Giraph

NeilButcher

Background• Giraph scalableplatformforimplementinggraphalgorithms

• DevelopedbyApache• Basedoff‘Pregel’• UtilizesHadoopMapReduceframeworktotargetgraphproblems

• OpenSource

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Advantages of Solving Problems with Giraph• Message-basedcommunication:nolocks• Globalsynchronization:nosemaphores• Simpletoprogram• Massivelyparallel:taskbasedprogramming• Faulttolerant:Savesintermediateresults

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Giraph Algorithms: Basic Idea• Algorithmsarewrittenfromtheperspectiveofavertex

• Verticessendmessagestoeachothertosharepertinentinformation

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How it Works• ’compute’ functionhasabilityto:– modifystateofvertexanditsoutgoingedges– Cansendmessagestoothervertices– Receivemessagessentinprevioussuperstep

• Thingsthathappenduringasuperstep:– A‘compute’functionisinvokedoneachvertexthatreceivedamessageintheprevioussuperstep

– Nextsuperstep beginsonly afterallverticeshavecompletedtheirwork

– Ifnomessagesareinflight,haltprogram4

Single Source Shortest Path Algorithm

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Readupdatesfromothervertices,findminimum

Senddistancetoothervertices

Single Source Shortest Path Example

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Single Source Shortest Path Example

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Single Source Shortest Path Example

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Single Source Shortest Path Example

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Single Source Shortest Path Example

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More Complex Example: PageRank

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Giraph Job Lifetime

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Implementing Algorithm in Giraph• DefineaVertex class– Subclassofexistingimplementations

• DefineaVertexInputFormat toreadthegraph• DefineVertexOutputFormat thatdefineshowtoextractresultbasedonVertexfinalstate

• Manyotherfeaturescanbeutilizedtoimproveperformance

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Aggregators• Eachvertexcanstorevaluesthatcanbereadbyallverticesinproceedingsuperstep

• Canmaintainvalues(sum,min,max,accumulate,userdefined,ect)

• Aggregatorsmustberegisteredonmaster

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Combiners• Userdefinedfunctiontocombinemessagesbeforebeingsentordelivered

• Savesonnetworkandmemory

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Checkpointing• Canbeexpensivebutnecessary• Ensuresnosinglepointoffailure• Storeworkatuserdefinedintervals• Restartonfailure

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Zookeeper Responsibilities: Computation State • Handlespartition/workermapping• Globalstate• Checkpointpaths,aggregatorvalues,statistics

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Master Responsibilties: Coordination

• Assignspartitionstoworkers– Hashmapping isdefault– Canbeuserdefined

• Monitorsworkers• Coordinatessupersteps (ending,startingect)

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Worker Responsibilities: Vertices

• Workersareassignedvertices• Performcompute• Passmessagesbetweenvertices• Computeslocalaggregationvalues

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