Messaging

Use Cases for a Message Bus

• Notifications
  • pipeline steps
    • single implementation (for most steps)
      • usage pattern: push-pull pipeline
      • exchange type: direct
      • queues: single queue for each pipeline step, with multiple consumers per queue
    • multiple implementations (e.g., user target resolvers)
      • usage pattern: push-pull pipeline
      • exchange type: direct
      • queues: different queue for each "type" of user-target resolver, with multiple consumers per queue
  • delivery channels
    • usage pattern: pub/sub or push-pull pipeline, depending on channel implementations
    • exchange type: fanout ( ? )
• Cross-process event handling
  • usage pattern: pub/sub
  • exchange type: direct and/or topic
• Course publish event (CMS → LMS)
  • usage pattern: request-reply RPC or pub/sub
  • exchange type: default or direct

Messaging Framework Properties

• message delivery guarantees: at-most-once, at-least-once, exactly-once
• message durability: persistent or transient
• message routing: automatic filtering
• competing or non-competing consumers
• usage patterns
  • request-reply RPC (pattern: task distribution, topology: service bus)
  • publish-subscribe (pattern: data distribution, topology: data distribution tree)
  • push-pull pipeline (pattern: parallel task distribution and collection, topology: parallelized pipeline)
• exchanges (per AMQP 0-9-1)
  • types: 
    • default (automatic routing to named queue)
    • direct (route based on message routing key) - usually for unicast in task distribution patterns
    • fanout (broadcast to multiple queues) - usually for broadcast routing in pub/sub patterns
    • topic (route based on match between routing key and queue binding pattern) - usually for multicast in pub/sub patterns
    • headers (route based on any/all match of header values) - "direct on steroids"
  • attributes: name, durability (survive broker restart), auto-delete (deleted when all queues have finished using it)
• queues
  • attributes: name, durable, exclusive, auto-delete
• consumers
  • push or pull APIs
  • message acks, rejects, bulk rejects
  • message prefetching
• messages
  • serialization format: JSON, thrift, protocol buffers, messagepack
  • durability: persistent or ephemeral

Notifications

Uses Cases for notifications

• Bazaar - time-based activities with intelligent prompting and synchronous chat for learners
• Discussions
  • forum activity - including notifying moderators and forum participants
  • social awareness
• Course Dates
• Course Announcements
• User Progress in course
• Analytics Use cases:
  • Instructors reaching out to learners
  • Automatically notifying/supporting struggling learners
  • Verified upgrade reminders (+ Jasper Ho Deen Abdul-Hathi (Deactivated))