mirror of
https://github.com/graphql-python/graphene.git
synced 2024-11-25 19:13:57 +03:00
5cb7d91aaa
* Revise documentation - Add missing reference to `flask-graphql` in integrations - align documentation for resolver arguments (use root for 1st argument instead of self) - explore use of `parent` instead of `root` for first argument - clarify resolvers and object type documentation - add documentation for Meta class options for ObjectType - expand quickstart documentation for first time users - streamline order of documentation for first time users (broad -> specific) - document resolver quirks * explict imports from graphene * rename doc refs for resolvers * suggestions typos and graphene import
119 lines
3.6 KiB
ReStructuredText
119 lines
3.6 KiB
ReStructuredText
Dataloader
|
|
==========
|
|
|
|
DataLoader is a generic utility to be used as part of your application's
|
|
data fetching layer to provide a simplified and consistent API over
|
|
various remote data sources such as databases or web services via batching
|
|
and caching.
|
|
|
|
|
|
Batching
|
|
--------
|
|
|
|
Batching is not an advanced feature, it's DataLoader's primary feature.
|
|
Create loaders by providing a batch loading function.
|
|
|
|
.. code:: python
|
|
|
|
from promise import Promise
|
|
from promise.dataloader import DataLoader
|
|
|
|
class UserLoader(DataLoader):
|
|
def batch_load_fn(self, keys):
|
|
# Here we return a promise that will result on the
|
|
# corresponding user for each key in keys
|
|
return Promise.resolve([get_user(id=key) for key in keys])
|
|
|
|
|
|
A batch loading function accepts a list of keys, and returns a ``Promise``
|
|
which resolves to a list of ``values``.
|
|
|
|
Then load individual values from the loader. ``DataLoader`` will coalesce all
|
|
individual loads which occur within a single frame of execution (executed once
|
|
the wrapping promise is resolved) and then call your batch function with all
|
|
requested keys.
|
|
|
|
|
|
.. code:: python
|
|
|
|
user_loader = UserLoader()
|
|
|
|
user_loader.load(1).then(lambda user: user_loader.load(user.best_friend_id))
|
|
|
|
user_loader.load(2).then(lambda user: user_loader.load(user.best_friend_id))
|
|
|
|
|
|
A naive application may have issued *four* round-trips to a backend for the
|
|
required information, but with ``DataLoader`` this application will make at most *two*.
|
|
|
|
Note that loaded values are one-to-one with the keys and must have the same
|
|
order. This means that if you load all values from a single query, you must
|
|
make sure that you then order the query result for the results to match the keys:
|
|
|
|
|
|
.. code:: python
|
|
|
|
class UserLoader(DataLoader):
|
|
def batch_load_fn(self, keys):
|
|
users = {user.id: user for user in User.objects.filter(id__in=keys)}
|
|
return Promise.resolve([users.get(user_id) for user_id in keys])
|
|
|
|
|
|
``DataLoader`` allows you to decouple unrelated parts of your application without
|
|
sacrificing the performance of batch data-loading. While the loader presents
|
|
an API that loads individual values, all concurrent requests will be coalesced
|
|
and presented to your batch loading function. This allows your application to
|
|
safely distribute data fetching requirements throughout your application and
|
|
maintain minimal outgoing data requests.
|
|
|
|
|
|
|
|
Using with Graphene
|
|
-------------------
|
|
|
|
DataLoader pairs nicely well with Graphene/GraphQL. GraphQL fields are designed
|
|
to be stand-alone functions. Without a caching or batching mechanism, it's easy
|
|
for a naive GraphQL server to issue new database requests each time a field is resolved.
|
|
|
|
Consider the following GraphQL request:
|
|
|
|
|
|
.. code::
|
|
|
|
{
|
|
me {
|
|
name
|
|
bestFriend {
|
|
name
|
|
}
|
|
friends(first: 5) {
|
|
name
|
|
bestFriend {
|
|
name
|
|
}
|
|
}
|
|
}
|
|
}
|
|
|
|
|
|
Naively, if ``me``, ``bestFriend`` and ``friends`` each need to request the backend,
|
|
there could be at most 13 database requests!
|
|
|
|
|
|
When using DataLoader, we could define the User type using our previous example with
|
|
leaner code and at most 4 database requests, and possibly fewer if there are cache hits.
|
|
|
|
|
|
.. code:: python
|
|
|
|
class User(graphene.ObjectType):
|
|
name = graphene.String()
|
|
best_friend = graphene.Field(lambda: User)
|
|
friends = graphene.List(lambda: User)
|
|
|
|
def resolve_best_friend(root, info):
|
|
return user_loader.load(root.best_friend_id)
|
|
|
|
def resolve_friends(root, info):
|
|
return user_loader.load_many(root.friend_ids)
|