Shift the burden of object approximation into the ApolloServerBase class.

The implementation of object-size approximation which is used for cache
eviction purposes in the `InMemoryLRUCache` implementation (via `lru-cache`)
was a short-term location for extensible logic which is better located
within `ApolloServerBase`.

This is particularly important since future logic may necessitate knowing or
understanding the current size (roughly, memory usage) of the in-memory
storage.  Effective immediately, this adds support for providing a `dispose`
function which is called when an object is purged from the cache to make
room for another.

packages/apollo-server-caching/src/InMemoryLRUCache.ts

@@ -1,32 +1,33 @@
 import LRU from 'lru-cache';
 import { KeyValueCache } from './KeyValueCache';
 
+function defaultLengthCalculation(item: any) {
+  if (Array.isArray(item) || typeof item === 'string') {
+    return item.length;
+  }
+
+  // Go with the lru-cache default "naive" size, in lieu anything better:
+  //   https://github.com/isaacs/node-lru-cache/blob/a71be6cd/index.js#L17
+  return 1;
+}
+
 export class InMemoryLRUCache<V = string> implements KeyValueCache<V> {
   private store: LRU.Cache<string, V>;
 
   // FIXME: Define reasonable default max size of the cache
-  constructor({ maxSize = Infinity }: { maxSize?: number } = {}) {
+  constructor({
+    maxSize = Infinity,
+    sizeCalculator = defaultLengthCalculation,
+    onDispose,
+  }: {
+    maxSize?: number;
+    sizeCalculator?: (value: V, key: string) => number;
+    onDispose?: (key: string, value: V) => void;
+  } = {}) {
     this.store = new LRU({
       max: maxSize,
-      length(item) {
-        if (Array.isArray(item) || typeof item === 'string') {
-          return item.length;
-        }
-
-        // If it's an object, we'll use JSON.stringify+Buffer.byteLength to
-        // approximate the size of what it would take to store.  It's certainly
-        // not 100% accurate, but it should be a fast implementation which
-        // doesn't require bringing in other dependencies or logic which we need
-        // to maintain.  In the future, we might consider something like:
-        // npm.im/object-sizeof, but this should be sufficient for now.
-        if (typeof item === 'object') {
-          return Buffer.byteLength(JSON.stringify(item), 'utf8');
-        }
-
-        // Go with the lru-cache default "naive" size, in lieu anything better:
-        //   https://github.com/isaacs/node-lru-cache/blob/a71be6cd/index.js#L17
-        return 1;
-      },
+      length: sizeCalculator,
+      dispose: onDispose,
     });
   }
 
@@ -43,4 +44,7 @@ export class InMemoryLRUCache<V = string> implements KeyValueCache<V> {
   async flush(): Promise<void> {
     this.store.reset();
   }
+  async getTotalSize() {
+    return this.store.length;
+  }
 }

packages/apollo-server-core/src/ApolloServer.ts

@@ -93,6 +93,10 @@ function getEngineServiceId(engine: Config['engine']): string | undefined {
 const forbidUploadsForTesting =
   process && process.env.NODE_ENV === 'test' && !supportsUploadsInNode;
 
+function approximateObjectSize<T>(obj: T): number {
+  return Buffer.byteLength(JSON.stringify(obj), 'utf8');
+}
+
 export class ApolloServerBase {
   public subscriptionsPath?: string;
   public graphqlPath: string = '/graphql';
@@ -496,13 +500,14 @@ export class ApolloServerBase {
   }
 
   private initializeDocumentStore(): void {
-    this.documentStore = new InMemoryLRUCache({
+    this.documentStore = new InMemoryLRUCache<DocumentNode>({
       // Create ~about~ a 30MiB InMemoryLRUCache.  This is less than precise
       // since the technique to calculate the size of a DocumentNode is
       // only using JSON.stringify on the DocumentNode (and thus doesn't account
       // for unicode characters, etc.), but it should do a reasonable job at
       // providing a caching document store for most operations.
       maxSize: Math.pow(2, 20) * 30,
+      sizeCalculator: approximateObjectSize,
     });
   }
 

packages/apollo-server-core/src/__tests__/runQuery.test.ts

@@ -573,7 +573,10 @@ describe('runQuery', () => {
         approximateObjectSize(parse(querySmall1)) +
         approximateObjectSize(parse(querySmall2));
 
-      const documentStore = new InMemoryLRUCache<DocumentNode>({ maxSize });
+      const documentStore = new InMemoryLRUCache<DocumentNode>({
+        maxSize,
+        sizeCalculator: approximateObjectSize,
+      });
 
       await runRequest({ plugins, documentStore, queryString: querySmall1 });
       expect(parsingDidStart.mock.calls.length).toBe(1);