Introduction

The Cepheus-broker component is a lightweight broker only supporting two kinds of operations:

  • requests forwarding by keeping track of which components register Context Entities.
  • pub/sub requests for Context Entities.

This keeps the implementation simple and sufficient for the use cases handled by the NGSI gateway.

The main goal of the Cepheus-broker is to sit between the IoT Agents or NGSI devices, forward their requests to a remote NGSI broker (like Orion) while allowing other NGSI components to subscribe to some the the updated Context Elements.

broker

Forwarding support

Request forwarding is based on the fact that all NGSI components will register their Context Entities on startup.

The broker will track these /registerContext requests (keeping a list of all providingApplication URLs) before forwarding them back to the remote broker.

Then when /updateContext or /queryContext requests arrive, they will be either forwarded to the remote broker if no providingApplication matches or forwarded to the corresponding providingApplication.

broker forward

The forwarding process is described in the Fiware-Orion project documentation: here

Pubsub support

Publish/subscribe is supported by the /subscribeContext requests to Context Entities that will trigger /notifyContext requests.

This feature is mainly used by Cepheus-CEP to track updates to Context Entities.

broker notify

Limitations

The broker has many limitations due to its simple design compared to a complete broker implementation.

  • Broker only supports registerContext, updateContext, queryContext, subscribeContext, unsubscribeContext and notifyContext operations from NGSI v1 API (json formated).
  • Subscriptions only support ONCHANGE as type of notification.
  • Subscriptions do not handle throttling.
  • All subscriptions and registrations are lost on restart of the application (not persisted on disk, kept in memory only).
  • If multiple NGSI providers register the same Context Entities, only the first provider will get the forwarded queryContext or updateContext requests.
  • When a queryContext or updateContext request contains references to multiple Context Entities, the request is forwarded only to the Context Provider of the first Context Entity.
  • Broker does not keep the any value of Context Entities, all requests will get forwarded to a Context Provider or the remote Broker.