cassandra summit 2014: interactive olap queries using apache cassandra and spark
DESCRIPTION
Presenter: Evan Chan, Principal Software Engineer at Socrata Inc. How do you rapidly derive complex insights on top of really big data sets in Cassandra? This session draws upon Evan's experience building a distributed, interactive, columnar query engine on top of Cassandra and Spark. We will start by surveying the existing query landscape of Cassandra and discuss ways to integrate Cassandra and Spark. We will dive into the design and architecture of a fast, column-oriented query architecture for Spark, and why columnar stores are so advantageous for OLAP workloads. I will present a schema for Parquet-like storage of analytical datasets on Cassandra. Find out why Cassandra and Spark are the perfect match for enabling fast, scalable, complex querying and storage of big analytical data.TRANSCRIPT
#CassandraSummit
OLAP WITH SPARK ANDCASSANDRA
EVAN CHANSEPT 2014
WHO AM I?Principal Engineer, @evanfchan
Creator of
Socrata, Inc.
http://github.com/velviaSpark Job Server
WE BUILD SOFTWARE TO MAKE DATA USEFUL TO MOREPEOPLE.
data.edmonton.ca finances.worldbank.org data.cityofchicago.orgdata.seattle.gov data.oregon.gov data.wa.govwww.metrochicagodata.org data.cityofboston.govinfo.samhsa.gov explore.data.gov data.cms.gov data.ok.govdata.nola.gov data.illinois.gov data.colorado.govdata.austintexas.gov data.undp.org www.opendatanyc.comdata.mo.gov data.nfpa.org data.raleighnc.gov dati.lombardia.itdata.montgomerycountymd.gov data.cityofnewyork.usdata.acgov.org data.baltimorecity.gov data.energystar.govdata.somervillema.gov data.maryland.gov data.taxpayer.netbronx.lehman.cuny.edu data.hawaii.gov data.sfgov.org
WE ARE SWIMMING IN DATA!
BIG DATA AT SOCRATATens of thousands of datasets, each one up to 30 million rowsCustomer demand for billion row datasetsWant to analyze across datasets
BIG DATA AT OOYALA2.5 billion analytics pings a day = almost a trillion events ayear.Roll up tables - 30 million rows per day
HOW CAN WE ALLOW CUSTOMERS TO QUERY AYEAR'S WORTH OF DATA?
Flexible - complex queries includedSometimes you can't denormalize your data enough
Fast - interactive speedsNear Real Time - can't make customers wait hours beforequerying new data
RDBMS? POSTGRES?Start hitting latency limits at ~10 million rowsNo robust and inexpensive solution for querying across shardsNo robust way to scale horizontally
PostGres runs query on single thread unless you partition(painful!)
Complex and expensive to improve performance (eg rolluptables, huge expensive servers)
OLAP CUBES?Materialize summary for every possible combinationToo complicated and brittleTakes forever to compute - not for real timeExplodes storage and memory
When in doubt, use brute force- Ken Thompson
CASSANDRAHorizontally scalableVery flexible data modelling (lists, sets, custom data types)Easy to operateNo fear of number of rows or documentsBest of breed storage technology, huge communityBUT: Simple queries only
APACHE SPARKHorizontally scalable, in-memory queriesFunctional Scala transforms - map, filter, groupBy, sortetc.SQL, machine learning, streaming, graph, R, many more pluginsall on ONE platform - feed your SQL results to a logisticregression, easy!THE Hottest big data platform, huge community, leavingHadoop in the dustDevelopers love it
SPARK PROVIDES THE MISSING FAST, DEEPANALYTICS PIECE OF CASSANDRA!
INTEGRATING SPARK AND CASSANDRAScala solutions:
Datastax integration:
(CQL-based)https://github.com/datastax/spark-cassandra-connectorCalliope
A bit more work:
Use traditional Cassandra client with RDDsUse an existing InputFormat, like CqlPagedInputFormat
Only reason to go here is probably you are not on CQL version ofCassandra, or you're using Shark/Hive.
A SPARK AND CASSANDRAOLAP ARCHITECTURE
SEPARATE STORAGE AND QUERY LAYERSCombine best of breed storage and query platformsTake full advantage of evolution of eachStorage handles replication for availabilityQuery can replicate data for scaling read concurrency -independent!
SCALE NODES, NOTDEVELOPER TIME!!
KEEPING IT SIMPLEMaximize row scan speedColumnar representation for efficiencyCompressed bitmap indexes for fast algebraFunctional transforms for easy memoization, testing,concurrency, composition
SPARK AS CASSANDRA'S CACHE
EVEN BETTER: TACHYON OFF-HEAP CACHING
INITIAL ATTEMPTSval rows = Seq( Seq("Burglary", "19xx Hurston", 10), Seq("Theft", "55xx Floatilla Ave", 5) )
sc.parallelize(rows) .map { values => (values[0], values) } .groupByKey .reduce(_[2] + _[2])
No existing generic query engine for Spark when we started(Shark was in infancy, had no indexes, etc.), so we built our ownFor every row, need to extract out needed columnsAbility to select arbitrary columns means using Seq[Any], notype safetyBoxing makes integer aggregation very expensive and memoryinefficient
COLUMNAR STORAGE AND QUERYING
The traditional row-based data storageapproach is dead- Michael Stonebraker
TRADITIONAL ROW-BASED STORAGESame layout in memory and on disk:
Name AgeBarak 46
Hillary 66
Each row is stored contiguously. All columns in row 2 come afterrow 1.
COLUMNAR STORAGE (MEMORY)Name column
0 10 1
Dictionary: {0: "Barak", 1: "Hillary"}
Age column
0 146 66
COLUMNAR STORAGE (CASSANDRA)Review: each physical row in Cassandra (e.g. a "partition key")stores its columns together on disk.
Schema CF
Rowkey TypeName StringDict
Age Int
Data CF
Rowkey 0 1Name 0 1
Age 46 66
ADVANTAGES OF COLUMNAR STORAGECompression
Dictionary compression - HUGE savings for low-cardinalitystring columnsRLE
Reduce I/OOnly columns needed for query are loaded from disk
Can keep strong types in memory, avoid boxingBatch multiple rows in one cell for efficiency
ADVANTAGES OF COLUMNAR QUERYINGCache locality for aggregating column of dataTake advantage of CPU/GPU vector instructions for ints /doublesavoid row-ifying until last possible momenteasy to derive computed columnsUse vector data / linear math libraries
COLUMNAR QUERY ENGINE VS ROW-BASED INSCALA
Custom RDD of column-oriented blocks of dataUses ~10x less heap10-100x faster for group by's on a single nodeScan speed in excess of 150M rows/sec/core for integeraggregations
SO, GREAT, OLAP WITH CASSANDRA ANDSPARK. NOW WHAT?
DATASTAX: CASSANDRA SPARK INTEGRATIONDatastax Enterprise now comes with HA Spark
HA master, that is.spark-cassandra-connector
SPARK SQLAppeared with Spark 1.0In-memory columnar storeCan read from Parquet and JSON now; direct Cassandraintegration comingQuerying is not column-based (yet)No indexesWrite custom functions in Scala .... take that Hive UDFs!!Integrates well with MLBase, Scala/Java/Python
CACHING A SQL TABLE FROM CASSANDRAval sqlContext = new org.apache.spark.sql.SQLContext(sc)
sc.cassandraTable[GDeltRow]("gdelt, "1979to2009") .registerAsTable("gdelt")sqlContext.cacheTable("gdelt")sqlContext.sql("SELECT Actor2Code, Actor2Name, Actor2CountryCode, AvgTone from gdelt ORDER BY AvgTone DESC LIMIT
Remember Spark is lazy, nothing is executed until thecollect()In Spark 1.1+: registerTempTable
SOME PERFORMANCE NUMBERSGDELT dataset, 117 million rows, 57 columns, ~50GBSpark 1.0.2, AWS 8 x c3.xlarge, cached in memory
Query Avgtime(sec)
SELECT count(*) FROM gdeltWHERE Actor2CountryCode ='CHN'
0.49
SELECT 4 columns Top K 1.51
SELECT Top countries by Avg Tone(Group By)
2.69
IMPORTANT - CACHINGBy default, queries will read data from source - Cassandra -every timeSpark RDD Caching - much faster, but big waste of memory(row oriented)Spark SQL table caching - fastest, memory efficient
WORK STILL NEEDEDIndexesColumnar querying for fast aggregationTachyon support for Cassandra/CQLEfficient reading from columnar storage formats
LESSONSExtremely fast distributed querying for these use cases
Data doesn't change much (and only bulk changes)Analytical queries for subset of columnsFocused on numerical aggregationsSmall numbers of group bys
For fast query performance, cache your data using Spark SQLConcurrent queries is a frontier with Spark. Use additionalSpark contexts.
THANK YOU!
EXTRA SLIDES
EXAMPLE CUSTOM INTEGRATION USINGASTYANAX
val cassRDD = sc.parallelize(rowkeys). flatMap { rowkey => columnFamily.get(rowkey).execute().asScala }
SOME COLUMNAR ALTERNATIVESMonetdb and Infobright - true columnar stores (storage +querying)Vertica and C-StoreGoogle BigQuery - columnar cloud database, Dremel basedAmazon RedShift