SO Scalability Requirements

SO scalability will be supported by managing multiple SO instances by utilizing OOM.
- In OOM, the number of SO component instances will be configured to control the number of active SO instances.
  - Target scalability will be supported. The MariaDB instance number could be different from the rest of SO components.
- Each BPMN execution engine will be configured for a shared database, so the engine can be scaled promptly and ready to handle assignments.
SO endpoints will be registered to MSB for communication load-balancing.
SO run-time scalability handling
- SO will have multiple Camunda Execution engine instances which share the centralized data store.
  - The centralized data store will be replicated, and the replication will be transparent to other SO components.
- The individual execution engine instances do not maintain session state across transactions.
  - The complete state is flushed out to the shared database when a process instance is complete or waiting for events (e.g., asynchronous event, message, human task, etc.).
  - Or, asynchronous continuations can be used during the workflow design when it is necessary to control save points actively (by design) and flush out the process instance states to the database.
  - Once a process instance is passivated, another engine instance can pick up and execute the remaining process instance flows.
- Multiple SDC distribution client instances will be instantiated.
  - A SDC notification will be routed to (or picked up by) one of the SDC notification client instances. Then, the assigned client instance will:
    - query for templates/models from SDC.
    - parse the template/models and store in the Catalog DB.
  - Due to less frequent templates/models changes and SDC notification client activities, a small number (2) of SDC distribution client instances can be configured.
- Multiple API handler instances will be instantiated, and all of the instances are active (active-active).
  - The requests from VID, External API and UUI towards the API handler instances will be distributed/routed via load-balancing. MSB is expected to handle their load-balancing.
  - An assigned API handler instance will communicate with the orchestration execution engine and Data store in a scalable manner.
    - Communications (invoking BPMN execution) with the orchestration execution engine will be done through MSB, no direct connection with hard-coded endpoints.
    - For storing requests and select recipes, the API Handler will communicate with the Data store (Request DB, Service Catalog), which is replicated.
- Multiple Resource/Controller Adapters will be instantiated for active-active operations.
  - The communications between the BPMN/TOSCA resource recipes and the adapter instances will be load-balanced through MSB.
- External communications with other ONAP components such as DACE, OOF, A&AI, SDNC, etc. will be done through MSB/DMaaP in a scalable manner like the above communication requirements.