Add Import/Export dev docs

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diff --git a/doc/development/README.md b/doc/development/README.md
index f22dde32de9..05715274a81 100644
--- a/doc/development/README.md
+++ b/doc/development/README.md
@@ -47,6 +47,7 @@ description: 'Learn how to contribute to GitLab.'
 - [Avoid modules with instance variables](module_with_instance_variables.md) if possible
 - [How to dump production data to staging](db_dump.md)
 - [Working with the GitHub importer](github_importer.md)
+- [Import/Export development documentation](import_export.md)
 - [Working with Merge Request diffs](diffs.md)
 - [Permissions](permissions.md)
 - [Prometheus metrics](prometheus_metrics.md)
diff --git a/doc/development/import_export.md b/doc/development/import_export.md
new file mode 100644
index 00000000000..71db1abb201
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+++ b/doc/development/import_export.md
@@ -0,0 +1,352 @@
+# Import/Export development documentation
+
+Troubleshooing and general development guidelines and tips for the [Import/Export feature](../user/project/settings/import_export.md).
+
+<i class="fa fa-youtube-play youtube" aria-hidden="true"></i> This document is originally based on the [Import/Export 201 presentation available on YouTube](https://www.youtube.com/watch?v=V3i1OfExotE).
+
+## Troubleshooting commands
+
+Finds information about the status of the import and further logs using the JID:
+
+```ruby
+# Rails console
+Project.find_by_full_path('group/project').import_state.slice(:jid, :status, :last_error)
+> {"jid"=>"414dec93f941a593ea1a6894", "status"=>"finished", "last_error"=>nil}
+```
+
+```bash
+# Logs
+grep JID /var/log/gitlab/sidekiq/current
+grep "Import/Export error" /var/log/gitlab/sidekiq/current
+grep "Import/Export backtrace" /var/log/gitlab/sidekiq/current
+```
+
+## Troubleshooting performance issues
+
+Read through the current performance problems using the Import/Export below.
+
+### OOM errors
+
+Out of memory (OOM) errors are normally caused by the [Sidekiq Memory Killer](https://docs.gitlab.com/ee/administration/operations/sidekiq_memory_killer.html):
+
+```bash
+SIDEKIQ_MEMORY_KILLER_MAX_RSS = 2GB in GitLab.com
+```
+
+An import status `started`, and the following sidekiq logs will signal a memory issue:
+
+```bash
+WARN: Work still in progress <struct with JID>
+```
+
+### Timeouts
+
+Timeout errors occur due to the `StuckImportJobsWorker` marking the process as failed:
+
+```ruby
+class StuckImportJobsWorker
+  include ApplicationWorker
+  include CronjobQueue
+
+  IMPORT_JOBS_EXPIRATION = 15.hours.to_i
+
+  def perform
+    import_state_without_jid_count = mark_import_states_without_jid_as_failed!
+    import_state_with_jid_count = mark_import_states_with_jid_as_failed!
+    ...
+```
+
+```bash
+Marked stuck import jobs as failed. JIDs: xyz
+```
+
+```                                         
+  +-----------+    +-----------------------------------+
+  |Export Job |--->| Calls ActiveRecord `as_json` and  |
+  +-----------+    | `to_json` on all project models   |
+                   +-----------------------------------+
+                   
+  +-----------+    +-----------------------------------+
+  |Import Job |--->| Loads all JSON in memory, then    |
+  +-----------+    | inserts into the DB in batches    |
+                   +-----------------------------------+
+```
+
+### Problems and solutions
+
+| Problem | Possible solutions |
+| -------- | -------- |
+| [Slow JSON](https://gitlab.com/gitlab-org/gitlab-ce/issues/54084) loading/dumping models from the database | [split the worker](https://gitlab.com/gitlab-org/gitlab-ce/issues/54085) |
+| | Batch export
+| | Optimize SQL
+| | Move away from `ActiveRecord` callbacks (difficult)
+| High memory usage (see also some [analysis](https://gitlab.com/gitlab-org/gitlab-ce/issues/35389) | DB Commit sweet spot that uses less memory |
+| | [Netflix Fast JSON API](https://github.com/Netflix/fast_jsonapi) may help |
+| | Batch reading/writing to disk and any SQL
+
+### Temporary solutions
+
+While the performance problems are not tackled, there is a process to workaround
+importing big projects, using a foreground import:
+
+[Foreground import](https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/5384) of big projects for customers.
+(Using the import template in the [infrastructure tracker](https://gitlab.com/gitlab-com/gl-infra/infrastructure/))
+
+## Security
+
+The Import/Export feature is constantly updated (adding new things to export), however
+the code hasn't been refactored in a long time. We should perform a [code audit](https://gitlab.com/gitlab-org/gitlab-ce/issues/42135)
+to make sure its dynamic nature does not increase the number of security concerns.
+
+### Security in the code
+
+Some of these classes provide a layer of security to the Import/Export.
+
+The `AttributeCleaner` removes any prohibited keys:
+
+```ruby
+# AttributeCleaner
+# Removes all `_ids` and other prohibited keys
+    class AttributeCleaner
+      ALLOWED_REFERENCES = RelationFactory::PROJECT_REFERENCES + RelationFactory::USER_REFERENCES + ['group_id']
+    
+      def clean
+        @relation_hash.reject do |key, _value|
+          prohibited_key?(key) || !@relation_class.attribute_method?(key) || excluded_key?(key)
+        end.except('id')
+      end
+      
+      ...
+
+```
+
+The `AttributeConfigurationSpec` checks and confirms the addition of new columns:
+
+```ruby
+# AttributeConfigurationSpec
+<<-MSG
+  It looks like #{relation_class}, which is exported using the project Import/Export, has new attributes:
+
+  Please add the attribute(s) to SAFE_MODEL_ATTRIBUTES if you consider this can be exported.
+  Otherwise, please blacklist the attribute(s) in IMPORT_EXPORT_CONFIG by adding it to its correspondent
+  model in the +excluded_attributes+ section.
+
+  SAFE_MODEL_ATTRIBUTES: #{File.expand_path(safe_attributes_file)}
+  IMPORT_EXPORT_CONFIG: #{Gitlab::ImportExport.config_file}
+MSG 
+```
+
+The `ModelConfigurationSpec` checks and confirms the addition of new models:
+
+```ruby
+# ModelConfigurationSpec
+<<-MSG
+  New model(s) <#{new_models.join(',')}> have been added, related to #{parent_model_name}, which is exported by
+  the Import/Export feature.
+
+  If you think this model should be included in the export, please add it to `#{Gitlab::ImportExport.config_file}`.
+
+  Definitely add it to `#{File.expand_path(ce_models_yml)}`
+  #{"or `#{File.expand_path(ee_models_yml)}` if the model/associations are EE-specific\n" if ee_models_hash.any?}
+  to signal that you've handled this error and to prevent it from showing up in the future.
+MSG
+```
+
+The `ExportFileSpec` detects encrypted or sensitive columns:
+
+```ruby
+# ExportFileSpec
+<<-MSG
+  Found a new sensitive word <#{key_found}>, which is part of the hash #{parent.inspect}    
+  If you think this information shouldn't get exported, please exclude the model or attribute in
+  IMPORT_EXPORT_CONFIG.
+
+  Otherwise, please add the exception to +safe_list+ in CURRENT_SPEC using #{sensitive_word} as the
+  key and the correspondent hash or model as the value.
+
+  Also, if the attribute is a generated unique token, please add it to RelationFactory::TOKEN_RESET_MODELS
+  if it needs to be reset (to prevent duplicate column problems while importing to the same instance).
+
+  IMPORT_EXPORT_CONFIG: #{Gitlab::ImportExport.config_file}
+  CURRENT_SPEC: #{__FILE__}
+MSG
+```
+
+## Versioning
+
+Import/Export does not use strict SemVer, since it has frequent constant changes
+during a single GitLab release. It does require an update when there is a breaking change.
+
+```ruby
+# ImportExport
+module Gitlab
+  module ImportExport
+    extend self
+
+    # For every version update, the version history in import_export.md has to be kept up to date.
+    VERSION = '0.2.4'
+```
+
+## Version history
+
+The [current version history](../user/project/settings/import_export.md) also displays the equivalent GitLab version
+and it is useful for knowing which versions won't be compatible between them.
+
+| GitLab version   | Import/Export version |
+| ---------------- | --------------------- |
+| 11.1 to current  | 0.2.4                 |
+| 10.8             | 0.2.3                 |
+| 10.4             | 0.2.2                 |
+| ...              | ...                   |
+| 8.10.3           | 0.1.3                 |
+| 8.10.0           | 0.1.2                 |
+| 8.9.5            | 0.1.1                 |
+| 8.9.0            | 0.1.0                 |
+
+### When to bump the version up
+
+We will have to bump the verision if we rename model/columns or perform any format
+modifications in the JSON structure or the file structure of the archive file.
+
+We do not need to bump the version up in any of the following cases:
+
+- Add a new column or a model
+- Remove a column or model (unless there is a DB constraint)
+- Export new things (such as a new type of upload)
+
+
+Every time we bump the version, the integration specs will fail and can be fixed with:
+
+```bash
+bundle exec rake gitlab:import_export:bump_version
+```
+
+### Renaming columns or models
+
+This is a relatively common occurence that will require a version bump.
+
+There is also the _RC problem_ - GitLab.com runs an RC, prior to any customers,
+meaning that we want to bump the version up in the next version (or patch release).
+
+For example:
+
+1. Add rename to `RelationRenameService` in X.Y
+2. Remove it from `RelationRenameService` in X.Y + 1 
+3. Bump Import/Export version in X.Y + 1
+
+```ruby
+module Gitlab
+  module ImportExport
+    class RelationRenameService
+      RENAMES = {
+        'pipelines' => 'ci_pipelines' # Added in 11.6, remove in 11.7
+      }.freeze
+```
+
+## A quick dive into the code
+
+### Import/Export configuration (`import_export.yml`)
+
+The main configuration `import_export.yml` defines what models can be exported/imported.
+
+Model relationships to be included in the project import/export:
+
+```yaml
+project_tree:
+  - labels:
+      :priorities
+  - milestones:
+    - events:
+      - :push_event_payload
+  - issues:
+    - events:
+    - ...
+```
+
+Only include the following attributes for the models specified:
+
+```yaml
+included_attributes:
+  user:
+    - :id
+    - :email
+  ...      
+      
+```
+
+Do not include the following attributes for the models specified:
+
+```yaml
+excluded_attributes:
+  project:
+    - :name
+    - :path
+    - ...
+```
+
+Extra methods to be called by the export:
+
+```yaml
+# Methods
+methods:
+  labels:
+    - :type
+  label:
+    - :type
+```
+
+### Import
+
+The import job status moves from `none` to `finished` or `failed` into different states:
+
+_import\_status_: none -> scheduled -> started -> finished/failed
+
+While the status is `started` the `Importer` code processes each step required for the import.
+
+```ruby
+# ImportExport::Importer
+module Gitlab
+  module ImportExport
+    class Importer
+      def execute
+        if import_file && check_version! && restorers.all?(&:restore) && overwrite_project
+          project_tree.restored_project
+        else
+          raise Projects::ImportService::Error.new(@shared.errors.join(', '))
+        end
+      rescue => e
+        raise Projects::ImportService::Error.new(e.message)
+      ensure
+        remove_import_file
+      end
+      
+      def restorers
+        [repo_restorer, wiki_restorer, project_tree, avatar_restorer,
+         uploads_restorer, lfs_restorer, statistics_restorer]
+      end
+```
+
+The export service, is similar to the `Importer`, restoring data instead of saving it.
+
+### Export
+
+```ruby
+# ImportExport::ExportService
+module Projects
+  module ImportExport
+    class ExportService < BaseService
+
+      def save_all!
+        if save_services
+          Gitlab::ImportExport::Saver.save(project: project, shared: @shared)
+          notify_success
+        else
+          cleanup_and_notify_error!
+        end
+      end
+
+      def save_services
+        [version_saver, avatar_saver, project_tree_saver, uploads_saver, repo_saver, 
+           wiki_repo_saver, lfs_saver].all?(&:save)
+      end
+```