accumulo summit 2015: accumulo in-depth: building bulk ingest [sponsored]
TRANSCRIPT
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!