Package org.apache.flink.cep.nfa

Class NFA<T>

java.lang.Object

org.apache.flink.cep.nfa.NFA<T>

Type Parameters:

T - Type of the processed events

public class NFA<T>
extends Object

Non-deterministic finite automaton implementation.

The CEP operator keeps one NFA per key, for keyed input streams, and a single global NFA for non-keyed ones. When an event gets processed, it updates the NFA's internal state machine.

An event that belongs to a partially matched sequence is kept in an internal buffer, which is a memory-optimized data-structure exactly for this purpose. Events in the buffer are removed when all the matched sequences that contain them are:

1. emitted (success)
2. discarded (patterns containing NOT)
3. timed-out (windowed patterns)

The implementation is strongly based on the paper "Efficient Pattern Matching over Event Streams".

See Also:

• https://people.cs.umass.edu/~yanlei/publications/sase-sigmod08.pdf

Constructor Summary

Constructors

Constructor	Description
NFA(Collection<State<T>> validStates, Map<String,Long> windowTimes, long windowTime, boolean handleTimeout)

Method Summary

Modifier and Type	Method	Description
org.apache.flink.api.java.tuple.Tuple2<Collection<Map<String,List<T>>>,Collection<org.apache.flink.api.java.tuple.Tuple2<Map<String,List<T>>,Long>>>	advanceTime(SharedBufferAccessor<T> sharedBufferAccessor, NFAState nfaState, long timestamp, AfterMatchSkipStrategy afterMatchSkipStrategy)	Prunes states assuming there will be no events with timestamp lower than the given one.
void	close()	Tear-down method for the NFA.
NFAState	createInitialNFAState()
Collection<State<T>>	getStates()
long	getWindowTime()
void	open(org.apache.flink.api.common.functions.RuntimeContext cepRuntimeContext, org.apache.flink.configuration.Configuration conf)	Initialization method for the NFA.
Collection<Map<String,List<T>>>	process(SharedBufferAccessor<T> sharedBufferAccessor, NFAState nfaState, T event, long timestamp, AfterMatchSkipStrategy afterMatchSkipStrategy, TimerService timerService)	Processes the next input event.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Details

NFA

public NFA(Collection<State<T>> validStates,
 Map<String,Long> windowTimes,
 long windowTime,
 boolean handleTimeout)

Method Details

getWindowTime

public long getWindowTime()

getStates

@VisibleForTesting
public Collection<State<T>> getStates()

createInitialNFAState

public NFAState createInitialNFAState()

open

public void open(org.apache.flink.api.common.functions.RuntimeContext cepRuntimeContext,
 org.apache.flink.configuration.Configuration conf)
          throws Exception

Initialization method for the NFA. It is called before any element is passed and thus suitable for one time setup work.

Parameters:

cepRuntimeContext - runtime context of the enclosing operator

conf - The configuration containing the parameters attached to the contract.

Throws:

Exception

close

public void close()
           throws Exception

Tear-down method for the NFA.

Throws:

Exception

process

public Collection<Map<String,List<T>>> process(SharedBufferAccessor<T> sharedBufferAccessor,
 NFAState nfaState,
 T event,
 long timestamp,
 AfterMatchSkipStrategy afterMatchSkipStrategy,
 TimerService timerService)
                                        throws Exception

Processes the next input event. If some of the computations reach a final state then the resulting event sequences are returned. If computations time out and timeout handling is activated, then the timed out event patterns are returned.

If computations reach a stop state, the path forward is discarded and currently constructed path is returned with the element that resulted in the stop state.

Parameters:

sharedBufferAccessor - the accessor to SharedBuffer object that we need to work upon while processing

nfaState - The NFAState object that we need to affect while processing

event - The current event to be processed or null if only pruning shall be done

timestamp - The timestamp of the current event

afterMatchSkipStrategy - The skip strategy to use after per match

timerService - gives access to processing time and time characteristic, needed for condition evaluation

Returns:

Tuple of the collection of matched patterns (e.g. the result of computations which have reached a final state) and the collection of timed out patterns (if timeout handling is activated)

Throws:

Exception - Thrown if the system cannot access the state.

advanceTime

public org.apache.flink.api.java.tuple.Tuple2<Collection<Map<String,List<T>>>,Collection<org.apache.flink.api.java.tuple.Tuple2<Map<String,List<T>>,Long>>> advanceTime(SharedBufferAccessor<T> sharedBufferAccessor,
 NFAState nfaState,
 long timestamp,
 AfterMatchSkipStrategy afterMatchSkipStrategy)
                                                                                                                                                                 throws Exception

Prunes states assuming there will be no events with timestamp lower than the given one. It clears the sharedBuffer and also emits all timed out partial matches.

Parameters:

sharedBufferAccessor - the accessor to SharedBuffer object that we need to work upon while processing

nfaState - The NFAState object that we need to affect while processing

timestamp - timestamp that indicates that there will be no more events with lower timestamp

Returns:

all pending matches and timed outed partial matches

Throws:

Exception - Thrown if the system cannot access the state.