# Elasticsearch knowledge **(STARTER ONLY)**

This area is to maintain a compendium of useful information when working with elasticsearch.

Information on how to enable Elasticsearch and perform the initial indexing is kept in ../integration/elasticsearch.md#enabling-elasticsearch

## Deep Dive

In June 2019, Mario de la Ossa hosted a [Deep Dive] on GitLab's [Elasticsearch integration] 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], and the slides on [Google Slides] and in [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.

[Deep Dive]: https://gitlab.com/gitlab-org/create-stage/issues/1
[Elasticsearch integration]: ../integration/elasticsearch.md
[recording on YouTube]: https://www.youtube.com/watch?v=vrvl-tN2EaA
[Google Slides]: https://docs.google.com/presentation/d/1H-pCzI_LNrgrL5pJAIQgvLX8Ji0-jIKOg1QeJQzChug/edit
[PDF]: https://gitlab.com/gitlab-org/create-stage/uploads/c5aa32b6b07476fa8b597004899ec538/Elasticsearch_Deep_Dive.pdf

## Initial installation on OS X

It is recommended to use the Docker image. After installing docker you can immediately spin up an instance with

```
docker run --name elastic56 -p 9200:9200 -p 9300:9300 -e "discovery.type=single-node" docker.elastic.co/elasticsearch/elasticsearch:5.6.12
```

and use `docker stop elastic56` and `docker start elastic56` to stop/start it.

### Installing on the host

We currently only support Elasticsearch [5.6 to 6.x](../integration/elasticsearch.md#version-requirements)

Version 5.6 is available on homebrew and is the recommended version to use in order to test compatibility.

```
brew install elasticsearch@5.6
```

There is no need to install any plugins

## New repo indexer (beta)

If you're interested on working with the new beta repo indexer, all you need to do is:

- git clone git@gitlab.com:gitlab-org/gitlab-elasticsearch-indexer.git
- make
- make install

this adds `gitlab-elasticsearch-indexer` to `$GOPATH/bin`, please make sure that is in your `$PATH`. After that GitLab will find it and you'll be able to enable it in the admin settings area.

**note:** `make` will not recompile the executable unless you do `make clean` beforehand

## Helpful rake tasks

- `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 repos or multiple forks for testing, please consider [following these instructions](rake_tasks.md#extra-project-seed-options)

## How does it work?

The Elasticsearch integration depends on an external indexer. We ship a [ruby indexer](https://gitlab.com/gitlab-org/gitlab-ee/blob/master/bin/elastic_repo_indexer) by default but are also working on 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_search.rb](https://gitlab.com/gitlab-org/gitlab-ee/blob/master/ee/app/models/concerns/elastic/application_search.rb).

All indexing after the initial one is done via `ElasticIndexerWorker` (sidekiq jobs).

Search queries are generated by the concerns found in [ee/app/models/concerns/elastic](https://gitlab.com/gitlab-org/gitlab-ee/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!

## Existing Analyzers/Tokenizers/Filters

These are all defined in <https://gitlab.com/gitlab-org/gitlab-ee/blob/master/ee/lib/elasticsearch/git/model.rb>

### Analyzers

#### `path_analyzer`

Used when indexing blobs' paths. Uses the `path_tokenizer` and the `lowercase` and `asciifolding` filters.

Please see the `path_tokenizer` explanation below for an example.

#### `sha_analyzer`

Used in blobs and commits. Uses the `sha_tokenizer` and the `lowercase` and `asciifolding` filters.

Please see the `sha_tokenizer` explanation later below for an example.

#### `code_analyzer`

Used when indexing a blob's filename and content. Uses the `whitespace` tokenizer and the filters: `code`, `edgeNGram_filter`, `lowercase`, and `asciifolding`

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.

#### `code_search_analyzer`

Not directly used for indexing, but rather used to transform a search input. Uses the `whitespace` tokenizer and the `lowercase` and `asciifolding` filters.

### Tokenizers

#### `sha_tokenizer`

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).

Example:

`240c29dc7e` becomes:

- `240c2`
- `240c29`
- `240c29d`
- `240c29dc`
- `240c29dc7`
- `240c29dc7e`

#### `path_tokenizer`

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.

Example:

`'/some/path/application.js'` becomes:

- `'/some/path/application.js'`
- `'some/path/application.js'`
- `'path/application.js'`
- `'application.js'`

### Filters

#### `code`

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.

Patterns:

- `"(\\p{Ll}+|\\p{Lu}\\p{Ll}+|\\p{Lu}+)"`: captures CamelCased and lowedCameCased strings as separate tokens
- `"(\\d+)"`: extracts digits
- `"(?=([\\p{Lu}]+[\\p{L}]+))"`: captures CamelCased strings recursively. Ex: `ThisIsATest` => `[ThisIsATest, IsATest, ATest, Test]`
- `'"((?:\\"|[^"]|\\")*)"'`: captures terms inside quotes, removing the quotes
- `"'((?:\\'|[^']|\\')*)'"`: same as above, for single-quotes
- `'\.([^.]+)(?=\.|\s|\Z)'`: separate terms with periods in-between
- `'\/?([^\/]+)(?=\/|\b)'`: separate path terms `like/this/one`

#### `edgeNGram_filter`

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`

## Gotchas

- 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

## Architecture

GitLab uses `elasticsearch-rails` for handling communication with Elasticsearch server. However, in order to achieve zero-downtime deployment during schema changes, an extra abstraction layer is built to allow:

* Indexing (writes) to multiple indexes, with different mappings
* Switching to different index for searches (reads) on the fly

Currently we are on the process of migrating models to this new design (e.g. `Snippet`), and it is hardwired to work with a single version for now.

Traditionally, `elasticsearch-rails` provides class and instance level `__elasticsearch__` proxy methods. If you call `Issue.__elasticsearch__`, you will get an instance of `Elasticsearch::Model::Proxy::ClassMethodsProxy`, and if you call `Issue.first.__elasticsearch__`, you will get an instance of `Elasticsearch::Model::Proxy::InstanceMethodsProxy`. These proxy objects would talk to Elasticsearch server directly.

In the new design, `__elasticsearch__` instead represents one extra layer of proxy. It would keep multiple versions of the actual proxy objects, and it would forward read and write calls to the proxy of the intended version.

The `elasticsearch-rails`'s way of specifying each model's mappings and other settings is to create a module for the model to include. However in the new design, each model would have its own corresponding subclassed proxy object, where the settings reside in. For example, snippet related setting in the past reside in `SnippetsSearch` module, but in the new design would reside in `SnippetClassProxy` (which is a subclass of `Elasticsearch::Model::Proxy::ClassMethodsProxy`). This reduces namespace pollution in model classes.

The global configurations per version are now in the `Elastic::(Version)::Config` class. You can change mappings there.

### Creating new version of schema

Currently GitLab would still work with a single version of setting. Once it is implemented, multiple versions of setting can exists in different folders (e.g. `ee/lib/elastic/v12p1` and `ee/lib/elastic/v12p3`). To keep a continuous git history, the latest version lives under the `/latest` folder, but is aliased as the latest version.

If the current version is `v12p1`, and we need to create a new version for `v12p3`, the steps are as follows:

1. Copy the entire folder of `v12p1` as `v12p3`
1. Change the namespace for files under `v12p3` folder from `V12p1` to `V12p3` (which are still aliased to `Latest`)
1. Delete `v12p1` folder
1. Copy the entire folder of `latest` as `v12p1`
1. Change the namespace for files under `v12p1` folder from `Latest` to `V12p1`
1. Make changes to `Latest` as needed

## Troubleshooting

### Getting `flood stage disk watermark [95%] exceeded`

You might get an error such as

```
[2018-10-31T15:54:19,762][WARN ][o.e.c.r.a.DiskThresholdMonitor] [pval5Ct]
   flood stage disk watermark [95%] exceeded on
   [pval5Ct7SieH90t5MykM5w][pval5Ct][/usr/local/var/lib/elasticsearch/nodes/0] free: 56.2gb[3%],
   all indices on this node will be marked read-only
```

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.

In addition, the `read_only_allow_delete` setting will be set to `true`.  It will block indexing, `forcemerge`, etc

```
curl "http://localhost:9200/gitlab-development/_settings?pretty"
```

Add this to your `elasticsearch.yml` file:

```
# turn off the disk allocator
cluster.routing.allocation.disk.threshold_enabled: false
```

_or_

```
# set your own limits
cluster.routing.allocation.disk.threshold_enabled: true
cluster.routing.allocation.disk.watermark.flood_stage: 5gb   # ES 6.x only
cluster.routing.allocation.disk.watermark.low: 15gb
cluster.routing.allocation.disk.watermark.high: 10gb
```

Restart Elasticsearch, and the `read_only_allow_delete` will clear on it's own.

_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.x/disk-allocator.html)_
