Move ReactiveCaching developer docs

Move the ReactiveCaching developer docs to its own file since it is
quite large.

doc/development/reactive_caching.md

+# `ReactiveCaching`
+
+> This doc refers to <https://gitlab.com/gitlab-org/gitlab/blob/master/app/models/concerns/reactive_caching.rb>.
+
+The `ReactiveCaching` concern is used for fetching some data in the background and store it
+in the Rails cache, keeping it up-to-date for as long as it is being requested.  If the
+data hasn't been requested for `reactive_cache_lifetime`, it will stop being refreshed,
+and then be removed.
+
+## Examples
+
+```ruby
+class Foo < ApplicationRecord
+  include ReactiveCaching
+
+  after_save :clear_reactive_cache!
+
+  def calculate_reactive_cache(param1, param2)
+    # Expensive operation here. The return value of this method is cached
+  end
+
+  def result
+    # Any arguments can be passed to `with_reactive_cache`. `calculate_reactive_cache`
+    # will be called with the same arguments.
+    with_reactive_cache(param1, param2) do |data|
+      # ...
+    end
+  end
+end
+```
+
+In this example, the first time `#result` is called, it will return `nil`. However,
+it will enqueue a background worker to call `#calculate_reactive_cache` and set an
+initial cache lifetime of 10 min.
+
+## How it works
+
+The first time `#with_reactive_cache` is called, a background job is enqueued and
+`with_reactive_cache` returns `nil`. The background job calls `#calculate_reactive_cache`
+and stores its return value. It also re-enqueues the background job to run again after
+`reactive_cache_refresh_interval`. Therefore, it will keep the stored value up to date.
+Calculations never run concurrently.
+
+Calling `#with_reactive_cache` while a value is cached will call the block given to
+`#with_reactive_cache`, yielding the cached value. It will also extend the lifetime
+of the cache by the `reactive_cache_lifetime` value.
+
+Once the lifetime has expired, no more background jobs will be enqueued and calling
+`#with_reactive_cache` will again return `nil` - starting the process all over again.
+
+## When to use
+
+- If we need to make a request to an external API (for example, requests to the k8s API).
+It is not advisable to keep the application server worker blocked for the duration of
+the external request.
+- If a model needs to perform a lot of database calls or other time consuming
+calculations.
+
+## How to use
+
+### In models and services
+
+The ReactiveCaching concern can be used in models as well as `project_services`
+(`app/models/project_services`).
+
+1. Include the concern in your model or service.
+
+   When including in a model:
+
+   ```ruby
+   include ReactiveCaching
+   ```
+
+   or when including in a `project_service`:
+
+   ```ruby
+   include ReactiveService
+   ```
+
+1. Implement the `calculate_reactive_cache` method in your model/service.
+1. Call `with_reactive_cache` in your model/service where the cached value is needed.
+
+### In controllers
+
+Controller endpoints that call a model or service method that uses `ReactiveCaching` should
+not wait until the background worker completes.
+
+- An API that calls a model or service method that uses `ReactiveCaching` should return
+`202 accepted` when the cache is being calculated (when `#with_reactive_cache` returns `nil`).
+- It should also
+[set the polling interval header](fe_guide/performance.md#realtime-components) with
+`Gitlab::PollingInterval.set_header`.
+- The consumer of the API is expected to poll the API.
+- You can also consider implementing [ETag caching](polling.md) to reduce the server
+load caused by polling.
+
+### Methods to implement in a model or service
+
+These are methods that should be implemented in the model/service that includes `ReactiveCaching`.
+
+#### `#calculate_reactive_cache` (required)
+
+- This method must be implemented. Its return value will be cached.
+- It will be called by `ReactiveCaching` when it needs to populate the cache.
+- Any arguments passed to `with_reactive_cache` will also be passed to `calculate_reactive_cache`.
+
+#### `#reactive_cache_updated` (optional)
+
+- This method can be implemented if needed.
+- It is called by the `ReactiveCaching` concern whenever the cache is updated.
+If the cache is being refreshed and the new cache value is the same as the old cache
+value, this method will not be called. It is only called if a new value is stored in
+the cache.
+- It can be used to perform an action whenever the cache is updated.
+
+### Methods called by a model or service
+
+These are methods provided by `ReactiveCaching` and should be called in
+the model/service.
+
+#### `#with_reactive_cache` (required)
+
+- `with_reactive_cache` must be called where the result of `calculate_reactive_cache`
+is required.
+- A block can be given to `with_reactive_cache`. `with_reactive_cache` can also take
+any number of arguments. Any arguments passed to `with_reactive_cache` will be
+passed to `calculate_reactive_cache`. The arguments passed to `with_reactive_cache`
+will be appended to the cache key name.
+- If `with_reactive_cache` is called when the result has already been cached, the
+block will be called, yielding the cached value and the return value of the block
+will be returned by `with_reactive_cache`. It will also reset the timeout of the
+cache to the `reactive_cache_lifetime` value.
+- If the result has not been cached as yet, `with_reactive_cache` will return nil.
+It will also enqueue a background job, which will call `calculate_reactive_cache`
+and cache the result.
+- Once the background job has completed and the result is cached, the next call
+to `with_reactive_cache` will pick up the cached value.
+- In the example below, `data` is the cached value which is yielded to the block
+given to `with_reactive_cache`.
+
+  ```ruby
+  class Foo < ApplicationRecord
+    include ReactiveCaching
+
+    def calculate_reactive_cache(param1, param2)
+      # Expensive operation here. The return value of this method is cached
+    end
+
+    def result
+      with_reactive_cache(param1, param2) do |data|
+        # ...
+      end
+    end
+  end
+  ```
+
+#### `#clear_reactive_cache!` (optional)
+
+- This method can be called when the cache needs to be expired/cleared. For example,
+it can be called in an `after_save` callback in a model so that the cache is
+cleared after the model is modified.
+- This method should be called with the same parameters that are passed to
+`with_reactive_cache` because the parameters are part of the cache key.
+
+#### `#without_reactive_cache` (optional)
+
+- This is a convenience method that can be used for debugging purposes.
+- This method calls `calculate_reactive_cache` in the current process instead of
+in a background worker.
+
+### Configurable options
+
+There are some `class_attribute` options which can be tweaked.
+
+#### `self.reactive_cache_key`
+
+- The value of this attribute is the prefix to the `data` and `alive` cache key names.
+The parameters passed to `with_reactive_cache` form the rest of the cache key names.
+- By default, this key uses the model's name and the ID of the record.
+
+  ```ruby
+  self.reactive_cache_key = -> (record) { [model_name.singular, record.id] }
+  ```
+
+- The `data` and `alive` cache keys in this case will be `"ExampleModel:1:arg1:arg2"`
+and `"ExampleModel:1:arg1:arg2:alive"` respectively, where `ExampleModel` is the
+name of the model, `1` is the ID of the record, `arg1` and `arg2` are parameters
+passed to `with_reactive_cache`.
+- If you're including this concern in a service instead, you will need to override
+the default by adding the following to your service:
+
+  ```ruby
+  self.reactive_cache_key = ->(service) { [service.class.model_name.singular, service.project_id] }
+  ```
+
+  If your reactive_cache_key is exactly like the above, you can use the existing
+  `ReactiveService` concern instead.
+
+#### `self.reactive_cache_lease_timeout`
+
+- `ReactiveCaching` uses `Gitlab::ExclusiveLease` to ensure that the cache calculation
+is never run concurrently by multiple workers.
+- This attribute is the timeout for the `Gitlab::ExclusiveLease`.
+- It defaults to 2 minutes, but can be overriden if a different timeout is required.
+
+```ruby
+self.reactive_cache_lease_timeout = 2.minutes
+```
+
+#### `self.reactive_cache_refresh_interval`
+
+- This is the interval at which the cache is refreshed.
+- It defaults to 1 minute.
+
+```ruby
+self.reactive_cache_lease_timeout = 1.minute
+```
+
+#### `self.reactive_cache_lifetime`
+
+- This is the duration after which the cache will be cleared if there are no requests.
+- The default is 10 minutes. If there are no requests for this cache value for 10 minutes,
+the cache will expire.
+- If the cache value is requested before it expires, the timeout of the cache will
+be reset to `reactive_cache_lifetime`.
+
+```ruby
+self.reactive_cache_lifetime = 10.minutes
+```
+
+#### `self.reactive_cache_worker_finder`
+
+- This is the method used by the background worker to find or generate the object on
+which `calculate_reactive_cache` can be called.
+- By default it uses the model primary key to find the object:
+
+  ```ruby
+  self.reactive_cache_worker_finder = ->(id, *_args) do
+    find_by(primary_key => id)
+  end
+  ```
+
+- The default behaviour can be overridden by defining a custom `reactive_cache_worker_finder`.
+
+  ```ruby
+  class Foo < ApplicationRecord
+    include ReactiveCaching
+
+    self.reactive_cache_worker_finder = ->(_id, *args) { from_cache(*args) }
+
+    def self.from_cache(var1, var2)
+      # This method will be called by the background worker with "bar1" and
+      # "bar2" as arguments.
+      new(var1, var2)
+    end
+
+    def initialize(var1, var2)
+      # ...
+    end
+
+    def calculate_reactive_cache(var1, var2)
+      # Expensive operation here. The return value of this method is cached
+    end
+
+    def result
+      with_reactive_cache("bar1", "bar2") do |data|
+        # ...
+      end
+    end
+  end
+  ```
+
+  - In this example, the primary key ID will be passed to `reactive_cache_worker_finder`
+  along with the parameters passed to `with_reactive_cache`.
+  - The custom `reactive_cache_worker_finder` calls `.from_cache` with the parameters
+  passed to `with_reactive_cache`.