Puneet Duggal created FLINK-32701:
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Summary: Potential Memory Leak in Flink CEP due to Persistent
Starting States in NFAState
Key: FLINK-32701
URL: https://issues.apache.org/jira/browse/FLINK-32701
Project: Flink
Issue Type: Bug
Components: Library / CEP
Affects Versions: 1.17.1, 1.17.0
Reporter: Puneet Duggal
Attachments: Screenshot 2023-07-26 at 11.45.06 AM.png, Screenshot
2023-07-26 at 11.50.28 AM.png
Our team has encountered a potential memory leak issue while working with the
Complex Event Processing (CEP) library in Flink v1.17.
h2. Context
The CEP Operator maintains a keyed state called NFAState, which holds two
queues: one for partial matches and one for completed matches. When a key is
first encountered, the CEP creates a starting computation state and stores it
in the partial matches queue. As more events occur that match the defined
conditions (e.g., a TAKE condition), additional computation states get added to
the queue, with their specific type (normal, pending, end) depending on the
pattern sequence.
However, I have noticed that the starting computation state remains in the
partial matches queue even after the pattern sequence has been completely
matched. This is also the case for keys that have already timed out. As a
result, the state gets stored for all keys that the CEP ever encounters,
leading to a continual increase in the checkpoint size.
h2. How to reproduce this
# Pattern Sequence - A not_followed_by B within 5 mins
# Time Characteristic - EventTime
# StateBackend - FsStateBackend
On my local machine, I started this pipeline and started sending events at the
rate of 10 events per second (only A) and as expected after 5 mins, CEP started
sending pattern matched output with the same rate. But the issue was that after
every 2 mins (checkpoint interval), checkpoint size kept on increasing.
Expectation was that after 5 mins (2-3 checkpoints), checkpoint size will
remain constant since any window of 5 mins will consist of the same number of
unique keys (older ones will get matched or timed out hence removed from
state). But as you can see below attached images, checkpoint size kept on
increasing till 40 checkpoints (around 1.5hrs).
P.S. - After 3 checkpoints (6 mins), the checkpoint size was around 1.78MB.
Hence assumption is that ideal checkpoint size for a 5 min window should be
less than 1.78MB.
As you can see after 39 checkpoints, I triggered a savepoint for this pipeline.
After that I used a savepoint reader to investigate what all is getting stored
in CEP states. Below code investigates NFAState of CEPOperator for potential
memory leak.
{code:java}
import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;
import org.apache.flink.api.common.state.ValueState;
import org.apache.flink.api.common.state.ValueStateDescriptor;
import org.apache.flink.cep.nfa.NFAState;
import org.apache.flink.cep.nfa.NFAStateSerializer;
import org.apache.flink.configuration.Configuration;
import org.apache.flink.runtime.state.filesystem.FsStateBackend;
import org.apache.flink.state.api.OperatorIdentifier;
import org.apache.flink.state.api.SavepointReader;
import org.apache.flink.state.api.functions.KeyedStateReaderFunction;
import org.apache.flink.streaming.api.datastream.DataStream;
import org.apache.flink.streaming.api.environment.StreamExecutionEnvironment;
import org.apache.flink.util.Collector;
import org.junit.jupiter.api.Test;
import java.io.Serializable;
import java.util.Objects;
public class NFAStateReaderTest {
private static final String NFA_STATE_NAME = "nfaStateName";
@Test
public void testNfaStateReader() throws Exception {
StreamExecutionEnvironment environment =
StreamExecutionEnvironment.getExecutionEnvironment();
SavepointReader savepointReader =
SavepointReader.read(environment,
"file:///opt/flink/savepoints/savepoint-093404-9bc0a38654df", new
FsStateBackend("file:///abc"));
DataStream<NFAStateOutput> stream =
savepointReader.readKeyedState(OperatorIdentifier.forUid("select_pattern_events"),
new NFAStateReaderTest.NFAStateReaderFunction());
stream.print();
environment.execute();
}
static class NFAStateReaderFunction extends
KeyedStateReaderFunction<DynamicTuple, NFAStateOutput> {
private ValueState<NFAState> computationStates;
private static Long danglingNfaCount = 0L;
private static Long newNfaCount = 0L;
private static Long minTimestamp = Long.MAX_VALUE;
private static Long minKeyForCurrentNfa = Long.MAX_VALUE;
private static Long minKeyForDanglingNfa = Long.MAX_VALUE;
private static Long maxKeyForDanglingNfa = Long.MIN_VALUE;
private static Long maxKeyForCurrentNfa = Long.MIN_VALUE;
@Override
public void open(Configuration parameters) {
computationStates = getRuntimeContext().getState(new
ValueStateDescriptor<>(NFA_STATE_NAME, new NFAStateSerializer()));
}
@Override
public void readKey(DynamicTuple key, Context ctx,
Collector<NFAStateOutput> out) throws Exception {
NFAState nfaState = computationStates.value();
if
(Objects.requireNonNull(nfaState.getPartialMatches().peek()).getStartTimestamp()
!= -1) {
minTimestamp = Math.min(minTimestamp,
nfaState.getPartialMatches().peek().getStartTimestamp());
minKeyForCurrentNfa = Math.min(minKeyForCurrentNfa,
Long.parseLong(key.getTuple().getField(0)));
maxKeyForCurrentNfa = Math.max(maxKeyForCurrentNfa,
Long.parseLong(key.getTuple().getField(0)));
newNfaCount++;
} else {
danglingNfaCount++;
minKeyForDanglingNfa = Math.min(minKeyForDanglingNfa,
Long.parseLong(key.getTuple().getField(0)));
maxKeyForDanglingNfa = Math.max(maxKeyForDanglingNfa,
Long.parseLong(key.getTuple().getField(0)));
}
NFAStateOutput nfaStateOutput =
new NFAStateOutput(
danglingNfaCount,
minTimestamp,
newNfaCount,
minKeyForCurrentNfa,
maxKeyForCurrentNfa,
minKeyForDanglingNfa,
maxKeyForDanglingNfa);
out.collect(nfaStateOutput);
}
}
@Data
@NoArgsConstructor
@AllArgsConstructor
static class NFAStateOutput implements Serializable {
private Long danglingNfaCount;
private Long minTimestamp;
private Long newNfaCount;
private Long minKeyForCurrentNfa;
private Long maxKeyForCurrentNfa;
private Long minKeyForDanglingNfa;
private Long maxKeyForDanglingNfa;
}
}
{code}
As an output it printed nfaStateOutput for each key but since all the
attributes in nfaStateOutput are aggregates, hence finalOutput printed was
{code:java}
NFAStateReaderTest.NFAStateOutput(danglingNfaCount=34391,
minTimestamp=1690359951958, newNfaCount=3000, minKeyForCurrentNfa=6244230,
maxKeyForCurrentNfa=6247229, minKeyForDanglingNfa=629818,
maxKeyForDanglingNfa=6244229){code}
As we can see, checkpoint is storing approximately 34391 dangling states (for
keys which have expired (matched or timed out) ) whereas there are only 3000
active keys (for which there are partial matches which are eligible for further
pattern sequence matching).
h2. Questions
Hence, I am curious about the reasoning behind this design choice, specifically
why the starting state remains in the partial matches queue for all keys, even
those that have either timed out or completed their matches.
Additionally, I am wondering what the implications would be if we were to
delete this starting state assuming that
# it is the only state left in the partial match queue.
# The completed match queue in nfaState is empty.
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