Generic (Golang 1.18) version of the generator https://github.com/vektah/dataloaden

dataloader.go

package storage

import (
	"sync"
	"time"
)

// LoaderConfig captures the config to create a new UserLoader
type LoaderConfig[Key comparable, Result any] struct {
	// Fetch is a method that provides the data for the loader
	Fetch func(keys []Key) ([]*Result, []error)

	// Wait is how long wait before sending a batch
	Wait time.Duration

	// MaxBatch will limit the maximum number of keys to send in one batch, 0 = not limit
	MaxBatch int
}

// NewLoader creates a new Loader given a fetch, wait, and maxBatch
func NewLoader[Key comparable, Result any](config LoaderConfig[Key, Result]) *Loader[Key, Result] {
	return &Loader[Key, Result]{
		fetch:    config.Fetch,
		wait:     config.Wait,
		maxBatch: config.MaxBatch,
	}
}

// Loader batches and caches requests
type Loader[Key comparable, Result any] struct {
	// this method provides the data for the loader
	fetch func(keys []Key) ([]*Result, []error)

	// how long to done before sending a batch
	wait time.Duration

	// this will limit the maximum number of keys to send in one batch, 0 = no limit
	maxBatch int

	// INTERNAL

	// lazily created cache
	cache map[Key]*Result

	// the current batch. keys will continue to be collected until timeout is hit,
	// then everything will be sent to the fetch method and out to the listeners
	batch *loaderBatch[Key, Result]

	// mutex to prevent races
	mu sync.Mutex
}

type loaderBatch[Key comparable, Result any] struct {
	keys    []Key
	data    []*Result
	error   []error
	closing bool
	done    chan struct{}
}

// Load a data by key, batching and caching will be applied automatically
func (l *Loader[Key, Result]) Load(key Key) (*Result, error) {
	return l.LoadThunk(key)()
}

// LoadThunk returns a function that when called will block waiting for a User.
// This method should be used if you want one goroutine to make requests to many
// different data loaders without blocking until the thunk is called.
func (l *Loader[Key, Result]) LoadThunk(key Key) func() (*Result, error) {
	l.mu.Lock()
	if it, ok := l.cache[key]; ok {
		l.mu.Unlock()
		return func() (*Result, error) {
			return it, nil
		}
	}
	if l.batch == nil {
		l.batch = &loaderBatch[Key, Result]{done: make(chan struct{})}
	}
	batch := l.batch
	pos := batch.keyIndex(l, key)
	l.mu.Unlock()

	return func() (*Result, error) {
		<-batch.done

		var data *Result
		if pos < len(batch.data) {
			data = batch.data[pos]
		}

		var err error
		// its convenient to be able to return a single error for everything
		if len(batch.error) == 1 {
			err = batch.error[0]
		} else if batch.error != nil {
			err = batch.error[pos]
		}

		if err == nil {
			l.mu.Lock()
			l.unsafeSet(key, data)
			l.mu.Unlock()
		}

		return data, err
	}
}

// LoadAll fetches many keys at once. It will be broken into appropriate sized
// sub batches depending on how the loader is configured
func (l *Loader[Key, Result]) LoadAll(keys []Key) ([]*Result, []error) {
	results := make([]func() (*Result, error), len(keys))

	for i, key := range keys {
		results[i] = l.LoadThunk(key)
	}

	users := make([]*Result, len(keys))
	errors := make([]error, len(keys))
	for i, thunk := range results {
		users[i], errors[i] = thunk()
	}
	return users, errors
}

// LoadAllThunk returns a function that when called will block waiting for a Users.
// This method should be used if you want one goroutine to make requests to many
// different data loaders without blocking until the thunk is called.
func (l *Loader[Key, Result]) LoadAllThunk(keys []Key) func() ([]*Result, []error) {
	results := make([]func() (*Result, error), len(keys))
	for i, key := range keys {
		results[i] = l.LoadThunk(key)
	}
	return func() ([]*Result, []error) {
		users := make([]*Result, len(keys))
		errors := make([]error, len(keys))
		for i, thunk := range results {
			users[i], errors[i] = thunk()
		}
		return users, errors
	}
}

// Prime the cache with the provided key and value. If the key already exists, no change is made
// and false is returned.
// (To forcefully prime the cache, clear the key first with loader.clear(key).prime(key, value).)
func (l *Loader[Key, Result]) Prime(key Key, value *Result) bool {
	l.mu.Lock()
	var found bool
	if _, found = l.cache[key]; !found {
		// make a copy when writing to the cache, its easy to pass a pointer in from a loop var
		// and end up with the whole cache pointing to the same value.
		cpy := *value
		l.unsafeSet(key, &cpy)
	}
	l.mu.Unlock()
	return !found
}

// Clear the value at key from the cache, if it exists
func (l *Loader[Key, Result]) Clear(key Key) {
	l.mu.Lock()
	delete(l.cache, key)
	l.mu.Unlock()
}

func (l *Loader[Key, Result]) unsafeSet(key Key, value *Result) {
	if l.cache == nil {
		l.cache = map[Key]*Result{}
	}
	l.cache[key] = value
}

// keyIndex will return the location of the key in the batch, if its not found
// it will add the key to the batch
func (b *loaderBatch[Key, Result]) keyIndex(l *Loader[Key, Result], key Key) int {
	for i, existingKey := range b.keys {
		if key == existingKey {
			return i
		}
	}

	pos := len(b.keys)
	b.keys = append(b.keys, key)
	if pos == 0 {
		go b.startTimer(l)
	}

	if l.maxBatch != 0 && pos >= l.maxBatch-1 {
		if !b.closing {
			b.closing = true
			l.batch = nil
			go b.end(l)
		}
	}

	return pos
}

func (b *loaderBatch[Key, Result]) startTimer(l *Loader[Key, Result]) {
	time.Sleep(l.wait)
	l.mu.Lock()

	// we must have hit a batch limit and are already finalizing this batch
	if b.closing {
		l.mu.Unlock()
		return
	}

	l.batch = nil
	l.mu.Unlock()

	b.end(l)
}

func (b *loaderBatch[Key, Result]) end(l *Loader[Key, Result]) {
	b.data, b.error = l.fetch(b.keys)
	close(b.done)
}

dataloader_test.go

package storage

import (
	"fmt"
	"strings"
	"sync"
	"testing"
	"time"

	"github.com/stretchr/testify/assert"
	"github.com/stretchr/testify/require"
)

// User is some kind of database backed model
type User struct {
	ID   string
	Name string
}

// NewUserLoader will collect user requests for 2 milliseconds and send them as a single batch to the fetch func
// normally fetch would be a database call.
func NewUserLoader() *Loader[string, User] {
	return NewLoader(LoaderConfig[string, User]{
		Fetch: func(keys []string) ([]*User, []error) {
			users := make([]*User, len(keys))
			errors := make([]error, len(keys))

			for i, key := range keys {
				users[i] = &User{ID: key, Name: "user " + key}
			}
			return users, errors
		},
		Wait:     2 * time.Millisecond,
		MaxBatch: 100,
	})
}

func TestUserLoader(t *testing.T) {
	var fetches [][]string
	var mu sync.Mutex

	dl := NewLoader(LoaderConfig[string, User]{
		Fetch: func(keys []string) ([]*User, []error) {
			mu.Lock()
			fetches = append(fetches, keys)
			mu.Unlock()

			users := make([]*User, len(keys))
			errors := make([]error, len(keys))

			for i, key := range keys {
				if strings.HasPrefix(key, "E") {
					errors[i] = fmt.Errorf("user not found")
				} else {
					users[i] = &User{ID: key, Name: "user " + key}
				}
			}
			return users, errors
		},
		Wait:     10 * time.Millisecond,
		MaxBatch: 5,
	})

	t.Run("fetch concurrent data", func(t *testing.T) {
		t.Run("load user successfully", func(t *testing.T) {
			t.Parallel()
			u, err := dl.Load("U1")
			require.NoError(t, err)
			require.Equal(t, u.ID, "U1")
		})

		t.Run("load failed user", func(t *testing.T) {
			t.Parallel()
			u, err := dl.Load("E1")
			require.Error(t, err)
			require.Nil(t, u)
		})

		t.Run("load many users", func(t *testing.T) {
			t.Parallel()
			u, err := dl.LoadAll([]string{"U2", "E2", "E3", "U4"})
			require.Equal(t, u[0].Name, "user U2")
			require.Equal(t, u[3].Name, "user U4")
			require.Error(t, err[1])
			require.Error(t, err[2])
		})

		t.Run("load thunk", func(t *testing.T) {
			t.Parallel()
			thunk1 := dl.LoadThunk("U5")
			thunk2 := dl.LoadThunk("E5")

			u1, err1 := thunk1()
			require.NoError(t, err1)
			require.Equal(t, "user U5", u1.Name)

			u2, err2 := thunk2()
			require.Error(t, err2)
			require.Nil(t, u2)
		})
	})

	t.Run("it sent two batches", func(t *testing.T) {
		mu.Lock()
		defer mu.Unlock()

		require.Len(t, fetches, 2)
		assert.Len(t, fetches[0], 5)
		assert.Len(t, fetches[1], 3)
	})

	t.Run("fetch more", func(t *testing.T) {
		t.Run("previously cached", func(t *testing.T) {
			t.Parallel()
			u, err := dl.Load("U1")
			require.NoError(t, err)
			require.Equal(t, u.ID, "U1")
		})

		t.Run("load many users", func(t *testing.T) {
			t.Parallel()
			u, err := dl.LoadAll([]string{"U2", "U4"})
			require.NoError(t, err[0])
			require.NoError(t, err[1])
			require.Equal(t, u[0].Name, "user U2")
			require.Equal(t, u[1].Name, "user U4")
		})
	})

	t.Run("no round trips", func(t *testing.T) {
		mu.Lock()
		defer mu.Unlock()

		require.Len(t, fetches, 2)
	})

	t.Run("fetch partial", func(t *testing.T) {
		t.Run("errors not in cache cache value", func(t *testing.T) {
			t.Parallel()
			u, err := dl.Load("E2")
			require.Nil(t, u)
			require.Error(t, err)
		})

		t.Run("load all", func(t *testing.T) {
			t.Parallel()
			u, err := dl.LoadAll([]string{"U1", "U4", "E1", "U9", "U5"})
			require.Equal(t, u[0].ID, "U1")
			require.Equal(t, u[1].ID, "U4")
			require.Error(t, err[2])
			require.Equal(t, u[3].ID, "U9")
			require.Equal(t, u[4].ID, "U5")
		})
	})

	t.Run("one partial trip", func(t *testing.T) {
		mu.Lock()
		defer mu.Unlock()

		require.Len(t, fetches, 3)
		require.Len(t, fetches[2], 3) // E1 U9 E2 in some random order
	})

	t.Run("primed reads dont hit the fetcher", func(t *testing.T) {
		dl.Prime("U99", &User{ID: "U99", Name: "Primed user"})
		u, err := dl.Load("U99")
		require.NoError(t, err)
		require.Equal(t, "Primed user", u.Name)

		require.Len(t, fetches, 3)
	})

	t.Run("priming in a loop is safe", func(t *testing.T) {
		users := []User{
			{ID: "Alpha", Name: "Alpha"},
			{ID: "Omega", Name: "Omega"},
		}
		for _, user := range users {
			dl.Prime(user.ID, &user)
		}

		u, err := dl.Load("Alpha")
		require.NoError(t, err)
		require.Equal(t, "Alpha", u.Name)

		u, err = dl.Load("Omega")
		require.NoError(t, err)
		require.Equal(t, "Omega", u.Name)

		require.Len(t, fetches, 3)
	})

	t.Run("cleared results will go back to the fetcher", func(t *testing.T) {
		dl.Clear("U99")
		u, err := dl.Load("U99")
		require.NoError(t, err)
		require.Equal(t, "user U99", u.Name)

		require.Len(t, fetches, 4)
	})

	t.Run("load all thunk", func(t *testing.T) {
		thunk1 := dl.LoadAllThunk([]string{"U5", "U6"})
		thunk2 := dl.LoadAllThunk([]string{"U6", "E6"})

		users1, err1 := thunk1()

		require.NoError(t, err1[0])
		require.NoError(t, err1[1])
		require.Equal(t, "user U5", users1[0].Name)
		require.Equal(t, "user U6", users1[1].Name)

		users2, err2 := thunk2()

		require.NoError(t, err2[0])
		require.Error(t, err2[1])
		require.Equal(t, "user U6", users2[0].Name)
	})
}