We used to send push notifications directly to APNS or GCM from our code. We were maintaining device_ids, OS types, and GCM/APNS certificates. This approach was complicated and hard to maintain.
We knew that an easier solution must exist. Then, we discovered Firebase.
Firebase is a service that does all the hard work for us. There is no need to store certificates on the server because Firebase handles them. Furthermore, we don't need to maintain OS types because Firebase knows which notification goes to APNS and which to GCM. Basically, Firebase handles the routing and delivery of push notifications to targeted devices. Firebase also has a dashboard with statistics that mobile developers can use for testing purposes.
There are many services (such as Amazon SNS) that provide push notifications, but we use Firebase because of its simplicity and reliability.
In this chapter, we will explain a few approaches to implementing push notifications and explain the pros and cons of each.
Before starting with development, read chapters about Firebase Cloud Messaging and app server development.
There are several approaches to sending push notifications using Firebase:
- sending directly to device_ids
- storing one device id per user
- storing multiple device ids per user
- sending to topics
- with persisted topics
- with non-persisted topics
- sending to device groups
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Using this approach means adding a new
device_idcolumn directly to a user model. -
This column will be overwritten on each login.
-
This column should be deleted on each logout.
-
PROS
- Easy to build.
-
CONS
- The user can get notifications only on the last device they logged in.
- Mobile developers need to be reminded to do a proper logout API call when the user logs out. They often forget to do this.
-
Usually, a user has multiple devices, so this approach is not recommended.
-
A new table for storing user device_ids needs to be implemented.
-
An index on device_id is mandatory.
-
PROS
- The user can get push notifications on multiple devices.
-
CONS
- We need to maintain the user's device_ids (create, delete).
class SessionsController < ApiController
def create
# login logic goes here
add_user_device
# render user
end
def destroy
# logout logic goes here
device.destroy if device.present?
# render user
end
private
def add_user_device
if device.exists?
device.update(user: current_user)
else
current_user.devices.create(device_id: params[:device_id])
end
end
def device
@device ||= Device.find_by(device_id: params[:device_id])
end
end module FirebaseCloudMessaging
class UserNotificationSender
attr_reader :message, :user_device_ids
# Firebase works with up to 1000 device_ids per call
MAX_USER_IDS_PER_CALL = 1000
def initialize(user_device_ids, message)
@user_device_ids = user_device_ids
@message = message
end
def call
user_device_ids.each_slice(MAX_USER_IDS_PER_CALL) do |device_ids|
fcm_client.send(device_ids, options)
end
end
private
def options
{
priority: 'high',
data: {
message: message
},
notification: {
body: message,
sound: 'default'
}
}
end
def fcm_client
@fcm_client ||= FCM.new(Rails.application.secrets.fcm['server_api_key'])
end
end
end-
Topic messaging is best suited for the content which is often sent to a group of users, for example, all users subscribed to weather information, some subreddits, etc.
-
Based on the publish/subscribe model, FCM topic messaging allows you to send a message to multiple devices that have opted into a particular topic. You compose topic messages as needed, and FCM handles routing and delivering the message reliably to the right devices.
-
Topic messaging supports unlimited topics and subscriptions for each app.
-
Persisted topics
-
With this approach, we need a way to create topic names and expose them through the API.
-
If Firebase doesn't have a topic, it will be created when the user subscribes for the first time.
-
CONS
- Topic names are being duplicated in Firebase as well as in our backend.
-
-
Non-persisted topics
-
We can agree on a convention for topic names with mobile developers.
-
Every topic name is parameterized (Firebase Push Notifications -> firebase-push-notifications).
-
PROS:
- With this approach, we don't need to store topic names in the backend.
- Easy to build.
-
CONS:
- Currently, it doesn't seem that Firebase has any limits on topics, but they might restrict this in the future just like Amazon SNS.
-
module FirebaseCloudMessaging
class UserNotificationSender
attr_reader :message, :topic
def initialize(topic, message)
@topic = topic
@message = message
end
def call
fcm_client.send_to_topic(topic, options)
end
private
def options
{
priority: 'high',
data: {
message: message
},
notification: {
body: message,
sound: 'default'
}
}
end
def fcm_client
@fcm_client ||= FCM.new(Rails.application.secrets.fcm['server_api_key'])
end
end
end- Firebase recently added support for managing topic subscriptions via a server. With this feature, we can have full control over subscriptions if necessary.
-
With device group messaging, you can send a single message to multiple instances of an app running on devices belonging to a group. Typically, a "group" refers to a set of different devices that belong to a single user. All devices in a group share a common notification key, which is the token that FCM uses to fan out messages to all devices in the group.
-
If you need to send messages to multiple devices per user, consider device group messaging for those cases of use.
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The maximum number of members allowed per one notification key is 20.
-
We haven't used device groups, but you can read more about this approach.
Since we are rather new at using Firebase, please talk to someone from the team before choosing a development approach.