DataLoader - automatically batch and cache repeated data loads


 use DataLoader;
 my $user_loader = DataLoader->new(sub {
    my @user_ids = @_;
    return getUsers(@user_ids);  # a Mojo::Promise

 # Now fetch your data whenever (asynchronously)
 my $data = Mojo::Promise->all(

 # getUsers is called only once - with (1,2)


DataLoader is a generic utility to be used as part of your application's data fetching layer. It provides a consistent API over various backends and reduces requests to those backends via automatic batching and caching of data.

It is primarily useful for GraphQL APIs where each resolver independently requests the object(s) it wants, then this loader can ensure requests are batched together and not repeated multiple times.

It is a port of the JavaScript version available at


To get started, create a batch loading function that maps a list of keys (typically strings/integers) to a Mojo::Promise that returns a list of values.

 my $user_loader = DataLoader->new(\&myBatchGetUsers);

Then load individual values from the loader. All individual loads that occur within a single tick of the event loop will be batched together.

     ->then(fun($user) { $user_loader->load($user->invitedById) })
     ->then(fun($invitedBy) { say "User 1 was invited by ", $invitedBy->name });
 # Somewhere else in the application
     ->then(fun($user) { $user_loader->load($user->lastInvitedId) })
     ->then(fun($lastInvited) { say "User 2 last invited ", $lastInvited->name }); 

A naive application may have issued four round-trips to the backend for the required information, but with DataLoader this application will make at most two.

Batch function

The batch loading function takes a list of keys as input, and returns a Mojo::Promise that resolves to a list of values. The ordering of the values should correspond to the ordering of the keys, with any missing values filled in with undef. For example, if the input is (2,9,6,1) and the backend service (e.g. database) returns:

 { id => 9, name => 'Chicago' }
 { id => 1, name => 'New York' }
 { id => 2, name => 'San Francisco' }

The backend has returned results in a different order than we requested, and omitted a result for key 6, presumably because no value exists for that key.

We need to re-order these results to match the original input (2,9,6,1), and include an undef result for 6:

   { id => 2, name => 'San Francisco' },
   { id => 9, name => 'Chicago' },
   { id => 1, name => 'New York' },

There are two typical error cases in the batch loading function. One is you get an error that invalidates the whole batch, for example you do a DB query for all input rows, and the DB fails to connect. In this case, simply die and the error will be passed through to all callers that are waiting for values included in this batch. In this case, the error is assumed to be transient, and nothing will be cached.

The second case is where some of the batch succeeds but some fails. In this case, use DataLoader->error to create error objects, and mix them in with the successful values:

   { id => 2, name => 'San Francisco' },      # this succeeded
   DataLoader->error("no permission"),        # this failed (id 9)
   undef,                                     # this item is missing (id 6)
   { id => 1, name => 'New York' },           # this succeeded

Now callers that have called load->(9) will get an exception. Callers for id 6 will receive undef and callers for ids 1 and 2 will get hashrefs of data. Additionally, these errors will be cached (see 'Caching Errors' below).


DataLoader provides a simple memoization cache for all loads that occur within a single request for your application. Multiple loads for the same value result in only one backend request, and additionally, the same object in memory is returned each time, reducing memory use.

 my $user_loader = DataLoader->new(...);
 my $promise1a = $user_loader->load(1);
 my $promise1b = $user_loader->load(1);
 is( refaddr($promise1a), refaddr($promise1b) );   # same object

Caching per-Request

The suggested way to use DataLoader is to create a new loader when a request (for example GraphQL request) begins, and destroy it once the request ends. This prevents duplicate backend operations and provides a consistent view of data across the request.

Using the same loader for multiple requests is not recommended as it may result in cached data being returned unexpectedly, or sensitive data being leaked to other users who should not be able to view it.

The default cache used by DataLoader is a simple hashref that stores all values for all keys loaded during the lifetime of the request; it is useful when request lifetime is short. If other behaviour is desired, see the cache_hashref constructor parameter.

Clearing Cache

It is sometimes necessary to clear values from the cache, for example after running an SQL UPDATE or similar, to prevent out of date values from being used. This can be done with the clear method.

Caching Errors

If the batch load fails (throws an exception or returns a rejected Promise), the requested values will not be cached. However, if the batch function returns a DataLoader::Error instance for individual value(s), those errors will be cached to avoid frequently loading the same error.

If you want to avoid this, you can catch the Promise error and clear the cache immediately afterwards, e.g.

 $user_loader->load(1)->catch(fun ($error) {
    if ($should_clear_error) {
    die $error;   # or whatever

Priming the Cache

It is also possible to prime the cache with data. For example if you fetch a user by ID, you could also prime a username-based cache:

 $user_by_id->load(1)->then(fun ($user) {
    $user_by_name->prime($user->name, $user);

If your backend query includes additional data, you could cache that too:

 for my $tag (@{$user->tags}) {
    $tag_loader->prime($tag->id, $tag->name);

If you update a value in the backend, you can update the cache to save queries later:

 $user = $user->update(favourite_color => 'red');
 $user_cache->clear($user->id)->prime($user->id, $user);

Using outside of GraphQL

DataLoader assumes the use of Mojolicious, specifically its promise implementation Mojo::Promise. The Mojo::Reactor::EV backend is recommended (and is automatically used provided you have EV installed) for optimal batching, although other backends will also work.

With the EV backend, DataLoader will work fine with any AnyEvent-based code. See the unit tests of this module for examples.

It would be possible to write a version of DataLoader that depends only on AnyEvent/EV and does not depend on Mojolicious. Let me know if there is interest.


new ( batch_load_function, %options )

Creates a public API for loading data from a particular back-end with unique keys, such as the id column of an SQL table. You must provide a batch loading function (described above).

Each instance gets, by default, a unique memoized cache of all loads made during the lifetime of the object. Consider a different cache for long-lived applications, and consider a new instance per request if each request has users with different access permissions or where fresh data is desired for each request.


batch (true)

Set to false to disable batching: the batch load function will be invoked once for each key.

max_batch_size (Infinity)

If set, limit the maximum number of items to pass to the batch load function at once.

If unset (undef or missing), there will be no limit.

cache (true)

Set to false to disable caching, which will create a new Promise and new key in the batch load function for every load of the same key. (This means the batch loader function may be called with duplicate keys).

cache_key_func (identity function)

Maps a load key $_ to a cache key. Useful when using objects as keys and two different objects should be considered equivalent, or to handle case- insensitivity, etc.

For example: cache_key_func => sub { lc } for case-insensitive comparisons

Compare objects as long as their id is the same:

 ... cache_key_func => sub { $_->{id} }

Compare the content of objects:

 use Storable;
 ... cache_key_func => sub { thaw($_) }
cache_hashref ({})

Pass a custom hashref for caching. You can tie this hashref to any tie module to get custom behaviour (such as LRU). (CHI support will be considered if there is interest)

load ( key )

Loads a key, returning a Mojo::Promise for the value represented by that key.

load_many ( @keys )

Loads multiple keys, returning a Promise that resolves a list of values.

Equivalent to DataLoader->all(map { $loader->load($_) } @keys).

clear ( key )

Clear the value at key from the cache, if it exists. Returns itself for method chaining.

clear_all ()

Clears the entire cache. To be used when some event results in unknown invalidations across this particular DataLoader. Returns itself for method chaining.

prime ( key, value )

Primes the cache with the provided key and value. If the key already exists, no change is made. (To forcefully prime the cache, clear the key first with $loader->clear($key)->prime($key, $value).) Returns itself for method chaining.

If you want to prime an error value, use DataLoader->error($message) as the second argument.

DataLoader->error( @message )

Shorthand for DataLoader::Error->new(@message). Should be used by the batch loading function to indicate particular items of data that could not be loaded. The error will be propogated to the load caller(s) for the data. Can also be used with prime.

DataLoader->all( @promises )

Alternative to Mojo::Promise's all that assumes all promises return a single argument only, and will return a list of single return values for all promises, in the same order as the promises.

For example:

 Mojo::Promise->all( Mojo::Promise->resolve(1), Mojo::Promise->resolve(2) );

resolves to [[1], [2]], but:

 DataLoader->all( Mojo::Promise->resolve(1), Mojo::Promise->resolve(2) );

resolves to [1, 2].

Additionally, Mojo::Promise->all() will die with "unable to call 'clone' on undefined value" (or similar), while DataLoader->all() returns a Promise that resolves to the empty list.

Throws an exception if any promise passed as an argument resolves to a list of more than one return value.