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---
layout: guide
search: true
section: Dataloader
title: Manual Parallelism
desc: Yield to Dataloader after starting work
index: 7
---
You can coordinate with {{ "GraphQL::Dataloader" | api_doc }} to run tasks in the background. To do this, call `dataloader.yield` inside `Source#fetch` after kicking off your task. For example:
```ruby
def fetch(ids)
# somehow queue up a background query,
# see examples below
future_result = async_query_for(ids)
# return control to the dataloader
dataloader.yield
# dataloader will come back here
# after calling other sources,
# now wait for the value
future_result.value
end
```
_Alternatively, you can use {% internal_link "AsyncDataloader", "/dataloader/async_dataloader" %} to automatically background I/O inside `Source#fetch` calls._
## Example: Rails load_async
You can use Rails's `load_async` method to load `ActiveRecord::Relation`s in the background. For example:
```ruby
class Sources::AsyncRelationSource < GraphQL::Dataloader::Source
def fetch(relations)
relations.each(&:load_async) # start loading them in the background
dataloader.yield # hand back to GraphQL::Dataloader
relations.each(&:load) # now, wait for the result, returning the now-loaded relation
end
end
```
You could call that source from a GraphQL field method:
```ruby
field :direct_reports, [Person]
def direct_reports
# prepare an ActiveRecord::Relation:
direct_reports = Person.where(manager: object)
# pass it off to the source:
dataloader
.with(Sources::AsyncRelationSource)
.load(direct_reports)
end
```
## Example: Rails async calculations
In a Dataloader source, you can run Rails async calculations in the background while other work continues. For example:
```ruby
class Sources::DirectReportsCount < GraphQL::Dataloader::Source
def fetch(users)
# Start the queries in the background:
promises = users.map { |u| u.direct_reports.async_count }
# Return to GraphQL::Dataloader:
dataloader.yield
# Now return the results, waiting if necessary:
promises.map(&:value)
end
end
```
Which could be used in a GraphQL field:
```ruby
field :direct_reports_count, Int
def direct_reports_count
dataloader.with(Sources::DirectReportsCount).load(object)
end
```
## Example: Concurrent::Future
You could use `concurrent-ruby` to put work in a background thread. For example, using `Concurrent::Future`:
```ruby
class Sources::ExternalDataSource < GraphQL::Dataloader::Source
def fetch(urls)
# Start some I/O-intensive work:
futures = urls.map do |url|
Concurrent::Future.execute {
# Somehow fetch and parse data:
get_remote_json(url)
}
end
# Yield back to GraphQL::Dataloader:
dataloader.yield
# Dataloader has done what it can,
# so now return the value, waiting if necessary:
futures.map(&:value)
end
end
```
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