Added BatchLoaderScheduler into the mix

Author: bbakerman

README.md

@@ -510,7 +510,50 @@ and there are also gains to this different mode of operation:
 However, with batch execution control comes responsibility! If you forget to make the call to `dispatch()` then the futures
 in the load request queue will never be batched, and thus _will never complete_! So be careful when crafting your loader designs.
 
-## Scheduled Dispatching
+## The BatchLoader Scheduler
+
+By default, when `dataLoader.dispatch()` is called the `BatchLoader` / `MappedBatchLoader` function will be invoked
+immediately.  
+
+However, you can provide your own `BatchLoaderScheduler` that allows this call to be done some time into
+the future.  
+
+You will be passed a callback (`ScheduledBatchLoaderCall` / `ScheduledMapBatchLoaderCall`) and you are expected
+to eventually call this callback method to make the batch loading happen.
+
+The following is a `BatchLoaderScheduler` that waits 10 milliseconds before invoking the batch loading functions.
+
+```java
+        new BatchLoaderScheduler() {
+
+            @Override
+            public <K, V> CompletionStage<List<V>> scheduleBatchLoader(ScheduledBatchLoaderCall<V> scheduledCall, List<K> keys, BatchLoaderEnvironment environment) {
+                return CompletableFuture.supplyAsync(() -> {
+                    snooze(10);
+                    return scheduledCall.invoke();
+                }).thenCompose(Function.identity());
+            }
+
+            @Override
+            public <K, V> CompletionStage<Map<K, V>> scheduleMappedBatchLoader(ScheduledMappedBatchLoaderCall<K, V> scheduledCall, List<K> keys, BatchLoaderEnvironment environment) {
+                return CompletableFuture.supplyAsync(() -> {
+                    snooze(10);
+                    return scheduledCall.invoke();
+                }).thenCompose(Function.identity());
+            }
+        };
+```
+
+You are given the keys to be loaded and an optional `BatchLoaderEnvironment` for informative purposes.  You can't change the list of 
+keys that will be loaded via this mechanism say.
+
+Also note, because there is a max batch size, it is possible for this scheduling to happen N times for a given `dispatch()`
+call.  The total set of keys will be sliced into batches themselves and then the `BatchLoaderScheduler` will be called for
+each batch of keys.  
+
+Do not assume that a single call to `dispatch()` results in a single call to `BatchLoaderScheduler`.
+
+## Scheduled Registry Dispatching
 
 `ScheduledDataLoaderRegistry` is a registry that allows for dispatching to be done on a schedule. It contains a
 predicate that is evaluated (per data loader contained within) when `dispatchAll` is invoked.

src/main/java/org/dataloader/DataLoaderHelper.java

@@ -3,6 +3,7 @@
 import org.dataloader.annotations.GuardedBy;
 import org.dataloader.annotations.Internal;
 import org.dataloader.impl.CompletableFutureKit;
+import org.dataloader.scheduler.BatchLoaderScheduler;
 import org.dataloader.stats.StatisticsCollector;
 import org.dataloader.stats.context.IncrementBatchLoadCountByStatisticsContext;
 import org.dataloader.stats.context.IncrementBatchLoadExceptionCountStatisticsContext;
@@ -417,10 +418,23 @@ CompletableFuture<List<V>> invokeLoader(List<K> keys, List<Object> keyContexts)
     @SuppressWarnings("unchecked")
     private CompletableFuture<List<V>> invokeListBatchLoader(List<K> keys, BatchLoaderEnvironment environment) {
         CompletionStage<List<V>> loadResult;
+        BatchLoaderScheduler batchLoaderScheduler = loaderOptions.getBatchLoaderScheduler();
         if (batchLoadFunction instanceof BatchLoaderWithContext) {
-            loadResult = ((BatchLoaderWithContext<K, V>) batchLoadFunction).load(keys, environment);
+            BatchLoaderWithContext<K, V> loadFunction = (BatchLoaderWithContext<K, V>) batchLoadFunction;
+            if (batchLoaderScheduler != null) {
+                BatchLoaderScheduler.ScheduledBatchLoaderCall<V> loadCall = () -> loadFunction.load(keys, environment);
+                loadResult = batchLoaderScheduler.scheduleBatchLoader(loadCall, keys, environment);
+            } else {
+                loadResult = loadFunction.load(keys, environment);
+            }
         } else {
-            loadResult = ((BatchLoader<K, V>) batchLoadFunction).load(keys);
+            BatchLoader<K, V> loadFunction = (BatchLoader<K, V>) batchLoadFunction;
+            if (batchLoaderScheduler != null) {
+                BatchLoaderScheduler.ScheduledBatchLoaderCall<V> loadCall = () -> loadFunction.load(keys);
+                loadResult = batchLoaderScheduler.scheduleBatchLoader(loadCall, keys, null);
+            } else {
+                loadResult = loadFunction.load(keys);
+            }
         }
         return nonNull(loadResult, () -> "Your batch loader function MUST return a non null CompletionStage").toCompletableFuture();
     }
@@ -434,10 +448,23 @@ private CompletableFuture<List<V>> invokeListBatchLoader(List<K> keys, BatchLoad
     private CompletableFuture<List<V>> invokeMapBatchLoader(List<K> keys, BatchLoaderEnvironment environment) {
         CompletionStage<Map<K, V>> loadResult;
         Set<K> setOfKeys = new LinkedHashSet<>(keys);
+        BatchLoaderScheduler batchLoaderScheduler = loaderOptions.getBatchLoaderScheduler();
         if (batchLoadFunction instanceof MappedBatchLoaderWithContext) {
-            loadResult = ((MappedBatchLoaderWithContext<K, V>) batchLoadFunction).load(setOfKeys, environment);
+            MappedBatchLoaderWithContext<K, V> loadFunction = (MappedBatchLoaderWithContext<K, V>) batchLoadFunction;
+            if (batchLoaderScheduler != null) {
+                BatchLoaderScheduler.ScheduledMappedBatchLoaderCall<K, V> loadCall = () -> loadFunction.load(setOfKeys, environment);
+                loadResult = batchLoaderScheduler.scheduleMappedBatchLoader(loadCall, keys, environment);
+            } else {
+                loadResult = loadFunction.load(setOfKeys, environment);
+            }
         } else {
-            loadResult = ((MappedBatchLoader<K, V>) batchLoadFunction).load(setOfKeys);
+            MappedBatchLoader<K, V> loadFunction = (MappedBatchLoader<K, V>) batchLoadFunction;
+            if (batchLoaderScheduler != null) {
+                BatchLoaderScheduler.ScheduledMappedBatchLoaderCall<K, V> loadCall = () -> loadFunction.load(setOfKeys);
+                loadResult = batchLoaderScheduler.scheduleMappedBatchLoader(loadCall, keys, null);
+            } else {
+                loadResult = loadFunction.load(setOfKeys);
+            }
         }
         CompletableFuture<Map<K, V>> mapBatchLoad = nonNull(loadResult, () -> "Your batch loader function MUST return a non null CompletionStage").toCompletableFuture();
         return mapBatchLoad.thenApply(map -> {

src/main/java/org/dataloader/DataLoaderOptions.java

@@ -18,6 +18,7 @@
 
 import org.dataloader.annotations.PublicApi;
 import org.dataloader.impl.Assertions;
+import org.dataloader.scheduler.BatchLoaderScheduler;
 import org.dataloader.stats.NoOpStatisticsCollector;
 import org.dataloader.stats.StatisticsCollector;
 
@@ -46,6 +47,7 @@ public class DataLoaderOptions {
     private Supplier<StatisticsCollector> statisticsCollector;
     private BatchLoaderContextProvider environmentProvider;
     private ValueCacheOptions valueCacheOptions;
+    private BatchLoaderScheduler batchLoaderScheduler;
 
     /**
      * Creates a new data loader options with default settings.
@@ -58,6 +60,7 @@ public DataLoaderOptions() {
         statisticsCollector = NoOpStatisticsCollector::new;
         environmentProvider = NULL_PROVIDER;
         valueCacheOptions = ValueCacheOptions.newOptions();
+        batchLoaderScheduler = null;
     }
 
     /**
@@ -77,6 +80,7 @@ public DataLoaderOptions(DataLoaderOptions other) {
         this.statisticsCollector = other.statisticsCollector;
         this.environmentProvider = other.environmentProvider;
         this.valueCacheOptions = other.valueCacheOptions;
+        batchLoaderScheduler = other.batchLoaderScheduler;
     }
 
     /**
@@ -304,4 +308,24 @@ public DataLoaderOptions setValueCacheOptions(ValueCacheOptions valueCacheOption
         this.valueCacheOptions = Assertions.nonNull(valueCacheOptions);
         return this;
     }
+
+    /**
+     * @return the {@link BatchLoaderScheduler} to use, which can be null
+     */
+    public BatchLoaderScheduler getBatchLoaderScheduler() {
+        return batchLoaderScheduler;
+    }
+
+    /**
+     * Sets in a new {@link BatchLoaderScheduler} that allows the call to a {@link BatchLoader} function to be scheduled
+     * to some future time.
+     *
+     * @param batchLoaderScheduler the scheduler
+     *
+     * @return the data loader options for fluent coding
+     */
+    public DataLoaderOptions setBatchLoaderScheduler(BatchLoaderScheduler batchLoaderScheduler) {
+        this.batchLoaderScheduler = batchLoaderScheduler;
+        return this;
+    }
 }

src/main/java/org/dataloader/scheduler/BatchLoaderScheduler.java

@@ -0,0 +1,74 @@
+package org.dataloader.scheduler;
+
+import org.dataloader.BatchLoader;
+import org.dataloader.BatchLoaderEnvironment;
+import org.dataloader.DataLoader;
+import org.dataloader.DataLoaderOptions;
+import org.dataloader.MappedBatchLoader;
+
+import java.util.List;
+import java.util.Map;
+import java.util.concurrent.CompletionStage;
+
+/**
+ * By default, when  {@link DataLoader#dispatch()} is called the {@link BatchLoader} / {@link MappedBatchLoader} function will be invoked
+ * immediately.  However, you can provide your own {@link BatchLoaderScheduler} that allows this call to be done some time into
+ * the future.  You will be passed a callback ({@link ScheduledBatchLoaderCall} / {@link ScheduledMappedBatchLoaderCall} and you are expected
+ * to eventually call this callback method to make the batch loading happen.
+ * <p>
+ * Note: Because there is a {@link DataLoaderOptions#maxBatchSize()} it is possible for this scheduling to happen N times for a given {@link DataLoader#dispatch()}
+ * call.  The total set of keys will be sliced into batches themselves and then the {@link BatchLoaderScheduler} will be called for
+ * each batch of keys.  Do not assume that a single call to {@link DataLoader#dispatch()} results in a single call to {@link BatchLoaderScheduler}.
+ */
+public interface BatchLoaderScheduler {
+
+
+    /**
+     * This represents a callback that will invoke a {@link BatchLoader} function under the covers
+     *
+     * @param <V> the value type
+     */
+    interface ScheduledBatchLoaderCall<V> {
+        CompletionStage<List<V>> invoke();
+    }
+
+    /**
+     * This represents a callback that will invoke a {@link MappedBatchLoader} function under the covers
+     *
+     * @param <K> the key type
+     * @param <V> the value type
+     */
+    interface ScheduledMappedBatchLoaderCall<K, V> {
+        CompletionStage<Map<K, V>> invoke();
+    }
+
+    /**
+     * This is called to schedule a {@link BatchLoader} call.
+     *
+     * @param scheduledCall the callback that needs to be invoked to allow the {@link BatchLoader} to proceed.
+     * @param keys          this is the list of keys that will be passed to the {@link BatchLoader}.
+     *                      This is provided only for informative reasons and you cant change the keys that are used
+     * @param environment   this is the {@link BatchLoaderEnvironment} in place,
+     *                      which can be null if it's a simple {@link BatchLoader} call
+     * @param <K>           the key type
+     * @param <V>           the value type
+     *
+     * @return a promise to the values that come from the {@link BatchLoader}
+     */
+    <K, V> CompletionStage<List<V>> scheduleBatchLoader(ScheduledBatchLoaderCall<V> scheduledCall, List<K> keys, BatchLoaderEnvironment environment);
+
+    /**
+     * This is called to schedule a {@link MappedBatchLoader} call.
+     *
+     * @param scheduledCall the callback that needs to be invoked to allow the {@link MappedBatchLoader} to proceed.
+     * @param keys          this is the list of keys that will be passed to the {@link MappedBatchLoader}.
+     *                      This is provided only for informative reasons and you cant change the keys that are used
+     * @param environment   this is the {@link BatchLoaderEnvironment} in place,
+     *                      which can be null if it's a simple {@link MappedBatchLoader} call
+     * @param <K>           the key type
+     * @param <V>           the value type
+     *
+     * @return a promise to the values that come from the {@link BatchLoader}
+     */
+    <K, V> CompletionStage<Map<K, V>> scheduleMappedBatchLoader(ScheduledMappedBatchLoaderCall<K, V> scheduledCall, List<K> keys, BatchLoaderEnvironment environment);
+}

src/test/java/org/dataloader/fixtures/TestKit.java

@@ -1,13 +1,19 @@
 package org.dataloader.fixtures;
 
 import org.dataloader.BatchLoader;
+import org.dataloader.BatchLoaderWithContext;
 import org.dataloader.DataLoader;
 import org.dataloader.DataLoaderFactory;
 import org.dataloader.DataLoaderOptions;
+import org.dataloader.MappedBatchLoader;
+import org.dataloader.MappedBatchLoaderWithContext;
 
 import java.util.ArrayList;
 import java.util.Collection;
+import java.util.HashMap;
 import java.util.List;
+import java.util.Map;
+import java.util.Set;
 import java.util.concurrent.CompletableFuture;
 
 import static java.util.stream.Collectors.toList;
@@ -19,6 +25,27 @@ public static <T> BatchLoader<T, T> keysAsValues() {
         return CompletableFuture::completedFuture;
     }
 
+    public static <T> BatchLoaderWithContext<T, T> keysAsValuesWithContext() {
+        return (keys, env) -> CompletableFuture.completedFuture(keys);
+    }
+
+    public static <K, V> MappedBatchLoader<K, V> keysAsMapOfValues() {
+        return keys -> mapOfKeys(keys);
+    }
+
+    public static <K, V> MappedBatchLoaderWithContext<K, V> keysAsMapOfValuesWithContext() {
+        return (keys, env) -> mapOfKeys(keys);
+    }
+
+    private static <K, V> CompletableFuture<Map<K, V>> mapOfKeys(Set<K> keys) {
+        Map<K, V> map = new HashMap<>();
+        for (K key : keys) {
+            //noinspection unchecked
+            map.put(key, (V) key);
+        }
+        return CompletableFuture.completedFuture(map);
+    }
+
     public static <K, V> BatchLoader<K, V> keysAsValues(List<List<K>> loadCalls) {
         return keys -> {
             List<K> ks = new ArrayList<>(keys);