Context in Modelica

Here’s the translation of the Go code to Modelica, formatted in Markdown suitable for Hugo:

Our example demonstrates how to create a simple HTTP server in Modelica. This server showcases the usage of a concept similar to context.Context for controlling cancellation. In Modelica, we don’t have an exact equivalent, but we can simulate similar behavior using timers and events.

model HttpServer
  import Modelica.Blocks.Interfaces.*;
  import Modelica.StateGraph;

  // Simulating HTTP server
  StateGraph.InitialStep serverStart(nIn=0, nOut=1);
  StateGraph.Step waitingForRequest(nIn=1, nOut=1);
  StateGraph.Step processingRequest(nIn=1, nOut=2);
  StateGraph.Step sendResponse(nIn=1, nOut=1);
  StateGraph.Step handleCancellation(nIn=1, nOut=1);

  // Transitions
  StateGraph.Transition T1(condition=time > 0);
  StateGraph.Transition T2(condition=newRequest.y);
  StateGraph.Transition T3(condition=time > processingStart.y + 10);
  StateGraph.Transition T4(condition=cancellationRequested.y);
  StateGraph.Transition T5(condition=true);
  StateGraph.Transition T6(condition=true);

  // Input signals
  BooleanInput newRequest;
  BooleanInput cancellationRequested;

  // Output signals
  BooleanOutput responseReady;
  BooleanOutput requestCancelled;

  // Helper components
  Modelica.Blocks.Sources.RealExpression processingStart;

equation
  connect(serverStart.outPort[1], T1.inPort);
  connect(T1.outPort, waitingForRequest.inPort[1]);
  connect(waitingForRequest.outPort[1], T2.inPort);
  connect(T2.outPort, processingRequest.inPort[1]);
  connect(processingRequest.outPort[1], T3.inPort);
  connect(processingRequest.outPort[2], T4.inPort);
  connect(T3.outPort, sendResponse.inPort[1]);
  connect(T4.outPort, handleCancellation.inPort[1]);
  connect(sendResponse.outPort[1], T5.inPort);
  connect(handleCancellation.outPort[1], T6.inPort);
  connect(T5.outPort, waitingForRequest.inPort[1]);
  connect(T6.outPort, waitingForRequest.inPort[1]);

  processingStart.y = if processingRequest.active then time else 0;
  responseReady = sendResponse.active;
  requestCancelled = handleCancellation.active;

  when processingRequest.activePort then
    Modelica.Utilities.Streams.print("server: hello handler started");
  end when;

  when pre(processingRequest.active) and not processingRequest.active then
    Modelica.Utilities.Streams.print("server: hello handler ended");
  end when;

  when handleCancellation.activePort then
    Modelica.Utilities.Streams.print("server: context canceled");
  end when;
end HttpServer;

In this Modelica model:

We use a StateGraph to simulate the server’s behavior.
The waitingForRequest step represents the server waiting for incoming requests.
When a request comes in (simulated by the newRequest input), the server moves to the processingRequest step.
If the processing completes within 10 seconds, it moves to the sendResponse step.
If a cancellation is requested (simulated by the cancellationRequested input) during processing, it moves to the handleCancellation step.
After either sending a response or handling cancellation, the server returns to the waiting state.

To use this model, you would need to create a simulation setup that provides inputs for newRequest and cancellationRequested, and observes the outputs responseReady and requestCancelled.

Note that Modelica, being a modeling language primarily used for physical systems and control systems, doesn’t have built-in support for creating actual HTTP servers. This model simulates the logical flow of an HTTP server handling requests and cancellations, but it wouldn’t actually serve HTTP requests in a real-world scenario.

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Context in Modelica