// -*- indent-tabs-mode: nil -*- // Counter.h #ifndef __Counter__ #define __Counter__ #include namespace Arc { /// A time very far in the future. extern const Glib::TimeVal ETERNAL; /// A time very far in the past. extern const Glib::TimeVal HISTORIC; // Some forward declarations. class Counter; class CounterTicket; class ExpirationReminder; /// A class defining a common interface for counters. /** This class defines a common interface for counters as well as some common functionality. The purpose of a counter is to provide housekeeping some resource such as e.g. disk space, memory or network bandwidth. The counter itself will not be aware of what kind of resource it limits the use of. Neither will it be aware of what unit is being used to measure that resource. Counters are thus very similar to semaphores. Furthermore, counters are designed to handle concurrent operations from multiple threads/processes in a consistent manner. Every counter has a limit, an excess limit and a value. The limit is a number that specify how many units are available for reservation. The value is the number of units that are currently available for reservation, i.e. has not already been reserved. The excess limit specify how many extra units can be reserved for high priority needs even if there are no normal units available for reservation. The excess limit is similar to the credit limit of e.g. a VISA card. The users of the resource must thus first call the counter in order to make a reservation of an appropriate amount of the resource, then allocate and use the resource and finally call the counter again to cancel the reservation. Typical usage is: \code // Declare a counter. Replace XYZ by some appropriate kind of // counter and provide required parameters. Unit is MB. XYZCounter memory(...); ... // Make a reservation of memory for 2000000 doubles. CounterTicket tick = memory.reserve(2*sizeof(double)); // Use the memory. double* A=new double[2000000]; doSomething(A); delete[] A; // Cancel the reservation. tick.cancel(); \endcode There are also alternative ways to make reservations, including self-expiring reservations, prioritized reservations and reservations that fail if they cannot be made fast enough. For self expiring reservations, a duration is provided in the reserve call: \code tick = memory.reserve(2*sizeof(double), Glib::TimeVal(1,0)); \endcode A self-expiring reservation can be cancelled explicitly before it expires, but if it is not cancelled it will expire automatically when the duration has passed. The default value for the duration is ETERNAL, which means that the reservation will not be cancelled automatically. Prioritized reservations may use the excess limit and succeed immediately even if there are no normal units available for reservation. The value of the counter will in this case become negative. A prioritized reservation looks like this: \code tick = memory.reserve(2*sizeof(double), Glib::TimeVal(1,0), true); \endcode Finally, a time out option can be provided for a reservation. If some task should be performed within two seconds or not at all, the reservation can look like this: \code tick = memory.reserve(2*sizeof(double), Glib::TimeVal(1,0), true, Glib::TimeVal(2,0)); if (tick.isValid()) doSomething(...); \endcode \ingroup common \headerfile Counter.h arc/Counter.h */ class Counter { protected: /// A typedef of identification numbers for reservation. /** This is a type that is used as identification numbers (keys) for referencing of reservations. It is used internally in counters for book keeping of reservations as well as in the CounterTicket class in order to be able to cancel and extend reservations. */ typedef unsigned long long int IDType; /// Default constructor. /** This is the default constructor. Since Counter is an abstract class, it should only be used by subclasses. Therefore it is protected. Furthermore, since the Counter class has no attributes, nothing needs to be initialized and thus this constructor is empty. */ Counter(); public: /// The destructor. /** This is the destructor of the Counter class. Since the Counter class has no attributes, nothing needs to be cleaned up and thus the destructor is empty. */ virtual ~Counter(); /// Returns the current limit of the counter. /** This method returns the current limit of the counter, i.e. how many units can be reserved simultaneously by different threads without claiming high priority. @return The current limit of the counter. */ virtual int getLimit() = 0; /// Sets the limit of the counter. /** This method sets a new limit for the counter. @param newLimit The new limit, an absolute number. @return The new limit. */ virtual int setLimit(int newLimit) = 0; /// Changes the limit of the counter. /** Changes the limit of the counter by adding a certain amount to the current limit. @param amount The amount by which to change the limit. @return The new limit. */ virtual int changeLimit(int amount) = 0; /// Returns the excess limit of the counter. /** Returns the excess limit of the counter, i.e. by how much the usual limit may be exceeded by prioritized reservations. @return The excess limit. */ virtual int getExcess() = 0; /// Sets the excess limit of the counter. /** This method sets a new excess limit for the counter. @param newExcess The new excess limit, an absolute number. @return The new excess limit. */ virtual int setExcess(int newExcess) = 0; /// Changes the excess limit of the counter. /** Changes the excess limit of the counter by adding a certain amount to the current excess limit. @param amount The amount by which to change the excess limit. @return The new excess limit. */ virtual int changeExcess(int amount) = 0; /// Returns the current value of the counter. /** Returns the current value of the counter, i.e. the number of unreserved units. Initially, the value is equal to the limit of the counter. When a reservation is made, the the value is decreased. Normally, the value should never be negative, but this may happen if there are prioritized reservations. It can also happen if the limit is decreased after some reservations have been made, since reservations are never revoked. @return The current value of the counter. */ virtual int getValue() = 0; /// Makes a reservation from the counter. /** This method makes a reservation from the counter. If the current value of the counter is too low to allow for the reservation, the method blocks until the reservation is possible or times out. @param amount The amount to reserve, default value is 1. @param duration The duration of a self expiring reservation, default is that it lasts forever. @param prioritized Whether this reservation is prioritized and thus allowed to use the excess limit. @param timeOut The maximum time to block if the value of the counter is too low, default is to allow "eternal" blocking. @return A CounterTicket that can be queried about the status of the reservation as well as for cancellations and extensions. */ virtual CounterTicket reserve(int amount = 1, Glib::TimeVal duration = ETERNAL, bool prioritized = false, Glib::TimeVal timeOut = ETERNAL) = 0; protected: /// Cancellation of a reservation. /** This method cancels a reservation. It is called by the CounterTicket that corresponds to the reservation. @param reservationID The identity number (key) of the reservation to cancel. */ virtual void cancel(IDType reservationID) = 0; /// Extension of a reservation. /** This method extends a reservation. It is called by the CounterTicket that corresponds to the reservation. @param reservationID Used for input as well as output. Contains the identification number of the original reservation on entry and the new identification number of the extended reservation on exit. @param expiryTime Used for input as well as output. Contains the expiry time of the original reservation on entry and the new expiry time of the extended reservation on exit. @param duration The time by which to extend the reservation. The new expiration time is computed based on the current time, NOT the previous expiration time. */ virtual void extend(IDType& reservationID, Glib::TimeVal& expiryTime, Glib::TimeVal duration = ETERNAL) = 0; /// Get the current time. /** Returns the current time. An "adapter method" for the assign_current_time() method in the Glib::TimeVal class. return The current time. */ Glib::TimeVal getCurrentTime(); /// Computes an expiry time. /** This method computes an expiry time by adding a duration to the current time. @param duration The duration. @return The expiry time. */ Glib::TimeVal getExpiryTime(Glib::TimeVal duration); /// A "relay method" for a constructor of the CounterTicket class. /** This method acts as a relay for one of the constructors of the CounterTicket class. That constructor is private, but needs to be accessible from the subclasses of Counter (but not from anywhere else). In order not to have to declare every possible subclass of Counter as a friend of CounterTicket, only the base class Counter is a friend and its subclasses access the constructor through this method. (If C++ had supported "package access", as Java does, this trick would not have been necessary.) @param reservationID The identity number of the reservation corresponding to the CounterTicket. @param expiryTime the expiry time of the reservation corresponding to the CounterTicket. @param counter The Counter from which the reservation has been made. @return The counter ticket that has been created. */ CounterTicket getCounterTicket(Counter::IDType reservationID, Glib::TimeVal expiryTime, Counter *counter); /// A "relay method" for the constructor of ExpirationReminder. /** This method acts as a relay for one of the constructors of the ExpirationReminder class. That constructor is private, but needs to be accessible from the subclasses of Counter (but not from anywhere else). In order not to have to declare every possible subclass of Counter as a friend of ExpirationReminder, only the base class Counter is a friend and its subclasses access the constructor through this method. (If C++ had supported "package access", as Java does, this trick would not have been necessary.) @param expTime the expiry time of the reservation corresponding to the ExpirationReminder. @param resID The identity number of the reservation corresponding to the ExpirationReminder. @return The ExpirationReminder that has been created. */ ExpirationReminder getExpirationReminder(Glib::TimeVal expTime, Counter::IDType resID); private: /// Copy constructor, should not be used. /** A private copy constructor, since Counters should never be copied. It should be impossible to use, but if that would happen by accident the program will exit with the EXIT_FAILURE code. */ Counter(const Counter& unique); /// Assignment operator, should not be used. /** A private assignment operator, since Counters should never be assigned. It should be impossible to use, but if that would happen by accident the program will exit with the EXIT_FAILURE code. */ void operator=(const Counter& unique); /// The CounterTicket class needs to be a friend. friend class CounterTicket; /// The ExpirationReminder class needs to be a friend. friend class ExpirationReminder; }; /// A class for "tickets" that correspond to counter reservations. /** This is a class for reservation tickets. When a reservation is made from a Counter, a ReservationTicket is returned. This ticket can then be queried about the validity of a reservation. It can also be used for cancellation and extension of reservations. Typical usage is: \code // Declare a counter. Replace XYZ by some appropriate kind of // counter and provide required parameters. Unit is MB. XYZCounter memory(...); ... // Make a reservation of memory for 2000000 doubles. CounterTicket tick = memory.reserve(2*sizeof(double)); // Use the memory. double* A=new double[2000000]; doSomething(A); delete[] A; // Cancel the reservation. tick.cancel(); \endcode \ingroup common \headerfile Counter.h arc/Counter.h */ class CounterTicket { public: /// The default constructor. /** This is the default constructor. It creates a CounterTicket that is not valid. The ticket object that is created can later be assigned a ticket that is returned by the reserve() method of a Counter. */ CounterTicket(); /// Returns the validity of a CounterTicket. /** This method checks whether a CounterTicket is valid. The ticket was probably returned earlier by the reserve() method of a Counter but the corresponding reservation may have expired. @return The validity of the ticket. */ bool isValid(); /// Extends a reservation. /** Extends a self-expiring reservation. In order to succeed the extension should be made before the previous reservation expires. @param duration The time by which to extend the reservation. The new expiration time is computed based on the current time, NOT the previous expiration time. */ void extend(Glib::TimeVal duration); /// Cancels a reservation. /** This method is called to cancel a reservation. It may be called also for self-expiring reservations, which will then be cancelled before they were originally planned to expire. */ void cancel(); private: /// A private constructor. /** This constructor creates an CounterTicket containing the specified expiry time and identity number of a reservation besides a pointer to the counter from which the reservation was made. In order to prevent unintended use, it is private. Because the Counter class must be able to use this constructor, it is declared to be a friend of this class. @param reservationID The identification number of the reservation. @param expiryTime The expiry time of the reservation. @param counter A pointer to the counter from which the reservation was made. */ CounterTicket(Counter::IDType reservationID, Glib::TimeVal expiryTime, Counter *counter); /// The identification number of the corresponding reservation. Counter::IDType reservationID; /// The expiry time of the corresponding reservation. Glib::TimeVal expiryTime; /// A pointer to the Counter from which the reservation was made. Counter *counter; //! The Counter class needs to be a friend. friend class Counter; }; /// A class intended for internal use within counters. /** This class is used for "reminder objects" that are used for automatic deallocation of self-expiring reservations. \ingroup common \headerfile Counter.h arc/Counter.h */ class ExpirationReminder { public: /// Less than operator, compares "soonness". /** This is the less than operator for the ExpirationReminder class. It compares the priority of such objects with respect to which reservation expires first. It is used when reminder objects are inserted in a priority queue in order to allways place the next reservation to expire at the top. */ bool operator<(const ExpirationReminder& other) const; /// Returns the expiry time. /** This method returns the expiry time of the reservation that this ExpirationReminder is associated with. @return The expiry time. */ Glib::TimeVal getExpiryTime() const; /// Returns the identification number of the reservation. /** This method returns the identification number of the self-expiring reservation that this ExpirationReminder is associated with. @return The identification number. */ Counter::IDType getReservationID() const; private: /// The constructor. /** This constructor creates an ExpirationReminder containing the specified expiry time and identity number of a reservation. In order to prevent unintended use, it is private. Because the Counter class must be able to use this constructor, it is declared to be a friend of this class. @param expiryTime The expiry time of the reservation. @param reservationID The identification number of the reservation. */ ExpirationReminder(Glib::TimeVal expiryTime, Counter::IDType reservationID); /// The expiry time of the corresponding reservation. Glib::TimeVal expiryTime; /// The identification number of t he corresponding reservation. Counter::IDType reservationID; /// The Counter class needs to be a friend. friend class Counter; }; } #endif