Architecture

W3C LDN

The overarching, logical architecture for COAR Notify is dictated by its reliance on the W3C Linked Data Notifications (LDN) standard.

LDN is designed to work in an architecture of 3 logical components:

• sender: sends notifications to the inbox of the receiver
• receiver: maintains the inbox and receives notifications there
• consumer: fetches notifications from the receiver's inbox

These components interact with each other via standard HTTP requests & responses.

Deployment

However, to achieve point-point communications between, for example a repository and a review service, it may be expedient to combine receiver and consumer into one logical component, to be embedded into the target system.

While this may be expedient, it does introduce a limitation, which is that the receiver's inbox is not necessarily accessible via HTTP. For many (possibly most) Notify use-cases, this limitation will not be important.

Although there are probably other permutations, I suggest that the following broad deployment architectures are likely to be considered. In each case I have used a repository as the example system - but clearly any other type of relevant system could be substituted.

This key applied to the following diagrams:

Option 1: Embedded and combined Consumer and Receiver

In this arrangement, the minimum development is done to allow the repository to both receive and send LDN notifications via HTTP. However, the Inbox is not exposed to HTTP Get requests for notifications received, because the combined Consumer/Receiver does not store the notifications at all. Instead, the Consumer processes the notifications as soon as they are received.

Advantages

• no reliance on external service (inbox provider)
• embedded and "in process" solution
• may be quicker, cheaper and easier to develop

Disadvantages

• both Receiver and Inbox components need to be developed
• no ability to fetch previous notifications
• tight coupling between repository and LDN Receiver creates potentially 'brittle' deployment - if the repository is offline, so is the Receiver and Inbox.
• no support for secondary consumers (e.g. aggregators, audit systems, backups, archiving etc.)
• may be difficult to scale to take advantage of more/disparate opportunities to interoperate with other LDN-enabled systems
• HTTP timeout calculations run the risk of including the time taken to process the notification once it is received, which distorts the whole network-timeout aspect.

Option 2: Embedded, but separate Consumer and Receiver with stored notifications

In this arrangement, a fully-functioning LDN Receiver and Inbox is embedded into the repository. The repository also embeds its own Consumer. In this example, the embedded consumer uses HTTP to fetch notifications from the Inbox, but it could perhaps use an "in process" method call of some kind.

Advantages

• notifications are persisted, allowing:
  • opportunity for more fault-tolerant behaviour
  • opportunity for repeated consumption of notifications by same or different services
• no reliance on external services (inbox provider)
• could support secondary consumers (e.g. aggregators, audit systems, backups, archiving etc.)
• HTTP timeout calculations are more accurate because they do not include the time taken to process the notification once it is received.

Disadvantages

• Receiver and Inbox components need to be developed
• tight coupling between repository and LDN Receiver creates potentially 'brittle' deployment - if the repository is offline, so is the Receiver and Inbox.

Option 3: Independent Receiver

In this arrangement, the Receiver and Inbox are deployed as a separate service. This creates the opportunity to use an off-the-shelf LDN Receiver rather than needing to develop this functionality in the repository system.

Advantages

• notifications are persisted, allowing:
  • opportunity for more fault-tolerant behaviour
  • opportunity for repeated consumption of notifications by same or different services
• Receiver and Inbox components may not need to be developed
• loose coupling between repository and LDN Receiver allowing:
  • development, maintenance and deployment to be managed separately
  • could support secondary consumers (e.g. aggregators, audit systems, backups, archiving etc.)

Disadvantages

• Reliance on external service (although this may not be perceived to be a problem)
• More complex deployment and maintenance
• Potentially duplication of security logic and/or configurations (e.g. the Receiver must know which notifications to accept on behalf of the Consumer)