This document explains how to use Flink’s state abstractions when developing an application.
There are two basic kinds of state in Flink:
Keyed State and
Keyed State is always relative to keys and can only be used in functions and operators on a
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.
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.
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.
Attention If your managed state needs custom serialization logic, please see the corresponding guide in order to ensure future compatibility. Flink’s default serializers don’t need special treatment.
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
KeyedStream, which can be created via
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
ListState<T>: This keeps a list of elements. You can append elements and retrieve an
over all currently stored elements. Elements are added using
addAll(List<T>), the Iterable can
be retrieved using
Iterable<T> get(). You can also override the existing list with
ReducingState<T>: This keeps a single value that represents the aggregation of all values
added to the state. The interface is similar to
ListState but elements added using
add(T) are reduced to an aggregate using a specified
AggregatingState<IN, OUT>: 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
add(IN) are aggregated using a specified
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 similar to
ListState but elements
add(T) are folded into an aggregate using a specified
MapState<UK, UV>: This keeps a list of mappings. You can put key-value pairs into the state and
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
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.
FoldingStateDescriptor have been deprecated in Flink 1.4 and will be completely removed in the future. Please use
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
FoldingStateDescriptor or a
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
is available in a
RichFunction has these methods for accessing state:
AggregatingState<IN, OUT> getAggregatingState(AggregatingState<IN, OUT>)
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:
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
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.
A time-to-live (TTL) can be assigned to the keyed state of any type. If a TTL is configured and a state value has expired, the stored value will be cleaned up on a best effort basis which is discussed in more detail below.
All state collection types support per-entry TTLs. This means that list elements and map entries expire independently.
In order to use state TTL one must first build a
StateTtlConfig configuration object. The TTL
functionality can then be enabled in any state descriptor by passing the configuration:
The configuration has several options to consider:
The first parameter of the
newBuilder method is mandatory, it is the time-to-live value.
The update type configures when the state TTL is refreshed (by default
StateTtlConfig.UpdateType.OnCreateAndWrite- only on creation and write access
StateTtlConfig.UpdateType.OnReadAndWrite- also on read access
The state visibility configures whether the expired value is returned on read access
if it is not cleaned up yet (by default
StateTtlConfig.StateVisibility.NeverReturnExpired- expired value is never returned
StateTtlConfig.StateVisibility.ReturnExpiredIfNotCleanedUp- returned if still available
In case of
NeverReturnExpired, the expired state behaves as if it does not exist anymore,
even if it still has to be removed. The option can be useful for use cases
where data has to become unavailable for read access strictly after TTL,
e.g. application working with privacy sensitive data.
ReturnExpiredIfNotCleanedUp allows to return the expired state before its cleanup.
The state backends store the timestamp of the last modification along with the user value, which means that enabling this feature increases consumption of state storage. Heap state backend stores an additional Java object with a reference to the user state object and a primitive long value in memory. The RocksDB state backend adds 8 bytes per stored value, list entry or map entry.
Only TTLs in reference to processing time are currently supported.
Trying to restore state, which was previously configured without TTL, using TTL enabled descriptor or vice versa
will lead to compatibility failure and
The TTL configuration is not part of check- or savepoints but rather a way of how Flink treats it in the currently running job.
The map state with TTL currently supports null user values only if the user value serializer can handle null values.
If the serializer does not support null values, it can be wrapped with
NullableSerializer at the cost of an extra byte in the serialized form.
Currently, expired values are only removed when they are read out explicitly,
e.g. by calling
Attention This means that by default if expired state is not read, it won’t be removed, possibly leading to ever growing state. This might change in future releases.
Additionally, you can activate the cleanup at the moment of taking the full state snapshot which
will reduce its size. The local state is not cleaned up under the current implementation
but it will not include the removed expired state in case of restoration from the previous snapshot.
It can be configured in
This option is not applicable for the incremental checkpointing in the RocksDB state backend.
More strategies will be added in the future for cleaning up expired state automatically in the background.
In addition to the interface described above, the Scala API has shortcuts for stateful
flatMap() functions with a single
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.
To use managed operator state, a stateful function can implement either the more general
interface, or the
ListCheckpointed<T extends Serializable> interface.
CheckpointedFunction interface provides access to non-keyed state with different
redistribution schemes. It requires the implementation of two methods:
Whenever a checkpoint has to be performed,
snapshotState() is called. The counterpart,
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
element2, when increasing the parallelism to 2,
element1 may end up in operator instance 0,
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
to buffer elements before sending them to the outside world. It demonstrates
the basic even-split redistribution list state:
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,
StateDescriptor that contains the state name and information
about the type of the value that the state holds:
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
If the method name does not contain the redistribution pattern, e.g.
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
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
ListCheckpointed interface is a more limited variant of
which only supports list-style state with even-split redistribution scheme on restore.
It also requires the implementation of two methods:
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
Collections.singletonList(MY_STATE) in the
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.
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