Zaqar’s notification semantics

One of Zaqar’s goals, from an API semantics perspective, is to provide support for push notifications - or subscriptions, if you will. After having added support for all the queuing related semantcis to Zaqar’s API, we can start adding support for other things that sit on top of the existing semantics like the ones mentioned in this spec.

Other references: - - - -

Problem description

Zaqar’s support for queuing semantics that allow users to publish and consume messages to and from the service. These semantics cover several use-cases but they leave out others like ‘listening’ for messages. There are, as well as for prod/cons scenarios, many use-cases for pub/sub that need to be supported. Some examples of such use-cases are: mobile push notifications, support for events triggering, meetering, etc.

In order to cover the above scenarios, Zaqar needs to provide an API that will allow users for subscribing to specific queues and get messages from there without requiring to poll. This will allow for a better architecture on the client side, a more reliable interaction between Zaqar and the client in terms of connection management and it also opens the doors for supporting other scenarios.

Proposed change

The proposal is to extend Zaqar’s API with support for subscriptions without adding a separate service - please see the section below with regards to both services being merged. This proposal introduces the high-level changes required for this feature but the detailed API specification has yet to be written.

Here’s a, non-exahustive, list of resources required to support notifications:

Subscriptions: Subscriptions define what the subscriber wants to listen on. It contains the data of the queues/topics, routing parameters, maximum retries and other configirations required to correctly filter messages and keep the high-level guarantees.

A subscription is used by subscribers to listen on things. A non persistent subscriber - webhooks, sms, mobile push protocols - are configured in the subscription itself, whereas other subscribers using persistent connection can load a specific subscription they want to listen on.

A subscription consists in a object containing the information of the source, destination, expiration and a custom field that contains information useful for the notifier.

POST /v2/queues/{queue_name}/subscriptions

    'subscriber': '',
    'ttl': 3600,
    'options': {}
  • source: The source queue for notifications
  • subscriber: The subscriber URI. This can be any of the supported subscriber URI and the implementation must be based on stevedore to allow external contributions.
  • ttl: time to live for this subscription. It’s a positive number that expresses, in seconds, how long a subscription should exist. If omitted the subscription won’t expire.
  • options: publisher specific options.

Publishers: Publishers are users and/or third-party resources publishing messages to the notification endpoints. Users publishing messages to Zaqar don’t need any specialized endpoint for this. Messages must go through the normal and already implemented message publishing API. However, third-party publishers require a specific configuration. Third-party publishers consume messages from other services and publish them in zaqar’s queues.

Workers: Publishing tasks need to be executed in a distributed - or local to the node - fashion, they need to be reliable, fault-tolerant and atomic. To do so, I’ve evaluated taskflow as a valid library for this job. Taskflow has support for local/distributed, blocking/concurrent task execution. It provides support for several execution models, persistence layers - including sqlite, mysql and zookeeper - and it has support for retries. The alternative to taskflow is writing our on implementation for this work, which doesn’t seem to be worth it at this stage.

Push Protocols

There are several types of push technologies that could be supported, therefore the implementation must be generic enough to allow for new implementations to be done easily and also as external plugins.

Nonetheless, this spec proposes starting with just the first of the plugins proposed below:

  • Webhooks
  • Email
  • Mobile push
  • SMS

Why not 2 services

Zaqar has stated since its inception that it provides a single common API for things that other cloud services like AWS have in 2 separate services. Nonetheless, I think the real motivations for these 2 API’s to be under the same service are:

  • Maintenance: It’s easier to maintain and scale just 1 service type than a group of different services.
  • Consistency: It’ll help reducing the code needed to implement the notification API since they both will be running under the same structure and users will know where to find the services easily.
  • Performance: Implementing the notification API in a separte service will require a communication between that service and the queuing service. At least, it’ll require a constant polling from the backend (if it doesn’t support push itself). By having everything under the same service, we can trigger notification for messages as they come in.

Future Improvements

  • Support for subscription’s confirmation


  • Split the above into 2 different services. As mentioned in the previous section, 2 services don’t seem to be the ideal solution for this.



Primary assignee:
Secondary assignees:


Target Milestone for completion:

Work Items

  • Work on the detailed API spec
  • Write storage code for notifications
  • Implement the API on top of the storage code.
  • Work on the publishers




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