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// Copyright (c) 2012 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.
// FieldTrial is a class for handling details of statistical experiments
// performed by actual users in the field (i.e., in a shipped or beta product).
// All code is called exclusively on the UI thread currently.
// The simplest example is an experiment to see whether one of two options
// produces "better" results across our user population. In that scenario, UMA
// data is uploaded to aggregate the test results, and this FieldTrial class
// manages the state of each such experiment (state == which option was
// pseudo-randomly selected).
// States are typically generated randomly, either based on a one time
// randomization (which will yield the same results, in terms of selecting
// the client for a field trial or not, for every run of the program on a
// given machine), or by a startup randomization (generated each time the
// application starts up, but held constant during the duration of the
// process), or by continuous randomization across a run (where the state
// can be recalculated again and again, many times during a process).
// Continuous randomization is not yet implemented.
// Example: Suppose we have an experiment involving memory, such as determining
// the impact of some pruning algorithm.
// We assume that we already have a histogram of memory usage, such as:
// HISTOGRAM_COUNTS("Memory.RendererTotal", count);
// Somewhere in main thread initialization code, we'd probably define an
// instance of a FieldTrial, with code such as:
// // FieldTrials are reference counted, and persist automagically until
// // process teardown, courtesy of their automatic registration in
// // FieldTrialList.
// // Note: This field trial will run in Chrome instances compiled through
// // 8 July, 2015, and after that all instances will be in "StandardMem".
// scoped_refptr<base::FieldTrial> trial(
// base::FieldTrialList::FactoryGetFieldTrial("MemoryExperiment", 1000,
// "StandardMem", 2015, 7, 8,
// NULL));
// const int high_mem_group =
// trial->AppendGroup("HighMem", 20); // 2% in HighMem group.
// const int low_mem_group =
// trial->AppendGroup("LowMem", 20); // 2% in LowMem group.
// // Take action depending of which group we randomly land in.
// if (trial->group() == high_mem_group)
// SetPruningAlgorithm(kType1); // Sample setting of browser state.
// else if (trial->group() == low_mem_group)
// SetPruningAlgorithm(kType2); // Sample alternate setting.
// We then, in addition to our original histogram, output histograms which have
// slightly different names depending on what group the trial instance happened
// to randomly be assigned:
// HISTOGRAM_COUNTS("Memory.RendererTotal", count); // The original histogram.
// static const bool memory_renderer_total_trial_exists =
// FieldTrialList::TrialExists("MemoryExperiment");
// if (memory_renderer_total_trial_exists) {
// HISTOGRAM_COUNTS(FieldTrial::MakeName("Memory.RendererTotal",
// "MemoryExperiment"), count);
// }
// The above code will create four distinct histograms, with each run of the
// application being assigned to of of the three groups, and for each group, the
// correspondingly named histogram will be populated:
// Memory.RendererTotal // 100% of users still fill this histogram.
// Memory.RendererTotal_HighMem // 2% of users will fill this histogram.
// Memory.RendererTotal_LowMem // 2% of users will fill this histogram.
// Memory.RendererTotal_StandardMem // 96% of users will fill this histogram.
#include <map>
#include <string>
#include <vector>
#include "base/base_export.h"
#include "base/gtest_prod_util.h"
#include "base/memory/ref_counted.h"
#include "base/observer_list_threadsafe.h"
#include "base/synchronization/lock.h"
#include "base/time.h"
namespace base {
class FieldTrialList;
class BASE_EXPORT FieldTrial : public RefCounted<FieldTrial> {
typedef int Probability; // Probability type for being selected in a trial.
// EntropyProvider is an interface for providing entropy for one-time
// randomized (persistent) field trials.
class BASE_EXPORT EntropyProvider {
virtual ~EntropyProvider();
// Returns a double in the range of [0, 1) based on |trial_name| that will
// be used for the dice roll for the specified field trial. A given instance
// should always return the same value given the same input |trial_name|.
virtual double GetEntropyForTrial(const std::string& trial_name) const = 0;
// A pair representing a Field Trial and its selected group.
struct ActiveGroup {
std::string trial_name;
std::string group_name;
typedef std::vector<ActiveGroup> ActiveGroups;
// A return value to indicate that a given instance has not yet had a group
// assignment (and hence is not yet participating in the trial).
static const int kNotFinalized;
// Changes the field trial to use one-time randomization, i.e. produce the
// same result for the current trial on every run of this client. Must be
// called right after construction.
void UseOneTimeRandomization();
// Disables this trial, meaning it always determines the default group
// has been selected. May be called immediately after construction, or
// at any time after initialization (should not be interleaved with
// AppendGroup calls). Once disabled, there is no way to re-enable a
// trial.
// TODO(mad):
// This doesn't properly reset to Default when a group was forced.
void Disable();
// Establish the name and probability of the next group in this trial.
// Sometimes, based on construction randomization, this call may cause the
// provided group to be *THE* group selected for use in this instance.
// The return value is the group number of the new group.
int AppendGroup(const std::string& name, Probability group_probability);
// Return the name of the FieldTrial (excluding the group name).
const std::string& trial_name() const { return trial_name_; }
// Return the randomly selected group number that was assigned, and notify
// any/all observers that this finalized group number has presumably been used
// (queried), and will never change. Note that this will force an instance to
// participate, and make it illegal to attempt to probabilistically add any
// other groups to the trial.
int group();
// If the group's name is empty, a string version containing the group number
// is used as the group name. This causes a winner to be chosen if none was.
const std::string& group_name();
// Helper function for the most common use: as an argument to specify the
// name of a HISTOGRAM. Use the original histogram name as the name_prefix.
static std::string MakeName(const std::string& name_prefix,
const std::string& trial_name);
// Enable benchmarking sets field trials to a common setting.
static void EnableBenchmarking();
// Set the field trial as forced, meaning that it was setup earlier than
// the hard coded registration of the field trial to override it.
// This allows the code that was hard coded to register the field trial to
// still succeed even though the field trial has already been registered.
// This must be called after appending all the groups, since we will make
// the group choice here. Note that this is a NOOP for already forced trials.
// And, as the rest of the FieldTrial code, this is not thread safe and must
// be done from the UI thread.
void SetForced();
// Allow tests to access our innards for testing purposes.
FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, Registration);
FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, AbsoluteProbabilities);
FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, RemainingProbability);
FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, FiftyFiftyProbability);
FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, MiddleProbabilities);
FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, OneWinner);
FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, DisableProbability);
FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, ActiveGroups);
FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, ActiveGroupsNotFinalized);
FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, DuplicateRestore);
FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, HashClientId);
FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, HashClientIdIsUniform);
FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, NameGroupIds);
FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, UseOneTimeRandomization);
friend class base::FieldTrialList;
friend class RefCounted<FieldTrial>;
// This is the group number of the 'default' group when a choice wasn't forced
// by a call to FieldTrialList::CreateFieldTrial. It is kept private so that
// consumers don't use it by mistake in cases where the group was forced.
static const int kDefaultGroupNumber;
FieldTrial(const std::string& name,
Probability total_probability,
const std::string& default_group_name);
virtual ~FieldTrial();
// Return the default group name of the FieldTrial.
std::string default_group_name() const { return default_group_name_; }
// Sets the chosen group name and number.
void SetGroupChoice(const std::string& group_name, int number);
// Ensures that a group is chosen, if it hasn't yet been. The field trial
// might yet be disabled, so this call will *not* notify observers of the
// status.
void FinalizeGroupChoice();
// Returns the trial name and selected group name for this field trial via
// the output parameter |active_group|, but only if the group has already
// been chosen and has been externally observed via |group()| and the trial
// has not been disabled. In that case, true is returned and |active_group|
// is filled in; otherwise, the result is false and |active_group| is left
// untouched.
bool GetActiveGroup(ActiveGroup* active_group) const;
// Returns the group_name. A winner need not have been chosen.
std::string group_name_internal() const { return group_name_; }
// The name of the field trial, as can be found via the FieldTrialList.
const std::string trial_name_;
// The maximum sum of all probabilities supplied, which corresponds to 100%.
// This is the scaling factor used to adjust supplied probabilities.
const Probability divisor_;
// The name of the default group.
const std::string default_group_name_;
// The randomly selected probability that is used to select a group (or have
// the instance not participate). It is the product of divisor_ and a random
// number between [0, 1).
Probability random_;
// Sum of the probabilities of all appended groups.
Probability accumulated_group_probability_;
int next_group_number_;
// The pseudo-randomly assigned group number.
// This is kNotFinalized if no group has been assigned.
int group_;
// A textual name for the randomly selected group. Valid after |group()|
// has been called.
std::string group_name_;
// When enable_field_trial_ is false, field trial reverts to the 'default'
// group.
bool enable_field_trial_;
// When forced_ is true, we return the chosen group from AppendGroup when
// appropriate.
bool forced_;
// Specifies whether the group choice has been reported to observers.
bool group_reported_;
// When benchmarking is enabled, field trials all revert to the 'default'
// group.
static bool enable_benchmarking_;
// Class with a list of all active field trials. A trial is active if it has
// been registered, which includes evaluating its state based on its probaility.
// Only one instance of this class exists.
class BASE_EXPORT FieldTrialList {
// Define a separator character to use when creating a persistent form of an
// instance. This is intended for use as a command line argument, passed to a
// second process to mimic our state (i.e., provide the same group name).
static const char kPersistentStringSeparator; // Currently a slash.
// Define expiration year in future. It is initialized to two years from Now.
static int kExpirationYearInFuture;
// Observer is notified when a FieldTrial's group is selected.
class BASE_EXPORT Observer {
// Notify observers when FieldTrials's group is selected.
virtual void OnFieldTrialGroupFinalized(const std::string& trial_name,
const std::string& group_name) = 0;
virtual ~Observer();
// This singleton holds the global list of registered FieldTrials.
// To support one-time randomized field trials, specify a non-NULL
// |entropy_provider| which should be a source of uniformly distributed
// entropy values. Takes ownership of |entropy_provider|. If one time
// randomization is not desired, pass in NULL for |entropy_provider|.
explicit FieldTrialList(const FieldTrial::EntropyProvider* entropy_provider);
// Destructor Release()'s references to all registered FieldTrial instances.
// Get a FieldTrial instance from the factory.
// |name| is used to register the instance with the FieldTrialList class,
// and can be used to find the trial (only one trial can be present for each
// name). |default_group_name| is the name of the default group which will
// be chosen if none of the subsequent appended groups get to be chosen.
// |default_group_number| can receive the group number of the default group as
// AppendGroup returns the number of the subsequence groups. |trial_name| and
// |default_group_name| may not be empty but |default_group_number| can be
// NULL if the value is not needed.
// Group probabilities that are later supplied must sum to less than or equal
// to the |total_probability|. Arguments |year|, |month| and |day_of_month|
// specify the expiration time. If the build time is after the expiration time
// then the field trial reverts to the 'default' group.
// Use this static method to get a startup-randomized FieldTrial or a
// previously created forced FieldTrial. If you want a one-time randomized
// trial, call UseOneTimeRandomization() right after creation.
static FieldTrial* FactoryGetFieldTrial(
const std::string& trial_name,
FieldTrial::Probability total_probability,
const std::string& default_group_name,
const int year,
const int month,
const int day_of_month,
int* default_group_number);
// The Find() method can be used to test to see if a named Trial was already
// registered, or to retrieve a pointer to it from the global map.
static FieldTrial* Find(const std::string& name);
// Returns the group number chosen for the named trial, or
// FieldTrial::kNotFinalized if the trial does not exist.
static int FindValue(const std::string& name);
// Returns the group name chosen for the named trial, or the
// empty string if the trial does not exist.
static std::string FindFullName(const std::string& name);
// Returns true if the named trial has been registered.
static bool TrialExists(const std::string& name);
// Creates a persistent representation of active FieldTrial instances for
// resurrection in another process. This allows randomization to be done in
// one process, and secondary processes can be synchronized on the result.
// The resulting string contains the name and group name pairs of all
// registered FieldTrials for which the group has been chosen and externally
// observed (via |group()|) and which have not been disabled, with "/" used
// to separate all names and to terminate the string. This string is parsed
// by |CreateTrialsFromString()|.
static void StatesToString(std::string* output);
// Fills in the supplied vector |active_groups| (which must be empty when
// called) with a snapshot of all registered FieldTrials for which the group
// has been chosen and externally observed (via |group()|) and which have
// not been disabled.
static void GetActiveFieldTrialGroups(
FieldTrial::ActiveGroups* active_groups);
// Use a state string (re: StatesToString()) to augment the current list of
// field trials to include the supplied trials, and using a 100% probability
// for each trial, force them to have the same group string. This is commonly
// used in a non-browser process, to carry randomly selected state in a
// browser process into this non-browser process, but could also be invoked
// through a command line argument to the browser process. The created field
// trials are marked as "used" for the purposes of active trial reporting.
static bool CreateTrialsFromString(const std::string& prior_trials);
// Create a FieldTrial with the given |name| and using 100% probability for
// the FieldTrial, force FieldTrial to have the same group string as
// |group_name|. This is commonly used in a non-browser process, to carry
// randomly selected state in a browser process into this non-browser process.
// It returns NULL if there is a FieldTrial that is already registered with
// the same |name| but has different finalized group string (|group_name|).
static FieldTrial* CreateFieldTrial(const std::string& name,
const std::string& group_name);
// Add an observer to be notified when a field trial is irrevocably committed
// to being part of some specific field_group (and hence the group_name is
// also finalized for that field_trial).
static void AddObserver(Observer* observer);
// Remove an observer.
static void RemoveObserver(Observer* observer);
// Notify all observers that a group has been finalized for |field_trial|.
static void NotifyFieldTrialGroupSelection(FieldTrial* field_trial);
// Return the number of active field trials.
static size_t GetFieldTrialCount();
// If one-time randomization is enabled, returns a weak pointer to the
// corresponding EntropyProvider. Otherwise, returns NULL.
static const FieldTrial::EntropyProvider*
// A map from FieldTrial names to the actual instances.
typedef std::map<std::string, FieldTrial*> RegistrationList;
// Helper function should be called only while holding lock_.
FieldTrial* PreLockedFind(const std::string& name);
// Register() stores a pointer to the given trial in a global map.
// This method also AddRef's the indicated trial.
// This should always be called after creating a new FieldTrial instance.
static void Register(FieldTrial* trial);
static FieldTrialList* global_; // The singleton of this class.
// This will tell us if there is an attempt to register a field
// trial or check if one-time randomization is enabled without
// creating the FieldTrialList. This is not an error, unless a
// FieldTrialList is created after that.
static bool used_without_global_;
// Lock for access to registered_.
base::Lock lock_;
RegistrationList registered_;
// Entropy provider to be used for one-time randomized field trials. If NULL,
// one-time randomization is not supported.
scoped_ptr<const FieldTrial::EntropyProvider> entropy_provider_;
// List of observers to be notified when a group is selected for a FieldTrial.
scoped_refptr<ObserverListThreadSafe<Observer> > observer_list_;
} // namespace base