Improve (and shorten) query polling implementation.

This implementation has the following benefits:

- It collapses the QueryScheduler abstraction into the QueryManager (which
  was always ultimately responsible for managing the lifetime of polling
  timers), thus simplifying the relationship between the QueryManager and
  its ObservableQuery objects.

- It's about 100 bytes smaller than the previous implementation, after
  minification and gzip.

- It uses setTimeout rather than setInterval, so event loop starvation
  never leads to a rapid succession of setInterval catch-up calls.

- It guarantees at most one timeout will be pending for an arbitrary
  number of polling queries, rather than a separate timer for every
  distinct polling interval.

- Fewer independent timers means better batching behavior, usually.

- Though there may be a delay between the desired polling time for a given
  query and the actual polling time, the delay is never greater than the
  minimum polling interval across all queries, which changes dynamically
  as polling queries are started and stopped.

packages/apollo-client/src/__mocks__/mockLinks.ts

@@ -3,6 +3,7 @@ import {
   ApolloLink,
   FetchResult,
   Observable,
+  GraphQLRequest,
   // Observer,
 } from 'apollo-link';
 
@@ -25,7 +26,7 @@ export function mockObservableLink(): MockSubscriptionLink {
 }
 
 export interface MockedResponse {
-  request: Operation;
+  request: GraphQLRequest;
   result?: FetchResult;
   error?: Error;
   delay?: number;
@@ -139,7 +140,7 @@ export class MockSubscriptionLink extends ApolloLink {
   }
 }
 
-function requestToKey(request: Operation): string {
+function requestToKey(request: GraphQLRequest): string {
   const queryString = request.query && print(request.query);
 
   return JSON.stringify({

packages/apollo-client/src/core/ObservableQuery.ts

@@ -2,11 +2,7 @@ import { isEqual, tryFunctionOrLogError, cloneDeep } from 'apollo-utilities';
 import { GraphQLError } from 'graphql';
 import { NetworkStatus, isNetworkRequestInFlight } from './networkStatus';
 import { Observable, Observer, Subscription } from '../util/Observable';
-
-import { QueryScheduler } from '../scheduler/scheduler';
-
 import { ApolloError } from '../errors/ApolloError';
-
 import { QueryManager } from './QueryManager';
 import { ApolloQueryResult, FetchType, OperationVariables } from './types';
 import {
@@ -68,10 +64,8 @@ export class ObservableQuery<
    */
   public variables: TVariables;
 
-  private isCurrentlyPolling: boolean;
   private shouldSubscribe: boolean;
   private isTornDown: boolean;
-  private scheduler: QueryScheduler<any>;
   private queryManager: QueryManager<any>;
   private observers: Observer<ApolloQueryResult<TData>>[];
   private subscriptionHandles: Subscription[];
@@ -81,11 +75,11 @@ export class ObservableQuery<
   private lastError: ApolloError;
 
   constructor({
-    scheduler,
+    queryManager,
     options,
     shouldSubscribe = true,
   }: {
-    scheduler: QueryScheduler<any>;
+    queryManager: QueryManager<any>;
     options: WatchQueryOptions<TVariables>;
     shouldSubscribe?: boolean;
   }) {
@@ -94,18 +88,16 @@ export class ObservableQuery<
     );
 
     // active state
-    this.isCurrentlyPolling = false;
     this.isTornDown = false;
 
     // query information
     this.options = options;
     this.variables = options.variables || ({} as TVariables);
-    this.queryId = scheduler.queryManager.generateQueryId();
+    this.queryId = queryManager.generateQueryId();
     this.shouldSubscribe = shouldSubscribe;
 
     // related classes
-    this.scheduler = scheduler;
-    this.queryManager = scheduler.queryManager;
+    this.queryManager = queryManager;
 
     // interal data stores
     this.observers = [];
@@ -520,11 +512,8 @@ export class ObservableQuery<
   }
 
   public stopPolling() {
-    if (this.isCurrentlyPolling) {
-      this.scheduler.stopPollingQuery(this.queryId);
-      this.options.pollInterval = undefined;
-      this.isCurrentlyPolling = false;
-    }
+    this.queryManager.stopPollingQuery(this.queryId);
+    this.options.pollInterval = undefined;
   }
 
   public startPolling(pollInterval: number) {
@@ -537,13 +526,8 @@ export class ObservableQuery<
       );
     }
 
-    if (this.isCurrentlyPolling) {
-      this.scheduler.stopPollingQuery(this.queryId);
-      this.isCurrentlyPolling = false;
-    }
     this.options.pollInterval = pollInterval;
-    this.isCurrentlyPolling = true;
-    this.scheduler.startPollingQuery(this.options, this.queryId);
+    this.queryManager.startPollingQuery(this.options, this.queryId);
   }
 
   private onSubscribe(observer: Observer<ApolloQueryResult<TData>>) {
@@ -594,8 +578,7 @@ export class ObservableQuery<
         );
       }
 
-      this.isCurrentlyPolling = true;
-      this.scheduler.startPollingQuery<TData>(this.options, this.queryId);
+      this.queryManager.startPollingQuery(this.options, this.queryId);
     }
 
     const observer: Observer<ApolloQueryResult<TData>> = {
@@ -623,11 +606,7 @@ export class ObservableQuery<
 
   private tearDownQuery() {
     this.isTornDown = true;
-
-    if (this.isCurrentlyPolling) {
-      this.scheduler.stopPollingQuery(this.queryId);
-      this.isCurrentlyPolling = false;
-    }
+    this.queryManager.stopPollingQuery(this.queryId);
 
     // stop all active GraphQL subscriptions
     this.subscriptionHandles.forEach(sub => sub.unsubscribe());

packages/apollo-client/src/core/QueryManager.ts

@@ -13,8 +13,6 @@ import {
   hasDirectives,
 } from 'apollo-utilities';
 
-import { QueryScheduler } from '../scheduler/scheduler';
-
 import { isApolloError, ApolloError } from '../errors/ApolloError';
 
 import { Observer, Subscription, Observable } from '../util/Observable';
@@ -54,7 +52,6 @@ export interface QueryInfo {
 }
 
 export class QueryManager<TStore> {
-  public scheduler: QueryScheduler<TStore>;
   public link: ApolloLink;
   public mutationStore: MutationStore = new MutationStore();
   public queryStore: QueryStore = new QueryStore();
@@ -66,6 +63,8 @@ export class QueryManager<TStore> {
 
   private onBroadcast: () => void;
 
+  private ssrMode: boolean;
+
   // let's not start at zero to avoid pain with bad checks
   private idCounter = 1;
 
@@ -104,7 +103,7 @@ export class QueryManager<TStore> {
     this.dataStore = store;
     this.onBroadcast = onBroadcast;
     this.clientAwareness = clientAwareness;
-    this.scheduler = new QueryScheduler({ queryManager: this, ssrMode });
+    this.ssrMode = ssrMode;
   }
 
   public mutate<T>({
@@ -662,7 +661,7 @@ export class QueryManager<TStore> {
     let transformedOptions = { ...options } as WatchQueryOptions;
 
     return new ObservableQuery<T>({
-      scheduler: this.scheduler,
+      queryManager: this,
       options: transformedOptions,
       shouldSubscribe: shouldSubscribe,
     });
@@ -1262,4 +1261,133 @@ export class QueryManager<TStore> {
       },
     };
   }
+
+  public checkInFlight(queryId: string) {
+    const query = this.queryStore.get(queryId);
+
+    return (
+      query &&
+      query.networkStatus !== NetworkStatus.ready &&
+      query.networkStatus !== NetworkStatus.error
+    );
+  }
+
+  // Map from client ID to { interval, options }.
+  public pollingInfoByQueryId = new Map<string, {
+    interval: number;
+    lastPollTimeMs: number;
+    options: WatchQueryOptions;
+  }>();
+
+  private nextPoll: {
+    time: number;
+    timeout: NodeJS.Timeout;
+  } | null = null;
+
+  public startPollingQuery(
+    options: WatchQueryOptions,
+    queryId: string,
+    listener?: QueryListener,
+  ): string {
+    const { pollInterval } = options;
+
+    if (!pollInterval) {
+      throw new Error(
+        'Attempted to start a polling query without a polling interval.',
+      );
+    }
+
+    // Do not poll in SSR mode
+    if (!this.ssrMode) {
+      this.pollingInfoByQueryId.set(queryId, {
+        interval: pollInterval,
+        // Avoid polling until at least pollInterval milliseconds from now.
+        // The -10 is a fudge factor to help with tests that rely on simulated
+        // timeouts via jest.runTimersToTime.
+        lastPollTimeMs: Date.now() - 10,
+        options: {
+          ...options,
+          fetchPolicy: 'network-only',
+        },
+      });
+
+      if (listener) {
+        this.addQueryListener(queryId, listener);
+      }
+
+      this.schedulePoll(pollInterval);
+    }
+
+    return queryId;
+  }
+
+  public stopPollingQuery(queryId: string) {
+    // Since the master polling interval dynamically adjusts to the contents of
+    // this.pollingInfoByQueryId, stopping a query from polling is as easy as
+    // removing it from the map.
+    this.pollingInfoByQueryId.delete(queryId);
+  }
+
+  // Calling this method ensures a poll will happen within the specified time
+  // limit, canceling any pending polls that would not happen in time.
+  private schedulePoll(timeLimitMs: number) {
+    const now = Date.now();
+
+    if (this.nextPoll) {
+      if (timeLimitMs < this.nextPoll.time - now) {
+        // The next poll will happen too far in the future, so cancel it, and
+        // fall through to scheduling a new timeout.
+        clearTimeout(this.nextPoll.timeout);
+      } else {
+        // The next poll will happen within timeLimitMs, so all is well.
+        return;
+      }
+    }
+
+    this.nextPoll = {
+      // Estimated time when the timeout will fire.
+      time: now + timeLimitMs,
+
+      timeout: setTimeout(() => {
+        this.nextPoll = null;
+        let nextTimeLimitMs = Infinity;
+
+        this.pollingInfoByQueryId.forEach((info, queryId) => {
+          // Pick next timeout according to current minimum interval.
+          if (info.interval < nextTimeLimitMs) {
+            nextTimeLimitMs = info.interval;
+          }
+
+          if (!this.checkInFlight(queryId)) {
+            // If this query was last polled more than interval milliseconds
+            // ago, poll it now. Note that there may be a small delay between
+            // the desired polling time and the actual polling time (equal to
+            // at most the minimum polling interval across all queries), but
+            // that's the tradeoff to batching polling intervals.
+            if (Date.now() - info.lastPollTimeMs >= info.interval) {
+              const updateLastPollTime = () => {
+                info.lastPollTimeMs = Date.now();
+              };
+              this.fetchQuery(queryId, info.options, FetchType.poll).then(
+                // Set info.lastPollTimeMs after the fetch completes, whether
+                // or not it succeeded. Promise.prototype.finally would be nice
+                // here, but we don't have a polyfill for that at the moment,
+                // and this code has historically silenced errors, which is not
+                // the behavior of .finally(updateLastPollTime).
+                updateLastPollTime,
+                updateLastPollTime
+              );
+            }
+          }
+        });
+
+        // If there were no entries in this.pollingInfoByQueryId, then
+        // nextTimeLimitMs will still be Infinity, so this.schedulePoll will
+        // not be called, thus ending the master polling interval.
+        if (isFinite(nextTimeLimitMs)) {
+          this.schedulePoll(nextTimeLimitMs);
+        }
+      }, timeLimitMs),
+    };
+  }
 }