balm_category.h

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/// @file balm_category.h
///
/// The content of this file has been pre-processed for Doxygen.
///
 
 
// balm_category.h                                                    -*-C++-*-
#ifndef INCLUDED_BALM_CATEGORY
#define INCLUDED_BALM_CATEGORY
 
#include <bsls_ident.h>
BSLS_IDENT("$Id: balm_category.h,v 1.4 2008/04/16 20:00:49 hversche Exp $")
 
/// @defgroup balm_category balm_category
/// @brief  Provide a representation of a metric category.
/// @addtogroup bal
/// @{
/// @addtogroup balm
/// @{
/// @addtogroup balm_category
/// @{
///
/// <h1> Outline </h1>
/// * <a href="#balm_category-purpose"> Purpose</a>
/// * <a href="#balm_category-classes"> Classes </a>
/// * <a href="#balm_category-description"> Description </a>
///   * <a href="#balm_category-alternative-systems-for-telemetry"> Alternative Systems for Telemetry </a>
///   * <a href="#balm_category-thread-safety"> Thread Safety </a>
///   * <a href="#balm_category-usage"> Usage </a>
///     * <a href="#balm_category-example-1-basic-usage"> Example 1: Basic Usage </a>
///
/// # Purpose {#balm_category-purpose}
/// Provide a representation of a metric category.
///
/// # Classes {#balm_category-classes}
///
/// -    balm::Category: a representation of a metric category
///
/// @see  balm_metricregistry, balm_metricid, balm_metricdescription
///
/// # Description {#balm_category-description}
/// This component provides a class, `balm::Category`, whose values
/// are used to categorize collected metrics.  A metric "category" is an
/// identifier (chosen by the application) that groups together one or more
/// metrics.  A `balm::Category` object contains the address of a string holding
/// the name of the category and a boolean value indicating whether the
/// category is currently enabled.  The `balm::Category` class suppresses copy
/// construction and assignment, and does not provide equality operators;
/// applications should use a *single* `balm::Category` object instance per
/// category (such as one provided by the *`balm::MetricRegistry`* component).
///
/// IMPORTANT: The category name, whose type is `const char *`, must remain
/// constant and valid throughout the lifetime of the `balm::Category` object.
///
/// ## Alternative Systems for Telemetry {#balm_category-alternative-systems-for-telemetry}
///
///
/// Bloomberg software may alternatively use the GUTS telemetry API, which is
/// integrated into Bloomberg infrastructure.
///
/// ## Thread Safety {#balm_category-thread-safety}
///
///
/// `balm::Category` is generally *const* *thread-safe*, meaning that accessors
/// may be invoked concurrently from different threads, but it is not safe to
/// access or modify a `balm::Category` in one thread while another thread
/// modifies the same object.  However, it is safe to *access* the `enabled`
/// property on one (or more) thread(s) while the object is being modified on
/// another thread.
///
/// ## Usage {#balm_category-usage}
///
///
/// This section illustrates intended use of this component.
///
/// ### Example 1: Basic Usage {#balm_category-example-1-basic-usage}
///
///
/// The following example demonstrates how to create, access, and modify a
/// `balm::Category` object.  We start by creating three category objects with
/// different names:
/// @code
/// balm::Category categoryA("A", true);
/// balm::Category categoryB("B", false);
/// balm::Category categoryC("C");
/// @endcode
/// Once the category objects have been created, we can use the `name` and
/// `enabled` methods to access their values:
/// @code
/// assert(0 == bsl::strcmp("A", categoryA.name()));
/// assert(0 == bsl::strcmp("B", categoryB.name()));
/// assert(0 == bsl::strcmp("C", categoryC.name()));
///
/// assert( categoryA.enabled());
/// assert(!categoryB.enabled());
/// assert( categoryC.enabled());
/// @endcode
/// Finally, we modify the enablement status of one of the categories, and then
/// write all three categories to the console:
/// @code
/// categoryC.setEnabled(false);
///
/// bsl::cout << "categoryA: " << categoryA << bsl::endl
///           << "categoryB: " << categoryB << bsl::endl
///           << "categoryC: " << categoryC << bsl::endl;
/// @endcode
/// With the resulting console output:
/// @code
/// categoryA: [ A ENABLED ]
/// categoryB: [ B DISABLED ]
/// categoryC: [ C DISABLED ]
/// @endcode
/// @}
/** @} */
/** @} */
 
/** @addtogroup bal
 * @{
 */
/** @addtogroup balm
 * @{
 */
/** @addtogroup balm_category
 * @{
 */
 
#include <balscm_version.h>
 
#include <bsls_atomic.h>
#include <bsls_libraryfeatures.h>
 
#include <bsl_iosfwd.h>
 
 
 
 
namespace balm {
 
class CategoryHolder;
 
                               // ==============
                               // class Category
                               // ==============
 
/// This class provides a mechanism for representing a category.  A category
/// is an identifier used to group related metrics.  A `Category` object
/// contains the address of a null-terminated string, `name`, holding the
/// name of the category and a boolean value, `enabled`, indicating whether
/// the category is currently enabled.
///
/// See @ref balm_category 
class Category {
 
    // DATA
    const char     *d_name_p;     // name of the category (held, not owned)
 
    bsls::AtomicInt d_enabled;    // whether the category is enabled
 
    CategoryHolder *d_holders_p;  // linked list of holders of this category
 
  private:
    // NOT IMPLEMENTED
    Category(const Category& );
    Category& operator=(const Category& );
 
  public:
    // CREATORS
 
    /// Create a category having the specified `name` address.  Optionally
    /// specify `enabledFlag`, the enabled status of the category; if
    /// `enabledFlag` is not specified, the `enabled` status is `true`.  
    /// 
    /// \pre The behavior is undefined   unless `name` remains valid and unmodified for
    /// the lifetime of this object.
    explicit Category(const char *name, bool enabledFlag = true);
 
    /// Destroy this category object.
    ~Category();
 
    // MANIPULATORS
 
    /// Set the name of this metric to the specified `name` address.  
    /// 
    /// \pre The behavior is undefined   unless `name` remains valid and unmodified
    /// for the lifetime of this object.
    void setName(const char *name);
 
    /// Set the `enabled` state of this category to the value of the
    /// specified `enabledFlag` and update any `CategoryHolder` objects registered with this category.  
    /// 
    /// \note Note that   this operation is *not*
    /// atomic, and other threads may simultaneously access the current
    /// enabled value while this operation is performed.  Also note that
    /// this operation has *linear* performance with respect to the number
    /// of registered category holders for `category`.
    void setEnabled(bool enabledFlag);
 
    /// Load into the specified `holder` the address of this category, its
    /// `enabled()` status, and the address of the next holder in the
    /// linked list of category holders maintained by this object
    /// (prepending `holder` to this category's linked list of category
    /// holders).  This category will update `holder->enabled()` when its
    /// enabled state changes, and will reset `holder` (i.e.,
    /// `holder->reset()`) when this category is destroyed.  
    /// 
    /// \pre The behavior is undefined   unless `holder` remains valid and *unmodified* (by the
    /// client) for the lifetime of this object and is *not* registered
    /// again with any category (including this one).
    void registerCategoryHolder(CategoryHolder *holder);
 
    // ACCESSORS
 
    /// Return the address of the non-modifiable null-terminated string
    /// containing the name of this category.
    const char *name() const;
 
    /// Report whether this category is enabled. This function is fully
    /// thread-safe.
    bool enabled() const;
 
    /// Return a *reference* to a const value indicating the enabled status
    /// of this category, allowing downstream uses to minimize latency by
    /// avoiding indirection through abstracted interfaces, albeit at some
    /// risk of object-lifetime violations.  The returned reference must not
    /// be allowed to outlive this category object.
    const bsls::AtomicInt& isEnabledRaw() const;
 
    /// Print this category to the specified output `stream` in some human
    /// readable form, and return the modifiable `stream`.
    bsl::ostream& print(bsl::ostream& stream) const;
};
 
// ============================================================================
//                            INLINE DEFINITIONS
// ============================================================================
 
// FREE OPERATORS
 
/// Output a formatted description of the specified `rhs` category to the
/// specified `stream`, and return the modifiable `stream`.
inline
bsl::ostream& operator<<(bsl::ostream& stream, const Category& rhs);
 
                            // ====================
                            // class CategoryHolder
                            // ====================
 
/// This class, informally referred to as a "category holder" (or simply
/// "holder"), holds a category, an enabled value, and a pointer to a
/// "next" holder.  Both the category and next pointer may be null.  The
/// intended use is as follows: (1) instances of this class are (only)
/// declared in contexts where collecting a metric occurs; (2) if the
/// enabled value is true, the category contains the address of a valid,
/// enabled, category; (3) if the next pointer is non-null, then the holder
/// pointed to holds the same category and threshold.  Instances of this
/// class must be *statically* initializable.  Hence, the data members are
/// `public`, and no constructors or destructor are defined.
///
/// This class should *not* be used directly by client code.  It is an
/// implementation detail of the `balm` metric collection system.
///
/// See @ref balm_category 
class CategoryHolder {
 
  private:
    // NOT IMPLEMENTED
    CategoryHolder& operator=(const CategoryHolder& rhs);
 
  public:
 
    // PUBLIC DATA MEMBERS
 
#ifdef BSLS_LIBRARYFEATURES_HAS_CPP17_BASELINE_LIBRARY
    bsls::AtomicBool                    d_enabled;     // whether the category
                                                       // is enabled
 
    bsls::AtomicPointer<const Category> d_category_p;  // held category (not
                                                       // owned)
 
    bsls::AtomicPointer<CategoryHolder> d_next_p;      // next category holder
                                                       // in linked list
#else
    bool            d_enabled;     // whether the category is enabled
    const Category *d_category_p;  // held category (not owned)
    CategoryHolder *d_next_p;      // next category holder in linked list
#endif
 
    // CREATORS
 
    // No constructors or destructors are declared in order to allow for static
    // initialization of instances of this class.
 
    // MANIPULATORS
 
    /// Reset this object to its default value.  The default value is:
    /// @code
    ///  { false, 0, 0 }
    /// @endcode
    void reset();
 
    /// Set the address of the category held by this holder to the specified
    /// `category`.
    void setCategory(const Category *category);
 
    /// Set the `enabled` state of this category to the value of the
    /// specified `enabledFlag`.
    void setEnabled(bool enabledFlag);
 
    /// Set this holder to point to the specified `holder`.
    void setNext(CategoryHolder *holder);
 
    // ACCESSORS
 
    /// Return the address of the non-modifiable category held by this
    /// holder.
    const Category *category() const;
 
    /// Return `true` if `category` is valid (i.e., non-null) and enabled,
    /// and `false` otherwise.
    bool enabled() const;
 
    /// Return the address of the modifiable holder pointed to by this
    /// holder.
    CategoryHolder *next() const;
};
 
// ============================================================================
//                        INLINE FUNCTION DEFINITIONS
// ============================================================================
 
                               // --------------
                               // class Category
                               // --------------
 
// CREATORS
inline
Category::Category(const char *name, bool enabledFlag)
: d_name_p(name)
, d_enabled(enabledFlag)
, d_holders_p(0)
{
}
 
// MANIPULATORS
inline
void Category::setName(const char *name)
{
    d_name_p = name;
}
 
// ACCESSORS
inline
const char *Category::name() const
{
    return d_name_p;
}
 
inline
bool Category::enabled() const
{
    return d_enabled;
}
 
inline
const bsls::AtomicInt& Category::isEnabledRaw() const
{
    return d_enabled;
}
 
                            // --------------------
                            // class CategoryHolder
                            // --------------------
 
// MANIPULATORS
inline
void CategoryHolder::setCategory(const Category *category)
{
    d_category_p = category;
}
 
inline
void CategoryHolder::setEnabled(bool enabledFlag)
{
    d_enabled = enabledFlag;
}
 
inline
void CategoryHolder::setNext(CategoryHolder *holder)
{
    d_next_p = holder;
}
 
// ACCESSORS
inline
const Category *CategoryHolder::category() const
{
    return d_category_p;
}
 
inline
bool CategoryHolder::enabled() const
{
    return d_enabled;
}
 
inline
CategoryHolder *CategoryHolder::next() const
{
    return d_next_p;
}
}  // close package namespace
 
// FREE OPERATORS
inline
bsl::ostream& balm::operator<<(bsl::ostream& stream, const Category& rhs)
{
    return rhs.print(stream);
}
 
 
 
#endif
 
// ----------------------------------------------------------------------------
// Copyright 2015 Bloomberg Finance L.P.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
// ----------------------------- END-OF-FILE ----------------------------------
 
/** @} */
/** @} */
/** @} */