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Serializing Pipelines

To be able to send object state over a network or store it in a file you have to first serialize it into raw bytes. Similarly, to be able to fetch an object state over a wire or read it from a persistent storage you have to deserialize it from raw bytes first. As Hazelcast is a distributed system by nature, serialization is integral part of it. Understanding, when it is involved, how does it support the pipelines and knowing differences between supported strategies is crucial to efficient usage of Hazelcast.

A typical pipeline involves lambda expressions. Since the whole pipeline definition must be serialized to be sent to the cluster, the lambda expressions must be serializable as well. The Java standard provides an essential building block: if the static type of the lambda is a subtype of Serializable you will automatically get a lambda instance that can serialize itself.

None of the functional interfaces in the JDK extend Serializable so we had to mirror the entire java.util.function package in our own com.hazelcast.function with all the interfaces subtyped and made Serializable. Each subtype has the name of the original with Ex appended. For example, a FunctionEx is just like Function but implements Serializable. We use these types everywhere in the Jet API.

As always with this kind of magic, auto-serializability of lambdas has its flipside: it is easy to overlook what’s going on.

If the lambda references a variable in the outer scope, the variable is captured and must also be serializable. If it references an instance variable of the enclosing class, it implicitly captures this so the entire class will be serialized. For example, this will fail because JetJob1 does not implement Serializable:

class JetJob1 {
    private String instanceVar;

    Pipeline buildPipeline() {
        Pipeline p = Pipeline.create();
          // Refers to `instanceVar`, capturing `this`, but `JetJob1` is not
          // `Serializable` so this call will fail.
          .filter(item -> item.equals(instanceVar));
        return p;

Just implementing Serializable for JetJob1 would be a viable workaround here. However, consider something just a bit different:

class JetJob2 implements Serializable {
    private String instanceVar;
    // A non-serializable field.
    private OutputStream fileOut;

    Pipeline buildPipeline() {
        Pipeline p = Pipeline.create();
         // Refers to `instanceVar`, capturing `this`. `JetJob2` is declared
         // as `Serializable`, but has a non-serializable field and this fails.
         .filter(item -> item.equals(instanceVar));
        return p;

Even though we never refer to fileOut, we are still capturing the entire JetJob2 instance. We might mark fileOut as transient, but the sane approach is to avoid referring to instance variables of the surrounding class. We can simply achieve this by assigning to a local variable, then referring to that variable inside the lambda:

class JetJob3 {
    private String instanceVar;

    Pipeline buildPipeline() {
        Pipeline p = Pipeline.create();
        // Declare a local variable that loads the value of the instance field.
        String findMe = instanceVar;
         // By referring to the local variable `findMe` we avoid
         // capturing `this` and the job runs fine.
         .filter(item -> item.equals(findMe));
        return p;

Another common pitfall is capturing an instance of DateTimeFormatter or a similar non-serializable class:

DateTimeFormatter formatter = DateTimeFormatter
Pipeline p = Pipeline.create();
BatchStage<Long> src = p.readFrom(Sources.list("input"));
// Captures the non-serializable formatter, so this fails.
src.map((Long tstamp) -> formatter.format(Instant.ofEpochMilli(tstamp)));

Sometimes we can get away by using one of the preconfigured formatters available in the JDK:

// Accesses the static final field `ISO_LOCAL_TIME`. Static fields are
// not subject to lambda capture, they are dereferenced when the code
// runs on the target machine.
src.map((Long tstamp) -> DateTimeFormatter.ISO_LOCAL_TIME

This refers to a static final field in the JDK, so the instance is available on any JVM. If this is not available, you may create a static final field in your own class, but you can also use mapUsingService(). In this case you provide a serializable factory that Hazelcast will ask to create an object on the target member. The object it returns does not have to be serializable. Here’s an example of that:

Pipeline p = Pipeline.create();
BatchStage<Long> src = p.readFrom(Sources.list("input"));
ServiceFactory<?, DateTimeFormatter> serviceFactory = nonSharedService(
        pctx -> DateTimeFormatter.ofPattern("HH:mm:ss.SSS")
        (formatter, tstamp) -> formatter.format(Instant.ofEpochMilli(tstamp)));

Serialization of Data Types

The objects you store in Hazelcast data structures must be serializable.

Another case that requires serializable objects is sending computation results between members, for example when grouping by key. To catch serialization issues early on, we recommend using a 2-member local cluster for development and testing.

Currently, Hazelcast supports 4 interfaces to serialize custom types:

The following table provides a comparison between them to help you in deciding which interface to use in your applications.

Serialization interface Advantages Drawbacks


Easy to start with, does not require implementation or registration

CPU intensive and space inefficient


Does not require registration, faster and more space efficient than Serializable

CPU intensive, space inefficient and requires implementation


Faster and more space efficient than Serializable. Supports versioning and partial deserialization

Requires implementation and registration


Fastest and lightest

Requires implementation and registration

Below you can find rough performance numbers you can expect when employing each of those strategies. A straightforward benchmark that continuously serializes and then deserializes this simple object:

class Person {
    private String firstName;
    private String lastName;
    private int age;
    private float height;

yields following throughputs:

# Processor: Intel(R) Core(TM) i7-4700HQ CPU @ 2.40GHz
# VM version: JDK 13, OpenJDK 64-Bit Server VM, 13+33

Benchmark                              Mode  Cnt  Score   Error   Units
SerializationBenchmark.serializable   thrpt    3  0.259 ± 0.087  ops/us
SerializationBenchmark.externalizable thrpt    3  0.846 ± 0.057  ops/us
SerializationBenchmark.portable       thrpt    3  1.171 ± 0.539  ops/us
SerializationBenchmark.stream         thrpt    3  4.828 ± 1.227  ops/us

Here are the sizes of the serialized form by each serializer:

Strategy                                        Number of Bytes  Overhead %
java.io.Serializable                                        162         523
java.io.Externalizable                                       87         234
com.hazelcast.nio.serialization.Portable                    104         300
com.hazelcast.nio.serialization.StreamSerializer             26           0

You can see that using plain Serializable can easily become a bottleneck in your application, as even with this simple data type it’s more than an order of magnitude slower than other serialization options, not to mention very wasteful with memory.

Write a Custom Serializer

For the best performance and simplest implementation we recommend using the Hazelcast StreamSerializer mechanism. Here is a sample implementation for a Person class:

class PersonSerializer implements StreamSerializer<Person> {

    private static final int TYPE_ID = 1;

    public int getTypeId() {
        return TYPE_ID;

    public void write(ObjectDataOutput out, Person person) throws IOException {

    public Person read(ObjectDataInput in) throws IOException {
        return new Person(in.readUTF(), in.readUTF(), in.readInt(), in.readFloat());

The type ID you use must be unique across all the serializers you register for a job, and additionally it must not clash with any global serializers you registered with the Hazelcast cluster.

Register a Serializer for a Single job

You can register a serializer in a job’s configuration object:

new JobConfig()
    .registerSerializer(Person.class, PersonSerializer.class)

Such a serializer is scoped: Its type ID doesn’t clash with the same type ID in another job. However, if you also use the serializer hook to register a global serializer on the Hazelcast cluster, a job-local ID would clash with it. The job-local serializer takes precedence, but it is best to avoid such clashes due to the potential for surprising behavior and hard-to-diagnose bugs.

The Jet engine uses the job-local serializer to serialize the objects as they travel through the pipeline (over distributed DAG edges) and get saved to snapshots.

Job-level serializers can also be used with sources and sinks that use in-memory data structures. You can read from/write to a local Observable, IList, IMap or ICache. We are working on adding the ability to read from an IMap using a user-defined predicate and projections, update an IMap, and read from EventJournal.

Register a Serializer with the Hazelcast cluster

You can register a serializer with the Hazelcast cluster, before starting it. For that you need a SerializerHook:

class PersonSerializerHook implements SerializerHook<Person> {

    public Class<Person> getSerializationType() {
        return Person.class;

    public Serializer createSerializer() {
        return new PersonSerializer();

    public boolean isOverwritable() {
        return true;

Hazelcast uses the Java service discovery mechanism to find your serializer hook. You should create a JAR with the serializer hook and its dependent classes, and the JAR should have a file META-INF/services/com.hazelcast.SerializerHook with the fully-qualified name of the serializer hook class:


Alternatively, you can add the following configuration to hazelcast.yaml:

        "type-class": "com.hazelcast.samples.jet.Person"
        "class-name": "com.hazelcast.samples.jet.PersonSerializer"

Put the JAR containing the serializer hook and related classes in the $HZ_HOME/lib directory. Make sure that each registered serializer has a unique type ID.

The advantage of a cluster-level serializer is that it is supported in all Hazelcast features.

3rd-Party Serialization Support

Google Protocol Buffers

Since the classes generated by Google Protocol Buffers (Protobuf) already implement java.io.Serializable, Hazelcast automatically supports them without a custom serializer. However, for best performance we encourage registering a Protobuf-specific serializer. There is a Jet extension module that simplifies this for Protobuf version 3.

If you want to use it locally within a job, add the extension as a dependency to your job’s project:

  • Gradle

  • Maven

compile "com.hazelcast.jet:hazelcast-jet-protobuf:5.0.5"

Implement the adapter by extending the provided class (where Person is of any Protobuf GeneratedMessageV3 type):

class PersonSerializer extends ProtobufSerializer<Person> {

    private static final int TYPE_ID = 1;

    PersonSerializer() {
        super(Person.class, TYPE_ID);

Then register it with the job:

new JobConfig()
    .registerSerializer(Person.class, PersonSerializer.class)

Also make sure that the Protobuf extension JAR is either on the cluster’s classpath or inlined into your job JAR by creating a fat JAR.

You can also install the serializer in the Hazelcast cluster by implementing and registering a serialization hook, as explained above.