4.3. Configure the Grouping API

Use the following steps to configure the Grouping API:
  1. Enable groups using either the declarative or programmatic method.
  2. Specify either an intrinsic or extrinsic group. For more information about these group types, see Section 4.1, “Grouping API Operations”
  3. Register all specified groupers.

4.3.1. Enable Groups

The first step to set up the Grouping API is to enable groups. The following example demonstrates how to enable Groups:
Configuration c = new ConfigurationBuilder().clustering().hash().groups().enabled().build();

4.3.2. Specify an Intrinsic Group

Use an intrinsic group with the Grouping API if:
  • the key class definition can be altered, that is if it is not part of an unmodifiable library.
  • if the key class is not concerned with the determination of a key/value pair group.
Use the @Group annotation in the relevant method to specify an intrinsic group. The group must always be a String, as illustrated in the example:

Example 4.2. Specifying an Intrinsic Group Example

class User {
   <!-- Additional configuration information here -->
   String office;
   <!-- Additional configuration information here -->
   public int hashCode() {
      // Defines the hash for the key, normally used to determine location
      <!-- Additional configuration information here -->
   // Override the location by specifying a group, all keys in the same
   // group end up with the same owner
   String getOffice() {
      return office;

4.3.3. Specify an Extrinsic Group

Specify an extrinsic group for the Grouping API if:
  • the key class definition cannot be altered, that is if it is part of an unmodifiable library.
  • if the key class is concerned with the determination of a key/value pair group.
An extrinsic group is specified using an implementation of the Grouper interface. This interface uses the computeGroup method to return the group.
In the process of specifying an extrinsic group, the Grouper interface acts as an interceptor by passing the computed value to computeGroup. If the @Group annotation is used, the group using it is passed to the first Grouper. As a result, using an intrinsic group provides even greater control.

Example 4.3. Specifying an Extrinsic Group Example

The following is an example that consists of a simple Grouper that uses the key class to extract the group from a key using a pattern. Any group information specified on the key class is ignored in such a situation.
public class KXGrouper implements Grouper<String> {
   // A pattern that can extract from a "kX" (e.g. k1, k2) style key
   // The pattern requires a String key, of length 2, where the first character is
   // "k" and the second character is a digit. We take that digit, and perform
   // modular arithmetic on it to assign it to group "1" or group "2".

   private static Pattern kPattern = Pattern.compile("(^k)(\\d)$");
    public String computeGroup(String key, String group) {
        Matcher matcher = kPattern.matcher(key);
        if (matcher.matches()) {
            String g = Integer.parseInt(matcher.group(2)) % 2 + "";
            return g;
        } else
            return null;
    public Class<String> getKeyType() {
        return String.class;

4.3.4. Register Groupers

After creation, each grouper must be registered to be used.

Example 4.4. Programmatically Register a Grouper

Configuration c = new ConfigurationBuilder().clustering().hash().groups().addGrouper(new KXGrouper()).enabled().build();