Store & handle push notification errors

[josephluck]

Use Expo's push tickets & receipts to enqueue retries for each failed push ticket.

Use cases

• User has revoked push notification permissions
• Expo has failed to deliver a push notification (server is down or something..)

Workflow:

Send some push notifications assuming everything's okay:

• Batch requests for push notifications
• Send push notifications
• Store response of push tickets / status in firebase for the batch, starting the retry count at 0

Retry and handle errors at intervals (30 mins, 1 hour?):

• Retrieve persisted response of push tickets / status from firebase (note that this is per-batch, so at most 100 tickets)
• Do a check against Expo's API for the current status of the push
• Enumerate and re-batch the push notifications
• Update the response of push tickets / status in firebase, incrementing the retry count by 1 - ensure that successful pushes are removed from the batch
• For any failed pushes that have been retried more than N times, remove the user's push token from their profile, assuming that they have not received the push

Notes

• During the retry phase, is it wise to check whether the user has turned off notifications?
• If we support multiple push intervals per day, how will we handle this?
• Do we definitely want to remove the users push token from their profile if they fail to receive a push? Might be that their phone is turned off etc. Should check what the failure actually represents
• Should we throw away batch data, or event drive it using something like firebase-queue?

Alternative architecture

Event-driven using a queue persisted in Firebase with cloud functions. Pseudo API for this:

type PushyOptions = {
  /**
   * Name of the firebase table to use to persist event state
   */
  tableName: 'push-notifications-events',

  /**
   * Number of times to retry sending before stopping.
   */  
  retryAttempts: 3,

  /**
   * Callback fired when any push attempt contains failures.
   */
  onAttemptFail: (failures, req, res) => {},

  /**
   * Callback fired when any push attempt is successful.
   */
  onAttemptSuccess: (successes, req, res) => {},

  /**
   * Specific event handler callback for when an entire job has failed to be
   * sent.
   * NB: can be used to remove user's PN token
   */
  onJobFail: (failures, req, res) => {},

  /**
   * Specific event handler callback for when an entire job has successfully
   * been sent.
   */
  onJobSuccess: (successes, req, res) => {},

  /**
   * Low level callback fired when any event handling starts. 
   * Can be used to intercept the event to modify it before it's handled.
   * Must return the event.
   */
  onEventStarted: (event) => {},

  /**
   * Low level callback fired when any event has been handled. 
   * Can be used to log or fire other events.
   * Must return void
   */
  onEventFinished: (event) => {},
}

type PushyAPI = {
  /**
   * Creates a push notification job and sends the initial batch of
   * notifications.
   */
  push: (notifications: any[]) => Promise<void>;

  /**
   * Method to retry any unsuccessful push notifications. If a job is already at
   * the maximum retry count, it will dispatch a failed event
   */
  retry: () => Promise<void>;
}

/**
 * Factory function to make an instance of Pushy
 */
type Pushy = (options: PushyOptions) => PushyAPI