org.axonframework.eventhandling.tokenstore

Interface TokenStore

All Known Implementing Classes:

InMemoryTokenStore, JdbcTokenStore, JpaTokenStore

public interface TokenStore

Describes a component capable of storing and retrieving event tracking tokens. An EventProcessor that is tracking an event stream can use the store to keep track of its position in the event stream. Tokens are stored by process name and segment index, enabling the same processor to be distributed over multiple processes or machines.

Author:

Rene de Waele, Allard Buijze

Method Summary

Modifier and Type	Method and Description
default void	deleteToken(String processorName, int segment) Deletes the token for the processor with given processorName and segment.
default void	extendClaim(String processorName, int segment) Extends the claim on the current token held by the this node for the given processorName and segment.
int[]	fetchSegments(String processorName) Returns an array of known segments for a given processorName.
TrackingToken	fetchToken(String processorName, int segment) Returns the last stored token for the given processorName and segment.
default void	initializeSegment(TrackingToken token, String processorName, int segment) Initializes a segment with given segment for the processor with given processorName to contain the given token.
default void	initializeTokenSegments(String processorName, int segmentCount) Initializes the given segmentCount number of segments for the given processorName to track its tokens.
default void	initializeTokenSegments(String processorName, int segmentCount, TrackingToken initialToken) Initializes the given segmentCount number of segments for the given processorName to track its tokens.
void	releaseClaim(String processorName, int segment) Release a claim of the token for given processorName and segment.
default boolean	requiresExplicitSegmentInitialization() Indicates whether this TokenStore instance requires segments to be explicitly initialized, before any tokens can be claimed for that segment.
default Optional<String>	retrieveStorageIdentifier() Returns a unique identifier that uniquely identifies the storage location of the tokens in this store.
void	storeToken(TrackingToken token, String processorName, int segment) Stores the given token in the store.

Method Detail

initializeTokenSegments

default void initializeTokenSegments(String processorName,
                                     int segmentCount)
                              throws UnableToClaimTokenException

Initializes the given segmentCount number of segments for the given processorName to track its tokens. This method should only be invoked when no tokens have been stored for the given processor, yet.

This method will initialize the tokens, but not claim them. It will create the segments ranging from 0 until segmentCount - 1.

The exact behavior when this method is called while tokens were already present, is undefined in case the token already present is not owned by the initializing process.

Parameters:

processorName - The name of the processor to initialize segments for

segmentCount - The number of segments to initialize

Throws:

UnableToClaimTokenException - when a segment has already been created

initializeTokenSegments

default void initializeTokenSegments(String processorName,
                                     int segmentCount,
                                     TrackingToken initialToken)
                              throws UnableToClaimTokenException

Initializes the given segmentCount number of segments for the given processorName to track its tokens. This method should only be invoked when no tokens have been stored for the given processor, yet.

This method will store initialToken for all segments as starting point for processor, but not claim them. It will create the segments ranging from 0 until segmentCount - 1.

The exact behavior when this method is called while tokens were already present, is undefined in case the token already present is not owned by the initializing process.

Parameters:

processorName - The name of the processor to initialize segments for

segmentCount - The number of segments to initialize

initialToken - The initial token which is used as a starting point for processor

Throws:

UnableToClaimTokenException - when a segment has already been created

storeToken

void storeToken(TrackingToken token,
                String processorName,
                int segment)
         throws UnableToClaimTokenException

Stores the given token in the store. The token marks the current position of the process with given processorName and segment. The given token may be null.

Any claims made by the current process have their timestamp updated.

This method should throw an UnableToClaimTokenException when the given segment has not been initialized with a Token (albeit null) yet. In that case, a segment must have been explicitly initialized. A TokenStore implementation's ability to do so is exposed by the requiresExplicitSegmentInitialization() method. If that method returns false, this method may implicitly initialize a token and return that token upon invocation.

Parameters:

token - The token to store for a given process and segment. May be null.

processorName - The name of the process for which to store the token

segment - The index of the segment for which to store the token

Throws:

UnableToClaimTokenException - when the token being updated has been claimed by another process.

fetchToken

TrackingToken fetchToken(String processorName,
                         int segment)
                  throws UnableToClaimTokenException

Returns the last stored token for the given processorName and segment. Returns null if the stored token for the given process and segment is null.

This method should throw an UnableToClaimTokenException when the given segment has not been initialized with a Token (albeit null) yet. In that case, a segment must have been explicitly initialized. A TokenStore implementation's ability to do so is exposed by the requiresExplicitSegmentInitialization() method. If that method returns false, this method may implicitly initialize a token and return that token upon invocation.

The token will be claimed by the current process (JVM instance), preventing access by other instances. To release the claim, use releaseClaim(String, int)

Parameters:

processorName - The process name for which to fetch the token

segment - The segment index for which to fetch the token

Returns:

The last stored TrackingToken or null if the store holds no token for given process and segment

Throws:

UnableToClaimTokenException - if there is a token for given processorName and segment, but they are claimed by another process.

extendClaim

default void extendClaim(String processorName,
                         int segment)
                  throws UnableToClaimTokenException

Extends the claim on the current token held by the this node for the given processorName and segment.

Parameters:

processorName - The process name for which to fetch the token

segment - The segment index for which to fetch the token

Throws:

UnableToClaimTokenException - if there is no token for given processorName and segment, or if it has been claimed by another process.

Implementation Requirements:

By default, this method invokes fetchToken(String, int), which also extends the claim if the token is held. TokenStore implementations may choose to implement this method if they can provide a more efficient way of extending this claim.

releaseClaim

void releaseClaim(String processorName,
                  int segment)

Release a claim of the token for given processorName and segment. If no such claim existed, nothing happens.

The caller must ensure not to use any streams opened based on the token for which the claim is released.

Parameters:

processorName - The name of the process owning the token (e.g. a TrackingEventProcessor name)

segment - the segment for which a token was obtained

initializeSegment

default void initializeSegment(TrackingToken token,
                               String processorName,
                               int segment)
                        throws UnableToInitializeTokenException

Initializes a segment with given segment for the processor with given processorName to contain the given token.

This method fails if a Token already exists for the given processor and segment, even if that token has been claimed by the active instance.

This method will not claim the initialized segment. Use fetchToken(String, int) to retrieve and claim the token.

Parameters:

token - The token to initialize the segment with

processorName - The name of the processor to create the segment for

segment - The identifier of the segment to initialize

Throws:

UnableToInitializeTokenException - if a Token already exists

UnsupportedOperationException - if this implementation does not support explicit initialization. See requiresExplicitSegmentInitialization().

deleteToken

default void deleteToken(String processorName,
                         int segment)
                  throws UnableToClaimTokenException

Deletes the token for the processor with given processorName and segment. The token must be owned by the current node, to be able to delete it.

Implementations should implement this method only when requiresExplicitSegmentInitialization() is overridden to return true. Deleting tokens using implementations that do not require explicit token initialization is unsafe, as a claim will automatically recreate the deleted token instance, which may result in concurrency issues.

Parameters:

processorName - The name of the processor to remove the token for

segment - The segment to delete

Throws:

UnableToClaimTokenException - if the token is not currently claimed by this node

UnsupportedOperationException - if this operation is not supported by this implementation

requiresExplicitSegmentInitialization

default boolean requiresExplicitSegmentInitialization()

Indicates whether this TokenStore instance requires segments to be explicitly initialized, before any tokens can be claimed for that segment.

Returns:

true if this instance requires tokens to be explicitly initialized, otherwise false.

See Also:

initializeTokenSegments(String, int), initializeTokenSegments(String, int, TrackingToken), initializeSegment(TrackingToken, String, int)

fetchSegments

int[] fetchSegments(String processorName)

Returns an array of known segments for a given processorName.

The segments returned are segments for which a token has been stored previously. When the TokenStore is empty, an empty array is returned.

Parameters:

processorName - The process name for which to fetch the segments

Returns:

an array of segment identifiers.

retrieveStorageIdentifier

default Optional<String> retrieveStorageIdentifier()
                                            throws UnableToRetrieveIdentifierException

Returns a unique identifier that uniquely identifies the storage location of the tokens in this store. Two token store implementations that share state, must return the same identifier. Two token store implementations that do not share a location, must return a different identifier (or an empty optional if identifiers are not supported).

Note that this method may require the implementation to consult its underlying storage. Therefore, a Transaction should be active when this method is called, similarly to invocations like fetchToken(String, int), fetchSegments(String), etc. When no Transaction is active, the behavior is undefined.

Returns:

an identifier to uniquely identify the storage location of tokens in this TokenStore.

Throws:

UnableToRetrieveIdentifierException - when the implementation was unable to determine its identifier