Accumulo Summit 2015: Accumulo In-Depth: Building Bulk Ingest [Sponsored]

Building Bulk ImportEric Newton

SW Complete, Inc.

Ingest 101

The life of a mutation:○ Send to server○ Write to Write-Ahead Log○ Store in memory○ Write memory to a file○ Merge, re-write as needed

At least two writes, small in-memory sort

Bulk Ingest

● Accumulo: heavy ingest● Use Map-Reduce efficiency● Pre-sort incoming data● Hand whole sorted files to accumulo● One write● Larger sorts

Version 1

importDirectory(String dir, String failDir)Client computes the servers that need the file:

■ Analysis of file■ Moves directory under Accumulo■ Retry logic (to handle splits, failures)

Version 1: problems

● Limited to the client’s computational power● Permission: client had to be all-knowing● Files could be added to servers many times● Defer file collection while bulk importing● Clients can fail

Version 1.1

● Clients hand bulk imports to the master● Fixed permission problems● Added a bulk import test to the Random

Walk test suit

The Test

Create a sorted file with a lot of 1’s:12345678 -> 1

Create an identical file, with lots of -1’s:12345678 -> -1

Add a summation iterator over the tableVerify: every entry should be zero:

12345678 -> 0

Random Walk 101

Randomly:○ Import files in random order○ Split the files into random sizes○ Split tablets and random points○ Kill tablet servers (agitate)

Under loadAt scale

Version 2.0

● Master only coordinates file processing● Distribute work to tablet servers● Master distributes files to tablet servers● Tablet serves

○ Analyze files for assignments○ Retry○ Communicate with destination tablet servers

FilesFilesFiles

Command Flow

Client Master TabletServer

TabletServer

TabletServer

TabletServer

Problems

Problems solved:○ Permissions controlled by Master○ File processing is distributed

Bulk import tested heavilyConsistency

○ Not so much

Version 3.0

Problem: file imported more than once○ Repeated reloading○ RPC timeouts○ Tablet migration○ Tablet split

Add flags to metadata table to prevent:○ file garbage collection○ repeated imports

Version 3.0

● Problem Solved○ Reduced Name Node ops○ Reduced trash laying around from failed imports○ Imports not repeated

● Does it stand up to the Random Walk Test?○ Not so much

Death by Slow Thread“Please take this file”

“OK… looks good, never saw this before”Sleep

“Hey, Please take this file, again”“OK… looks good, never saw this before”“Thanks!”Compact!Wakeup! Time to import that file from the 1st request!

Zookeeper to the Rescue

Add a 3rd party negotiator● Define a session● Add a file only while session is active● Store session in zookeeper● Get agreement about the session at each critical point,

including metadata table updatesSession guides clean-up of markersSession closes only after all agree

Session

“Take this file, for session 123”“OK, working on session 123”“Never saw this file before”Sleep

“Repeat, file for session 123”“Never saw this file before”

“Anybody working on session 123?”

Session

“Yes, I’m processing session 123!”“OK, finish up.”

Double check on session 123, import file“Anybody working on session 123?”

“What’s session 123?”Remove markers in metadata

Bulk Import

● Problem solved:○ Distributed processing○ Permissions○ Files imported once, and only once○ Markers are cleaned up in the face of failures

● Performance○ Not so much

Master as BottleneckBulk import thousands of filesEvery 15 minutesMaster renames files and puts them under /accumulo

BottleneckOne master competes for NN ops with N tablet servers

Master, Go Faster

Add configurable thread poolPush more move requests to the NNCompete more fiercely for resources

Bulk Import

More efficient data ingestEasy: just a file to the right tabletsHard: consistency in a distributed systemTesting is your friendNothing prepares you for large-scale problems!