Elasticsearch Under the Hood
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Elasticsearch is quite common tool nowadays. Usually as a part of ELK stack, but in some cases to support main feature of the system as search engine. Documentation on regular use cases and on usage in general is pretty good, but how it really works, how it behaves beneath the surface of the API? This talk is about that, we will look under the hood of Elasticsearch and dive deep in the largely unknown implementation details. Talk covers cluster behaviour, communication with Lucene and Lucene internals to literally bits and pieces. Come and see Elasticsearch dissected.
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Elasticsearch Under the Hood
- 3. Disclaimer ● Elasticsearch 6.3 ● Lucene 7.3
- 4. What is it? What is it good for? ● Full text search ● Scalable and robust full text search
- 5. What is Lucene? What it has to do with Elasticsearch? ● Full text search library ● Elasticsearch is just a wrapper around it which provides scalability, durability and REST API
- 6. API level Index document POST doc/default { "class": "ConfigurationParser", "method": "build", "description": "<p class=’paragraph’>Creates an instance of <link>ObjectGenerator</link> based on provided configuration. Resulting <link>ObjectGenerator</link> will try to convert configured output to specified <code>objectType</code>.</p>" }
- 7. API level Update document POST doc/default/1/_update { "doc": { "description": "New description value" } }
- 8. API level Delete document DELETE doc/default/1
- 9. API level Search GET doc/default/_search { "query": { "match": { "title": { "query": "ObjectGenerator" } } } }
- 10. Cluster level
- 11. How does that look like? Transaction Log LuceneREST ES node UUID name ES ES Cluster “Elasticsearch” Transaction Log LuceneREST ES node UUID name ES Transaction Log LuceneREST ES node UUID name ES Transaction Log LuceneREST ES node UUID name ES
- 12. All nodes are of the same type? ● Master node ● Data node ● Ingest node ● Tribe node ● Coordinator node ● Default case
- 13. Shards Index Doc1 Doc2 Doc3 Doc4 Doc5 Doc6 Doc7 Doc8 Doc9 Doc10 Doc11 Doc12 Node Node Node Node Node Node Index - S1 Doc1 Doc4 Doc8 Index - S5 Doc9 Doc12 Index - S4 Doc7 Doc10 Index - S2 Doc3 Doc5 Doc6 Index - S3 Doc2 Doc11
- 14. Replicas Node Node Index - S1 R1 Doc1 Doc4 Doc8 Node Node Node Index - S1 R2 Doc1 Doc4 Doc8 Index - S1 R3 Doc1 Doc4 Doc8 Index - S1 R4 Doc1 Doc4 Doc8 Index - S1 P Doc1 Doc4 Doc8
- 15. Scaling ● Single node cluster with 3 shards and 1 replica ● Unassigned shards problem Node 1 S1 P S2 P S3 P Single node cluster State:
- 16. Scaling out ● Two node cluster with 3 shards and 1 replica ● To prevent unassigned shards: number of nodes > number of replicas + 1 Node 1 S1 P S2 P S3 P Two node cluster State: Node 2 S1 R S2 R S3 R
- 17. Scaling out ● Three node cluster with 3 shards and 1 replica ● Load spread across all nodes Node 1 S1 P S2 P Three node cluster State: Node 2 S2 R Node 3 S1 R S3 P S3 R
- 18. Scaling out ● Seven node cluster with 3 shards and 1 replica, one node unused ● Increase number of shards (not possible) ● Increase replication factor (possible on running cluster) Node 1 S1 P Seven node cluster State: Node 2 S2 P Node 3 S3 P Node 4 S1 R Node 5 S2 R Node 6 S3 R Node 7
- 19. Scaling in ● Make sure you have enough nodes to support replication factor when node is killed ● Wait for green status ● If necessary, lower the replication factor
- 20. Replacing the node ● Same as scale out / scale in
- 21. How does the write look like? Node 2 S2 P S3 R Node 4 S1 R S4 P Node 3 S1 ISR S2 ISR Node 5 S1 ISR S4 ISR Node 6 S1 P S3 ISR S4 ISR Node 1 S2 ISR S3 P ES Cluster Generate doc ID if not present Hash ID to determine replication group (routing param) Coordinator node
- 22. How does the read look like? Node 2 S2 P S3 R Node 4 S1 R S4 P Node 3 S1 ISR S2 ISR Node 5 S1 ISR S4 ISR Node 6 S1 P S3 ISR S4 ISR Node 1 S2 ISR S3 P ES Cluster Coordinator node Resolve search request to relevant shards Combine the results
- 23. What if something goes wrong? ● Network partition (one master) ● Network partition (two masters) ● Network partition (three masters) ● Primary shard node failure (in sync replicas available) ● Primary shard node failure (no in sync replicas available) ● Write replication (replica write failure) ● Node failure (read)
- 24. What if something goes wrong? Network partition (one master) ● discovery.zen.no_master_block Shard 2 P Shard 2 R Shard 1 P Shard 2 RShard 1 R Master ES Cluster
- 25. What if something goes wrong? Network partition (two masters) ● discovery.zen.minimum_master_nodes Shard 2 P Master Shard 1 P Shard 2 RShard 1 R Master ES Cluster
- 26. What if something goes wrong? Network partition (three masters) ● discovery.zen.minimum_master_nodes Shard 2 P Master Shard 1 P MasterShard 1 R Master ES Cluster
- 27. What if something goes wrong? Primary shard node failure (in sync replicas available) Shard 2 P Shard 1 ISR Shard 1 P Shard 2 ISRShard 1 R Master ES Cluster Shard 1 P (new)
- 28. What if something goes wrong? Primary shard node failure (no in sync replicas available) Shard 2 P Shard 1 R Shard 1 P Shard 2 ISRShard 1 R Master ES Cluster allocate_stale_primary cmd Shard 1 ISR
- 29. What if something goes wrong? Write replication (replica write failure) Coordinator node Shard 1 ISR Shard 1 P Shard 1 RShard 1 ISR Master ES Cluster Shard 1 R
- 30. What if something goes wrong? Node failure (read) Coordinator node Shard 1 P Shard 1 R Shard 2 PShard 2 R Master ES Cluster
- 31. Lucene level
- 32. How is write processed? Elasticsearch Lucene Character filters Tokenizer Token filters File storage
- 33. How is write processed? ● Mapping character filter ● HTML strip character filter ● Pattern replace character filter Character fiters <p class=’paragraph’>Creates an instance of <link>ObjectGenerator</link> based on provided configuration. Resulting <link>ObjectGenerator</link> will try to convert configured output to specified <code>objectType</code>.</p> Creates an instance of ObjectGenerator based on provided configuration. Resulting ObjectGenerator will try to convert configured output to specified objectType.
- 34. How is write processed? ● Standard tokenizer ● Keyword tokenizer ● Letter tokenizer ● Lowercase tokenizer ● N gram tokenizer ● Edge n gram tokenizer ● Regular expression pattern tokenizer Tokenizer Creates an instance of ObjectGenerator based on provided configuration. Resulting ObjectGenerator will try to convert configured output to specified objectType. [Creates, an, instance, of, ObjectGenerator, based, on, provided, configuration, Resulting, ObjectGenerator, will, try, to, convert, configured, output, to, specified, objectType]
- 35. How is write processed? ● Lower case filter ● English possessive filter ● Stop filter ● Synonym filter ● Reversed wildcard filter ● English minimal stem filter Token filters [Creates, an, instance, of, ObjectGenerator, based, on, provided, configuration, Resulting, ObjectGenerator, will, try, to, convert, configured, output, to, specified, objectType] [creates, instance, ObjectGenerator, based, provided, configuration, resulting, try, convert, configured, output, specified, objectType]
- 36. Inverted index File storage - Logical view ● Difference between forward and inverted index ● Doc1: “Peter has a brown dog and a white cat” ● Doc2: “Mike has a black dog” ● Doc3: “Rachel has a brown cat” Forward index Doc1 brown, cat, dog, peter, white Doc2 black, dog, mike Doc3 brown, cat, rachel Inverted index 0 black 1 1 brown 0, 2 2 cat 0, 2 3 dog 0, 1 4 mike 1 5 peter 0 6 rachel 2 7 white 0 Documents 0 Doc1 (Peter has a brown dog and a white cat.) 1 Doc2 (Mike has a black dog.) 2 Doc3 (Rachel has a brown cat.) term ordinal terms dict postings list doc id document Segment
- 37. Inverted index File storage - Logical view (deleting documents) Inverted index 0 black 1 1 brown 0, 2 2 cat 0, 2 3 dog 0, 1 4 mike 1 5 peter 0 6 rachel 2 7 white 0 Documents 0 Doc1 (Peter has a brown dog and a white cat.) 1 Doc2 (Mike has a black dog.) 2 Doc3 (Rachel has a brown cat.) term ordinal terms dict postings list doc id document Segment Live documents 0 1 2
- 38. Inverted index File storage - Logical view (merge segments) Inverted index 0 black 1 1 brown 0, 2 2 cat 0, 2 3 dog 0, 1 4 mike 1 5 peter 0 6 rachel 2 7 white 0 Documents 0 Doc1 1 Doc2 2 Doc3 Segment 1 Inverted index 0 balloon 3 1 boy 3, 4 2 brown 4 3 cat 4 4 little 3, 4 5 red 3 Documents 3 Doc4 4 Doc5 Segment 2 Inverted index 0 balloon 3 1 black 1 2 boy 3, 4 3 brown 2, 4 4 cat 2, 4 5 little 3, 4 6 mike 1 7 rachel 2 8 red 3 Documents 1 Doc2 2 Doc3 3 Doc4 4 Doc5 Merged segment Live documents 1 2 3 4 Live documents 0 1 2 Live documents 1 2 3 4
- 39. Elasticsearch How does the write look like? Logical view (write path, refresh & commit) Lucene write Memory Disk Seg 1 Seg 2 Seg 3 Seg 4 write In-mem buffer flush Transaction log Commit point Seg 3 Seg 5 Seg 4
- 40. How does the read look like? Lucene Memory read merge Seg1 Disk Seg 1 Seg 2 Commit point Seg 3 Seg 4 Seg2 Seg3 Seg4 response Logical view (read path)
- 41. Compaction Lucene Disk Seg1 Seg2 Seg3 Seg4 Compact Seg5 Commit point
- 42. Lucene low level
- 43. Lucene codecs ● Abstraction over data format within files ● Keeps low level details away from Lucene ● File formats are codec-dependent
- 44. File formats ● Each index in separate UUD-named dir (Elasticsearch’s doing, prevent index corruption when recreating) ● Segments file (segments_1, segments_2, …) ● Lock file (write.lock) Per-index files
- 45. File formats ● Segment info (.si) - lucene ver, num of docs, os, os ver, java ver, files included ● Term index (.tip) - Index into the Term Dictionary ● Term Dictionary (.tim) - Stores term info ● Postings (.pos) - Stores information where document is located within stored fields ● Field index (.fdx) - Index into Field Data ● Field Data (.fdt) - Stored fields for documents (real values) ● ... Per-segment file
- 46. How does the read look like? Term query Term = tomato
- 47. How does the read look like? File view - Term Index ● FST (Finite State Transducer) ● Stores term prefixes ● Map String -> Object t / 1 b / 7 o / 1 r / 2 h / 3 e / 6 to = 2 thr = 6 br = 9 the = 10 be = 13
- 48. How does the read look like? File view - Term Dictionary ● Jump to the given block (offset) ● Number of items within the block (25 - 48) ... ... ... [Prefix = to] Suffix Frequency Offset ad 1 102 ast 1 135 aster 2 167 day 7 211 e 2 233 ilette 3 251 mato 8 287 nic 5 309 oth 3 355 Jump here Not found Not found Not found Not found Not found Not found Found
- 49. How does the read look like? File view - Postings lists ● Jump to the given offset in the postings list ● Encoded using modified FOR (Frame of Reference) delta ○ delta-encode ○ split into blocks of N = 128 values ○ bit packing per block ○ if remaining, encode with vInt Example with N = 4 Offset Document ids ... ... 287 1, 3, 4, 6, 8, 20, 22, 26, 30, 31 ... ... Delta encode: 1, 2, 1, 2, 2, 12, 2, 4, 4, 1 Split to blocks: [1, 2, 1, 2] [2, 12, 2, 4] 4, 1 2 bits per value total: 1 byte 4 bits per value total: 2 bytes vInt encoded 1 byte per value total: 2 bytes Uncompressed: 40 (10 * 4) bytes Compressed: 5 (1 + 2 + 2) bytes
- 50. How does the read look like? File view - Field Index & Field Data ● Stored sequentially ● Compressed using LZ4 in 16+KB blocks Starting Doc id Offset 0 33 4 188 5 312 7 423 12 605 13 811 20 934 25 1084 Field Index Field Data [Offset = 33] Doc 0 Doc 1 Doc 2 Doc 3 [Offset = 188] Doc 4 [Offset = 312] Doc 5 Doc 6 [Offset = 423] Doc 7 ... 16KB 16KB 16KB 16KB
- 51. References ● https://www.elastic.co/guide/en/elasticsearch/reference/6.3/index.html ● https://lucene.apache.org/core/7_3_1/index.html ● https://www.elastic.co/blog/tracking-in-sync-shard-copies ● https://www.elastic.co/blog/found-elasticsearch-from-the-bottom-up ● https://www.youtube.com/watch?v=T5RmMNDR5XI ● https://www.youtube.com/watch?v=c9O5_a50aOQ ● https://www.elastic.co/guide/en/elasticsearch/guide/current/distributed-cluster.html ● http://blog.mikemccandless.com/2010/12/using-finite-state-transducers-in.html
- 52. Thank you