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Compact Serialization (BETA)

As an enhancement to existing serialization methods, Hazelcast offers a BETA version of the Compact serialization, with the following main features.

  • Separates the schema from the data and stores it per type, not per object which results in less memory and bandwidth usage compared to other formats

  • Does not require a class to implement an interface or change the source code of the class in any way

  • Supports schema evolution which permits adding or removing fields, or changing the types of fields

  • Can work with no configuration or any kind of factory/serializer registration

  • Platform and language independent

  • Supports partial deserialization of fields, without deserializing the whole objects during queries or indexing

Hazelcast achieves these features by having a well-known schema of objects and replicating them across the cluster which enables members and clients to fetch schemas they don’t have in their local registries. Each serialized object carries just a schema identifier and relies on the schema distribution service or configuration to match identifiers with the actual schema. Once the schemas are fetched, they are cached locally on the members and clients so that the next operations that use the schema do not incur extra costs.

Schemas help Hazelcast to identify the locations of the fields on the serialized binary data. With this information, Hazelcast can deserialize individual fields of the data, without reading the whole binary. This results in a better query and indexing performance.

Schemas can evolve freely by adding or removing fields. Even, the types of the fields can be changed. Multiple versions of the schema may live in the same cluster and both the old and new readers may read the compatible parts of the data. This feature is especially useful in rolling upgrade scenarios.

The Compact serialization does not require any changes in the user classes as it doesn’t need a class to implement a particular interface. Serializers might be implemented and registered separately from the classes.

It also supports zero-configuration use cases by automatically extracting schemas out of the classes using reflection, which is cached and reused later, with no extra cost.

The underlying format of the compact serialized objects is platform and language independent. Native client supports will be added shortly after promoting this feature to stable status.

Note that, currently, the feature is in BETA state and Hazelcast does not guarantee behavior or API compatibility.

During the BETA period, Compact serialization has to be enabled explicitly as shown in the CompactSerializationConfig section.

Using Compact Serialization With Zero-Configuration

Compact Serialization can be used without a configuration or serializer registration. As described in the Serialization Interface Types, Hazelcast tries to find a serializer for any object. Before this feature, if there were no serializers associated with a certain class, we were throwing an exception indicating that there is no suitable serializer for it. Now, as a last effort, Hazelcast tries to use Compact serialization. To do this, Hazelcast tries to extract a schema out of the class using reflection. If successful, it registers the reflective serializer associated with the extracted schema and uses it while serializing/deserializing instances of that class. If the automatic schema extraction fails, Hazelcast throws an exception as before.

Currently, Hazelcast supports extracting schemas out of the classes that have the following field types.

  • Primitive types: boolean, byte, short, char, integer, long, float, and double.

  • String

  • java.time.LocalDate, java.time.LocalTime, java.time.LocalDateTime, and java.time.OffsetDateTime

  • java.math.BigDecimal

  • Arrays of the types shown above

  • Nested classes that contain the fields above and arrays of them

Assume that we have the following Employee class.

public class Employee {
    private long id;
    private String name;

    public Employee() {
    }

    public Employee(long id, String name) {
        this.id = id;
        this.name = name;
    }

    public long getId() {
        return id;
    }

    public String getName() {
        return name;
    }
}

If we don’t perform any kind of configuration change and use the instances of the class directly, there won’t be any exceptions thrown. Hazelcast will generate a schema out of the Employee class the first time we try to serialize an object, cache it, and reuse it for the subsequent serializations/deserializations.

For reflective schema extraction and serializer to work, the class must have an empty public constructor.

ClientConfig config = new ClientConfig();
config.getSerializationConfig()
        .getCompactSerializationConfig()
        .setEnabled(true); // Required during BETA

HazelcastInstance client = HazelcastClient.newHazelcastClient(config);
IMap<Long, Employee> map = client.getMap("employees");
Employee employee = new Employee(1, "John Doe");
map.set(1L, employee);
Employee employeeFromMap = map.get(1L);
Since the Compact Serialization feature is in BETA, to use zero-configuration, ironically, you have to enable it with the CompactSerializationConfig as shown below. This limitation will be removed once the BETA period ends, and Hazelcast will enable Compact serialization by default. Once this happens, zero-configuration will work as promised, without requiring any kind of configuration.

Implementing CompactSerializer

Another way to use Compact serialization is to implement the CompactSerializer interface for a class and register it to the configuration.

For example, assume that we have the same Employee class as above. Then, a Compact serializer can be implemented as such.

class EmployeeSerializer implements CompactSerializer<Employee> {
    @Override
    public Employee read(CompactReader reader) {
        long id = reader.readLong("id");
        String name = reader.readString("name");
        return new Employee(id, name);
    }

    @Override
    public void write(CompactWriter writer, Employee employee) {
        writer.writeLong("id", employee.getId());
        writer.writeString("name", employee.getName());
    }
}

The last step is to register the serializer to the CompactSerializationConfig. Below is the programmatic and declarative configurations for this step.

Programmatic Configuration:

SerializationConfig serializationConfig = new SerializationConfig();
serializationConfig.
        getCompactSerializationConfig()
        .setEnabled(true) // Required during BETA
        .register(Employee.class, "employee", new EmployeeSerializer());

Declarative Configuration:

  • XML

  • YAML

<hazelcast>
    ...
    <serialization>
        <compact-serialization enabled="true">
            <registered-classes>
                <class type-name="employee" serializer="com.example.EmployeeSerializer">
                    com.example.Employee
                </class>
            </registered-classes>
        </compact-serialization>
    </serialization>
    ...
</hazelcast>
hazelcast:
  serialization:
    compact-serialization:
      enabled: true
      registered-classes:
        - class: com.example.Employee
          type-name: employee
          serializer: com.example.EmployeeSerializer

A schema will be created from the serializer, and a unique schema identifier will be assigned to it automatically.

From now on, Hazelcast will serialize instances of the Employee class using the EmployeeSerializer.

Schema Evolution

Compact serialization permits schemas and classes to evolve by adding or removing fields, or by changing the types of fields. More than one version of a class may live in the same cluster and different clients or members might use different versions of the class.

Hazelcast handles the versioning internally. So, you don’t have to change anything in the classes or serializers apart from the added, removed, or changed fields.

Hazelcast achieves this by identifying each version of the class by a unique fingerprint. Any change in a class results in a different fingerprint. Hazelcast uses 64 bits Rabin Fingerprint to assign identifiers to schemas, which has an extremely low collision rate.

Different versions of the schema with different identifiers are replicated in the cluster and can be fetched by clients or members internally. That allows old readers to read fields of the classes they know when they try to read data serialized by a new writer. Similarly, new readers might read fields of the classes available in the data, when they try to read data serialized by an old writer.

Assume that the two versions of the following Employee class lives in the cluster.

class Employee {
    long id;
    String name;
}
class Employee {
    private long id;
    private String name;
    private int age; // Newly added field
}

Then, when faced with binary data serialized by the new writer, old readers will be able to read the following fields.

public Employee read(CompactReader reader) {
    long id = reader.readLong("id");
    String name = reader.readString("name");
    // The new "age" field is there, but the old reader does not
    // know anything about it. Hence, it will simply ignore that field.
    return new Employee(id, name);
}

Then, when faced with binary data serialized by the old writer, new readers will be able to read the following fields. Also, Hazelcast provides convenient APIs to read default values when there is no such field in the data.

public Employee read(CompactReader reader) {
    long id = reader.readLong("id");
    String name = reader.readString("name");
    // Read the "age" if it exists, or the default value 0.
    // reader.readInt("age") would throw if the "age" field
    // does not exist in data.
    int age = reader.readInt("age", 0);
    return new Employee(id, name, age);
}

Note that, when an old reader reads data written by an old writer, or a new reader reads a data written by a new writer, they will be able to read all fields.

CompactSerializationConfig

Since the Compact serialization is in BETA, and the configuration API might change in the future, we didn’t define it in our XSDs. If you want to configure Compact serialization with XML or YAML configurations, you have to disable schema validation, by setting the hazelcast.config.schema.validation.enabled system property to false.

During the BETA period, Compact serialization has to be enabled explicitly as shown below.

Programmatic Configuration:

SerializationConfig serializationConfig = new SerializationConfig();
serializationConfig.
        getCompactSerializationConfig()
        .setEnabled(true);

Declarative Configuration:

  • XML

  • YAML

<hazelcast>
    ...
    <serialization>
        <compact-serialization enabled="true" />
    </serialization>
    ...
</hazelcast>
hazelcast:
  serialization:
    compact-serialization:
      enabled: true

Apart from that, the configuration can be used to register either

  • an explicit CompactSerializer

  • a reflective serializer for a class.

In case of an explicit serializer, you have to supply a type name for the class.

Choosing a type name will associate that name with the schema and will make the polyglot use cases where there are multiple clients from different languages easier.

When a class is serialized using the reflective serializer, Hazelcast will choose the fully qualified class name as the type name automatically.

Below is the way to register an explicit serializer for a certain class.

Programmatic Configuration:

SerializationConfig serializationConfig = new SerializationConfig();
serializationConfig.
        getCompactSerializationConfig()
        .setEnabled(true)
        .register(Foo.class, "foo", new FooSerializer()); // Use the "foo" as the type name

Declarative Configuration:

  • XML

  • YAML

<hazelcast>
    ...
    <serialization>
        <compact-serialization enabled="true">
            <registered-classes>
                <class type-name="foo" serializer="com.example.FooSerializer">
                    com.example.Foo
                </class>
            </registered-classes>
        </compact-serialization>
    </serialization>
    ...
</hazelcast>
hazelcast:
  serialization:
    compact-serialization:
      enabled: true
      registered-classes:
        - class: com.example.Foo
          type-name: foo
          serializer: com.example.FooSerializer

Lastly, the following is a sample configuration that registers reflective serializer for a certain class, without implementing an explicit serializer.

Programmatic Configuration:

SerializationConfig serializationConfig = new SerializationConfig();
serializationConfig.
        getCompactSerializationConfig()
        .setEnabled(true)
        .register(Bar.class); // Uses the fully qualified class name as the type name

Declarative Configuration:

  • XML

  • YAML

<hazelcast>
    ...
    <serialization>
        <compact-serialization enabled="true">
            <registered-classes>
                <class>com.example.Bar</class>
            </registered-classes>
        </compact-serialization>
    </serialization>
    ...
</hazelcast>
hazelcast:
  serialization:
    compact-serialization:
      enabled: true
      registered-classes:
        - class: com.example.Bar

If you want to override the serialization mechanism used for Serializable or Externalizable classes and use Compact serialization; you must register reflective serializers for them.

GenericRecord Representation

As described in the Accessing Domain Objects Without Domain Classes section, compact serialized objects can also be represented by a GenericRecord, without requiring the class in the classpath.