Don't write to persisted query cache until execution will begin.

Since we already write to the persisted query cache asynchronously (and
intentionally do not await the Promise) this won't have any affect on the
current behavior of when the persisted query cache is written to in the
event of an executable request, but if an operation comes in and doesn't
parse or validate, we'll avoid wasting cache space on an operation that will
never execute.

Additionally, in a similar light, if a plugin throws an error which stops
execution, we can avoid the side-effect of writing to the persisted query
cache.

In terms of the APQ behavior, while this could cause additional round-trips
for a client which repeatedly sends an invalid operation, that operation is
never going to successfully finish anyway.  While developer tooling will
help avoid this problem in the first place, this refusal to store an invalid
operation in the APQ cache could actually help diagnose a failure since the
full operation (rather than just the SHA256 of that document) will appear
in the browser's dev-tools on the retry.

If we're looking to spare parsing and validating documents which we know are
going to fail, I think that's going to be a better use of the (new)
`documentStore` cache (#2111), since its in-memory and can accommodate a
more complex data structure which includes the validation errors from the
previous attempt which will, of course, be the same when validating the same
operation against the same schema again.  As the PR that introduced that
feature shows, sparing those additional parses and validations (cached APQ
documents still needs to be parsed and validated, currently) will provide a
more successful performance win overall.

Ref: #2111

CHANGELOG.md

@@ -3,6 +3,7 @@
 ### vNEXT
 
 - Switch from `json-stable-stringify` to `fast-json-stable-stringify`. [PR #2065](https://github.com/apollographql/apollo-server/pull/2065)
+- Don't write to the persisted query cache until execution will begin. [PR #2227](https://github.com/apollographql/apollo-server/pull/2227)
 
 ### v2.3.1
 

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

@@ -44,6 +44,7 @@ import {
 } from 'apollo-server-plugin-base';
 
 import { Dispatcher } from './utils/dispatcher';
+import { KeyValueCache } from 'apollo-server-caching';
 
 export {
   GraphQLRequest,
@@ -102,6 +103,7 @@ export async function processGraphQLRequest<TContext>(
 
   let queryHash: string;
 
+  let persistedQueryCache: KeyValueCache | undefined;
   let persistedQueryHit = false;
   let persistedQueryRegister = false;
 
@@ -116,10 +118,14 @@ export async function processGraphQLRequest<TContext>(
       );
     }
 
+    // We'll store a reference to the persisted query cache so we can actually
+    // do the write at a later point in the request pipeline processing.
+    persistedQueryCache = config.persistedQueries.cache;
+
     queryHash = extensions.persistedQuery.sha256Hash;
 
     if (query === undefined) {
-      query = await config.persistedQueries.cache.get(`apq:${queryHash}`);
+      query = await persistedQueryCache.get(`apq:${queryHash}`);
       if (query) {
         persistedQueryHit = true;
       } else {
@@ -134,11 +140,11 @@ export async function processGraphQLRequest<TContext>(
         );
       }
 
+      // We won't write to the persisted query cache until later.
+      // Defering the writing gives plugins the ability to "win" from use of
+      // the cache, but also have their say in whether or not the cache is
+      // written to (by interrupting the request with an error).
       persistedQueryRegister = true;
-
-      Promise.resolve(
-        config.persistedQueries.cache.set(`apq:${queryHash}`, query),
-      ).catch(console.warn);
     }
   } else if (query) {
     // FIXME: We'll compute the APQ query hash to use as our cache key for
@@ -212,6 +218,16 @@ export async function processGraphQLRequest<TContext>(
       >,
     );
 
+    // Now that we've gone through the pre-execution phases of the request
+    // pipeline, and given plugins appropriate ability to object (by throwing
+    // an error) and not actually write, we'll write to the cache if it was
+    // determined earlier in the request pipeline that we should do so.
+    if (persistedQueryRegister && persistedQueryCache) {
+      Promise.resolve(persistedQueryCache.set(`apq:${queryHash}`, query)).catch(
+        console.warn,
+      );
+    }
+
     const executionDidEnd = await dispatcher.invokeDidStartHook(
       'executionDidStart',
       requestContext as WithRequired<