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author | GitLab Bot <gitlab-bot@gitlab.com> | 2020-02-04 21:08:50 +0300 |
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committer | GitLab Bot <gitlab-bot@gitlab.com> | 2020-02-04 21:08:50 +0300 |
commit | ca05512007cea51e05d3431b2c8bd7228c754370 (patch) | |
tree | 5202d429acd68c071445aff9e352379173ec9c0b /doc/development/reactive_caching.md | |
parent | 6b833f1e0340e00fdee074da9c42c0d4e07a46d2 (diff) |
Add latest changes from gitlab-org/gitlab@master
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diff --git a/doc/development/reactive_caching.md b/doc/development/reactive_caching.md new file mode 100644 index 00000000000..de93a5aa1d0 --- /dev/null +++ b/doc/development/reactive_caching.md @@ -0,0 +1,276 @@ +# `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`. |