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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.
In August 2020, a second Deep Dive was hosted, focusing on [GitLab's specific architecture for multi-indices support](#zero-downtime-reindexing-with-multiple-indices). The [recording on YouTube](https://www.youtube.com/watch?v=0WdPR9oB2fg) and the [slides](https://lulalala.gitlab.io/gitlab-elasticsearch-deepdive/) are available. Everything covered in this deep dive was accurate as of GitLab 13.3.
-`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.
-`gitlab:elastic:test:index_size_change`: Outputs index size, reindexes, and outputs index size again. Useful when testing improvements to indexing size.
Additionally, if you need large repositories or multiple forks for testing, please consider [following these instructions](rake_tasks.md#extra-project-seed-options)
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).
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).
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!
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.
Please see the `code` filter for an explanation on how tokens are split.
Not directly used for indexing, but rather used to transform a search input. Uses the `whitespace` tokenizer and the `lowercase` and `asciifolding` filters.
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 searchable by any sub-set of it (minimum of 5 chars).
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.
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.
- Searches can have their own analyzers. Remember to check when editing analyzers
-`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
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.
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.:
-`Issue.__elasticsearch__` returns an instance of `Elasticsearch::Model::Proxy::ClassMethodsProxy`
-`Issue.first.__elasticsearch__` returns an instance of `Elasticsearch::Model::Proxy::InstanceMethodsProxy`.
These proxy objects would talk to Elasticsearch server directly (see top half of the diagram).
In the planned new design, each model would have a pair of corresponding sub-classed 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).
`__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:
-`model.__elasticsearch__.search` would be forwarded to the one stable index, since it is a read operation.
-`model.__elasticsearch__.update_document` would be forwarded to all indices, to keep all indices up-to-date.
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`).
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.
This only supported for indices created with GitLab 13.0 or greater.
Migrations are stored in the [`ee/elastic/migrate/`](https://gitlab.com/gitlab-org/gitlab/-/tree/master/ee/elastic/migrate) folder with `YYYYMMDDHHMMSS_migration_name.rb`
file name format, which is similar to Rails database migrations:
```ruby
# frozen_string_literal: true
class MigrationName <Elastic::Migration
# Important: Any update to the Elastic index mappings should be replicated in Elastic::Latest::Config
def migrate
end
# Check if the migration has completed
# Return true if completed, otherwise return false
def completed?
end
end
```
Applied migrations are stored in `gitlab-#{RAILS_ENV}-migrations` index. All unexecuted migrations
are applied by the [`Elastic::MigrationWorker`](https://gitlab.com/gitlab-org/gitlab/blob/master/ee/app/workers/elastic/migration_worker.rb)
cron worker sequentially.
Any update to the Elastic index mappings should be replicated in [`Elastic::Latest::Config`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/lib/elastic/latest/config.rb).
### Migration options supported by the [`Elastic::MigrationWorker`](https://gitlab.com/gitlab-org/gitlab/blob/master/ee/app/workers/elastic/migration_worker.rb)
-`batched!` - Allow the migration to run in batches. If set, the [`Elastic::MigrationWorker`](https://gitlab.com/gitlab-org/gitlab/blob/master/ee/app/workers/elastic/migration_worker.rb)
