histogram_synchronizer.h | Explore in Territory

// Copyright 2012 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef CONTENT_BROWSER_METRICS_HISTOGRAM_SYNCHRONIZER_H_
#define CONTENT_BROWSER_METRICS_HISTOGRAM_SYNCHRONIZER_H_

#include <string>
#include <vector>

#include "base/functional/callback.h"
#include "base/memory/singleton.h"
#include "base/synchronization/lock.h"
#include "base/task/task_runner.h"
#include "base/time/time.h"
#include "components/metrics/histogram_subscriber.h"

namespace content {

// This class maintains state that is used to upload histogram data from the
// various child processes, into the browser process. Such transactions are
// usually instigated by the browser. In general, a child process will respond
// by gathering snapshots of all internal histograms, calculating what has
// changed since its last upload, and transmitting a pickled collection of
// deltas.
//
// There are actually two modes of update request.  One is synchronous (and
// blocks the UI thread, waiting to populate an about:histograms tab) and the
// other is asynchronous, and used by the metrics services in preparation for a
// log upload.
//
// To assure that all the processes have responded, a counter is maintained to
// indicate the number of pending (not yet responsive) processes. To avoid
// confusion about a response (i.e., is the process responding to a current
// request for an update, or to an old request for an update) we tag each group
// of requests with a sequence number. When an update arrives we can ignore it
// (relative to the counter) if it does not relate to a current outstanding
// sequence number.
//
// There is one final mode of use, where a renderer spontaneously decides to
// transmit a collection of histogram data.  This is designed for use when the
// renderer is terminating.  Unfortunately, renders may be terminated without
// warning, and the best we can do is periodically acquire data from a tab, such
// as when a page load has completed.  In this mode, the renderer uses a
// reserved sequence number, different from any sequence number that might be
// specified by a browser request.  Since this sequence number can't match an
// outstanding sequence number, the pickled data is accepted into the browser,
// but there is no impact on the counters.

class HistogramSynchronizer : public metrics::HistogramSubscriber { … };

}  // namespace content

#endif  // CONTENT_BROWSER_METRICS_HISTOGRAM_SYNCHRONIZER_H_
chromium/content/browser/metrics/histogram_synchronizer.h
