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Push notification API

Centrifugo excels in delivering real-time in-app messages to online users. Sometimes though you need a way to engage offline users to come back to your app. Or trigger some update in the app while it's running in the background. That's where push notifications may be used. Push notifications delivered over battery-efficient platform-dependent transport.

With Centrifugo PRO push notifications may be delivered to all popular application platforms:

  • Android devices
  • iOS devices
  • Web browsers which support Web Push API (Chrome, Firefox, see this matrix)

Centrifugo PRO provides API to manage user device tokens, device topic subscriptions and API to send push notifications towards registered devices and group of devices (subscribed to a topic).

Push

To deliver push notifications to devices Centrifugo PRO integrates with the following providers:

FCM, HMS, APNs handle the frontend and transport aspects of notification delivery. Device token storage, management and efficient push notification broadcasting is managed by Centrifugo PRO. Tokens are stored in a PostgreSQL database. To facilitate efficient push notification broadcasting towards devices, Centrifugo PRO includes worker queues based on Redis streams (and also provides and option to use PostgreSQL-based queue).

Integration with FCM means that you can use existing Firebase messaging SDKs to extract push notification token for a device on different platforms (iOS, Android, Flutter, web browser) and setting up push notification listeners. The same for HMS and APNs - just use existing native SDKs and best practices on the frontend. Only a couple of additional steps required to integrate frontend with Centrifugo PRO device token and device topic storage. After doing that you will be able to send push notification towards single device, or towards group of devices subscribed to a topic. For example, with a simple Centrifugo API call like this:

curl -X POST http://localhost:8000/api/send_push_notification \
-H "Authorization: apikey <KEY>" \
-d @- <<'EOF'

{
"recipient": {
"filter": {
"topics": ["test"]
}
},
"notification": {
"fcm": {
"message": {
"notification": {"title": "Hello", "body": "How are you?"}
}
}
}
}
EOF

In addition, Centrifugo PRO includes a helpful web UI for inspecting registered devices and sending push notifications:

Motivation and design choices

We tried to be practical with our Push Notification API, let's look at its design choices and implementation properties we were able to achieve.

Storage for tokens

To start delivering push notifications in the application, developers usually need to integrate with providers such as FCM, HMS, and APNs. This integration typically requires the storage of device tokens in the application database and the implementation of sending push messages to provider push services.

Centrifugo PRO simplifies the process by providing a backend for device token storage, following best practices in token management. It reacts to errors and periodically removes stale devices/tokens to maintain a working set of device tokens based on provider recommendations.

Efficient queuing

Additionally, Centrifugo PRO provides an efficient, scalable queuing mechanism for sending push notifications. Developers can send notifications from the app backend to Centrifugo API with minimal latency and let Centrifugo process sending to FCM, HMS, APNs concurrently using built-in workers. In our tests, we achieved several millions pushes per minute.

Centrifugo PRO also supports delayed push notifications feature – to queue push for a later delivery, so for example you can send notification based on user time zone and let Centrifugo PRO send it when needed.

Unified secure topics

FCM and HMS have a built-in way of sending notification to large groups of devices over topics mechanism (the same for HMS). One problem with native FCM or HMS topics though is that client can subscribe to any topic from the frontend side without any permission check. In today's world this is usually not desired. So Centrifugo PRO re-implements FCM, HMS topics by introducing an additional API to manage device subscriptions to topics.

tip

In some cases you may have real-time channels and device subscription topics with matching names – to send messages to both online and offline users. Though it's up to you.

Centrifugo PRO device topic subscriptions also add a way to introduce the missing topic semantics for APNs.

Centrifugo PRO additionally provides an API to create persistent bindings of user to notification topics. Then – as soon as user registers a device – it will be automatically subscribed to its own topics. As soon as user logs out from the app and you update user ID of the device - user topics binded to the device automatically removed/switched. This design solves one of the issues with FCM – if two different users use the same device it's becoming problematic to unsubscribe the device from large number of topics upon logout. Also, as soon as user to topic binding added (using user_topic_update API) – it will be synchronized across all user active devices. You can still manage such persistent subscriptions on the application backend side if you prefer and provide the full list inside device_register call.

Non-obtrusive proxying

Unlike other solutions that combine different provider push sending APIs into a unified API, Centrifugo PRO provides a non-obtrusive proxy for all the mentioned providers. Developers can send notification payloads in a format defined by each provider.

It's also possible to send notifications into native FCM, HMS topics or send to raw FCM, HMS, APNs tokens using Centrifugo PRO's push API, allowing them to combine native provider primitives with those added by Centrifugo (i.e., sending to a list of device IDs or to a list of topics).

Builtin analytics

Furthermore, Centrifugo PRO offers the ability to inspect sent push notifications using ClickHouse analytics. Providers may also offer their own analytics, such as FCM, which provides insight into push notification delivery. Centrifugo PRO also offers a way to analyze push notification delivery and interaction using the update_push_status API.

Steps to integrate

  1. Add provider SDK on the frontend side, follow provider instructions for your platform to obtain a push token for a device. For example, for FCM see instructions for iOS, Android, Flutter, Web Browser). The same for HMS or APNs – frontend part should be handled by their native SDKs.
  2. Call Centrifugo PRO backend API with the obtained token. From the application backend call Centrifugo device_register API to register the device in Centrifugo PRO storage. Optionally provide list of topics to subscribe device to.
  3. Centrifugo returns a registered device object. Pass a generated device ID to the frontend and save it on the frontend together with a token received from FCM.
  4. Call Centrifugo send_push_notification API whenever it's time to deliver a push notification.

At any moment you can inspect device storage by calling device_list API.

Once user logs out from the app, you can detach user ID from device by using device_update or remove device with device_remove API.

Configuration

In Centrifugo PRO you can configure one push provider or use all of them – this choice is up to you.

FCM

As mentioned above Centrifigo uses PostgreSQL for token storage. To enable push notifications make sure database section defined in the configration and fcm is in the push_notifications.enabled_providers list. Centrifugo PRO uses Redis for queuing push notification requests, so Redis address should be configured also. Finally, to integrate with FCM a path to the credentials file must be provided (see how to create one in this instruction). So the full configuration to start sending push notifications over FCM may look like this:

{
...
"database": {
"dsn": "postgresql://postgres:[email protected]:5432/postgres"
},
"push_notifications": {
"redis_address": "localhost:6379",
"enabled_providers": ["fcm"],
"fcm_credentials_file_path": "/path/to/service/account/credentials.json"
}
}
tip

Actually, PostgreSQL database configuration is optional here – you can use push notifications API without it. In this case you will be able to send notifications to FCM, HMS, APNs raw tokens, FCM and HMS native topics and conditions. I.e. using Centrifugo as an efficient proxy for push notifications (for example if you already keep tokens in your database). But sending to device ids and topics, and token/topic management APIs won't be available for usage.

HMS

{
...
"database": {
"dsn": "postgresql://postgres:[email protected]:5432/postgres"
},
"push_notifications": {
"redis_address": "localhost:6379",
"enabled_providers": ["hms"],
"hms_app_id": "<your_app_id>",
"hms_app_secret": "<your_app_secret>",
}
}
tip

See example how to get app id and app secret here.

APNs

{
...
"database": {
"dsn": "postgresql://postgres:[email protected]:5432/postgres"
},
"push_notifications": {
"redis_address": "localhost:6379",
"enabled_providers": ["apns"],
"apns_endpoint": "development",
"apns_bundle_id": "com.example.your_app",
"apns_auth": "token",
"apns_token_auth_key_path": "/path/to/auth/key/file.p8",
"apns_token_key_id": "<your_key_id>",
"apns_token_team_id": "your_team_id",
}
}

We also support auth over p12 certificates with the following options:

  • push_notifications.apns_cert_p12_path
  • push_notifications.apns_cert_p12_b64
  • push_notifications.apns_cert_p12_password

Other options

push_notifications.max_inactive_device_days

This integer option configures the number of days to keep device without updates. By default Centrifugo does not remove inactive devices.

push_notifications.enable_redis_delayed_scheduler

Boolean option which enables Redis scheduler to process delayed push notifications. It's off by default since produces additional requests to Redis. When using PostgreSQL as push notifications queue engine you don't need to enable sheduler explicitly.

push_notifications.dry_run

Boolean option, when true Centrifugo PRO does not send push notifications to FCM, APNs, HMS providers but instead just print logs. Useful for development.

push_notifications.dry_run_latency

Duration. When set together with push_notifications.dry_run every dry-run request will cause some delay in workers emulating real-world latency. Useful for development.

Use PostgreSQL as queue

Centrifugo PRO utilizes Redis Streams as the default queue engine for push notifications. However, it also offers the option to employ PostgreSQL for queuing. It's as simple as:

config.json
{
...
"database": {
"dsn": "postgresql://postgres:[email protected]:5432/postgres"
},
"push_notifications": {
"queue_engine": "database",
// rest of the options...
}
}
tip

Queue based on Redis streams is generally more efficient, so if you start with PostgreSQL based queue – you have an option to switch to a more performant implementation later. Though in-flight and currently queued push notifications will be lost during a switch.

API description

device_register

Registers or updates device information.

device_register request

FieldTypeRequiredDescription
idstringNoID of the device being registered (provide it when updating).
providerstringYesProvider of the device token (valid choices: fcm, hms, apns).
tokenstringYesPush notification token for the device.
platformstringYesPlatform of the device (valid choices: ios, android, web).
userstringNoUser associated with the device.
topicsarray of stringsNoDevice topic subscriptions. This should be a full list which replaces all the topics previously accociated with the device. User topics managed by UserTopic model will be automatically attached.
metamap<string, string>NoAdditional custom metadata for the device

device_register result

Field NameTypeRequiredDescription
idstringYesThe device ID that was registered/updated.

device_update

Call this method to update device. For example, when user logs out the app and you need to detach user ID from the device.

device_update request

FieldTypeRequiredDescription
idsrepeated stringNoDevice ids to filter
usersrepeated stringNoDevice users filter
user_updateDeviceUserUpdateNoOptional user update object
meta_updateDeviceMetaUpdateNoOptional device meta update object
topics_updateDeviceTopicsUpdateNoOptional topics update object

DeviceUserUpdate:

FieldTypeRequiredDescription
userstringYesUser to set

DeviceMetaUpdate:

FieldTypeRequiredDescription
metamap<string, string>YesMeta to set

DeviceTopicsUpdate:

FieldTypeRequiredDescription
opstringYesOperation to make: add, remove or set
topicsrepeated stringYesTopics for the operation

device_update result

Empty object.

device_remove

Removes device from storage. This may be also called when user logs out the app and you don't need its device token after that.

device_remove request

Field NameTypeRequiredDescription
idsrepeated stringNoA list of device IDs to be removed
usersrepeated stringNoA list of device user IDs to filter devices to remove

device_remove result

Empty object.

device_list

Returns a paginated list of registered devices according to request filter conditions.

device_list request

FieldTypeRequiredDescription
filterDeviceFilterYesHow to filter results
cursorstringNoCursor for pagination (last device id in previous batch, empty for first page).
limitint32NoMaximum number of devices to retrieve.
include_total_countboolNoFlag indicating whether to include total count for the current filter.
include_topicsboolNoFlag indicating whether to include topics information for each device.
include_metaboolNoFlag indicating whether to include meta information for each device.

DeviceFilter:

FieldTypeRequiredDescription
idsrepeated stringNoList of device IDs to filter results.
providersrepeated stringNoList of device token providers to filter results.
platformsrepeated stringNoList of device platforms to filter results.
usersrepeated stringNoList of device users to filter results.
topicsrepeated stringNoList of topics to filter results.

device_list result

Field NameTypeRequiredDescription
itemsrepeated DeviceYesA list of devices
next_cursorstringNoCursor string for retreiving the next page, if not set - then no next page exists
total_countintegerNoTotal count value (if include_total_count used)

Device:

Field NameTypeRequiredDescription
idstringYesThe device's ID.
providerstringYesThe device's token provider.
tokenstringYesThe device's token.
platformstringYesThe device's platform.
userstringNoThe user associated with the device.
topicsarray of stringsNoOnly included if include_topics was true
metamap<string, string>NoOnly included if include_meta was true

device_topic_update

Manage mapping of device to topics.

device_topic_update request

FieldTypeRequiredDescription
device_idstringYesDevice ID.
opstringYesadd or remove or set
topicsrepeated stringNoList of topics.

device_topic_update result

Empty object.

device_topic_list

List device to topic mapping.

device_topic_list request

FieldTypeRequiredDescription
filterDeviceTopicFilterNoList of device IDs to filter results.
cursorstringNoCursor for pagination (last device id in previous batch, empty for first page).
limitint32NoMaximum number of devices to retrieve.
include_deviceboolNoFlag indicating whether to include Device information for each object.
include_total_countboolNoFlag indicating whether to include total count info to response.

DeviceTopicFilter:

FieldTypeRequiredDescription
device_idsrepeated stringNoList of device IDs to filter results.
device_providersrepeated stringNoList of device token providers to filter results.
device_platformsrepeated stringNoList of device platforms to filter results.
device_usersrepeated stringNoList of device users to filter results.
topicsrepeated stringNoList of topics to filter results.
topic_prefixstringNoTopic prefix to filter results.

device_topic_list result

Field NameTypeRequiredDescription
itemsrepeated DeviceTopicYesA list of DeviceChannel objects
next_cursorstringNoCursor string for retreiving the next page, if not set - then no next page exists
total_countintegerNoTotal count value (if include_total_count used)

DeviceTopic:

FieldTypeRequiredDescription
idstringYesID of DeviceTopic object
device_idstringYesDevice ID
topicstringYesTopic

user_topic_update

Manage mapping of topics with users. These user topics will be automatically attached to user devices upon registering. And removed from device upon deattaching user.

user_topic_update request

FieldTypeRequiredDescription
userstringYesUser ID.
opstringYesadd or remove or set
topicsrepeated stringNoList of topics.

user_topic_update result

Empty object.

user_topic_list

List user to topic mapping.

user_topic_list request

FieldTypeRequiredDescription
flterUserTopicFilterNoFilter object.
cursorstringNoCursor for pagination (last id in previous batch, empty for first page).
limitint32NoMaximum number of UserTopic objects to retrieve.
include_total_countboolNoFlag indicating whether to include total count info to response.

UserTopicFilter:

FieldTypeRequiredDescription
usersrepeated stringNoList of users to filter results.
topicsrepeated stringNoList of topics to filter results.
topic_prefixstringNoChannel prefix to filter results.

user_topic_list result

Field NameTypeDescription
itemsrepeated UserTopicA list of UserTopic objects
next_cursorstringNo
total_countintegerNo

UserTopic:

FieldTypeRequiredDescription
idstringYesID of UserTopic
userstringYesUser ID
topicstringYesTopic

send_push_notification

Send push notification to specific device_ids, or to topics, or native provider identifiers like fcm_tokens, or to fcm_topic. Request will be queued by Centrifugo, consumed by Centrifugo built-in workers and sent to the provider API.

send_push_notification request

Field nameTypeRequiredDescription
recipientPushRecipientYesRecipient of push notification
notificationPushNotificationYesPush notification to send
uidstringNoUnique send id, used for Centrifugo builtin analytics or to cancel delayed push. We recommend using UUID v4 for it
send_atint64NoOptional Unix time in the future (in seconds) when to send push notification, push will be queued until that time.

PushRecipient (you must set only one of the following fields):

FieldTypeRequiredDescription
filterDeviceFilterNoSend to device IDs based on Centrifugo device storage filter
fcm_tokensrepeated stringNoSend to a list of FCM native tokens
fcm_topicstringNoSend to a FCM native topic
fcm_conditionstringNoSend to a FCM native condition
hms_tokensrepeated stringNoSend to a list of HMS native tokens
hms_topicstringNoSend to a HMS native topic
hms_conditionstringNoSend to a HMS native condition
apns_tokensrepeated stringNoSend to a list of APNs native tokens

PushNotification:

FieldTypeRequiredDescription
expire_atint64NoUnix timestamp when Centrifugo stops attempting to send this notification. Note, it's Centrifugo specific and does not relate to notification TTL fields. We generally recommend to always set this to a reasonable value to protect your app from old push notifications sending
fcmFcmPushNotificationNoNotification for FCM
hmsHmsPushNotificationNoNotification for HMS
apnsApnsPushNotificationNoNotification for APNs

FcmPushNotification:

FieldTypeRequiredDescription
messageJSON objectYesFCM Message described in FCM docs.

HmsPushNotification:

FieldTypeRequiredDescription
messageJSON objectYesHMS Message described in HMS Push Kit docs.

ApnsPushNotification:

FieldTypeRequiredDescription
headersmap<string, string>NoAPNs headers
payloadJSON objectYesAPNs payload

send_push_notification result

Field NameTypeDescription
uidstringUnique send id, matches uid in request if it was provided

cancel_push

Cancel delayed push notification (which was sent with custom send_at value).

update_push_status request

FieldTypeRequiredDescription
uidstringYesuid of push notification to cancel

update_push_status result

Empty object.

update_push_status

This API call is experimental, some changes may happen here.

Centrifugo PRO also allows tracking status of push notification delivery and interaction. It's possible to use update_push_status API to save the updated status of push notification to the notifications analytics table. Then it's possible to build insights into push notification effectiveness by querying the table.

The update_push_status API supposes that you are using uid field with each notification sent and you are using Centrifugo PRO generated device IDs (as described in steps to integrate).

This is a part of server API at the moment, so you need to proxy requests to this endpoint over your backend. We can consider making this API suitable for requests from the client side – please reach out if your use case requires it.

update_push_status request

FieldTypeRequiredDescription
uidstringYesuid (unique send id) from send_push_notification
statusstringYesStatus of push notification - delivered or interacted
device_idstringYesDevice ID
msg_idstringNoMessage ID

update_push_status result

Empty object.

Exposed metrics

Several metrics are available to monitor the state of Centrifugo push worker system:

centrifugo_push_notification_count

  • Type: Counter
  • Labels: provider, recipient_type, platform, success, err_code
  • Description: Total count of push notifications.
  • Usage: Helps in tracking the number and success rate of push notifications sent, providing insights for optimization and troubleshooting.

centrifugo_push_queue_consuming_lag

  • Type: Gauge
  • Labels: provider, queue
  • Description: Queue consuming lag in seconds.
  • Usage: Useful for monitoring the delay in processing jobs from the queue, helping identify potential bottlenecks and ensuring timely processing.

centrifugo_push_consuming_inflight_jobs

  • Type: Gauge
  • Labels: provider, queue
  • Description: Number of inflight jobs being consumed.
  • Usage: Helps in tracking the load on the job processing system, ensuring that resources are being utilized efficiently.

centrifugo_push_job_duration_seconds

  • Type: Summary
  • Labels: provider, recipient_type
  • Description: Duration of push processing job in seconds.
  • Usage: Useful for monitoring the performance of job processing, helping in performance tuning and issue resolution.

Further reading and tutorials

Coming soon.