Resiliency in Elasticsearch & Lucene

As one of the most popular search engines based on Apache Lucene, Elasticsearch recognizes the crucial importance of being resilient to hardware and network failure. This is why Elasticsearch invests a lot to enable Elasticsearch and Apache Lucene detect and cope with increasingly complex failures. Elasticsearch’s lead developer, Boaz Leskes, will cover the recent highlights and future plans of the company’s resiliency strategy. He will explain all aspects of Elasticsearch, ranging from the lowest level of a single file, through network connection of a single node, and all the way up to distributed failures on the cluster level. Even though the talk is about possible failures and various coping strategies, participants will also get an interesting peek under the hood and learn about the inner workings of Elasticsearch.

Talk given at the Tel Aviv Elastic Meetup

Transcript

1. Resiliency in Elasticsearch and Lucene Boaz Leskes and Igor Motov four months later
2. { } CC-BY-ND 4.0 Resiliency noun 1. the power or ability to return to the original form, position, etc., after being bent, compressed, or stretched; elasticity. 2. ability to recover readily from illness, depression, adversity, or the like; buoyancy. 2 re·sil·ience
3. { } CC-BY-ND 4.0 Failures happen 3
4. { } CC-BY-ND 4.0 Why is it (even more) important now? 4
5. { } CC-BY-ND 4.0 Average Elasticsearch cluster growth 5 Large cluster circa 2011
6. { } CC-BY-ND 4.0 Average Elasticsearch cluster growth 6 Large cluster circa 2011 Large cluster circa 2015
7. { } CC-BY-ND 4.0 Failure rates X number of nodes 7
8. { } CC-BY-ND 4.0 8 Slow Fast Node Cluster kill -9 dead disk corruption long GC master gone network disconnects timeouts
9. { } CC-BY-ND 4.0 9 Slow Fast Node Cluster kill -9 dead disk corruption long GC master gone network disconnects timeouts SOFTWARE BUGS
10. { } CC-BY-ND 4.0 10 Work in progress
11. { } CC-BY-ND 4.0 Pulling the plug 11
12. { } CC-BY-ND 4.0 Replicas and Transaction Log 12 Replica Shard Lucene Index Transaction Log Lucene Buffer Primary Shard Lucene Index Transaction Log Lucene Buffer
13. { } CC-BY-ND 4.0 Replicas and Transaction Log 13 Replica Shard Lucene Index Transaction Log Lucene Buffer Primary Shard Lucene Index Transaction Log Lucene Buffer
14. { } CC-BY-ND 4.0 Replicas and Transaction Log 14 Replica Shard Lucene Index Transaction Log Lucene Buffer Primary Shard Lucene Index Transaction Log Lucene Buffer
15. { } CC-BY-ND 4.0 Replicas and Transaction Log 15 Replica Shard Lucene Index Transaction Log Lucene Buffer Primary Shard Lucene Index Transaction Log Lucene Buffer
16. { } CC-BY-ND 4.0 Transaction Log • Transaction Log – stores every operation (create/update/delete) – fsync-ed every 5 sec (configurable) • every request (default - #11011, coming v2.0) • Lucene Segments – fsync-ed when transaction log is full (every 30 min or 512mb) 16
17. { } CC-BY-ND 4.0 Hard Disk Failures 17
18. { } CC-BY-ND 4.0 Hard Drive Failures • Complete failure • Running out of disk space • Data corruption 18
19. { } CC-BY-ND 4.0 Complete Disk Failures • Automatic shard failover • Replicas 19
20. { } CC-BY-ND 4.0 Multi data paths 20 Disk 1 Disk 2 Disk3 Disk 4 shard 1 shard 2 shard 3 shard 4
21. { } CC-BY-ND 4.0 Multi data paths 21 Disk 1 Disk 2 Disk3 Disk 4 shard 1 shard 2 shard 3 shard 4
22. { } CC-BY-ND 4.0 Multi data paths 22 Disk 1 Disk 2 Disk3 Disk 4 shard 1 shard 2 shard 3 shard 4
23. { } CC-BY-ND 4.0 Complete Disk Failures (WIP) • Current multi-path setup strips shard data across multiple disks • Disk loss impacts all shards on node • Reduce failure impact by using one disk per shard 
 (v2.0, #9498) 23
24. { } CC-BY-ND 4.0 Running out of Disk Space • Can lead to truncated files and thus corruption • Easy to anticipate by monitoring • Disk-space aware allocation decider – added in 0.90.4 – enabled by default since 1.3.5 • Check-pointed transaction log (v2.0 , #11143) 24
25. { } CC-BY-ND 4.0 file content (10GB) Data Corruption - the bit in the haystack 25
26. { } CC-BY-ND 4.0 Data Corruption - Checksums 26 footer + checksum file content (10gb)
27. { } CC-BY-ND 4.0 Data Corruption - Checksums • Elasticsearch automatically checks checksums of – Small files on index open (since v1.3.0) – All files during replication, relocation, snapshot, and restore (since v1.3.3 - v1.4.0) – Transaction log (since v1.4.0) – Use checksum to identify entire segments to reduce chance of hash collisions (since v1.4.0) 27
28. { } CC-BY-ND 4.0 Data Corruption (WIP) • Checksums of Metadata files (coming in v1.5.0, #8010) • Support validation of checksum on all files when node starts (v2.0.0, #9183) • Make validation during merge operation more efficient (v2.0.0, LUCENE-5894) • Add per-segment/per-commit ids (v2.0.0, LUCENE-5895) • Prevent use of known-bad java versions (coming in v1.5.0, #7580) 28
29. { } CC-BY-ND 4.0 Cluster Issues 29
30. { } CC-BY-ND 4.0 Why nodes leave the cluster? • Complete node failure • Unresponsive nodes • Network Failures 30
31. { } CC-BY-ND 4.0 Unresponsive Nodes 31
32. { } CC-BY-ND 4.0 Biggest Memory User- Field Data (v1.x) • sorting • aggregations • doc[“foo”] in scripts • Parent-child id cache – has_child/has_parent queries 32
33. { } CC-BY-ND 4.0 Circuit Breakers • Estimate size of the field data for each query and fail the query if it tries to load too much data – field data (since v1.0.0) – parent-child (since v1.1.0) – some aggregation structures (since v1.4.0) 33
34. { } CC-BY-ND 4.0 Doc values • On-disk low memory alternative to field data • Significant performance improvements in v1.4.0 • Enabled by default for all numeric and non-analyzed fields (#10209, v2.0) 34
35. { } CC-BY-ND 4.0 OOM Resiliency (WIP) • Add hard limit on from/size (coming in v1.5.0, #9311) • Add hit size circuit breaker (coming in v2.0, #9310) • Prevent combinatorial explosion in aggregations (TBD, #8081, #9825) • Smarter filter query caching (LUCENE-6303, v2.0, #10897) 35
36. { } CC-BY-ND 4.0 Dedicate Master Nodes 36 node 1 node.master: false node.data: true node 1 node 1 node 1 node 1 node 1 node 1 node 1 node 1 node 1 node 1 data 1 node.master: true node.data: false master 1 master 2 master 3
37. { } CC-BY-ND 4.0 Network Issues 37
38. { } CC-BY-ND 4.0 Partitions & partial knowledge 38 node M node node node node
39. { } CC-BY-ND 4.0 Remember to set minimum_master_nodes • Set discovery.zen.minumum_master_nodes:(N/2 +1) in elasticsearch.yml 39
40. { } CC-BY-ND 4.0 Partitions & partial knowledge 40 node M node node node node
41. { } CC-BY-ND 4.0 Improved Zen Discovery • Significant improvements in v1.4.0 – Gossip on master loss – bigger ping outreach – resiliency to stale gossip – better two-masters resolution – faster failure detection 41
42. { } CC-BY-ND 4.0 Improving Zen Discovery (WIP) • Prevent setting incorrect minimum_master_nodes (coming in v1.5.0, #8321, #9051) • Refuse revived master (coming in v1.5.0, #9632) • Diff based ClusterState publishing (v2.0 #10212) 42
43. { } CC-BY-ND 4.0 External pressure 43
44. { } CC-BY-ND 4.0 External pressure - bounded queues and thread pools (<0.20) - time out long running queries (WIP, PR #9156) - index throttling (v1.2.0, #6066) 44
45. { } CC-BY-ND 4.0 Known Unknowns 45
46. { } CC-BY-ND 4.0 Known Unknowns - Simulate disruptions (v1.4.0, #7492) - Simulate corruption (v1.3.0, #5924) - Reproducible evil - Users info is critical 46
47. { } CC-BY-ND 4.0 It’s an ongoing effort • Check the progress on our resiliency status page – http://www.elasticsearch.org/guide/en/elasticsearch/ resiliency/current/ • Or search for issues labeled “resiliency” on github – https://github.com/elasticsearch/elasticsearch/ 47
48. { } Thank you! boaz@elastic.co, igor@elastic.co @bleskes, @imotov
49. { } This work is licensed under the Creative Commons Attribution-NoDerivatives 4.0 International License. To view a copy of this license, visit: http://creativecommons.org/licenses/by-nd/4.0/ or send a letter to: Creative Commons PO Box 1866 Mountain View, CA 94042 USA CC-BY-ND 4.0