RESTful Notification
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This document proposes design patterns for robust asynchronous notification using REST. It introduces the Monitor pattern which creates a managed observe relationship between resources using a defined link relation and parameters. It also introduces Events, which are represented as REST resources within an observable collection, to handle state changes that require more than simple notification, such as alerts that have a lifecycle of being generated, acknowledged, and cleared. Examples of using Monitor links to update resources based on changes in other resources, subscribe to MQTT topics, and publish updates to topics are also provided.
RESTful Notification
- 1. REST Design Patterns for Robust Asynchronous Notification Using simple observe/notify to build a robust and reusable design pattern for asynchronous notifications Michael J Koster
- 2. Problems • Observe is not a well managed relationship – The list of observers is hidden server state – Client can't be certain if it is still in the list – Conditional Observe is difficult to manage • Events have life cycle beyond one notification – Alerts are generated, acknowledged, and eventually cleared – Use cases for asynchronous Event delivery, polling, and batch Event processing
- 3. Design Patterns • Monitor – Create a managed Observe relationship using a REST resource with a defined link relation and parameter set • Events – REST resource to represent an Event instance – Maintain Event instances in an observable collection
- 4. Monitor • Use the IANA registered "monitor" link relation – Description: Refers to a resource that can be used to monitor changes in an HTTP resource (RFC5989) – Similar to "boundto" (dynlink) but defines a unidirectional state update from context to target. • A Monitor may use Observe on the server to obtain state changes of the context resource • A Monitor may implement conditional notification using filter parameters (dynlink) as well as defining transfer methods and formats • A Monitor may support multiple source and target protocols based on URI scheme (mqtt, coap, http) • Monitor parameters may be encoded as link attributes or as properties of a monitor configuration resource
- 5. Monitor Monitored Resource Observer FIlter Updater Monitor Resource Observe Notify Update Reply Context Monitor Target accept accept-method accept-schema accept-interface pmin, pmax lt, gt, st, eq nbul, nbll, init content-type target-method target-schema target-interface
- 6. Monitor Patterns Monitored Resource A Monitor Monitor Resource B Observe Notify Update Monitored Resource A Monitor Monitor Resource BNotify Monitored Resource A Monitor Monitor Resource B Observe Notify Update Reply Reply Update Reply Observe <>;anchor=A;rel=monitor<B>;rel=monitor A Pushes State To B B Observes State From A Remote Update Remote Observe
- 7. Monitor Link Examples Update a monitor resource when context is updated "rel": "monitor", "href": "monitor" } Update the context when a remote resource is updated { "anchor": "coap://0m2m.net:5683/example/test", "rel": "monitor", "href": "" }
- 8. Monitor Link Examples Subscribe to an MQTT topic and update a resource { "anchor": "mqtt://0m2m.net/example/topic", "rel": "monitor", "href": "updated-on-mqtt-notify" } Publish updates on a resource to an MQTT topic { "anchor": "publish-updates-to-mqtt", "rel": "monitor", "href": "mqtt://0m2m.net/example/topic" }
- 9. Events • State changes that require more than simple notification may be handled as Events • Events may have a life cycle, like log records, alerts, etc. • A monitor may add state change notifications to a collection of Event instances using CREATE • The Event collection is Observable and transmits newly created Event instances as notifications
- 10. Monitor Link to Event Collection Create new event instances when events occur { "anchor": "/example/resource/event-emitter", "rel": "monitor", "href": "events", "target-method": "create" } Push event notifications to a MQTT topic { "anchor": "events", "rel": "monitor", "href": "mqtt://0m2m.net/example/topic" {