CODEWITHSUNDEEP
javafirebasefirebase-cloud-messaging
thechosenone
thechosenone

Reputation: 75

Send a Notification to a Group of Devices with FCM

I want to be able to call a function like this: (from client)

void sendNotificationToGroupMembers(String groupId, String message)

with a DB Structure like This

/userTokens/userId:userInstanceId
/groupMembers/groupId/[userIds...]
/serverRequests/...

Now, I after research I think I have an idea on how to approach it, but I don't know if it's the right approach and it is very involved for something I consider should be a very simple task, so I was wondering if there is an easier/simpler way to approach my problem.

This is how I would do it

[Backend]

  1. Register ChildEventListener on node /serverRequets/ inside a Servlet running on a Google Cloud Flexible

[Client]

  1. Push to node /serverRequests/push/groupId:message

[Backend]

  1. Get /groupMembers/groupId/ and save userIds in a list
  2. For each userId, get his userInstanceId in /userTokens/userId and save to a list
  3. Subscribe every userInstanceId to topic "groupId" using ":batchAdd"
  4. Send a notification to topic "groupId"

But I see a problem at steps 4. Because to get the userInstanceId I must make a request with ValueEventListener for each userId, but since they are asynchronous, how do I determine when all the instanceId's have been downloaded? Do I set a timer of 5 seconds? Do I check how large the userId list is, and then use something like LinkedBlockingQueue to publish to a Thread each time an instanceId is downloaded, and only if it reaches the size, do the step 5?

Because I don't find a multiple .equalTo query, and I can't download the entire userTokens node...

Upvotes: 2

Views: 778

Answers (1)

Chris
Chris

Reputation: 8020

First off, I would use Cloud Functions for this, not a custom listener you set up. You can see an example of a notification function here

Next, I would consider using topic messaging, since it would cover the use case you are describing (sending a message to members of a group). I employed this myself and it works a charm.

You flow would now be like this:

  • Client tells FCM it would like to subscribe to topic groupID.

  • At some point there is a message pushed to an endpoint in your /groupId/messages/ (or however you want to structure it, remember, denormalization of a NoSQL database is good!)

  • Then have a cloud function listen to /{groupId}/messages/{messageId} and post a new message to your topic /groupId

I hope this makes sense and I understood your question correctly

Upvotes: 2

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