What's new with Apache Spark's Structured Streaming?

What’s new with Apache
Spark’s Structured
Streaming?
Miklos Christine
3/21/2017

$ whoami
Solutions Architect @ Databricks
• Apache Spark Advocate
• Build and architect big data platforms for streaming and batch processing
Previously:
• Sales Engineer @ Cloudera
• Software Engineer @ Cisco

building robust
stream processing
apps is hard

Complexities in stream processing
Complex Data
Diverse data formats
(json, avro, binary, …)
Data can be dirty,
late, out-of-order
Complex Systems
Diverse storage
systems and formats
(SQL, NoSQL, parquet, ... )
System failures
Complex
Workloads
Event time processing
Combining streaming
with interactive queries,
machine learning

Spark Streaming 1.x APIs (DStreams)
Difficulties:
• Separate SparkStreamingContext()
• Additional library packages and dependencies
• Kafka / Kinesis libraries
• State management / window functions
• Serialization issues & code changes (upgrade issues)
• 1 SparkStreamingContext() per Application

Spark Streaming 1.x (aka DStreams)
// Function to create a new StreamingContext and set it up
def creatingFunc(): StreamingContext = {
// Create a StreamingContext
val ssc = new StreamingContext(sc, Seconds(batchIntervalSeconds))
// Get the input stream from the source
val topic1_Stream = createKafkaStream(ssc, kafkaTopic1, kafkaBrokers)
// … … … logic
// To make sure data is not deleted by the time we query it interactively
ssc.remember(Minutes(1))
println("Creating function called to create new StreamingContext")
ssc
}

Spark Streaming 1.x (aka DStreams)
// Get or create a streaming context.
val ssc = StreamingContext.getActiveOrCreate(creatingFunc)
// This starts the streaming context in the background.
ssc.start()
// This is to ensure that we wait for some time before the background streaming job
starts. This will put this cell on hold for 5 times the batchIntervalSeconds.
ssc.awaitTerminationOrTimeout(batchIntervalSeconds * 5 * 1000)

Structured Streaming
stream processing on Spark SQL engine
fast, scalable, fault-tolerant
rich, unified, high level APIs
deal with complex data and complex workloads
rich ecosystem of data sources
integrate with many storage systems

you
should not have to
reason about streaming

you
should write simple batch queries
&
Spark
should automatically streamify
them

Treat Streams as Unbounded Tables
11
data stream unbounded input table
new data in the
data stream
=
new rows appended
to a unbounded table

New Model Trigger: every 1 sec
Time
Input data up
to t = 3
Query
Input: data from source as an
append-only table
Trigger: how frequently to check
input for new data
Query: operations on input
usual map/filter/reduce
new window, session ops
t=1 t=2 t=3
data up
to t = 1
data up
to t = 2

New Model
result up
to t = 1
Result
Query
Time
data up
to t = 1
Input data up
to t = 2
result up
to t = 2
data up
to t = 3
result up
to t = 3
Result: final operated table
updated after every trigger
Output: what part of result to write
to storage after every trigger
Output
[complete mode]
write all rows in result table to storage
t=1 t=2 t=3
Complete output: write full result table every
time

New Model
Query
Time
data up
to t = 1
Input data up
to t = 2
data up
to t = 3
Result: final operated table
updated after every trigger
Output: what part of result to write
to storage after every trigger
Complete output: write full result table every
time
Append output: write only new rows that got
added to result table since previous batch
t=1 t=2 t=3
Result result
up to t =
3
Output
[append mode] write new rows since last trigger to storage
result
up to t =
1
result
up to t =
2

static data =
bounded table
streaming data =
unbounded table
API - Dataset/DataFrame
Single API !

Batch Queries with DataFrames
input = spark.read
.format("json")
.load("source-path")
result = input
.select("device", "signal")
.where("signal > 15")
result.write
.format("parquet")
.save("dest-path")
Read from Json file
Select some devices
Write to parquet file

Streaming Queries with DataFrames
input = spark.readStream
.format("json")
result = input
result.writeStream
.format("parquet")
.start("dest-path")
Read from Json file stream
Replace read with readStream
Select some devices
Code does not change
Write to Parquet file stream
Replace save() with start()

DataFrames,
Datasets,
SQL
Logical Plan
Streaming
Source
Project
device, signal
Filter
signal > 15
Streaming
Sink
Spark automatically streamifies!
Spark SQL converts batch-like query to a series of
incremental execution plans operating on new batches of
data
Series of
Incremental
Execution Plans
process
newfiles
t = 1 t = 2 t = 3
process
newfiles
process
newfiles
input = spark.readStream
.format("json")
result = input
result.writeStream
.format("parquet")
.start("dest-path")

Fault-tolerance with Checkpointing
Checkpointing - metadata
(e.g. offsets) of current batch stored
in a write ahead log in HDFS/S3
Query can be restarted from the log
Streaming sources can replay the
exact data range in case of failure
Streaming sinks can dedup reprocessed
data when writing, idempotent by design
end-to-end
exactly-once
guarantees
process
newfiles
t = 1 t = 2 t = 3
process
newfiles
process
newfiles
write
ahead
log

Traditional ETL
Raw, dirty, un/semi-structured is data dumped as files
Periodic jobs run every few hours to convert raw data
to structured data ready for further analytics
file
dump
seconds hours
table
10101010

Traditional ETL
Hours of delay before taking decisions on latest data
Unacceptable when time is of essence
[intrusion detection, anomaly detection, etc.]
file
dump
seconds hours
table
10101010

Streaming ETL w/ Structured Streaming
Structured Streaming enables raw data to be available
as structured data as soon as possible
table
seconds10101010

Streaming ETL w/ Structured Streaming
24
Example
- Json data being received in
Kafka
- Parse nested json and flatten it
- Store in structured Parquet
table
- Get end-to-end failure
guarantees
val rawData = spark.readStream
.format("kafka")
.option("subscribe", "topic")
.option("kafka.boostrap.servers",...)
.load()
val parsedData = rawData
.selectExpr("cast (value as string) as json"))
.select(from_json("json").as("data"))
.select("data.*")
val query = parsedData.writeStream
.option("checkpointLocation", "/checkpoint")
.partitionBy("date")
.format("parquet")

Reading from Kafka [Spark 2.1]
Support Kafka 0.10.0.1
Specify options to configure
How?
kafka.boostrap.servers => broker1
What?
subscribe => topic1,topic2,topic3 // fixed list of topics
subscribePattern => topic* // dynamic list of topics
assign => {"topicA":[0,1] } // specific partitions
Where?
startingOffsets => latest(default)
/ earliest / {"topicA":{"0":23,"1":345} }
.format("kafka")
.load()

Reading from Kafka
rawData dataframe has
the following columns
key value topic partition offset timestamp
[binary] [binary] "topicA" 0 345 1486087873
[binary] [binary] "topicB" 3 2890 1486086721
.format("kafka")
.load()

Transforming Data
Cast binary value to string
Name it column json
.selectExpr("cast (value as string) as json")
.select("data.*")

Transforming Data
.select("data.*")
Name it column json
Parse json string and expand into
nested columns, name it data
json
{ "timestamp": 1486087873, "device":
"devA", …}
{ "timestamp": 1486082418, "device":
"devX", …}
data (nested)
timestamp device …
1486087873 devA …
1486086721 devX …
from_json("json")
as "data"

Transforming Data
.select("data.*")
data (nested)
1486087873 devA …
1486086721 devX …
1486087873 devA …
1486086721 devX …
select("data.*")
(not nested)
Name it column json
Parse json string and expand
into nested columns, name it
data
Flatten the nested columns

Transforming Data
Name it column json
Parse json string and expand
into nested columns, name it
data
Flatten the nested columns
powerful built-in APIs to
perform complex data
transformations
from_json, to_json, explode, ...
100s of functions
(see our blog post)
.select("data.*")

Save parsed data as Parquet
table in the given path
Partition files by date so that
future queries on time slices
of data is fast
e.g. query on last 48 hours of
data
Writing to Parquet table
.option("checkpointLocation", ...)
.format("parquet")
.start("/parquetTable")

Checkpointing
Enable checkpointing by
setting the checkpoint
location to save offset logs
start actually starts a
continuous running
StreamingQuery in the
Spark cluster
.format("parquet")
.start("/parquetTable/")

Streaming Query
query is a handle to the continuously
running StreamingQuery
Used to monitor and manage the
execution
StreamingQuery
.format("parquet")
.start("/parquetTable/")
processnew
data
t = 1 t = 2 t = 3
processnew
data
processnew
data

Data Consistency on Ad-hoc Queries
Data available for complex, ad-hoc analytics within
seconds
Parquet table is updated atomically, ensures prefix
integrity
Even if distributed, ad-hoc queries will see either all updates from
streaming query or none, read more in our blog
https://databricks.com/blog/2016/07/28/structured-streaming-in-apache-spark.html
seconds!
complex, ad-hoc
queries on
latest
data

Event time Aggregations
Many use cases require aggregate statistics by event time
E.g. what's the #errors in each system in the 1 hour windows?
Many challenges
Extracting event time from data, handling late, out-of-order data
DStream APIs were insufficient for event-time stuff

Windowing is just another type of grouping in Struct.
Streaming
number of records every hour
Support UDAFs!
parsedData
.groupBy(window("timestamp","1 hour"))
.count()
avg signal strength of each
device every 10 mins
Event time Aggregations
parsedData
.groupBy(
"device",
window("timestamp","10 mins"))
.avg("signal")

Stateful Processing for Aggregations
Aggregates has to be saved as
distributed state between triggers
Each trigger reads previous state and
writes updated state
State stored in memory,
backed by write ahead log in HDFS/S3
Fault-tolerant, exactly-once guarantee!
process
newdata
t = 1
sink
src
t = 2
process
newdata
sink
src
t = 3
process
newdata
sink
src
state state
write
ahead
log
state updates
are written to
log for checkpointing
state

Watermarking and Late Data
Watermark [Spark 2.1] - threshold
on how late an event is expected
to be in event time
Trails behind max seen event time
Trailing gap is configurable
event time
max event
time
watermark data older
than
watermark
not expected
12:30 PM
12:20 PM
trailing gap
of 10 mins

Data newer than watermark may
be late, but allowed to aggregate
Data older than watermark is "too
late" and dropped
Windows older than watermark
automatically deleted to limit the
amount of intermediate state
event time
max event
time
watermark data too
late,
dropped
12:30 PM
late data
allowed to
aggregate

event time
max event
time
watermark data older
than
watermark
not expected
parsedData
.withWatermark("timestamp", "10 minutes")
.groupBy(window("timestamp","5 minutes"))
.count()
late data
allowed to
aggregate
allowed
lateness
of 10 mins

Watermarking to Limit State [Spark 2.1]
data too late,
ignored in counts,
state dropped
Processing Time12:00
12:05
12:10
12:15
12:10 12:15
12:20
12:07
12:13
12:08
EventTime
12:15
12:18
12:04
watermark updated to
12:14 - 10m = 12:04
for next trigger,
state < 12:04 deleted
data is late, but
considered in counts
parsedData
.withWatermark("timestamp", "10 minutes")
.groupBy(window("timestamp","5 minutes"))
.count()
system tracks max
observed event time
12:08
wm = 12:04
10min
12:14
more details in online
programming guide

mapGroupsWithState
allows any user-defined
stateful ops to a
user-defined state
--
supports timeouts
--
fault-tolerant,
exactly-once
--
supports Scala and Java
Arbitrary Stateful Operations [Spark 2.2]
dataset
.groupByKey(groupingFunc)
.mapGroupsWithState(mappingFunc)
def mappingFunc(
key: K,
values: Iterator[V],
state: KeyedState[S]): U = {
// update or remove state
// set timeouts
// return mapped value
}

Many more updates!
StreamingQueryListener [Spark 2.1]
Receive of regular progress heartbeats for health and perf monitoring
Automatic in Databricks, come to the Databricks booth for a demo!!
Kafka Batch Queries [Spark 2.2]
Run batch queries on Kafka just like a file system
Kafka Sink [Spark 2.2]
Write to Kafka, can only give at-least-once guarantee as
Kafka doesn't support transactional updates
Kinesis Source
Read from Amazon Kinesis
44

More Info
Structured Streaming Programming Guide
http://spark.apache.org/docs/latest/structured-streaming-programming-guide.html
Databricks blog posts for more focused discussions
https://databricks.com/blog/2016/07/28/continuous-applications-evolving-streaming-in-apache-spark-2-0.html
https://databricks.com/blog/2016/07/28/structured-streaming-in-apache-spark.html
https://databricks.com/blog/2017/01/19/real-time-streaming-etl-structured-streaming-apache-spark-2-1.html
https://databricks.com/blog/2017/02/23/working-complex-data-formats-structured-streaming-apache-spark-2-1.html
and more to come, stay tuned!!
45

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Comparison with Other Engines
48
Read our blog to understand this table

What's new with Apache Spark's Structured Streaming?

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