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Image RemovedThe Publish-Subscribe (pub/sub) messaging pattern is used among all the components in the Apache Stratos architecture. In this messaging pattern the components that send the messages are named as publishers; while, the components that receive the message are named as subscribers. The publishers do not define where the published messages, which are namely topics, need to be routed. Instead, the publishers maintain the topics, so that any subscribers who are interested in a specific topic can subscribe to it.
All components (mainly the topic based pub/sub communications) use Message Broker (MB) for its communication in Apache Stratos 4.0.0. This helps to integrate all the components in loosely coupled manner; while, providing the advantage of adding or removing components without affecting to overall system. All components need to know only one well-known-member (WKA), which is the Message Broker. As a result, this enables Apache Stratos to be configured easily. In addition, the pub/sub communication enables Apache Stratos to scale effortlessly scale into multi-cloud effortlessly..
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As depicted in the above diagram, the message flow that takes place among Apache Stratos components has been explained as follows:

  1. The user creates an application in Stratos Manager.
    The user initially selects a cartridge together with an auto-scaling policy and deployment policy that will be used for the application and also specifies the repository for the application. Thereafter, based on the information provided by the user, Stratos Manager will generate a subscription key for the subscription and store it against the repository information.
  2. Stratos Manager will send the new subscription details to the Cloud Controller.
    The Cloud Controller will send the subscription details, which include the cartridges, policies and subscription key, to the Cloud Controller via a service call. 
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Topology topic

The topology topic, which is inside the Apache Stratos architecture, is the main topic that contains all the member information. Cloud Controller will populate/publish to this topic; while, all other components subscribe and use it.

The topology topic contains the cluster details, member details (e.g., IP address), ports, member status (e.g., member starting and member active), partition details, Load Balancer details, and more. At any given time when looking at the topology, it is possible to get a full picture of the system.Thereby, when Auto-scaler is taking decisions on scale up or down, it uses the topology to get an accurate state of a particular cluster. The Stratos Manager UI will also retrieve information from the topology when displaying the graphical topology for given cluster. The Load Balancers use the same topology topic to add members to its load balancing cluster or clusters.