dataloader v1.0.6 Dataloader View Source


Dataloader provides an easy way efficiently load data in batches. It’s inspired by, although it makes some small API changes to better suit Elixir use cases.

Central to Dataloader is the idea of a source. A single Dataloader struct can have many different sources, which represent different ways to load data.

Here’s an example of a data loader using an ecto source, and then loading some organization data.

source =

# setup the loader
loader = |> Dataloader.add_source(:db, source)

# load some things
loader =
  |> Dataloader.load(:db, Organization, 1)
  |> Dataloader.load_many(:db, Organization, [4, 9])

# actually retrieve them
loader =

# Now we can get whatever values out we want
organizations = Dataloader.get_many(loader, :db, Organization, [1,4])

This will do a single SQL query to get all organizations by ids 1,4, and 9. You can load multiple batches from multiple sources, and then when run/1 is called batch will be loaded concurrently.

Here we named the source :db within our dataloader. More commonly though if you’re using Phoenix you’ll want to name it after one of your contexts, and have a different source used for each context. This provides an easy way to enforce data access rules within each context. See the DataLoader.Ecto moduledocs for more details


There are two configuration options:

  • timeout - The maximum timeout to wait for running a source, defaults to 1s more than the maximum timeout of all added sources. Set with care, timeouts should really only be set on sources.
  • get_policy - This configures how the dataloader will behave when fetching data which may have errored when we tried to load it.

These can be set as part of the new/1 call. So, for example, to configure a dataloader that returns nil on error with a 5s timeout:

loader =
    get_policy: :return_nil_on_error,
    timeout: :timer.seconds(5)


There are three implemented behaviours for this:

  • raise_on_error (default)- If successful, calling get/4 or get_many/4 will return the value. If there was an exception when trying to load any of the data, it will raise that exception
  • return_nil_on_error - Behaves similar to raise_on_error but will just return nil instead of raising. It will still log errors
  • tuples - This will return {:ok, value}/{:error, reason} tuples depending on a successful or failed load, allowing for more fine-grained error handling if required

Link to this section Summary


This is a helper method to run a set of async tasks in a separate supervision tree which

Create a new Dataloader instance

This function is depreacted in favour of async_safely/3

This helper function will call fun on all items asynchronously, returning a map of :ok/:error tuples, keyed off the items. For example

Link to this section Types

Link to this type option() View Source
option() :: {:timeout, pos_integer()} | {:get_policy, atom()}
Link to this type source_name() View Source
source_name() :: any()
Link to this type t() View Source
t() :: %Dataloader{
  options: [option()],
  sources: %{optional(source_name()) => Dataloader.Source.t()}

Link to this section Functions

Link to this function add_source(loader, name, source) View Source
add_source(t(), source_name(), Dataloader.Source.t()) :: t()
Link to this function async_safely(mod, fun, args \\ []) View Source
async_safely(module(), atom(), list()) :: any()

This is a helper method to run a set of async tasks in a separate supervision tree which:

  1. Is run by a supervisor linked to the main process. This ensures any async tasks will get killed if the main process is killed.
  2. Spawns a separate task which traps exits for running the provided function. This ensures we will always have some output, but are not setting :trap_exit on the main process.

NOTE: The provided fun must accept a Task.Supervisor as its first argument, as this function will prepend the relevant supervisor to args

See run_tasks/4 for an example of a fun implementation, this will return whatever that returns.

Link to this function get(loader, source, batch_key, item_key) View Source
get(t(), source_name(), any(), any()) :: any() | no_return()
Link to this function get_many(loader, source, batch_key, item_keys) View Source
get_many(t(), source_name(), any(), any()) ::
  [any()] | {:ok, [any()]} | no_return()
Link to this function load(loader, source_name, batch_key, val) View Source
load(t(), source_name(), any(), any()) :: t() | no_return()
Link to this function load_many(loader, source_name, batch_key, vals) View Source
load_many(t(), source_name(), any(), [any()]) :: t() | no_return()
Link to this function new(opts \\ []) View Source
new([option()]) :: t()

Create a new Dataloader instance.

See moduledoc for available options

Link to this function pending_batches?(loader) View Source
pending_batches?(t()) :: boolean()
Link to this function pmap(items, fun, opts \\ []) View Source
pmap(list(), (... -> any()), keyword()) :: map()

This function is depreacted in favour of async_safely/3

This used to be used by both the Dataloader module for running multiple source queries concurrently, and the KV and Ecto sources to actually run separate batch fetches (e.g. for Posts and Users at the same time).

The problem was that the behaviour between the sources and the parent Dataloader was actually slightly different. The Dataloader-specific behaviour has been pulled out into run_tasks/4

Please use async_safely instead of this for fetching data from sources

Link to this function put(loader, source_name, batch_key, item_key, result) View Source
Link to this function run(dataloader) View Source
run(t()) :: t() | no_return()
Link to this function run_tasks(items, fun, opts \\ []) View Source
run_tasks(list(), (... -> any()), keyword()) :: map()

This helper function will call fun on all items asynchronously, returning a map of :ok/:error tuples, keyed off the items. For example:

iex> Dataloader.run_tasks([1,2,3], fn x -> x * x end, [])
  1 => {:ok, 1},
  2 => {:ok, 4},
  3 => {:ok, 9}

Similarly, for errors:

iex> Dataloader.run_tasks([1,2,3], fn _x -> Process.sleep(5) end, [timeout: 1])
  1 => {:error, :timeout},
  2 => {:error, :timeout},
  3 => {:error, :timeout}