geode-docs/developing/delta_propagation/delta_propagation_example.html.md.erb

---
title:  Delta Propagation Example
---

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This topic provides an example of delta propagation.

<a id="delta_propagation_example__section_F6700EB7F305462D9F0DEE3080497A14"></a>
In this example, the feeder client is connected to the first server, and the receiver client is connected to the second. The servers are peers to each other.

<img src="../../images/DeltaPropagation-3.gif" id="delta_propagation_example__image_E20DAE5925C44EBDA03D1AC8325D1122" class="image" />

The example demonstrates the following operations:

1.  In the Feeder client, the application updates the entry object and puts the entry. In response to the `put`, <%=vars.product_name%> calls `hasDelta`, which returns true, so <%=vars.product_name%> calls `toDelta` and forwards the extracted delta to the server. If `hasDelta` returned false, <%=vars.product_name%> would distribute the full entry value.
2.  In Server1, <%=vars.product_name%> applies the delta to the cache, distributes the received delta to the server's peers, and forwards it to any other clients with interest in the entry (there are no other clients to Server1 in this example)
3.  In Server2, <%=vars.product_name%> applies the delta to the cache and forwards it to its interested clients, which in this case is just the Receiver client.

<a id="delta_propagation_example__section_185444FC51FB467587A62DFEC07C9C7D"></a>

This example shows the basic approach to programming a `Delta` implementation.

``` pre
package delta;

import org.apache.geode.Delta;
import org.apache.geode.InvalidDeltaException;
import java.io.DataInput;
import java.io.DataOutput;
import java.io.IOException;
import java.io.Serializable;

/**
 * Sample implementation of Delta
 * 
 * @author GemStone Systems, Inc.
 * @since 6.1
 */
public class SimpleDelta implements Delta, Serializable {

// Original object fields
    private int intVal;
    private double doubleVal;

    // Added for delta - one boolean per field to track changed status
    private transient boolean intFldChd = false;
    private transient boolean dblFldChd = false;

    public SimpleDelta(){}

    public SimpleDelta(int i, double d){
        this.intVal = i;
        this.doubleVal = d;
    }

    public boolean hasDelta() {
        return this.intFldChd || this.dblFldChd;
    }

    public void toDelta(DataOutput out) throws IOException {
        System.out.println("Extracting delta from " + this.toString());
    // Write information on what has changed to the 
    // data stream, so fromDelta knows what it's getting 
        out.writeBoolean(intFldChd);
        if (intFldChd) {
    // Write just the changes into the data stream
            out.writeInt(this.intVal);
    // Once the delta information is written, reset the delta status field
            this.intFldChd = false;
            System.out.println(" Extracted delta from field 'intVal' = "
                                                + this.intVal);
        }
        out.writeBoolean(dblFldChd);
        if (dblFldChd) {
            out.writeDouble(this.doubleVal);
            this.dblFldChd = false;
            System.out.println(" Extracted delta from field 'doubleVal' = "
                                                + this.doubleVal);
        }
    }

    public void fromDelta(DataInput in) throws IOException, InvalidDeltaException {
        System.out.println("Applying delta to " + this.toString());
        // For each field, read whether there is a change 
        if (in.readBoolean()) {
            // Read the change and apply it to the object 
            this.intVal = in.readInt();
            System.out.println(" Applied delta to field 'intVal' = "
                        + this.intVal);
        }
        if (in.readBoolean()) {
            this.doubleVal = in.readDouble();
            System.out.println(" Applied delta to field 'doubleVal' = "
                        + this.doubleVal);
        }
    }
    // In the setter methods, add setting of delta-related 
    // fields indicating what has changed 
    public void setIntVal(int anIntVal) {
        this.intFldChd = true;
        this.intVal = anIntVal;
    }

    public void setDoubleVal(double aDoubleVal) {
        this.dblFldChd = true;
        this.doubleVal = aDoubleVal;
    }

    public String toString() {
        return "SimpleDelta [ hasDelta = " + hasDelta() + ", intVal = " + 
                this.intVal + ", doubleVal = {" + this.doubleVal + "} ]";
    }
}
```