This document discusses scaling SolrCloud to support a large number of collections. It identifies four main problems in scaling: 1) large cluster state size, 2) overseer performance issues with thousands of collections, 3) difficulty moving data between collections, and 4) limitations in exporting full result sets. The document outlines solutions implemented to each problem, including splitting the cluster state, optimizing the overseer, improving data management between collections, and enabling distributed deep paging to export full result sets. Testing showed the ability to support 30 hosts, 120 nodes, 1000 collections, over 6 billion documents, and sustained performance targets.

2. Scaling SolrCloud to a Large
Number of Collections
Shalin Shekhar Mangar
Lucidworks Inc.

3. Apache Solr has tremendous momentum
Solr is both established & growing
8M+ total
downloads
250,000+
monthly downloads
Largest community of developers.
2500+
open Solr jobs.
The most widely used search
solution on the planet.
Lucene/Solr Revolution
world’s largest open source
user conference dedicated
to Lucene/Solr.
You use
Solr everyday.
Solr has tens of thousands
of applications in production.

4. Solr Scalability is unmatched

5. The traditional search use-case
• One large index distributed across multiple nodes
• A large number of users searching on the same data
• Searches happen across the entire cluster
Example: eCommerce Product Catalogue

6. “The limits of the possible can only be defined by
going beyond them into the impossible.”
—Arthur C. Clarke

7. Analyze, measure and optimize
•Analyze and find missing features
•Setup a performance testing environment on AWS
•Devise tests for stability and performance
•Find bugs and bottlenecks and fixes

8. Problem #1: Cluster state and updates
•The SolrCloud cluster state has information about all
collections, their shards and replicas
•All nodes and (Java) clients watch the cluster state
•Every state change is notified to all nodes
•Limited to (slightly less than) 1MB by default
•1 node restart triggers a few 100 watcher fires and pulls
from ZK for a 100 node cluster (three states: down,
recovering and active)

9. Solution: Split cluster state and scale
• Each collection gets it’s own state node in ZK
• Nodes selectively watch only those states
which they are a member of
• Clients cache state and use smart cache
updates instead of watching nodes
• http://issues.apache.org/jira/browse/
SOLR-5473

10. Problem #2: Overseer Performance
• Thousands of collections create a lot of state
updates
• Overseer falls behind and replicas can’t
recover or can’t elect a leader
• Under high indexing/search load, GC pauses
can cause overseer queue to back up

11. Solution - Improve the overseer
• Optimize polling for new items in overseer
queue (SOLR-5436)
• Dedicated overseers nodes (SOLR-5476)
• New Overseer Status API (SOLR-5749)
• Asynchronous execution of collection
commands (SOLR-5477, SOLR-5681)

12. Problem #3: Moving data around
• Not all users are born equal - A tenant may have a
few very large users
• We wanted to be able to scale an individual user’s
data — maybe even as it’s own collection
• SolrCloud can split shards with no downtime but it
only splits in half
• No way to ‘extract’ user’s data to another collection
or shard

13. Solution: Improved data management
• Shard can be split on arbitrary hash ranges
(SOLR-5300)
• Shard can be split by a given key (SOLR-5338,
SOLR-5353)
• A new ‘migrate’ API to move a user’s data to
another (new) collection without downtime
(SOLR-5308)

14. Problem #4: Exporting data
• Lucene/Solr is designed for finding top-N
search results
• Trying to export full result set brings down the
system due to high memory requirements as
you go deeper

15. Solution: Distributed deep paging

16. Testing scale at scale
• Performance goals: 6 billion documents, 4000
queries/sec, 400 updates/sec, 2 seconds NRT
sustained performance
• 5% large collections (50 shards), 15% medium (10
shards), 85% small (1 shard) with replication factor of 3
• Target hardware: 24 CPUs, 126G RAM, 7 SSDs (460G)
+ 1 HDD (200G)
• 80% traffic served by 20% of the tenants

17. Test Infrastructure

18. Logging

20. How to manage large clusters?
• Tim Potter wrote the Solr Scale Toolkit
• Fabric based tool to setup and manage
SolrCloud clusters in AWS complete with
collectd and SiLK
• Backup/Restore from S3. Parallel clone
commands.
• Open source!
• https://github.com/LucidWorks/solr-scale-tk

21. Gathering metrics and analyzing logs
• Lucidworks SiLK (Solr + Logstash + Kibana)
• collectd daemons on each host
• rabbitmq to queue messages before delivering to log stash
• Initially started with Kafka but discarded thinking it is overkill
• Not happy with rabbitmq — crashes/unstable
• Might try Kafka again soon
• http://www.lucidworks.com/lucidworks-silk

22. Generating data and load
• Custom randomized data generator (re-producible
using a seed)
• JMeter for generating load
• Embedded CloudSolrServer (Solr Java client)
using JMeter Java Action Sampler
• JMeter distributed mode was itself a bottleneck!
• Not open source (yet) but we’re working on it!

23. Numbers
•30 hosts, 120 nodes, 1000 collections, 6B+ docs,
15000 queries/second, 2000 writes/second, 2
second NRT sustained over 24-hours
•More than 3x the numbers we needed
•Unfortunately, we had to stop testing at that
point :(
•Our biggest cluster cost us just $120/hour :)

24. Not over yet
• We continue to test performance at scale
• Published indexing performance benchmark, working on others
• 15 nodes, 30 shards, 1 replica, 157195 docs/sec
• 15 nodes, 30 shards, 2 replicas, 61062 docs/sec
• http://searchhub.org/introducing-the-solr-scale-toolkit/
• Setting up an internal performance testing environment
• Jenkins CI
• Jepsen tests
• Single node benchmarks
• Cloud tests
• Stay tuned!

25. Pushing the limits

26. Not over yet
• SolrCloud continues to be improved
• SOLR-6220 - Replica placement strategy
• SOLR-6273 - Cross data center replication
• SOLR-5656 - Auto-add replicas on HDFS
• SOLR-5986 - Don’t allow runaway queries to harm the cluster
• SOLR-5750 - Backup/Restore API for SolrCloud
• Many, many more

27. Thank you!
shalin@apache.org
@shalinmangar