server/src/main/java/com/orientechnologies/orient/server/OSystemDatabase.java
public void createCluster(final String className, final String clusterName) {
    final ODatabaseDocumentInternal currentDB = ODatabaseRecordThreadLocal.instance().getIfDefined();
    try {
      final ODatabaseDocumentInternal sysdb = openSystemDatabase();
      try {

        if (!sysdb.existsCluster(clusterName)) {
          OSchema schema = sysdb.getMetadata().getSchema();
          OClass cls = schema.getClass(className);

          if (cls != null) {
            cls.addCluster(clusterName);
          } else {
            OLogManager.instance().error(this, "createCluster() Class name %s does not exist", null, className);
          }
        }

      } finally {
        sysdb.close();
      }

    } finally {
      if (currentDB != null)
        ODatabaseRecordThreadLocal.instance().set(currentDB);
      else
        ODatabaseRecordThreadLocal.instance().remove();
    }
  }
public void createCluster(final String className, final String clusterName) {
    final ODatabaseDocumentInternal currentDB = ODatabaseRecordThreadLocal.instance().getIfDefined();
    try {
      final ODatabaseDocumentInternal sysdb = openSystemDatabase();
      try {

        if (!sysdb.existsCluster(clusterName)) {
          OSchema schema = sysdb.getMetadata().getSchema();
          OClass cls = schema.getClass(className);

          if (cls != null) {
            cls.addCluster(clusterName);
          } else {
            OLogManager.instance().error(this, "createCluster() Class name %s does not exist", null, className);
          }
        }

      } finally {
        sysdb.close();
      }

    } finally {
      if (currentDB != null)
        ODatabaseRecordThreadLocal.instance().set(currentDB);
      else
        ODatabaseRecordThreadLocal.instance().remove();
    }
  }
Adds the specified cluster to the class, if it doesn't already exist.