Croquet — Replicated Computation
Teresa Lasarow
20.07.2021
Software Architectures HPI
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# Once upon a time - Croquet
- Croquet as shared virtual 3D environment for collaboration of multi-users
- Supports communication, highly scalable collaboration, resource sharing, and synchronous computation
- TeaTime architecture → basis for object-object communication and world/object synchronization, including initial content synchronization while user joins a world
https://theses.liacs.nl/pdf/2009-06SjaakWolff.pdf - Figure 1
Paper: Croquet - A collaboration system architecture - Figure 4
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# Distributed Computing Systems
### Communication
- Communication as central part of computing experience, e.g., multi-player games, multi-user apps etc.
- Distributed System → collection of independent components located on different machines (nodes) that share messages with each other in order to achieve common goals
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# Distributed Computing Systems
### Client-Server
- Most popular architecture for distributed systems
- Use-Case: Cheating → "never trust the client"
- One single node (Server) is in charge of determining game state advancements
- Drawback: Server could be a bottleneck
https://www.gabrielgambetta.com/client-side-prediction-server-reconciliation.html
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### Demo
##### Simulation in Croquet
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# "Don't repeat yourself" - Or do you?
- Distributed systems → often makes a lot of sense to have many of one thing
- Redundancy → duplicate components of a system
- Doesn't really help if a redundant node is out of synch with original
- Replication → is what makes duplicated values actually usefull
- Ensures that the redundant node (a replica) is identical to all of its other copies
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# "Don't repeat yourself" - Or do you?
### Replicated Computation
- A process of keeping copies of data at various locations or in serialized time
- require processes to handle incoming events
- Pro's: Reduce user waiting time, increased availability and increased performance
- Challenge: Consistency through sharing of information among redundant resources such as software and hardware
- Replica is deterministic → Produce the same result
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# Today is - Croquet
- No more 3D-virtual environment, no more portals, no more avatars
- A bit-identical synchronization system
- Applications are developed as though they are local, single-user experiences → Only one client
- Replicator serves as event mirror
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# Demo
#### Dice Example
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# Croquet - Software Participants
## The Model - Replicated Object
- Instances created with create() but new
- Initialized via init() but constructor() → only initialized the first time the object comes into existence in the session
```javascript
class RootModel extends Croquet.Model {
init(options={}) {
super.init();
this.sceneModel = options.sceneModel;
}
}
```
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# Croquet - Software Participants
## The View - Non-Replicated Object
- Independent local view
- Provides mechanism to hook into the replicated model simulation
```javascript
class DiceView extends Croquet.View {
constructor(model) {
super(model);
this.model = model;
}
}
```
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# Croquet - Software Participants
## The Reflector
- Stateless, public, message-passing services located in the cloud
- Received events from a user are mirrored to all session joined users
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# Croquet - Software Participants
## Events
Communication between Model and View
- publish - sending events
- subscribe - receiving events
- event-handling ...
- Model-to-Model: event-handler is executed immediately
- Model-to-View: event is queued and handled by local view
- View-to-View: event is queued and handled in the same update cycle
- View-to-Model: transmitted to the reflector and mirrored to all users
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# Croquet - Software Participants
## The Session
- Instantiating the root model (for new session) or resuming a snapshot
```javascript
Croquet.Session.join({
appId: "org.lively.croquet.dice",
name: "DiceByModel",
password: "super_safe",
model: RootModel,
view: RootView,
});
```
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# Dice Architecture
#### User starts a session → Model and View is initialized
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# Dice Architecture
#### Other users join the same session → Use of the same session-id
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# Dice Architecture
#### User3 clicks dice to roll it → publish the event to the reflector
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# Dice Architecture
#### Reflector mirrors the incoming event to all replica of the Model in the current session
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# Dice Architecture
#### Model publish calculation result to corresponding view
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# Futur Work
- Stress test
- Benchmark
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# Merci
#### Thank you for your attention!
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