Working with State

Stateful functions and operators store data across the processing of individual elements/events, making state a critical building block for any type of more elaborate operation. For example:

  • When an application searches for certain event patterns, the state will store the sequence of events encountered so far.
  • When aggregating events per minute, the state holds the pending aggregates.
  • When training a machine learning model over a stream of data points, the state holds the current version of the model parameters.

In order to make state fault tolerant, Flink needs to be aware of the state and checkpoint it. In many cases, Flink can also manage the state for the application, meaning Flink deals with the memory management (possibly spilling to disk if necessary) to allow applications to hold very large state.

This document explains how to use Flink’s state abstractions when developing an application.

Keyed State and Operator State

There are two basic kinds of state in Flink: Keyed State and Operator State.

Keyed State

Keyed State is always relative to keys and can only be used in functions and operators on a KeyedStream.

You can think of Keyed State as Operator State that has been partitioned, or sharded, with exactly one state-partition per key. Each keyed-state is logically bound to a unique composite of <parallel-operator-instance, key>, and since each key “belongs” to exactly one parallel instance of a keyed operator, we can think of this simply as <operator, key>.

Keyed State is further organized into so-called Key Groups. Key Groups are the atomic unit by which Flink can redistribute Keyed State; there are exactly as many Key Groups as the defined maximum parallelism. During execution each parallel instance of a keyed operator works with the keys for one or more Key Groups.

Operator State

With Operator State (or non-keyed state), each operator state is bound to one parallel operator instance. The Kafka Connector is a good motivating example for the use of Operator State in Flink. Each parallel instance of the Kafka consumer maintains a map of topic partitions and offsets as its Operator State.

The Operator State interfaces support redistributing state among parallel operator instances when the parallelism is changed. There can be different schemes for doing this redistribution.

Raw and Managed State

Keyed State and Operator State exist in two forms: managed and raw.

Managed State is represented in data structures controlled by the Flink runtime, such as internal hash tables, or RocksDB. Examples are “ValueState”, “ListState”, etc. Flink’s runtime encodes the states and writes them into the checkpoints.

Raw State is state that operators keep in their own data structures. When checkpointed, they only write a sequence of bytes into the checkpoint. Flink knows nothing about the state’s data structures and sees only the raw bytes.

All datastream functions can use managed state, but the raw state interfaces can only be used when implementing operators. Using managed state (rather than raw state) is recommended, since with managed state Flink is able to automatically redistribute state when the parallelism is changed, and also do better memory management.

Using Managed Keyed State

The managed keyed state interface provides access to different types of state that are all scoped to the key of the current input element. This means that this type of state can only be used on a KeyedStream, which can be created via stream.keyBy(…).

Now, we will first look at the different types of state available and then we will see how they can be used in a program. The available state primitives are:

  • ValueState<T>: This keeps a value that can be updated and retrieved (scoped to key of the input element as mentioned above, so there will possibly be one value for each key that the operation sees). The value can be set using update(T) and retrieved using T value().

  • ListState<T>: This keeps a list of elements. You can append elements and retrieve an Iterable over all currently stored elements. Elements are added using add(T), the Iterable can be retrieved using Iterable<T> get().

  • ReducingState<T>: This keeps a single value that represents the aggregation of all values added to the state. The interface is the same as for ListState but elements added using add(T) are reduced to an aggregate using a specified ReduceFunction.

  • FoldingState<T, ACC>: This keeps a single value that represents the aggregation of all values added to the state. Contrary to ReducingState, the aggregate type may be different from the type of elements that are added to the state. The interface is the same as for ListState but elements added using add(T) are folded into an aggregate using a specified FoldFunction.

  • MapState<UK, UV>: This keeps a list of mappings. You can put key-value pairs into the state and retrieve an Iterable over all currently stored mappings. Mappings are added using put(UK, UV) or putAll(Map<UK, UV>). The value associated with a user key can be retrieved using get(UK). The iterable views for mappings, keys and values can be retrieved using entries(), keys() and values() respectively.

All types of state also have a method clear() that clears the state for the currently active key, i.e. the key of the input element.

Attention FoldingState will be deprecated in one of the next versions of Flink and will be completely removed in the future. A more general alternative will be provided.

It is important to keep in mind that these state objects are only used for interfacing with state. The state is not necessarily stored inside but might reside on disk or somewhere else. The second thing to keep in mind is that the value you get from the state depends on the key of the input element. So the value you get in one invocation of your user function can differ from the value in another invocation if the keys involved are different.

To get a state handle, you have to create a StateDescriptor. This holds the name of the state (as we will see later, you can create several states, and they have to have unique names so that you can reference them), the type of the values that the state holds, and possibly a user-specified function, such as a ReduceFunction. Depending on what type of state you want to retrieve, you create either a ValueStateDescriptor, a ListStateDescriptor, a ReducingStateDescriptor, a FoldingStateDescriptor or a MapStateDescriptor.

State is accessed using the RuntimeContext, so it is only possible in rich functions. Please see here for information about that, but we will also see an example shortly. The RuntimeContext that is available in a RichFunction has these methods for accessing state:

  • ValueState<T> getState(ValueStateDescriptor<T>)
  • ReducingState<T> getReducingState(ReducingStateDescriptor<T>)
  • ListState<T> getListState(ListStateDescriptor<T>)
  • FoldingState<T, ACC> getFoldingState(FoldingStateDescriptor<T, ACC>)
  • MapState<UK, UV> getMapState(MapStateDescriptor<UK, UV>)

This is an example FlatMapFunction that shows how all of the parts fit together:

public class CountWindowAverage extends RichFlatMapFunction<Tuple2<Long, Long>, Tuple2<Long, Long>> {

    /**
     * The ValueState handle. The first field is the count, the second field a running sum.
     */
    private transient ValueState<Tuple2<Long, Long>> sum;

    @Override
    public void flatMap(Tuple2<Long, Long> input, Collector<Tuple2<Long, Long>> out) throws Exception {

        // access the state value
        Tuple2<Long, Long> currentSum = sum.value();

        // update the count
        currentSum.f0 += 1;

        // add the second field of the input value
        currentSum.f1 += input.f1;

        // update the state
        sum.update(currentSum);

        // if the count reaches 2, emit the average and clear the state
        if (currentSum.f0 >= 2) {
            out.collect(new Tuple2<>(input.f0, currentSum.f1 / currentSum.f0));
            sum.clear();
        }
    }

    @Override
    public void open(Configuration config) {
        ValueStateDescriptor<Tuple2<Long, Long>> descriptor =
                new ValueStateDescriptor<>(
                        "average", // the state name
                        TypeInformation.of(new TypeHint<Tuple2<Long, Long>>() {}), // type information
                        Tuple2.of(0L, 0L)); // default value of the state, if nothing was set
        sum = getRuntimeContext().getState(descriptor);
    }
}

// this can be used in a streaming program like this (assuming we have a StreamExecutionEnvironment env)
env.fromElements(Tuple2.of(1L, 3L), Tuple2.of(1L, 5L), Tuple2.of(1L, 7L), Tuple2.of(1L, 4L), Tuple2.of(1L, 2L))
        .keyBy(0)
        .flatMap(new CountWindowAverage())
        .print();

// the printed output will be (1,4) and (1,5)
class CountWindowAverage extends RichFlatMapFunction[(Long, Long), (Long, Long)] {

  private var sum: ValueState[(Long, Long)] = _

  override def flatMap(input: (Long, Long), out: Collector[(Long, Long)]): Unit = {

    // access the state value
    val tmpCurrentSum = sum.value

    // If it hasn't been used before, it will be null
    val currentSum = if (tmpCurrentSum != null) {
      tmpCurrentSum
    } else {
      (0L, 0L)
    }

    // update the count
    val newSum = (currentSum._1 + 1, currentSum._2 + input._2)

    // update the state
    sum.update(newSum)

    // if the count reaches 2, emit the average and clear the state
    if (newSum._1 >= 2) {
      out.collect((input._1, newSum._2 / newSum._1))
      sum.clear()
    }
  }

  override def open(parameters: Configuration): Unit = {
    sum = getRuntimeContext.getState(
      new ValueStateDescriptor[(Long, Long)]("average", createTypeInformation[(Long, Long)])
    )
  }
}


object ExampleCountWindowAverage extends App {
  val env = StreamExecutionEnvironment.getExecutionEnvironment

  env.fromCollection(List(
    (1L, 3L),
    (1L, 5L),
    (1L, 7L),
    (1L, 4L),
    (1L, 2L)
  )).keyBy(_._1)
    .flatMap(new CountWindowAverage())
    .print()
  // the printed output will be (1,4) and (1,5)

  env.execute("ExampleManagedState")
}

This example implements a poor man’s counting window. We key the tuples by the first field (in the example all have the same key 1). The function stores the count and a running sum in a ValueState. Once the count reaches 2 it will emit the average and clear the state so that we start over from 0. Note that this would keep a different state value for each different input key if we had tuples with different values in the first field.

State in the Scala DataStream API

In addition to the interface described above, the Scala API has shortcuts for stateful map() or flatMap() functions with a single ValueState on KeyedStream. The user function gets the current value of the ValueState in an Option and must return an updated value that will be used to update the state.

val stream: DataStream[(String, Int)] = ...

val counts: DataStream[(String, Int)] = stream
  .keyBy(_._1)
  .mapWithState((in: (String, Int), count: Option[Int]) =>
    count match {
      case Some(c) => ( (in._1, c), Some(c + in._2) )
      case None => ( (in._1, 0), Some(in._2) )
    })

Using Managed Operator State

To use managed operator state, a stateful function can implement either the more general CheckpointedFunction interface, or the ListCheckpointed<T extends Serializable> interface.

CheckpointedFunction

The CheckpointedFunction interface provides access to non-keyed state with different redistribution schemes. It requires the implementation of two methods:

void snapshotState(FunctionSnapshotContext context) throws Exception;

void initializeState(FunctionInitializationContext context) throws Exception;

Whenever a checkpoint has to be performed, snapshotState() is called. The counterpart, initializeState(), is called every time the user-defined function is initialized, be that when the function is first initialized or be that when the function is actually recovering from an earlier checkpoint. Given this, initializeState() is not only the place where different types of state are initialized, but also where state recovery logic is included.

Currently, list-style managed operator state is supported. The state is expected to be a List of serializable objects, independent from each other, thus eligible for redistribution upon rescaling. In other words, these objects are the finest granularity at which non-keyed state can be redistributed. Depending on the state accessing method, the following redistribution schemes are defined:

  • Even-split redistribution: Each operator returns a List of state elements. The whole state is logically a concatenation of all lists. On restore/redistribution, the list is evenly divided into as many sublists as there are parallel operators. Each operator gets a sublist, which can be empty, or contain one or more elements. As an example, if with parallelism 1 the checkpointed state of an operator contains elements element1 and element2, when increasing the parallelism to 2, element1 may end up in operator instance 0, while element2 will go to operator instance 1.

  • Union redistribution: Each operator returns a List of state elements. The whole state is logically a concatenation of all lists. On restore/redistribution, each operator gets the complete list of state elements.

Below is an example of a stateful SinkFunction that uses CheckpointedFunction to buffer elements before sending them to the outside world. It demonstrates the basic even-split redistribution list state:

public class BufferingSink
        implements SinkFunction<Tuple2<String, Integer>>,
                   CheckpointedFunction,
                   CheckpointedRestoring<ArrayList<Tuple2<String, Integer>>> {

    private final int threshold;

    private transient ListState<Tuple2<String, Integer>> checkpointedState;

    private List<Tuple2<String, Integer>> bufferedElements;

    public BufferingSink(int threshold) {
        this.threshold = threshold;
        this.bufferedElements = new ArrayList<>();
    }

    @Override
    public void invoke(Tuple2<String, Integer> value) throws Exception {
        bufferedElements.add(value);
        if (bufferedElements.size() == threshold) {
            for (Tuple2<String, Integer> element: bufferedElements) {
                // send it to the sink
            }
            bufferedElements.clear();
        }
    }

    @Override
    public void snapshotState(FunctionSnapshotContext context) throws Exception {
        checkpointedState.clear();
        for (Tuple2<String, Integer> element : bufferedElements) {
            checkpointedState.add(element);
        }
    }

    @Override
    public void initializeState(FunctionInitializationContext context) throws Exception {
        ListStateDescriptor<Tuple2<String, Integer>> descriptor =
            new ListStateDescriptor<>(
                "buffered-elements",
                TypeInformation.of(new TypeHint<Tuple2<Long, Long>>() {}));

        checkpointedState = context.getOperatorStateStore().getListState(descriptor);

        if (context.isRestored()) {
            for (Tuple2<String, Integer> element : checkpointedState.get()) {
                bufferedElements.add(element);
            }
        }
    }

    @Override
    public void restoreState(ArrayList<Tuple2<String, Integer>> state) throws Exception {
        // this is from the CheckpointedRestoring interface.
        this.bufferedElements.addAll(state);
    }
}
class BufferingSink(threshold: Int = 0)
  extends SinkFunction[(String, Int)]
    with CheckpointedFunction
    with CheckpointedRestoring[List[(String, Int)]] {

  @transient
  private var checkpointedState: ListState[(String, Int)] = null

  private val bufferedElements = ListBuffer[(String, Int)]()

  override def invoke(value: (String, Int)): Unit = {
    bufferedElements += value
    if (bufferedElements.size == threshold) {
      for (element <- bufferedElements) {
        // send it to the sink
      }
      bufferedElements.clear()
    }
  }

  override def snapshotState(context: FunctionSnapshotContext): Unit = {
    checkpointedState.clear()
    for (element <- bufferedElements) {
      checkpointedState.add(element)
    }
  }

  override def initializeState(context: FunctionInitializationContext): Unit = {
    val descriptor = new ListStateDescriptor[(String, Int)](
      "buffered-elements",
      TypeInformation.of(new TypeHint[(String, Int)]() {})
    )

    checkpointedState = context.getOperatorStateStore.getListState(descriptor)

    if(context.isRestored) {
      for(element <- checkpointedState.get()) {
        bufferedElements += element
      }
    }
  }

  override def restoreState(state: List[(String, Int)]): Unit = {
    bufferedElements ++= state
  }
}

The initializeState method takes as argument a FunctionInitializationContext. This is used to initialize the non-keyed state “containers”. These are a container of type ListState where the non-keyed state objects are going to be stored upon checkpointing.

Note how the state is initialized, similar to keyed state, with a StateDescriptor that contains the state name and information about the type of the value that the state holds:

ListStateDescriptor<Tuple2<String, Integer>> descriptor =
    new ListStateDescriptor<>(
        "buffered-elements",
        TypeInformation.of(new TypeHint<Tuple2<Long, Long>>() {}));

checkpointedState = context.getOperatorStateStore().getListState(descriptor);
val descriptor = new ListStateDescriptor[(String, Long)](
    "buffered-elements",
    TypeInformation.of(new TypeHint[(String, Long)]() {})
)

checkpointedState = context.getOperatorStateStore.getListState(descriptor)

The naming convention of the state access methods contain its redistribution pattern followed by its state structure. For example, to use list state with the union redistribution scheme on restore, access the state by using getUnionListState(descriptor). If the method name does not contain the redistribution pattern, e.g. getListState(descriptor), it simply implies that the basic even-split redistribution scheme will be used.

After initializing the container, we use the isRestored() method of the context to check if we are recovering after a failure. If this is true, i.e. we are recovering, the restore logic is applied.

As shown in the code of the modified BufferingSink, this ListState recovered during state initialization is kept in a class variable for future use in snapshotState(). There the ListState is cleared of all objects included by the previous checkpoint, and is then filled with the new ones we want to checkpoint.

As a side note, the keyed state can also be initialized in the initializeState() method. This can be done using the provided FunctionInitializationContext.

ListCheckpointed

The ListCheckpointed interface is a more limited variant of CheckpointedFunction, which only supports list-style state with even-split redistribution scheme on restore. It also requires the implementation of two methods:

List<T> snapshotState(long checkpointId, long timestamp) throws Exception;

void restoreState(List<T> state) throws Exception;

On snapshotState() the operator should return a list of objects to checkpoint and restoreState has to handle such a list upon recovery. If the state is not re-partitionable, you can always return a Collections.singletonList(MY_STATE) in the snapshotState().

Stateful Source Functions

Stateful sources require a bit more care as opposed to other operators. In order to make the updates to the state and output collection atomic (required for exactly-once semantics on failure/recovery), the user is required to get a lock from the source’s context.

public static class CounterSource
        extends RichParallelSourceFunction<Long>
        implements ListCheckpointed<Long> {

    /**  current offset for exactly once semantics */
    private Long offset;

    /** flag for job cancellation */
    private volatile boolean isRunning = true;

    @Override
    public void run(SourceContext<Long> ctx) {
        final Object lock = ctx.getCheckpointLock();

        while (isRunning) {
            // output and state update are atomic
            synchronized (lock) {
                ctx.collect(offset);
                offset += 1;
            }
        }
    }

    @Override
    public void cancel() {
        isRunning = false;
    }

    @Override
    public List<Long> snapshotState(long checkpointId, long checkpointTimestamp) {
        return Collections.singletonList(offset);
    }

    @Override
    public void restoreState(List<Long> state) {
        for (Long s : state)
            offset = s;
    }
}
class CounterSource
       extends RichParallelSourceFunction[Long]
       with ListCheckpointed[Long] {

  @volatile
  private var isRunning = true

  private var offset = 0L

  override def run(ctx: SourceFunction.SourceContext[Long]): Unit = {
    val lock = ctx.getCheckpointLock

    while (isRunning) {
      // output and state update are atomic
      lock.synchronized({
        ctx.collect(offset)

        offset += 1
      })
    }
  }

  override def cancel(): Unit = isRunning = false

  override def restoreState(state: util.List[Long]): Unit =
    for (s <- state) {
      offset = s
    }

  override def snapshotState(checkpointId: Long, timestamp: Long): util.List[Long] =
    Collections.singletonList(offset)

}

Some operators might need the information when a checkpoint is fully acknowledged by Flink to communicate that with the outside world. In this case see the org.apache.flink.runtime.state.CheckpointListener interface.

Custom Serialization for Managed State

This section is targeted as a guideline for users who require the use of custom serialization for their state, covering how to provide a custom serializer and how to handle upgrades to the serializer for compatibility. If you’re simply using Flink’s own serializers, this section is irrelevant and can be skipped.

Using custom serializers

As demonstrated in the above examples, when registering a managed operator or keyed state, a StateDescriptor is required to specify the state’s name, as well as information about the type of the state. The type information is used by Flink’s type serialization framework to create appropriate serializers for the state.

It is also possible to completely bypass this and let Flink use your own custom serializer to serialize managed states, simply by directly instantiating the StateDescriptor with your own TypeSerializer implementation:

public class CustomTypeSerializer extends TypeSerializer<Tuple2<String, Integer>> {...};

ListStateDescriptor<Tuple2<String, Integer>> descriptor =
    new ListStateDescriptor<>(
        "state-name",
        new CustomTypeSerializer());

checkpointedState = getRuntimeContext().getListState(descriptor);
class CustomTypeSerializer extends TypeSerializer[(String, Integer)] {...}

val descriptor = new ListStateDescriptor[(String, Integer)](
    "state-name",
    new CustomTypeSerializer)
)

checkpointedState = getRuntimeContext.getListState(descriptor);

Note that Flink writes state serializers along with the state as metadata. In certain cases on restore (see following subsections), the written serializer needs to be deserialized and used. Therefore, it is recommended to avoid using anonymous classes as your state serializers. Anonymous classes do not have a guarantee on the generated classname, varying across compilers and depends on the order that they are instantiated within the enclosing class, which can easily cause the previously written serializer to be unreadable (since the original class can no longer be found in the classpath).

Handling serializer upgrades and compatibility

Flink allows changing the serializers used to read and write managed state, so that users are not locked in to any specific serialization. When state is restored, the new serializer registered for the state (i.e., the serializer that comes with the StateDescriptor used to access the state in the restored job) will be checked for compatibility, and is replaced as the new serializer for the state.

A compatible serializer would mean that the serializer is capable of reading previous serialized bytes of the state, and the written binary format of the state also remains identical. The means to check the new serializer’s compatibility is provided through the following two methods of the TypeSerializer interface:

public abstract TypeSerializerConfigSnapshot snapshotConfiguration();
public abstract CompatibilityResult ensureCompatibility(TypeSerializerConfigSnapshot configSnapshot);

Briefly speaking, every time a checkpoint is performed, the snapshotConfiguration method is called to create a point-in-time view of the state serializer’s configuration. The returned configuration snapshot is stored along with the checkpoint as the state’s metadata. When the checkpoint is used to restore a job, that serializer configuration snapshot will be provided to the new serializer of the same state via the counterpart method, ensureCompatibility, to verify compatibility of the new serializer. This method serves as a check for whether or not the new serializer is compatible, as well as a hook to possibly reconfigure the new serializer in the case that it is incompatible.

Note that Flink’s own serializers are implemented such that they are at least compatible with themselves, i.e. when the same serializer is used for the state in the restored job, the serializer’s will reconfigure themselves to be compatible with their previous configuration.

The following subsections illustrate guidelines to implement these two methods when using custom serializers.

Implementing the snapshotConfiguration method

The serializer’s configuration snapshot should capture enough information such that on restore, the information carried over to the new serializer for the state is sufficient for it to determine whether or not it is compatible. This could typically contain information about the serializer’s parameters or binary format of the serialized data; generally, anything that allows the new serializer to decide whether or not it can be used to read previous serialized bytes, and that it writes in the same binary format.

How the serializer’s configuration snapshot is written to and read from checkpoints is fully customizable. The below is the base class for all serializer configuration snapshot implementations, the TypeSerializerConfigSnapshot.

public abstract TypeSerializerConfigSnapshot extends VersionedIOReadableWritable {
  public abstract int getVersion();
  public void read(DataInputView in) {...}
  public void write(DataOutputView out) {...}
}

The read and write methods define how the configuration is read from and written to the checkpoint. The base implementations contain logic to read and write the version of the configuration snapshot, so it should be extended and not completely overridden.

The version of the configuration snapshot is determined through the getVersion method. Versioning for the serializer configuration snapshot is the means to maintain compatible configurations, as information included in the configuration may change over time. By default, configuration snapshots are only compatible with the current version (as returned by getVersion). To indicate that the configuration is compatible with other versions, override the getCompatibleVersions method to return more version values. When reading from the checkpoint, you can use the getReadVersion method to determine the version of the written configuration and adapt the read logic to the specific version.

Attention The version of the serializer’s configuration snapshot is not related to upgrading the serializer. The exact same serializer can have different implementations of its configuration snapshot, for example when more information is added to the configuration to allow more comprehensive compatibility checks in the future.

One limitation of implementing a TypeSerializerConfigSnapshot is that an empty constructor must be present. The empty constructor is required when reading the configuration snapshot from checkpoints.

Implementing the ensureCompatibility method

The ensureCompatibility method should contain logic that performs checks against the information about the previous serializer carried over via the provided TypeSerializerConfigSnapshot, basically doing one of the following:

  • Check whether the serializer is compatible, while possibly reconfiguring itself (if required) so that it may be compatible. Afterwards, acknowledge with Flink that the serializer is compatible.

  • Acknowledge that the serializer is incompatible and that state migration is required before Flink can proceed with using the new serializer.

The above cases can be translated to code by returning one of the following from the ensureCompatibility method:

  • CompatibilityResult.compatible(): This acknowledges that the new serializer is compatible, or has been reconfigured to be compatible, and Flink can proceed with the job with the serializer as is.

  • CompatibilityResult.requiresMigration(): This acknowledges that the serializer is incompatible, or cannot be reconfigured to be compatible, and requires a state migration before the new serializer can be used. State migration is performed by using the previous serializer to read the restored state bytes to objects, and then serialized again using the new serializer.

  • CompatibilityResult.requiresMigration(TypeDeserializer deserializer): This acknowledgement has equivalent semantics to CompatibilityResult.requiresMigration(), but in the case that the previous serializer cannot be found or loaded to read the restored state bytes for the migration, a provided TypeDeserializer can be used as a fallback resort.

Attention Currently, as of Flink 1.3, if the result of the compatibility check acknowledges that state migration needs to be performed, the job simply fails to restore from the checkpoint as state migration is currently not available. The ability to migrate state will be introduced in future releases.

Managing TypeSerializer and TypeSerializerConfigSnapshot classes in user code

Since TypeSerializers and TypeSerializerConfigSnapshots are written as part of checkpoints along with the state values, the availability of the classes within the classpath may affect restore behaviour.

TypeSerializers are directly written into checkpoints using Java Object Serialization. In the case that the new serializer acknowledges that it is incompatible and requires state migration, it will be required to be present to be able to read the restored state bytes. Therefore, if the original serializer class no longer exists or has been modified (resulting in a different serialVersionUID) as a result of a serializer upgrade for the state, the restore would not be able to proceed. The alternative to this requirement is to provide a fallback TypeDeserializer when acknowledging that state migration is required, using CompatibilityResult.requiresMigration(TypeDeserializer deserializer).

The class of TypeSerializerConfigSnapshots in the restored checkpoint must exist in the classpath, as they are fundamental components to compatibility checks on upgraded serializers and would not be able to be restored if the class is not present. Since configuration snapshots are written to checkpoints using custom serialization, the implementation of the class is free to be changed, as long as compatibility of the configuration change is handled using the versioning mechanisms in TypeSerializerConfigSnapshot.