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https://github.com/exaspark/batch-loader
:zap: Powerful tool for avoiding N+1 DB or HTTP queries
https://github.com/exaspark/batch-loader
batching dataloader gem graphql graphql-ruby n-plus-1 nplus1 ruby
Last synced: 12 days ago
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:zap: Powerful tool for avoiding N+1 DB or HTTP queries
- Host: GitHub
- URL: https://github.com/exaspark/batch-loader
- Owner: exAspArk
- License: mit
- Created: 2017-07-31T14:59:54.000Z (over 7 years ago)
- Default Branch: main
- Last Pushed: 2024-04-24T15:23:08.000Z (7 months ago)
- Last Synced: 2024-05-17T16:49:23.814Z (6 months ago)
- Topics: batching, dataloader, gem, graphql, graphql-ruby, n-plus-1, nplus1, ruby
- Language: Ruby
- Homepage: https://engineering.universe.com/batching-a-powerful-way-to-solve-n-1-queries-every-rubyist-should-know-24e20c6e7b94
- Size: 275 KB
- Stars: 1,021
- Watchers: 13
- Forks: 51
- Open Issues: 11
-
Metadata Files:
- Readme: README.md
- Changelog: CHANGELOG.md
- License: LICENSE.txt
- Code of conduct: CODE_OF_CONDUCT.md
Awesome Lists containing this project
- awesome-ruby - batch-loader
README
# BatchLoader
[](https://coveralls.io/github/exAspArk/batch-loader)
[](https://codeclimate.com/github/exAspArk/batch-loader/maintainability)
[](https://rubygems.org/gems/batch-loader)
[](https://rubygems.org/gems/batch-loader)
This gem provides a generic lazy batching mechanism to avoid N+1 DB queries, HTTP queries, etc.
Developers from these companies use `BatchLoader`:
## Contents
* [Highlights](#highlights)
* [Usage](#usage)
* [Why?](#why)
* [Basic example](#basic-example)
* [How it works](#how-it-works)
* [RESTful API example](#restful-api-example)
* [GraphQL example](#graphql-example)
* [Loading multiple items](#loading-multiple-items)
* [Batch key](#batch-key)
* [Caching](#caching)
* [Replacing methods](#replacing-methods)
* [Installation](#installation)
* [API](#api)
* [Related tools](#related-tools)
* [Implementation details](#implementation-details)
* [Development](#development)
* [Contributing](#contributing)
* [Alternatives](#alternatives)
* [License](#license)
* [Code of Conduct](#code-of-conduct)
## Highlights
* Generic utility to avoid N+1 DB queries, HTTP requests, etc.
* Adapted Ruby implementation of battle-tested tools like [Haskell Haxl](https://github.com/facebook/Haxl), [JS DataLoader](https://github.com/facebook/dataloader), etc.
* Batching is isolated and lazy, load data in batch where and when it's needed.
* Automatically caches previous queries (identity map).
* Thread-safe (`loader`).
* No need to share batching through variables or custom defined classes.
* No dependencies, no monkey-patches, no extra primitives such as Promises.
## Usage
### Why?
Let's have a look at the code with N+1 queries:
```ruby
def load_posts(ids)
Post.where(id: ids)
end
posts = load_posts([1, 2, 3]) # Posts SELECT * FROM posts WHERE id IN (1, 2, 3)
# _ ↓ _
# ↙ ↓ ↘
users = posts.map do |post| # U ↓ ↓ SELECT * FROM users WHERE id = 1
post.user # ↓ U ↓ SELECT * FROM users WHERE id = 2
end # ↓ ↓ U SELECT * FROM users WHERE id = 3
# ↘ ↓ ↙
# ¯ ↓ ¯
puts users # Users
```
The naive approach would be to preload dependent objects on the top level:
```ruby
# With ORM in basic cases
def load_posts(ids)
Post.where(id: ids).includes(:user)
end
# But without ORM or in more complicated cases you will have to do something like:
def load_posts(ids)
# load posts
posts = Post.where(id: ids)
user_ids = posts.map(&:user_id)
# load users
users = User.where(id: user_ids)
user_by_id = users.each_with_object({}) { |user, memo| memo[user.id] = user }
# map user to post
posts.each { |post| post.user = user_by_id[post.user_id] }
end
posts = load_posts([1, 2, 3]) # Posts SELECT * FROM posts WHERE id IN (1, 2, 3)
# _ ↓ _ SELECT * FROM users WHERE id IN (1, 2, 3)
# ↙ ↓ ↘
users = posts.map do |post| # U ↓ ↓
post.user # ↓ U ↓
end # ↓ ↓ U
# ↘ ↓ ↙
# ¯ ↓ ¯
puts users # Users
```
But the problem here is that `load_posts` now depends on the child association and knows that it has to preload data for future use. And it'll do it every time, even if it's not necessary. Can we do better? Sure!
### Basic example
With `BatchLoader` we can rewrite the code above:
```ruby
def load_posts(ids)
Post.where(id: ids)
end
def load_user(post)
BatchLoader.for(post.user_id).batch do |user_ids, loader|
User.where(id: user_ids).each { |user| loader.call(user.id, user) }
end
end
posts = load_posts([1, 2, 3]) # Posts SELECT * FROM posts WHERE id IN (1, 2, 3)
# _ ↓ _
# ↙ ↓ ↘
users = posts.map do |post| # BL ↓ ↓
load_user(post) # ↓ BL ↓
end # ↓ ↓ BL
# ↘ ↓ ↙
# ¯ ↓ ¯
puts users # Users SELECT * FROM users WHERE id IN (1, 2, 3)
```
As we can see, batching is isolated and described right in a place where it's needed.
### How it works
In general, `BatchLoader` returns a lazy object. Each lazy object knows which data it needs to load and how to batch the query. As soon as you need to use the lazy objects, they will be automatically loaded once without N+1 queries.
So, when we call `BatchLoader.for` we pass an item (`user_id`) which should be collected and used for batching later. For the `batch` method, we pass a block which will use all the collected items (`user_ids`):
BatchLoader.for(post.user_id).batch do |user_ids, loader|
...
end
Inside the block we execute a batch query for our items (`User.where`). After that, all we have to do is to call `loader` by passing an item which was used in `BatchLoader.for` method (`user_id`) and the loaded object itself (`user`):
BatchLoader.for(post.user_id).batch do |user_ids, loader|
User.where(id: user_ids).each { |user| loader.call(user.id, user) }
end
When we call any method on the lazy object, it'll be automatically loaded through batching for all instantiated `BatchLoader`s:
puts users # => SELECT * FROM users WHERE id IN (1, 2, 3)
For more information, see the [Implementation details](#implementation-details) section.
### RESTful API example
Now imagine we have a regular Rails app with N+1 HTTP requests:
```ruby
# app/models/post.rb
class Post < ApplicationRecord
def rating
HttpClient.request(:get, "https://example.com/ratings/#{id}")
end
end
# app/controllers/posts_controller.rb
class PostsController < ApplicationController
def index
posts = Post.limit(10)
serialized_posts = posts.map { |post| {id: post.id, rating: post.rating} } # N+1 HTTP requests for each post.rating
render json: serialized_posts
end
end
```
As we can see, the code above will make N+1 HTTP requests, one for each post. Let's batch the requests with a gem called [parallel](https://github.com/grosser/parallel):
```ruby
class Post < ApplicationRecord
def rating_lazy
BatchLoader.for(post).batch do |posts, loader|
Parallel.each(posts, in_threads: 10) { |post| loader.call(post, post.rating) }
end
end
# ...
end
```
`loader` is thread-safe. So, if `HttpClient` is also thread-safe, then with `parallel` gem we can execute all HTTP requests concurrently in threads (there are some benchmarks for [concurrent HTTP requests](https://github.com/exAspArk/concurrent_http_requests) in Ruby). Thanks to Matz, MRI releases GIL when thread hits blocking I/O – HTTP request in our case.
In the controller, all we have to do is to replace `post.rating` with the lazy `post.rating_lazy`:
```ruby
class PostsController < ApplicationController
def index
posts = Post.limit(10)
serialized_posts = posts.map { |post| {id: post.id, rating: post.rating_lazy} }
render json: serialized_posts
end
end
```
`BatchLoader` caches the loaded values. To ensure that the cache is purged between requests in the app add the following middleware to your `config/application.rb`:
```ruby
config.middleware.use BatchLoader::Middleware
```
See the [Caching](#caching) section for more information.
### GraphQL example
Batching is particularly useful with GraphQL. Using such techniques as preloading data in advance to avoid N+1 queries can be very complicated, since a user can ask for any available fields in a query.
Let's take a look at the simple [graphql-ruby](https://github.com/rmosolgo/graphql-ruby) schema example:
```ruby
class MyProjectSchema < GraphQL::Schema
query Types::QueryType
end
module Types
class QueryType < Types::BaseObject
field :posts, [PostType], null: false
def posts
Post.all
end
end
end
module Types
class PostType < Types::BaseObject
name "Post"
field :user, UserType, null: false
def user
object.user # N+1 queries
end
end
end
module Types
class UserType < Types::BaseObject
name "User"
field :name, String, null: false
end
end
```
If we want to execute a simple query like the following, we will get N+1 queries for each `post.user`:
```ruby
query = "
{
posts {
user {
name
}
}
}
"
MyProjectSchema.execute(query)
```
To avoid this problem, all we have to do is to change the resolver to return `BatchLoader::GraphQL` ([#32](https://github.com/exAspArk/batch-loader/pull/32) explains why not just `BatchLoader`):
```ruby
module Types
class PostType < Types::BaseObject
name "Post"
field :user, UserType, null: false
def user
BatchLoader::GraphQL.for(object.user_id).batch do |user_ids, loader|
User.where(id: user_ids).each { |user| loader.call(user.id, user) }
end
end
end
end
```
And setup GraphQL to use the built-in `lazy_resolve` method:
```ruby
class MyProjectSchema < GraphQL::Schema
query Types::QueryType
use BatchLoader::GraphQL
end
```
---
If you need to use BatchLoader with ActiveRecord in multiple places, you can use this `preload:` helper shared by [Aha!](https://www.aha.io/engineering/articles/automatically-avoiding-graphql-n-1s):
```rb
field :user, UserType, null: false, preload: :user
# ^^^^^^^^^^^^^^
# Simply add this instead of defining custom `user` method with BatchLoader
```
And add this custom field resolver that uses ActiveRecord's preload functionality with BatchLoader:
```rb
# app/graphql/types/base_object.rb
field_class Types::PreloadableField
# app/graphql/types/preloadable_field.rb
class Types::PreloadableField < Types::BaseField
def initialize(*args, preload: nil, **kwargs, &block)
@preloads = preload
super(*args, **kwargs, &block)
end
def resolve(type, args, ctx)
return super unless @preloads
BatchLoader::GraphQL.for(type).batch(key: self) do |records, loader|
ActiveRecord::Associations::Preloader.new(records: records.map(&:object), associations: @preloads).call
records.each { |r| loader.call(r, super(r, args, ctx)) }
end
end
end
```
### Loading multiple items
For batches where there is no item in response to a call, we normally return `nil`. However, you can use `:default_value` to return something else instead:
```ruby
BatchLoader.for(post.user_id).batch(default_value: NullUser.new) do |user_ids, loader|
User.where(id: user_ids).each { |user| loader.call(user.id, user) }
end
```
For batches where the value is some kind of collection, such as an Array or Hash, `loader` also supports being called with a block, which yields the _current_ value, and returns the _next_ value. This is extremely useful for 1:Many (`has_many`) relationships:
```ruby
BatchLoader.for(user.id).batch(default_value: []) do |user_ids, loader|
Comment.where(user_id: user_ids).each do |comment|
loader.call(comment.user_id) { |memo| memo << comment }
end
end
```
### Batch key
It's possible to reuse the same `BatchLoader#batch` block for loading different types of data by specifying a unique `key`.
For example, with polymorphic associations:
```ruby
def lazy_association(post)
id = post.association_id
key = post.association_type
BatchLoader.for(id).batch(key: key) do |ids, loader, args|
model = Object.const_get(args[:key])
model.where(id: ids).each { |record| loader.call(record.id, record) }
end
end
post1 = Post.save(association_id: 1, association_type: 'Tag')
post2 = Post.save(association_id: 1, association_type: 'Category')
lazy_association(post1) # SELECT * FROM tags WHERE id IN (1)
lazy_association(post2) # SELECT * FROM categories WHERE id IN (1)
```
It's also required to pass custom `key` when using `BatchLoader` with metaprogramming (e.g. `eval`).
### Caching
By default `BatchLoader` caches the loaded values. You can test it by running something like:
```ruby
def user_lazy(id)
BatchLoader.for(id).batch do |ids, loader|
User.where(id: ids).each { |user| loader.call(user.id, user) }
end
end
puts user_lazy(1) # SELECT * FROM users WHERE id IN (1)
# => <#User:...>
puts user_lazy(1) # no request
# => <#User:...>
```
Usually, it's just enough to clear the cache between HTTP requests in the app. To do so, simply add the middleware:
```ruby
use BatchLoader::Middleware
```
To drop the cache manually you can run:
```ruby
puts user_lazy(1) # SELECT * FROM users WHERE id IN (1)
puts user_lazy(1) # no request
BatchLoader::Executor.clear_current
puts user_lazy(1) # SELECT * FROM users WHERE id IN (1)
```
In some rare cases it's useful to disable caching for `BatchLoader`. For example, in tests or after data mutations:
```ruby
def user_lazy(id)
BatchLoader.for(id).batch(cache: false) do |ids, loader|
# ...
end
end
puts user_lazy(1) # SELECT * FROM users WHERE id IN (1)
puts user_lazy(1) # SELECT * FROM users WHERE id IN (1)
```
If you set `cache: false`, it's likely you also want `replace_methods: false` (see below section).
### Replacing methods
By default, `BatchLoader` replaces methods on its instance by calling `#define_method` after batching to copy methods from the loaded value.
This consumes some time but allows to speed up any future method calls on the instance.
In some cases, when there are a lot of instances with a huge number of defined methods, this initial process of replacing the methods can be slow.
You may consider avoiding the "up front payment" and "pay as you go" with `#method_missing` by disabling the method replacement:
```ruby
BatchLoader.for(id).batch(replace_methods: false) do |ids, loader|
# ...
end
```
## Installation
Add this line to your application's Gemfile:
```ruby
gem 'batch-loader'
```
And then execute:
$ bundle
Or install it yourself as:
$ gem install batch-loader
## API
```ruby
BatchLoader.for(item).batch(
default_value: default_value,
cache: cache,
replace_methods: replace_methods,
key: key
) do |items, loader, args|
# ...
end
```
| Argument Key | Default | Description |
| --------------- | --------------------------------------------- | ------------------------------------------------------------- |
| `item` | - | Item which will be collected and used for batching. |
| `default_value` | `nil` | Value returned by default after batching. |
| `cache` | `true` | Set `false` to disable caching between the same executions. |
| `replace_methods` | `true` | Set `false` to use `#method_missing` instead of replacing the methods after batching. |
| `key` | `nil` | Pass custom key to uniquely identify the batch block. |
| `items` | - | List of collected items for batching. |
| `loader` | - | Lambda which should be called to load values loaded in batch. |
| `args` | `{default_value: nil, cache: true, replace_methods: true, key: nil}` | Arguments passed to the `batch` method. |
## Related tools
These gems are built by using `BatchLoader`:
* [decidim-core](https://github.com/decidim/decidim/) – participatory democracy framework made with Ruby on Rails.
* [ams_lazy_relationships](https://github.com/Bajena/ams_lazy_relationships/) – ActiveModel Serializers add-on for eliminating N+1 queries.
* [batch-loader-active-record](https://github.com/mathieul/batch-loader-active-record/) – ActiveRecord lazy association generator to avoid N+1 DB queries.
`BatchLoader` in other programming languages:
* [batch_loader](https://github.com/exaspark/batch_loader) - Elixir implementation.
## Implementation details
See the [slides](https://speakerdeck.com/exaspark/batching-a-powerful-way-to-solve-n-plus-1-queries) [37-42].
## Development
After checking out the repo, run `bin/setup` to install dependencies. Then, run `rake spec` to run the tests. You can also run `bin/console` for an interactive prompt that will allow you to experiment.
To install this gem onto your local machine, run `bundle exec rake install`. To release a new version, update the version number in `version.rb`, and then run `bundle exec rake release`, which will create a git tag for the version, push git commits and tags, and push the `.gem` file to [rubygems.org](https://rubygems.org).
## Contributing
Bug reports and pull requests are welcome on GitHub at https://github.com/exAspArk/batch-loader. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the [Contributor Covenant](http://contributor-covenant.org) code of conduct.
## Alternatives
There are some other Ruby implementations for batching such as:
* [shopify/graphql-batch](https://github.com/shopify/graphql-batch)
* [sheerun/dataloader](https://github.com/sheerun/dataloader)
However, `batch-loader` has some differences:
* It is implemented for general usage and can be used not only with GraphQL. In fact, we use it for RESTful APIs and GraphQL on production at the same time.
* It doesn't try to mimic implementations in other programming languages which have an asynchronous nature. So, it doesn't load extra dependencies to bring such primitives as Promises, which are not very popular in Ruby community.
Instead, it uses the idea of lazy objects, which are included in the [Ruby standard library](https://ruby-doc.org/core-2.4.1/Enumerable.html#method-i-lazy). These lazy objects allow one to return the necessary data at the end when it's necessary.
* It doesn't force you to share batching through variables or custom defined classes, just pass a block to the `batch` method.
* It doesn't require to return an array of the loaded objects in the same order as the passed items. I find it difficult to satisfy these constraints: to sort the loaded objects and add `nil` values for the missing ones. Instead, it provides the `loader` lambda which simply maps an item to the loaded object.
* It doesn't depend on any other external dependencies. For example, no need to load huge external libraries for thread-safety, the gem is thread-safe out of the box.
## License
The gem is available as open source under the terms of the [MIT License](http://opensource.org/licenses/MIT).
## Code of Conduct
Everyone interacting in the Batch::Loader project’s codebases, issue trackers, chat rooms and mailing lists is expected to follow the [code of conduct](https://github.com/exAspArk/batch-loader/blob/master/CODE_OF_CONDUCT.md).