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Trail: RMI

## Creating a Client Program

> The compute engine is a pretty simple program: it runs tasks that are handed to it. The clients for the compute engine are more complex. A client needs to call the compute engine, but it also has to define the task to be performed by the compute engine.

> Two separate classes make up the client in our example. The first class, `ComputePi`, looks up and calls a `Compute` object. The second class, `Pi`, implements the `Task` interface and defines the work to be done by the compute engine. The job of the `Pi` class is to compute the value of to some number of decimal places.

> As you recall, the nonremote ``Task`` interface is defined as follows:

> >     package compute;

> >     public interface Task extends java.io.Serializable {
> >         Object execute();
> >     }

> The `Task` interface extends `java.io.Serializable` so that an object that implements the interface can be serialized by the RMI runtime and sent to a remote virtual machine as part of a remote method invocation. We could have chosen to have our implementation classes implement both the `Task` interface and the `Serializable` interface and gotten the same effect. However, the whole purpose of the `Task` interface is to allow implementations of that interface to be passed to a `Compute` object, so having a class that implements the `Task` interface that does not also implement the `Serializable` interface doesn't make sense. Therefore we associate the two interfaces explicitly in the type system, ensuring that all `Task` objects are serializable.

> The code that calls a `Compute` object's methods must obtain a reference to that object, create a `Task` object, and then request that the task be executed. The definition of the task `Pi` is shown later. A `Pi` object is constructed with a single argument, the desired precision of the result. The result of the task execution is a `java.math.BigDecimal` representing calculated to the specified precision.

> The client class ``client.ComputePi`` is as follows.

> >     package client;

> >     import java.rmi.*;
> >     import java.math.*;
> >     import compute.*;

> >     public class ComputePi {
> >         public static void main(String args[]) {
> >             if (System.getSecurityManager() == null) {
> >                 System.setSecurityManager(new RMISecurityManager());
> >             }
> >             try {
> >                 String name = "//" + args[0] + "/Compute";
> >                 Compute comp = (Compute) Naming.lookup(name);
> >                 Pi task = new Pi(Integer.parseInt(args[1]));
> >                 BigDecimal pi = (BigDecimal) (comp.executeTask(task));
> >                 System.out.println(pi);
> >             } catch (Exception e) {
> >                 System.err.println("ComputePi exception: " +
> >                                    e.getMessage());
> >                 e.printStackTrace();
> >             }
> >         }
> >     }

> Like the `ComputeEngine` server, the client begins by installing a security manager. This is necessary because RMI could be downloading code to the client. In this example the `ComputeEngine`'s stub is downloaded to the client. Any time code is downloaded by RMI, a security manager must be present. As with the server, the client uses the security manager provided by the RMI system for this purpose.

> After installing a security manager, the client constructs a name used to look up a `Compute` remote object. The value of the first command line argument, `args[0]`, is the name of the remote host on which the `Compute` object runs. The client uses the `Naming.lookup` method to look up the remote object by name in the remote host's registry. When doing the name lookup, the code creates a URL that specifies the host where the compute server is running. The name passed in the `Naming.lookup` call has the same URL syntax as the name passed in the `Naming.rebind` call, which was discussed earlier.

> Next, the client creates a new `Pi` object, passing to the `Pi` constructor the second command line argument, `args[1]`, which indicates the number of decimal places to use in the calculation. Finally, the client invokes the `executeTask` method of the `Compute` remote object. The object passed into the `executeTask` call returns an object of type `java.math.BigDecimal`, so the program casts the result to that type and stores the return value in the variable `result`. Then, the program prints out the result. The following figure depicts the flow of messages among the `ComputePi` client, the `rmiregistry`, and the `ComputeEngine`.

> Finally, let's look at the reason for all of this in the first place: the `Pi` class. This class implements the `Task` interface and computes the value of to a specified number of decimal places. For this example the actual algorithm is unimportant except, of course, for the accuracy of the computation. All that is important is that the computation is numerically rather expensive and thus the sort of thing that you would want to have occur on a more capable server.

> Here is the code for the class `client.Pi`, which implements `Task`.

> >     package client;

> >     import compute.*;
> >     import java.math.*;

> >     public class Pi implements Task {

> >         /** constants used in pi computation */
> >         private static final BigDecimal ZERO =
> >             BigDecimal.valueOf(0);
> >         private static final BigDecimal  ONE =
> >             BigDecimal.valueOf(1);
> >         private static final BigDecimal FOUR =
> >             BigDecimal.valueOf(4);

> >         /** rounding mode to use during pi computation */
> >         private static final int roundingMode =
> >             BigDecimal.ROUND_HALF_EVEN;

> >         /** digits of precision after the decimal point */
> >         private int digits;

> >         /**
> >          * Construct a task to calculate pi to the specified
> >          * precision.
> >          */
> >         public Pi(int digits) {
> >             this.digits = digits;
> >         }

> >         /**
> >          * Calculate pi.
> >          */
> >         public Object execute() {
> >             return computePi(digits);
> >         }

> >         /**
> >          * Compute the value of pi to the specified number of
> >          * digits after the decimal point.  The value is
> >          * computed using Machin's formula:
> >          *
> >          *          pi/4 = 4*arctan(1/5) - arctan(1/239)
> >          *
> >          * and a power series expansion of arctan(x) to
> >          * sufficient precision.
> >          */
> >         public static BigDecimal computePi(int digits) {
> >             int scale = digits + 5;
> >             BigDecimal arctan1_5 = arctan(5, scale);
> >             BigDecimal arctan1_239 = arctan(239, scale);
> >             BigDecimal pi = arctan1_5.multiply(FOUR).subtract(
> >                                       arctan1_239).multiply(FOUR);
> >             return pi.setScale(digits,
> >                                BigDecimal.ROUND_HALF_UP);
> >         }
> >         /**
> >          * Compute the value, in radians, of the arctangent of
> >          * the inverse of the supplied integer to the speficied
> >          * number of digits after the decimal point.  The value
> >          * is computed using the power series expansion for the
> >          * arc tangent:
> >          *
> >          * arctan(x) = x - (x^3)/3 + (x^5)/5 - (x^7)/7 +
> >          *     (x^9)/9 ...
> >          */
> >         public static BigDecimal arctan(int inverseX,
> >                                       int scale)
> >         {
> >             BigDecimal result, numer, term;
> >             BigDecimal invX = BigDecimal.valueOf(inverseX);
> >             BigDecimal invX2 =
> >                 BigDecimal.valueOf(inverseX * inverseX);

> >             numer = ONE.divide(invX, scale, roundingMode);

> >             result = numer;
> >             int i = 1;
> >             do {
> >                 numer =
> >                     numer.divide(invX2, scale, roundingMode);
> >                 int denom = 2 * i + 1;
> >                 term =
> >                     numer.divide(BigDecimal.valueOf(denom),
> >                                  scale, roundingMode);
> >                 if ((i % 2) != 0) {
> >                     result = result.subtract(term);
> >                 } else {
> >                     result = result.add(term);
> >                 }
> >                 i++;
> >             } while (term.compareTo(ZERO) != 0);
> >             return result;
> >         }
> >     }

> The most interesting feature of this example is that the `Compute` object never needs `Pi`'s class definition until a `Pi` object is passed in as an argument to the `executeTask` method. At that point the code for the class is loaded by RMI into the `Compute` object's virtual machine, the `execute` method is called, and the task's code is executed. The resulting `Object`, which in the case of the `Pi` task is a `java.math.BigDecimal` object, is handed back to the calling client, where it is used to print out the result of the calculation.

> The fact that the supplied `Task` object computes the value of `Pi` is irrelevant to the `ComputeEngine` object. You could also implement a task that, for example, generated a random prime number by using a probabilistic algorithm. That would also be numerically intensive and therefore a candidate for being shipped over to the `ComputeEngine`, but it would involve very different code. This code could also be downloaded when the `Task` object was passed to a `Compute` object. In just the way that the algorithm for computing `Pi` is brought in when needed, the code that generates the random prime would be brought in when needed. The `Compute` object knows only that each object it receives implements the `execute` method; it does not know, and does not need to know, what the implementation does.

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