CommunitySolidServer/documentation/markdown/architecture/features/initialization.md

Server initialization

When starting the server, multiple Initializers trigger to set up everything correctly, the last one of which starts listening to the specified port. Similarly, when stopping the server several Finalizers trigger to clean up where necessary, although the latter only happens when starting the server through code.

App

flowchart TD
  App("<strong>App</strong><br>App")
  App --> AppArgs

  subgraph AppArgs[" "]
    Initializer("<strong>Initializer</strong><br><i>Initializer</i>")
    AppFinalizer("<strong>Finalizer</strong><br><i>Finalizer</i>")
  end

App (urn:solid-server:default:App) is the main component that gets instantiated by Components.js. Every other component should be able to trace an instantiation path back to it if it also wants to be instantiated.

It's only function is to contain an Initializer and Finalizer which get called by calling start/stop respectively.

Initializer

flowchart TD
  Initializer("<strong>Initializer</strong><br>SequenceHandler")
  Initializer --> InitializerArgs

  subgraph InitializerArgs[" "]
    direction LR
    LoggerInitializer("<strong>LoggerInitializer</strong><br>LoggerInitializer")
    PrimaryInitializer("<strong>PrimaryInitializer</strong><br>ProcessHandler")
    WorkerInitializer("<strong>WorkerInitializer</strong><br>ProcessHandler")
  end

  LoggerInitializer --> PrimaryInitializer
  PrimaryInitializer --> WorkerInitializer

The very first thing that needs to happen is initializing the logger. Before this other classes will be unable to use logging.

The PrimaryInitializer will only trigger once, in the primary worker thread, while the WorkerInitializer will trigger for every worker thread. Although if your server setup is single-threaded, which is the default, there is no relevant difference between those two.

PrimaryInitializer

flowchart TD
  PrimaryInitializer("<strong>PrimaryInitializer</strong><br>ProcessHandler")
  PrimaryInitializer --> PrimarySequenceInitializer("<strong>PrimarySequenceInitializer</strong><br>SequenceHandler")
  PrimarySequenceInitializer --> PrimarySequenceInitializerArgs

  subgraph PrimarySequenceInitializerArgs[" "]
    direction LR
    CleanupInitializer("<strong>CleanupInitializer</strong><br>SequenceHandler")
    PrimaryParallelInitializer("<strong>PrimaryParallelInitializer</strong><br>ParallelHandler")
    WorkerManager("<strong>WorkerManager</strong><br>WorkerManager")
  end

  CleanupInitializer --> PrimaryParallelInitializer
  PrimaryParallelInitializer --> WorkerManager

The above is a simplification of all the initializers that are present in the PrimaryInitializer as there are several smaller initializers that also trigger but are less relevant here.

The CleanupInitializer is an initializer that cleans up anything that might have remained from a previous server start and could impact behaviour. Relevant components in other parts of the configuration are responsible for adding themselves to this array if needed. An example of this is file-based locking components which might need to remove any dangling locking files.

The PrimaryParallelInitializer can be used to add any initializers to that have to happen in the primary process. This makes it easier for users to add initializers by being able to append to its handlers.

The WorkerManager is responsible for setting up the worker threads, if any.

WorkerInitializer

flowchart TD
  WorkerInitializer("<strong>WorkerInitializer</strong><br>ProcessHandler")
  WorkerInitializer --> WorkerSequenceInitializer("<strong>WorkerSequenceInitializer</strong><br>SequenceHandler")
  WorkerSequenceInitializer --> WorkerSequenceInitializerArgs

  subgraph WorkerSequenceInitializerArgs[" "]
    direction LR
    WorkerParallelInitializer("<strong>WorkerParallelInitializer</strong><br>ParallelHandler")
    ServerInitializer("<strong>ServerInitializer</strong><br>ServerInitializer")
  end

  WorkerParallelInitializer --> ServerInitializer

The WorkerInitializer is quite similar to the PrimaryInitializer but triggers once per worker thread. Like the PrimaryParallelInitializer, the WorkerParallelInitializer can be used to add any custom initializers that need to run.

ServerInitializer

The ServerInitializer is the initializer that finally starts up the server by listening to the relevant port, once all the initialization described above is finished. This is an example of a component that differs based on some of the choices made during configuration.

flowchart TD
  ServerInitializer("<strong>ServerInitializer</strong><br>ServerInitializer")
  ServerInitializer --> WebSocketServerFactory("<strong>ServerFactory</strong><br>WebSocketServerFactory")
  WebSocketServerFactory --> BaseHttpServerFactory("<br>BaseHttpServerFactory")
  BaseHttpServerFactory --> HttpHandler("<strong>HttpHandler</strong><br><i>HttpHandler</i>")

  ServerInitializer2("<strong>ServerInitializer</strong><br>ServerInitializer")
  ServerInitializer2 ---> BaseHttpServerFactory2("<strong>ServerFactory</strong><br>BaseHttpServerFactory")
  BaseHttpServerFactory2 --> HttpHandler2("<strong>HttpHandler</strong><br><i>HttpHandler</i>")

Depending on if the configurations necessary for websockets are imported or not, the urn:solid-server:default:ServerFactory identifier will point to a different resource. There will always be a BaseHttpServerFactory that starts the HTTP(S) server, but there might also be a WebSocketServerFactory wrapped around it to handle websocket support. Although not indicated here, the parameters for initializing the BaseHttpServerFactory might also differ in case an HTTPS configuration is imported.

The HttpHandler it takes as input is responsible for how HTTP requests get resolved.