Stateful Distributed Stream Processing

Stateful distributed stream
processing
Gyula Fóra
gyfora@apache.org
@GyulaFora

This talk
§ Stateful processing by example
§ Definition and challenges
§ State in current open-source systems
§ State in Apache Flink
§ Closing
2Apache Flink Meetup @ MapR2015-‐‑08-‐‑27

Stateful processing by example
§ Window aggregations
• Total number of customers
in the last 10 minutes
• State: Current aggregate
§ Machine learning
• Fitting trends to the evolving
stream
• State: Model

Stateful processing by example
§ Pattern recognition
• Detect suspicious financial
activity
• State: Matched prefix
§ Stream-stream joins
• Match ad views and
impressions
• State: Elements in the window

Stateful operators
§ All these examples use a common processing
pattern
§ Stateful operator (in essence):
𝒇:
𝒊𝒏, 𝒔𝒕𝒂𝒕𝒆 ⟶ 𝒐𝒖𝒕, 𝒔𝒕𝒂𝒕𝒆.
§ State hangs around and can be read and
modified as the stream evolves
§ Goal: Get as close as possible while
maintaining scalability and fault-tolerance

State-of-the-art systems
§ Most systems allow developers to
implement stateful programs
§ Trick is to limit the scope of 𝒇 (state access)
while maintaining expressivity
§ Issues to tackle:
• Expressivity
• Exactly-once semantics
• Scalability to large inputs
• Scalability to large states

§ States available only in Trident API
§ Dedicated operators for state updates and
queries
§ State access methods
• stateQuery(…)
• partitionPersist(…)
• persistentAggregate(…)
§ It’s very difficult to
implement transactional
states
Exactly-‐‑once guarantee

Storm Word Count

§ Stateless runtime by design
• No continuous operators
• UDFs are assumed to be stateless
§ State can be generated as a stream of
RDDs: updateStateByKey(…)
𝒇:
𝑺𝒆𝒒[𝒊𝒏 𝒌], 𝒔𝒕𝒂𝒕𝒆 𝒌 ⟶ 𝒔𝒕𝒂𝒕𝒆.
𝒌
§ 𝒇 is scoped to a specific key
§ Exactly-once semantics

val stateDstream = wordDstream.updateStateByKey[Int](
newUpdateFunc,
new HashPartitioner(ssc.sparkContext.defaultParallelism),
true,
initialRDD)
val updateFunc = (values: Seq[Int], state: Option[Int]) => {
val currentCount = values.sum
val previousCount = state.getOrElse(0)
Some(currentCount + previousCount)
}
Spark Streaming Word Count

§ Stateful dataflow operators
(Any task can hold state)
§ State changes are stored
as a log by Kafka
§ Custom storage engines can
be plugged in to the log
§ 𝒇 is scoped to a specific task
§ At-least-once processing
semantics

Samza Word Count
public class WordCounter implements StreamTask, InitableTask {
//Some omitted details…
private KeyValueStore<String, Integer> store;
public void process(IncomingMessageEnvelope envelope,
MessageCollector collector,
TaskCoordinator coordinator) {
//Get the current count
String word = (String) envelope.getKey();
Integer count = store.get(word);
if (count == null) count = 0;
//Increment, store and send
count += 1;
store.put(word, count);
collector.send(
new OutgoingMessageEnvelope(OUTPUT_STREAM, word ,count));
}
}

What can we say so far?
§ Trident
+ Consistent state accessible from outside
– Only works well with idempotent states
– States are not part of the operators
§ Spark
+ Integrates well with the system guarantees
– Limited expressivity
– Immutability increases update complexity
§ Samza
+ Efficient log based state updates
+ States are well integrated with the operators
– Lack of exactly-once semantics
– State access is not fully transparent

§ Take what’s good, make it work + add
some more
§ Clean and powerful abstractions
• Local (Task) state
• Partitioned (Key) state
§ Proper API integration
• Java: OperatorState interface
• Scala: mapWithState, flatMapWithState…
§ Exactly-once semantics by checkpointing

Flink Word Count
words.keyBy(x => x).mapWithState {
(word, count: Option[Int]) =>
{
val newCount = count.getOrElse(0) + 1
val output = (word, newCount)
(output, Some(newCount))
}
}

Local State
§ Task scoped state access
§ Can be used to implement
custom access patterns
§ Typical usage:
• Source operators (offset)
• Machine learning models
• Use cyclic flows to simulate
global state access

Local State Example (Java)
public class MySource extends RichParallelSourceFunction {
// Omitted details
private OperatorState<Long> offset;
@Override
public void run(SourceContext ctx) {
Object checkpointLock = ctx.getCheckpointLock();
isRunning = true;
while (isRunning) {
synchronized (checkpointLock) {
offset.update(offset.value() + 1);
// ctx.collect(next);
}
}
}
}

Partitioned State
§ Key scoped state access
§ Highly scalable
§ Allows for incremental
backup/restore
§ Typical usage:
• Any per-key operation
• Grouped aggregations
• Window buffers

Partitioned State Example (Scala)
// Compute the current average of each city's temperature
temps.keyBy("city").mapWithState {
(in: Temp, state: Option[(Double, Long)]) =>
{
val current = state.getOrElse((0.0, 0L))
val updated = (current._1 + in.temp, current._2 + 1)
val avg = Temp(in.city, updated._1 / updated._2)
(avg, Some(updated))
}
}
case class Temp(city: String, temp: Double)

Exactly-once semantics
§ Based on consistent global snapshots
§ Algorithm designed for stateful dataflows
Detailed mechanism

Exactly-once semantics
§ Low runtime overhead
§ Checkpointing logic is separated from
application logic
Blogpost on streaming fault-‐‑tolerance

Summary
§ State is essential to many applications
§ Fault-tolerant streaming state is a hard
problem
§ There is a trade-off between expressivity vs
scalability/fault-tolerance
§ Flink tries to hit the sweet spot with…
• Providing very flexible abstractions
• Keeping good scalability and exactly-once
semantics

Stateful Distributed Stream Processing

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