Introduce a request throttler for limiting requests in mobile NTP.

components/ntp_snippets/request_counter.h

Index: components/ntp_snippets/request_counter.h
diff --git a/components/ntp_snippets/request_counter.h b/components/ntp_snippets/request_counter.h
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+++ b/components/ntp_snippets/request_counter.h
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+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef COMPONENTS_NTP_SNIPPETS_REQUEST_COUNTER_H_
+#define COMPONENTS_NTP_SNIPPETS_REQUEST_COUNTER_H_
+
+#include <string>
+
+#include "base/macros.h"
+
+class PrefRegistrySimple;
+class PrefService;
+
+namespace base {
+class HistogramBase;
+}  // namespace base
+
+namespace ntp_snippets {
+
+struct RequestTypeInfo;
+
+// Counts requests to external services, compares them to a daily quota, reports
+// them to UMA. In the application code, create one local instance for each type
+// of requests, identified by the RequestType. The request counter is based on:
+//  - daily quota from a variation param "quota_|type|" in the NTPSnippets trial
+//  - pref "ntp.requests_quota.|type|.count" to store the current counter value,
+//  - pref "ntp.requests_quota.|type|.day" to store current day to which the
+//    current counter value applies.
+// Furthermore the counter reports to histograms:
+//  - "NewTabPage.RequestCounter.RequestStatus_|type|" - status of each request;
+//  - "NewTabPage.RequestCounter.PerDay_|type|" - the daily number of requests.
+// Thus, when you add a new RequestType of RequestCounter, you must also:

[tschumann, 2016/07/21 11:37:09]

it would be nice to clearly separate implementation notes for further extensions
from the usage documentation.
Maybe something like:
// Implementation notes: When extending this class for a new RequestType, ...

[jkrcal, 2016/07/21 18:17:57]

Done.

+//  1) define in request_counter.cc in kRequestTypeInfo
+//     a) the string value for your |type| and
+//     b) constants for day/count prefs;
+//  2) define a new RequestCounterTypes histogram suffix in histogram.xml (with
+//     the same string value as in 1a)).
+class RequestCounter {
+ public:
+  // Enumeration listing all current applications of the request counter.
+  enum class RequestType {
+    CONTENT_SUGGESTION_FETCHER
+  };
+
+  // Enumeration listing all possible outcomes for fetch attempts. Used for UMA
+  // histogram, so do not change existing values. Insert new values at the end,
+  // and update the histogram definition.
+  enum class RequestStatus {
+    FORCED,
+    QUOTA_GRANTED,
+    QUOTA_EXCEEDED,
+    REQUEST_STATUS_COUNT
+  };
+
+  RequestCounter(PrefService* pref_service,
+                 RequestType type,
+                 int default_quota);
+
+  // Registers profile prefs for all RequestTypes.
+  static void RegisterProfilePrefs(PrefRegistrySimple* registry);
+
+  // Returns whether quota is available for another request and reports this
+  // information to UMA.
+  bool DemandQuotaForRequest();
+
+  // Reports a forced request to UMA (without checking the quota).
+  void ReportForcedRequest() const;

[tschumann, 2016/07/21 11:37:09]

as discussed offline, it would be nice to have a more uniform API, e.g. also
have a demand functionality for forced requests and hide the reporting logic
from the public interface.

[Marc Treib, 2016/07/21 11:43:50]

+1, I like this idea.

[jkrcal, 2016/07/21 18:17:58]

Done. I documented the current behaviour. I guess that in the future, we may
want to be more restrictive w.r.t. forced requests.

+
+ private:
+  // Also emits the PerDay histogram if the day changed.
+  void ResetCounterIfDayChanged();
+
+  std::string GetRequestTypeAsString() const;
+
+  int GetCount() const;
+  void SetCount(int count);
+  int GetDay() const;
+  void SetDay(int day);
+  bool HasDay() const;
+
+  PrefService* pref_service_;
+  const RequestTypeInfo& type_info_;
+
+  // It comes from a variation parameter or |default_quota| as a fallback.
+  int quota_;
+
+  // The histograms for reporting the requests of the given |type_|.
+  base::HistogramBase* histogram_request_status_;
+  base::HistogramBase* histogram_per_day_;
+
+  DISALLOW_COPY_AND_ASSIGN(RequestCounter);
+};
+
+}  // namespace ntp_snippets
+
+#endif  // COMPONENTS_NTP_SNIPPETS_REQUEST_COUNTER_H_