Package io.camunda.zeebe.logstreams.impl.flowcontrol

Class FlowControl

java.lang.Object

io.camunda.zeebe.logstreams.impl.flowcontrol.FlowControl

All Implemented Interfaces:

LogStorage.AppendListener

public final class FlowControl
extends Object
implements LogStorage.AppendListener

Maintains a view of in-flight entries as they are being appended, written, committed and finally processed.

If enabled, a write rate limiter is used to limit the rate of appends to the log storage. Additionally, a request limiter is used to limit the amount of unprocessed user commands to ensure fast response times.

Thread safety

Access patterns:

1. Calls to tryAcquire(WriteContext, List) from the sequencer, serialized through the sequencers write lock.
2. Calls to onAppend(InFlightEntry, long) from the sequencer, serialized through the sequencers write lock.
3. Calls to onWrite(long, long) from the log storage, serialized through the single raft thread.
4. Calls to onCommit(long, long) from the log storage, serialized through the single raft thread.
5. Calls to onProcessed(long) from the stream processor, serialized through the stream processor actor.

The order in which these methods are called is weakly constrained:

• tryAcquire(WriteContext, List) and onAppend(InFlightEntry, long) are always called in that order and before any other methods.
• onWrite(long, long) and onCommit(long, long) and onProcessed(long) can be called in any order.

The weak ordering forces us to program quite defensively and carefully choose where and how we modify internal state.

The inFlight map is only modified in the onAppend(InFlightEntry, long) method. All other methods only read from it.

A volatile field lastProcessedPosition is only modified in onProcessed(long) and used in onAppend(InFlightEntry, long) to clean up old entries.

The RateMeasurement#observe method only returns true when a new observation value is available. This way we prevent updating the metrics too often with repeated values. We use the RateMeasurements to update the cluster load and the exporting rate metrics.

Nested Class Summary

Nested Classes

Modifier and Type	Class	Description
static enum	FlowControl.Rejection

Constructor Summary

Constructors

Constructor	Description
FlowControl(LogStreamMetrics metrics)
FlowControl(LogStreamMetrics metrics, com.netflix.concurrency.limits.Limit requestLimit, RateLimit writeRateLimit)

Method Summary

Modifier and Type	Method	Description
com.netflix.concurrency.limits.Limit	getRequestLimit()
RateLimit	getWriteRateLimit()
void	onAppend(InFlightEntry entry, long highestPosition)
void	onCommit(long index, long highestPosition)	Called when the entry has been successfully committed.
void	onExported(long position)
void	onProcessed(long position)
void	onWrite(long index, long highestPosition)	Called when the entry has been successfully written to the local storage.
void	setRequestLimit(com.netflix.concurrency.limits.Limit requestLimit)
void	setWriteRateLimit(RateLimit writeRateLimit)
Either<FlowControl.Rejection,InFlightEntry>	tryAcquire(WriteContext context, List<LogAppendEntryMetadata> batchMetadata)	Tries to acquire a free in-flight spot, applying backpressure as needed.

Methods inherited from class java.lang.Object

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

Constructor Details

FlowControl

public FlowControl(LogStreamMetrics metrics)

FlowControl

public FlowControl(LogStreamMetrics metrics,
 com.netflix.concurrency.limits.Limit requestLimit,
 RateLimit writeRateLimit)

Method Details

tryAcquire

public Either<FlowControl.Rejection,InFlightEntry> tryAcquire(WriteContext context,
 List<LogAppendEntryMetadata> batchMetadata)

Tries to acquire a free in-flight spot, applying backpressure as needed.

Returns:

An Optional containing a InFlightEntry if append was accepted, an empty Optional otherwise.

onAppend

public void onAppend(InFlightEntry entry,
 long highestPosition)

onWrite

public void onWrite(long index,
 long highestPosition)

Description copied from interface: LogStorage.AppendListener

Called when the entry has been successfully written to the local storage.

Specified by:

onWrite in interface LogStorage.AppendListener

Parameters:

index - the index of the written entry

onCommit

public void onCommit(long index,
 long highestPosition)

Description copied from interface: LogStorage.AppendListener

Called when the entry has been successfully committed.

Specified by:

onCommit in interface LogStorage.AppendListener

Parameters:

index - the index of the committed entry

highestPosition - the highest position (i.e. last position) of the records within the entry that was committed.

onProcessed

public void onProcessed(long position)

onExported

public void onExported(long position)

getRequestLimit

public com.netflix.concurrency.limits.Limit getRequestLimit()

setRequestLimit

public void setRequestLimit(com.netflix.concurrency.limits.Limit requestLimit)

getWriteRateLimit

public RateLimit getWriteRateLimit()

setWriteRateLimit

public void setWriteRateLimit(RateLimit writeRateLimit)