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Parameterized Communication

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Reflection/Inspection

Before a protocol can be executed, it must be examined to learn what input it needs (keys, names, IP addresses), and which results it yields (if any).

Obol scripts use the metacommands [input] and [returns] to specify their input requirements and return values. All input requirements must be satisfied before execution proceeds. Applications can access the set of unset input requirements, and the set of script-set return values at any time. There is a simple event-notification mechanism allowing applications to be notified (or they can poll) of state changes, such as a new input requirement, results becoming available, or that an error has occurred. Listings One and Two show how this mechanism is used by client and server applications, respectively.

import lang.API;
import lang.Runtime;
import lang.ScriptHandle;
import lang.ReturnValue;
import lang.ObolException;
 ...
API lobo = Runtime.getInstance();
ScriptHandle script = lobo.getScriptInstance(lobo.loadScript("./server.obol"));
script.setSymbol("portNo", 1234);
script.startExecution();
try {
   while(true) {
      int status = script.waitForStatus(ScriptHandle.STATUS_DONE);
      if (0 != (status & ScriptHandle.STATUS_RESULT_AVAILABLE)) {
         System.out.println("Received \"" + 
           script.getSymbol("data").getValue() + "\"");
      }
      if (0 != (status & ScriptHandle.STATUS_DONE) {
         break;
      }
   }
} catch (ObolException e) {
   System.err.println(e);
}
Listing One
import lang.API;
import lang.Runtime;
import lang.ScriptHandle;
import lang.ObolException;
 ...
API lobo = Runtime.getInstance();
ScriptHandle script = lobo.getScriptInstance(
lobo.loadScript("./client.obol"));
script.setSymbol("portNo", 1234);
script.startExecution();
try {
   while(true) {
      int status = script.waitForStatus(ScriptHandle.STATUS_DONE);
      if (0 != (status & ScriptHandle.STATUS_DONE) {
         break;
      }
   }
} catch (ObolException e) {
   System.err.println(e);
}
Listing Two

Scripts have access to this mechanism, and can invoke other scripts via the [use] metacommand, binding local symbols to input/return symbols defined by the invoked script. Unresolved input requirements are eventually forwarded to the application. Listing Three (available at http://www.ddj.com/code/) is an example of how to examine a script's input requirements; Listing Four (also available at http://www.ddj.com/code/) presents a much quicker way of setting known inputs. The approach is similar for scripts' return values, using the getResultSpecification() and getIntermediateResults() API methods.


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