The ARGO Messaging Service (AMS)  is a Publish/Subscribe Service, which implements the Google PubSub protocol. Instead of focusing on a single Messaging API specification for handling the logic of publishing/subscribing to the broker network the API focuses on creating nodes of Publishers and Subscribers as a Service. It provides an HTTP API that enables Users/Systems to implement message oriented service using the Publish/Subscribe Model over plain HTTP.

Features 

• Ease of use: It supports an HTTP API and a python library so as to easily integrate with the AMS. 
• Push Delivery: ΑΜS instantly pushes asynchronous event notifications when messages are published to the message topic. Subscribers are notified when a message is available.
• Replay messages: replay messages that have been acknowledged by seeking to a timestamp. 
• Schema Support: on demand mechanism that enables a)  the definition of the expected payload schema, b)  the definition of the expected set of attributes and values and c) the validation for each message if the requirements are met and immediately notify client
• Replicate messages on multiple topics: Republisher script that consumes and publishes messages for specific topics (ex. SITES) 

Architectural aspect:

• Durability: provide very high durability, and at-least-once delivery, by storing copies of the same message on multiple servers.

• Scalability: It can handle increases in load without noticeable degradation of latency or availability
• Latency: A high performance service that can serve more than 1 billion messages per year 
• Availability:  it deals with different types of issues, gracefully failing over in a way that is unnoticeable to end users. Failures can occur in hardware, in software, and due to load.  

Fundamentals

In the Publish/Subscribe paradigm, Publishers are users/systems that can send messages to named-channels called Topics. Subscribers are users/systems that create Subscriptions to specific topics and receive messages.

• Topics: Topics are resources that can hold messages. Publishers (users/systems) can create topics on demand and name them (Usually with names that make sense and express the class of messages delivered in the topic)

• Subscriptions: In order for a user to be able to consume messages, he must first create a subscription. Subscriptions are resources that can be created by users on demand and are attached to specific topics. Each topic can have multiple subscriptions but each subscription can be attached to just one topic. Subscriptions allows Subscribers to incrementally consume messages, at their own pace, while the progress is automatically tracked for each subscription.
• Message: The combination of data and (optional) attributes that a publisher sends to a topic and is eventually delivered to subscribers.
• Message attribute: A key-value pair that a publisher can define for a message. 

Pull vs Push Subscriptions

Pub/Sub supports both push and pull message delivery. In push delivery, the Pub/Sub initiates requests to your subscriber application to deliver messages. In pull delivery, your subscription application initiates requests to the Pub/Sub server to retrieve messages.

Pull subscriptions

Pull subscriptions can be configured to require that message deliveries are acknowledged by the Subscribers. If an acknowledgement is made, subscription can resume progressing and send the next available messages. If no acknowledgement is made subscription pauses progressing and re-sends the same messages.

In a pull subscription, the subscribing application explicitly calls the API pull method, which requests delivery of a message in the subscription queue. The Pub/Sub server responds with the message (or an error if the queue is empty), and an ack ID. The subscriber then explicitly calls the acknowledge method, using the returned ack ID, to acknowledge receipt.

Push subscriptions

In a push subscription, the Pub/Sub server sends a request to the subscriber application, at a preconfigured endpoint. The subscriber's HTTP response serves as an implicit acknowledgement: a success response indicates that the message has been successfully processed and the Pub/Sub system can delete it from the subscription; a non-success response indicates that the Pub/Sub server should resend it (implicit "nack"). To ensure that subscribers can handle the message flow, the Pub/Sub dynamically adjusts the flow of requests and uses an algorithm to rate-limit retries.