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+# Value Stream Analytics development guide
+
+Value stream analytics calculates the time between two arbitrary events recorded on domain objects and provides aggregated statistics about the duration.
+
+For information on how to configure Value Stream Analytics in GitLab, see our [analytics documentation](../user/analytics/value_stream_analytics.md).
+
+## Stage
+
+During development, events occur that move issues and merge requests through different stages of progress until they are considered finished. These stages can be expressed with the `Stage` model.
+
+Example stage:
+
+- Name: Development
+- Start event: Issue created
+- End event: Issue first mentioned in commit
+- Parent: `Group: gitlab-org`
+
+### Events
+
+Events are the smallest building blocks of the value stream analytics feature. A stage consists of two events:
+
+- Start
+- End
+
+These events play a key role in the duration calculation.
+
+Formula: `duration = end_event_time - start_event_time`
+
+To make the duration calculation flexible, each `Event` is implemented as a separate class. They're responsible for defining a timestamp expression that will be used in the calculation query.
+
+#### Implementing an `Event` class
+
+There are a few methods that are required to be implemented, the `StageEvent` base class describes them in great detail. The most important ones are:
+
+- `object_type`
+- `timestamp_projection`
+
+The `object_type` method defines which domain object will be queried for the calculation. Currently two models are allowed:
+
+- `Issue`
+- `MergeRequest`
+
+For the duration calculation the `timestamp_projection` method will be used.
+
+```ruby
+def timestamp_projection
+  # your timestamp expression comes here
+end
+
+# event will use the issue creation time in the duration calculation
+def timestamp_projection
+  Issue.arel_table[:created_at]
+end
+```
+
+NOTE: **Note:**
+More complex expressions are also possible (e.g. using `COALESCE`). Look at the existing event classes for examples.
+
+In some cases, defining the `timestamp_projection` method is not enough. The calculation query should know which table contains the timestamp expression. Each `Event` class is responsible for making modifications to the calculation query to make the `timestamp_projection` work. This usually means joining an additional table.
+
+Example for joining the `issue_metrics` table and using the `first_mentioned_in_commit_at` column as the timestamp expression:
+
+```ruby
+def object_type
+  Issue
+end
+
+def timestamp_projection
+  IssueMetrics.arel_table[:first_mentioned_in_commit_at]
+end
+
+def apply_query_customization(query)
+  # in this case the query attribute will be based on the Issue model: `Issue.where(...)`
+  query.joins(:metrics)
+end
+```
+
+### Validating start and end events
+
+Some start/end event pairs are not "compatible" with each other. For example:
+
+- "Issue created" to "Merge Request created": The event classes are defined on different domain models, the `object_type` method is different.
+- "Issue closed" to "Issue created": Issue must be created first before it can be closed.
+- "Issue closed" to "Issue closed": Duration is always 0.
+
+The `StageEvents` module describes the allowed `start_event` and `end_event` pairings (`PAIRING_RULES` constant). If a new event is added, it needs to be registered in this module.
+​To add a new event:​
+
+1. Add an entry in `ENUM_MAPPING` with a unique number, it'll be used in the `Stage` model as `enum`.
+1. Define which events are compatible with the event in the `PAIRING_RULES` hash.
+
+Supported start/end event pairings:
+
+```mermaid
+graph LR;
+  IssueCreated --> IssueClosed;
+  IssueCreated --> IssueFirstAddedToBoard;
+  IssueCreated --> IssueFirstAssociatedWithMilestone;
+  IssueCreated --> IssueFirstMentionedInCommit;
+  IssueCreated --> IssueLastEdited;
+  IssueCreated --> IssueLabelAdded;
+  IssueCreated --> IssueLabelRemoved;
+  MergeRequestCreated --> MergeRequestMerged;
+  MergeRequestCreated --> MergeRequestClosed;
+  MergeRequestCreated --> MergeRequestFirstDeployedToProduction;
+  MergeRequestCreated --> MergeRequestLastBuildStarted;
+  MergeRequestCreated --> MergeRequestLastBuildFinished;
+  MergeRequestCreated --> MergeRequestLastEdited;
+  MergeRequestCreated --> MergeRequestLabelAdded;
+  MergeRequestCreated --> MergeRequestLabelRemoved;
+  MergeRequestLastBuildStarted --> MergeRequestLastBuildFinished;
+  MergeRequestLastBuildStarted --> MergeRequestClosed;
+  MergeRequestLastBuildStarted --> MergeRequestFirstDeployedToProduction;
+  MergeRequestLastBuildStarted --> MergeRequestLastEdited;
+  MergeRequestLastBuildStarted --> MergeRequestMerged;
+  MergeRequestLastBuildStarted --> MergeRequestLabelAdded;
+  MergeRequestLastBuildStarted --> MergeRequestLabelRemoved;
+  MergeRequestMerged --> MergeRequestFirstDeployedToProduction;
+  MergeRequestMerged --> MergeRequestClosed;
+  MergeRequestMerged --> MergeRequestFirstDeployedToProduction;
+  MergeRequestMerged --> MergeRequestLastEdited;
+  MergeRequestMerged --> MergeRequestLabelAdded;
+  MergeRequestMerged --> MergeRequestLabelRemoved;
+  IssueLabelAdded --> IssueLabelAdded;
+  IssueLabelAdded --> IssueLabelRemoved;
+  IssueLabelAdded --> IssueClosed;
+  IssueLabelRemoved --> IssueClosed;
+  IssueFirstAddedToBoard --> IssueClosed;
+  IssueFirstAddedToBoard --> IssueFirstAssociatedWithMilestone;
+  IssueFirstAddedToBoard --> IssueFirstMentionedInCommit;
+  IssueFirstAddedToBoard --> IssueLastEdited;
+  IssueFirstAddedToBoard --> IssueLabelAdded;
+  IssueFirstAddedToBoard --> IssueLabelRemoved;
+  IssueFirstAssociatedWithMilestone --> IssueClosed;
+  IssueFirstAssociatedWithMilestone --> IssueFirstAddedToBoard;
+  IssueFirstAssociatedWithMilestone --> IssueFirstMentionedInCommit;
+  IssueFirstAssociatedWithMilestone --> IssueLastEdited;
+  IssueFirstAssociatedWithMilestone --> IssueLabelAdded;
+  IssueFirstAssociatedWithMilestone --> IssueLabelRemoved;
+  IssueFirstMentionedInCommit --> IssueClosed;
+  IssueFirstMentionedInCommit --> IssueFirstAssociatedWithMilestone;
+  IssueFirstMentionedInCommit --> IssueFirstAddedToBoard;
+  IssueFirstMentionedInCommit --> IssueLastEdited;
+  IssueFirstMentionedInCommit --> IssueLabelAdded;
+  IssueFirstMentionedInCommit --> IssueLabelRemoved;
+  IssueClosed --> IssueLastEdited;
+  IssueClosed --> IssueLabelAdded;
+  IssueClosed --> IssueLabelRemoved;
+  MergeRequestClosed --> MergeRequestFirstDeployedToProduction;
+  MergeRequestClosed --> MergeRequestLastEdited;
+  MergeRequestClosed --> MergeRequestLabelAdded;
+  MergeRequestClosed --> MergeRequestLabelRemoved;
+  MergeRequestFirstDeployedToProduction --> MergeRequestLastEdited;
+  MergeRequestFirstDeployedToProduction --> MergeRequestLabelAdded;
+  MergeRequestFirstDeployedToProduction --> MergeRequestLabelRemoved;
+  MergeRequestLastBuildFinished --> MergeRequestClosed;
+  MergeRequestLastBuildFinished --> MergeRequestFirstDeployedToProduction;
+  MergeRequestLastBuildFinished --> MergeRequestLastEdited;
+  MergeRequestLastBuildFinished --> MergeRequestMerged;
+  MergeRequestLastBuildFinished --> MergeRequestLabelAdded;
+  MergeRequestLastBuildFinished --> MergeRequestLabelRemoved;
+  MergeRequestLabelAdded --> MergeRequestLabelAdded;
+  MergeRequestLabelAdded --> MergeRequestLabelRemoved;
+  MergeRequestLabelRemoved --> MergeRequestLabelAdded;
+  MergeRequestLabelRemoved --> MergeRequestLabelRemoved;
+```
+
+### Parent
+
+Teams and organizations might define their own way of building software, thus stages can be completely different. For each stage, a parent object needs to be defined.
+
+Currently supported parents:
+
+- `Project`
+- `Group`
+
+#### How parent relationship it work
+
+1. User navigates to the value stream analytics page.
+1. User selects a group.
+1. Backend loads the defined stages for the selected group.
+1. Additions and modifications to the stages will be persisted within the selected group only.
+
+### Default stages
+
+The [original implementation](https://gitlab.com/gitlab-org/gitlab/issues/847) of value stream analytics defined 7 stages. These stages are always available for each parent, however altering these stages is not possible.
+​
+To make things efficient and reduce the number of records created, the default stages are expressed as in-memory objects (not persisted). When the user creates a custom stage for the first time, all the stages will be persisted. This behaviour is implemented in the value stream analytics service objects.
+​
+The reason for this was that we'd like to add the abilities to hide and order stages later on.
+
+## Data Collector
+
+`DataCollector` is the central point where the data will be queried from the database. The class always operates on a single stage and consists of the following components:
+
+- `BaseQueryBuilder`:
+  - Responsible for composing the initial query.
+  - Deals with `Stage` specific configuration: events and their query customizations.
+  - Parameters coming from the UI: date ranges.
+- `Median`: Calculates the median duration for a stage using the query from  `BaseQueryBuilder`.
+- `RecordsFetcher`: Loads relevant records for a stage using the query from  `BaseQueryBuilder` and specific `Finder` classes to apply visibility rules.
+- `DataForDurationChart`: Loads calculated durations with the finish time (end event timestamp) for the scatterplot chart.
+
+For a new calculation or a query, implement it as a new method call in the `DataCollector` class.
+
+## Database query
+
+Structure of the database query:
+
+```sql
+SELECT (customized by: Median or RecordsFetcher or DataForDurationChart)
+FROM OBJECT_TYPE (Issue or MergeRequest)
+INNER JOIN (several JOIN statements, depending on the events)
+WHERE
+  (Filter by the PARENT model, example: filter Issues from Project A)
+  (Date range filter based on the OBJECT_TYPE.created_at)
+  (Check if the START_EVENT is earlier than END_EVENT, preventing negative duration)
+```
+
+Structure of the `SELECT` statement for `Median`:
+
+```sql
+SELECT (calculate median from START_EVENT_TIME-END_EVENT_TIME)
+```
+
+Structure of the `SELECT` statement for `DataForDurationChart`:
+
+```sql
+SELECT (START_EVENT_TIME-END_EVENT_TIME) as duration, END_EVENT.timestamp
+```
+
+## High-level overview
+
+- Rails Controller (`Analytics::CycleAnalytics` module): Value stream analytics exposes its data via JSON endpoints, implemented within the `analytics` workspace. Configuring the stages are also implements JSON endpoints (CRUD).
+- Services (`Analytics::CycleAnalytics` module): All `Stage` related actions will be delegated to respective service objects.
+- Models (`Analytics::CycleAnalytics` module): Models are used to persist the `Stage` objects `ProjectStage` and `GroupStage`.
+- Feature classes (`Gitlab::Analytics::CycleAnalytics` module):
+  - Responsible for composing queries and define feature specific busines logic.
+  - `DataCollector`, `Event`, `StageEvents`, etc.
+
+## Testing
+
+Since we have a lots of events and possible pairings, testing each pairing is not possible. The rule is to have at least one test case using an `Event` class.
+
+Writing a test case for a stage using a new `Event` can be challenging since data must be created for both events. To make this a bit simpler, each test case must be implemented in the `data_collector_spec.rb` where the stage is tested through the `DataCollector`. Each test case will be turned into multiple tests, covering the following cases:
+
+- Different parents: `Group` or `Project`
+- Different calculations: `Median`, `RecordsFetcher` or `DataForDurationChart`