218 lines
12 KiB
Markdown
218 lines
12 KiB
Markdown
# Elasticsearch knowledge **(STARTER ONLY)**
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This area is to maintain a compendium of useful information when working with Elasticsearch.
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Information on how to enable Elasticsearch and perform the initial indexing is in
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the [Elasticsearch integration documentation](../integration/elasticsearch.md#enabling-elasticsearch).
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## Deep Dive
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In June 2019, Mario de la Ossa hosted a Deep Dive (GitLab team members only: `https://gitlab.com/gitlab-org/create-stage/issues/1`) on GitLab's [Elasticsearch integration](../integration/elasticsearch.md) to share his domain specific knowledge with anyone who may work in this part of the code base in the future. You can find the [recording on YouTube](https://www.youtube.com/watch?v=vrvl-tN2EaA), and the slides on [Google Slides](https://docs.google.com/presentation/d/1H-pCzI_LNrgrL5pJAIQgvLX8Ji0-jIKOg1QeJQzChug/edit) and in [PDF](https://gitlab.com/gitlab-org/create-stage/uploads/c5aa32b6b07476fa8b597004899ec538/Elasticsearch_Deep_Dive.pdf). Everything covered in this deep dive was accurate as of GitLab 12.0, and while specific details may have changed since then, it should still serve as a good introduction.
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## Supported Versions
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See [Version Requirements](../integration/elasticsearch.md#version-requirements).
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Developers making significant changes to Elasticsearch queries should test their features against all our supported versions.
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## Setting up development environment
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See the [Elasticsearch GDK setup instructions](https://gitlab.com/gitlab-org/gitlab-development-kit/blob/master/doc/howto/elasticsearch.md)
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## Helpful Rake tasks
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- `gitlab:elastic:test:index_size`: Tells you how much space the current index is using, as well as how many documents are in the index.
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- `gitlab:elastic:test:index_size_change`: Outputs index size, reindexes, and outputs index size again. Useful when testing improvements to indexing size.
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Additionally, if you need large repos or multiple forks for testing, please consider [following these instructions](rake_tasks.md#extra-project-seed-options)
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## How does it work?
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The Elasticsearch integration depends on an external indexer. We ship an [indexer written in Go](https://gitlab.com/gitlab-org/gitlab-elasticsearch-indexer). The user must trigger the initial indexing via a Rake task but, after this is done, GitLab itself will trigger reindexing when required via `after_` callbacks on create, update, and destroy that are inherited from [/ee/app/models/concerns/elastic/application_versioned_search.rb](https://gitlab.com/gitlab-org/gitlab/blob/master/ee/app/models/concerns/elastic/application_versioned_search.rb).
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After initial indexing is complete, create, update, and delete operations for all models except projects (see [#207494](https://gitlab.com/gitlab-org/gitlab/issues/207494)) are tracked in a Redis [`ZSET`](https://redis.io/topics/data-types#sorted-sets). A regular `sidekiq-cron` `ElasticIndexBulkCronWorker` processes this queue, updating many Elasticsearch documents at a time with the [Bulk Request API](https://www.elastic.co/guide/en/elasticsearch/reference/current/docs-bulk.html).
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Search queries are generated by the concerns found in [ee/app/models/concerns/elastic](https://gitlab.com/gitlab-org/gitlab/tree/master/ee/app/models/concerns/elastic). These concerns are also in charge of access control, and have been a historic source of security bugs so please pay close attention to them!
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## Existing Analyzers/Tokenizers/Filters
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These are all defined in [ee/lib/elastic/latest/config.rb](https://gitlab.com/gitlab-org/gitlab/blob/master/ee/lib/elastic/latest/config.rb)
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### Analyzers
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#### `path_analyzer`
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Used when indexing blobs' paths. Uses the `path_tokenizer` and the `lowercase` and `asciifolding` filters.
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Please see the `path_tokenizer` explanation below for an example.
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#### `sha_analyzer`
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Used in blobs and commits. Uses the `sha_tokenizer` and the `lowercase` and `asciifolding` filters.
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Please see the `sha_tokenizer` explanation later below for an example.
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#### `code_analyzer`
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Used when indexing a blob's filename and content. Uses the `whitespace` tokenizer and the filters: [`code`](#code), [`edgeNGram_filter`](#edgengram_filter), `lowercase`, and `asciifolding`
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The `whitespace` tokenizer was selected in order to have more control over how tokens are split. For example the string `Foo::bar(4)` needs to generate tokens like `Foo` and `bar(4)` in order to be properly searched.
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Please see the `code` filter for an explanation on how tokens are split.
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NOTE: **Known Issues**:
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Currently the [Elasticsearch code_analyzer doesn't account for all code cases](../integration/elasticsearch.md#known-issues).
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#### `code_search_analyzer`
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Not directly used for indexing, but rather used to transform a search input. Uses the `whitespace` tokenizer and the `lowercase` and `asciifolding` filters.
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### Tokenizers
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#### `sha_tokenizer`
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This is a custom tokenizer that uses the [`edgeNGram` tokenizer](https://www.elastic.co/guide/en/elasticsearch/reference/5.5/analysis-edgengram-tokenizer.html) to allow SHAs to be searcheable by any sub-set of it (minimum of 5 chars).
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Example:
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`240c29dc7e` becomes:
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- `240c2`
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- `240c29`
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- `240c29d`
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- `240c29dc`
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- `240c29dc7`
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- `240c29dc7e`
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#### `path_tokenizer`
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This is a custom tokenizer that uses the [`path_hierarchy` tokenizer](https://www.elastic.co/guide/en/elasticsearch/reference/5.5/analysis-pathhierarchy-tokenizer.html) with `reverse: true` in order to allow searches to find paths no matter how much or how little of the path is given as input.
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Example:
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`'/some/path/application.js'` becomes:
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- `'/some/path/application.js'`
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- `'some/path/application.js'`
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- `'path/application.js'`
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- `'application.js'`
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### Filters
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#### `code`
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Uses a [Pattern Capture token filter](https://www.elastic.co/guide/en/elasticsearch/reference/5.5/analysis-pattern-capture-tokenfilter.html) to split tokens into more easily searched versions of themselves.
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Patterns:
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- `"(\\p{Ll}+|\\p{Lu}\\p{Ll}+|\\p{Lu}+)"`: captures CamelCased and lowedCameCased strings as separate tokens
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- `"(\\d+)"`: extracts digits
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- `"(?=([\\p{Lu}]+[\\p{L}]+))"`: captures CamelCased strings recursively. Ex: `ThisIsATest` => `[ThisIsATest, IsATest, ATest, Test]`
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- `'"((?:\\"|[^"]|\\")*)"'`: captures terms inside quotes, removing the quotes
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- `"'((?:\\'|[^']|\\')*)'"`: same as above, for single-quotes
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- `'\.([^.]+)(?=\.|\s|\Z)'`: separate terms with periods in-between
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- `'\/?([^\/]+)(?=\/|\b)'`: separate path terms `like/this/one`
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#### `edgeNGram_filter`
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Uses an [Edge NGram token filter](https://www.elastic.co/guide/en/elasticsearch/reference/5.5/analysis-edgengram-tokenfilter.html) to allow inputs with only parts of a token to find the token. For example it would turn `glasses` into permutations starting with `gl` and ending with `glasses`, which would allow a search for "`glass`" to find the original token `glasses`
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## Gotchas
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- Searches can have their own analyzers. Remember to check when editing analyzers
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- `Character` filters (as opposed to token filters) always replace the original character, so they're not a good choice as they can hinder exact searches
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## Zero downtime reindexing with multiple indices
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Currently GitLab can only handle a single version of setting. Any setting/schema changes would require reindexing everything from scratch. Since reindexing can take a long time, this can cause search functionality downtime.
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To avoid downtime, GitLab is working to support multiple indices that
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can function at the same time. Whenever the schema changes, the admin
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will be able to create a new index and reindex to it, while searches
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continue to go to the older, stable index. Any data updates will be
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forwarded to both indices. Once the new index is ready, an admin can
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mark it active, which will direct all searches to it, and remove the old
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index.
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This is also helpful for migrating to new servers, e.g. moving to/from AWS.
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Currently we are on the process of migrating to this new design. Everything is hardwired to work with one single version for now.
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### Architecture
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The traditional setup, provided by `elasticsearch-rails`, is to communicate through its internal proxy classes. Developers would write model-specific logic in a module for the model to include in (e.g. `SnippetsSearch`). The `__elasticsearch__` methods would return a proxy object, e.g.:
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- `Issue.__elasticsearch__` returns an instance of `Elasticsearch::Model::Proxy::ClassMethodsProxy`
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- `Issue.first.__elasticsearch__` returns an instance of `Elasticsearch::Model::Proxy::InstanceMethodsProxy`.
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These proxy objects would talk to Elasticsearch server directly (see top half of the diagram).
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In the planned new design, each model would have a pair of corresponding subclassed proxy objects, in which model-specific logic is located. For example, `Snippet` would have `SnippetClassProxy` and `SnippetInstanceProxy` (being subclass of `Elasticsearch::Model::Proxy::ClassMethodsProxy` and `Elasticsearch::Model::Proxy::InstanceMethodsProxy`, respectively).
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`__elasticsearch__` would represent another layer of proxy object, keeping track of multiple actual proxy objects. It would forward method calls to the appropriate index. For example:
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- `model.__elasticsearch__.search` would be forwarded to the one stable index, since it is a read operation.
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- `model.__elasticsearch__.update_document` would be forwarded to all indices, to keep all indices up-to-date.
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The global configurations per version are now in the `Elastic::(Version)::Config` class. You can change mappings there.
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### Creating new version of schema
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NOTE: **Note:** this is not applicable yet as multiple indices functionality is not fully implemented.
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Folders like `ee/lib/elastic/v12p1` contain snapshots of search logic from different versions. To keep a continuous Git history, the latest version lives under `ee/lib/elastic/latest`, but its classes are aliased under an actual version (e.g. `ee/lib/elastic/v12p3`). When referencing these classes, never use the `Latest` namespace directly, but use the actual version (e.g. `V12p3`).
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The version name basically follows GitLab's release version. If setting is changed in 12.3, we will create a new namespace called `V12p3` (p stands for "point"). Raise an issue if there is a need to name a version differently.
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If the current version is `v12p1`, and we need to create a new version for `v12p3`, the steps are as follows:
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1. Copy the entire folder of `v12p1` as `v12p3`
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1. Change the namespace for files under `v12p3` folder from `V12p1` to `V12p3` (which are still aliased to `Latest`)
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1. Delete `v12p1` folder
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1. Copy the entire folder of `latest` as `v12p1`
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1. Change the namespace for files under `v12p1` folder from `Latest` to `V12p1`
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1. Make changes to files under the `latest` folder as needed
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## Troubleshooting
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### Getting `flood stage disk watermark [95%] exceeded`
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You might get an error such as
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```plaintext
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[2018-10-31T15:54:19,762][WARN ][o.e.c.r.a.DiskThresholdMonitor] [pval5Ct]
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flood stage disk watermark [95%] exceeded on
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[pval5Ct7SieH90t5MykM5w][pval5Ct][/usr/local/var/lib/elasticsearch/nodes/0] free: 56.2gb[3%],
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all indices on this node will be marked read-only
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```
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This is because you've exceeded the disk space threshold - it thinks you don't have enough disk space left, based on the default 95% threshold.
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In addition, the `read_only_allow_delete` setting will be set to `true`. It will block indexing, `forcemerge`, etc
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```shell
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curl "http://localhost:9200/gitlab-development/_settings?pretty"
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```
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Add this to your `elasticsearch.yml` file:
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```yaml
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# turn off the disk allocator
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cluster.routing.allocation.disk.threshold_enabled: false
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```
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_or_
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```yaml
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# set your own limits
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cluster.routing.allocation.disk.threshold_enabled: true
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cluster.routing.allocation.disk.watermark.flood_stage: 5gb # ES 6.x only
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cluster.routing.allocation.disk.watermark.low: 15gb
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cluster.routing.allocation.disk.watermark.high: 10gb
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```
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Restart Elasticsearch, and the `read_only_allow_delete` will clear on it's own.
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_from "Disk-based Shard Allocation | Elasticsearch Reference" [5.6](https://www.elastic.co/guide/en/elasticsearch/reference/5.6/disk-allocator.html#disk-allocator) and [6.x](https://www.elastic.co/guide/en/elasticsearch/reference/6.7/disk-allocator.html)_
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