the app, if the user unsubscribes from a topic on one device and the second device app is closed, the second app will keep receiving notifications as it's registration token is still related to that topic and will only unsubscribe on app restart.

Here the service has been made START_STICKY, has a listener inside onCreate and has a backup alarm set to restart the service in onDestroy method. Thus the only case where this won't work is an app force stop or the listener being in a backoff mode due to long non connectivity.

When the app launches it also syncs once with the location and subs the left out topics. Service will stop on signout and unsub from all the previous user's topics. I have kept a keepSynced on the location to ensure it stays in sync

Potential issues are -> service killed by force stop. May cause some ram usage. Sync might not be instantaneous depending on how long the connectivity was lost and how long it takes firebase to resync with the db. Resync and sub unsub all topics in each service restart.

2) This is a relatively more complicated approach:

Save the firebase token from the instance id service for each user in the app server.

On each signout, instead of calling delete iid, keep the same token but overwrite the user attached with the new logged in uid.

On each subscription, do a bulk subscribe to the topic using the Instance Id server api using all the registration tokens for one user.

Each time a user signs out call unsubscribe on all the topics one by one using the instance id api for that device's particular token. Same process for token refresh.

Do to same on unsubscription of each topic.

This case will need one synchronous call for all the topics subscriptions and unsubscriptions. It will also require the token to be constantly updated in the app server.

Please suggest any other better scenario to solve this issue.

If my scenarios have any flaws or needs modifications, please do suggest. I'm stuck and can't think of anything else at the moment.

Answer 1

I'm assuming that you're keeping track of each of the tokens for a user's devices. So the code on your server should be able to know, for a given user, how to send a message to all of their devices, regardless of subscriptions.

Try thinking of user topic subscriptions in the same way that you think of the data broadcasted within those topic. When topic subscriptions change for a user, that event could be broadcast to all of their devices with FCM, telling your app to read the subscriptions from your database and set subscriptions again as necessary. Your app will only wake a receive a notice to change subscriptions when the user requests it.

Then, when it comes time to publish a message to the topic, you can be sure that all the devices that checked in like this will receive the message to the topic.

To boil it down: 1. When a user's subscriptions change in your app, notify the server of that change. 2. In your server, update the user's subscriptions to your database. 3. Then, send an FCM data message to all of the user's devices to reload subscriptions. 4. When that message is received in your app, read the subscription updates out of your database and update the FCM subscriptions accordingly. Your app will have to arrange to stay alive for as long as it takes to do this.