SAM and Cerebral

• Example implementation of the SAM pattern (home).
• Uses the Cerebral framework (home).
• Project folder structure follows the Control-Entity-Boundary (ECB) pattern (blog post).

Quick Start

• npm installs all dependencies.
• npm run build sets up static dependencies for the website.
• npm start starts the Budo development-webserver and serves the directory (watch + rebuild + autoreload).
• npm run lint formats the code with Prettier and checks few Eslint rules.

SAM pattern and Cerebral

• A Cerebral module may be implemented as a SAM container. Each Cerebral signal is a SAM-step action, and this ensures that the model is updated in a precisely defined step (as in TLA+).
• Triggering an action may execute a step, if there is a proposal.
• A proposal in turn may trigger an indeterminate (see Actor Model) amount of automatic next-actions via the next-action-predicate function.
• Only the accept function has write access to the model.
• The computeStateRepresentation function derives the state-representation (view-model). It also gives a hint in form of a list of allowed actions, so that buttons, etc. can be disabled in the UI.
• Although technically wrong, in this implementation, the model is saved in the Cerebral state-tree as well, just to be visible in Cerebral's debugger. This also allows the model or parts of it to be used as view-model, which requires less code for this exercise.
• Helper functions allow communication between SAM-step-containers. If one container calls an action of another, it should wait for the other to compute its state-representation, which is the (asynchronous) return value of the action-call.

SAM and the Entity Control Boundary pattern

The SAM pattern maps to the ECB pattern too:

• boundary --> View component
• control --> Cerebral module, actions, third party APIs
• entity --> model (accept), compute state, compute next action

ToDo

• App features

  • Add blog post example (conflict-free example). No replies without parent should be shown because of eventual consistency.

    • post posts
    • reply to posts
    • like posts and show counter per user (publicly visible)

  • Add Git-like example (with conflict management)? Track concurrent changes and show causal chain for conflict resolution by user.

  • Use control state for page change (routing) example?

  • Add session control state example (log in <-> log out on authenticated route)?

  • Use security token.

  • Add notification service when remote changes arrive.

  • Prevent automatic example post by more than one user?

• Storage layer

  • Add PouchDB provider to Cerebral context
  • Save model to DB
  • Specify Bolt-on Shim Layer in TLA+
  • Implement Bolt-on Shim Layer
    • Globally unique identifier for local store
    • Local Lamport time-stamp sequence number for writes
    • As external service?
    • Remove SAM container semantics?
    • Refactor shim module to be sub-module?

• SAM container

  • Allow for next action to not block step until complete (blockStep = true)? -> No
  • Queue action to NAP when in progress (optionally)
  • Compute: Mark action specific cancel buttons as disabled.

• Check allowed action in step.

• Add tests.

• Create Proxy to hide state API?

• Defer updating the view until end of SAM step? (State updates may trigger rerending immediately currently.)

Consider

• Prevent or at least warn about concurrent mutations while a step is in progress?

Notes

• Async Ticker is not a simple next action example. Ticker could be a singleton.