struct::pool(n)               Tcl Data Structures              struct::pool(n)



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NAME
       struct::pool - Create and manipulate pool objects (of discrete items)

SYNOPSIS
       package require Tcl  8.2

       package require struct::pool  ?1.2.1?

       ::struct::pool ?poolName? ?maxsize?

       poolName option ?arg arg ...?

       poolName add itemName1 ?itemName2 itemName3 ...?

       poolName clear ?-force?

       poolName destroy ?-force?

       poolName info type ?arg?

       poolName maxsize ?maxsize?

       poolName release itemName

       poolName remove itemName ?-force?

       poolName request itemVar ?options?

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DESCRIPTION
       This package provides pool objects which can be used to manage finite
       collections of discrete items.

       ::struct::pool ?poolName? ?maxsize?
              Creates a new pool object. If no poolName is supplied, then the
              new pool will be named poolX, where X is a positive integer.
              The optional second argument maxsize has to be a positive
              integer indicating the maximum size of the pool; this is the
              maximum number of items the pool may hold. The default for this
              value is 10.

              The pool object has an associated global Tcl command whose name
              is poolName. This command may be used to invoke various
              configuration operations on the report. It has the following
              general form:

              poolName option ?arg arg ...?
                     Option and the args determine the exact behavior of the
                     command. See section POOL OBJECT COMMAND for a detailed
                     list of options and their behaviour.

POOLS AND ALLOCATION
       The purpose of the pool command and the pool object command that it
       generates, is to manage pools of discrete items.  Examples of a pool of
       discrete items are:

       o      the seats in a cinema, theatre, train etc.. for which
              visitors/travelers can  make a reservation;

       o      the dynamic IP-addresses that an ISP can dole out  to
              subscribers;

       o      a car rental's collection of cars, which can be rented by
              customers;

       o      the class rooms in a school building, which need to be
              scheduled;

       o      the database connections available to client-threads in a web-
              server application;

       o      the books in a library that customers can borrow;

       o      etc ...

       The common denominator in the examples is that there is a more or less
       fixed number of items (seats, IP-addresses, cars, ...) that are
       supposed to be allocated on a more or less regular basis. An item can
       be allocated only once at a time. An item that is allocated, must be
       released before it can be re-allocated.  While several items in a pool
       are being allocated and released continuously, the total number of
       items in the pool remains constant.

       Keeping track of which items are allocated, and by whom, is the purpose
       of the pool command and its subordinates.

       Pool parlance: If we say that an item is allocated, it means that the
       item is busy, owned or occupied; it is not available anymore. If an
       item is free, it is available. Deallocating an item is equivalent to
       setting free or releasing an item. The person or entity to which the
       item has been allotted is said to own the item.

ITEMS
       Discrete items

       The pool command is designed for discrete items only. Note that there
       are pools where allocation occurs on a non-discrete basis, for example
       computer memory. There are also pools from which the shares that are
       doled out are not expected to be returned, for example a charity fund
       or a pan of soup from which you may receive a portion. Finally, there
       are even pools from which nothing is ever allocated or returned, like a
       swimming pool or a cesspool.

       Unique item names

       A pool cannot manage duplicate item names. Therefore, items in a pool
       must have unique names.

       Item equivalence

       From the point of view of the manager of a pool, items are equivalent.
       The manager of a pool is indifferent about which entity/person occupies
       a given item. However, clients may have preferences for a particular
       item, based on some item property they know.

       Preferences

       A future owner may have a preference for a particular item. Preference
       based allocation is supported (see the -prefer option to the request
       subcommand). A preference for a particular item is most likely to
       result from variability among features associated with the items. Note
       that the pool commands themselves are not designed to manage such item
       properties. If item properties play a role in an application, they
       should be managed separately.

POOL OBJECT COMMAND
       The following subcommands and corresponding arguments are available to
       any pool object command.

       poolName add itemName1 ?itemName2 itemName3 ...?
              This command adds the items on the command line to the pool. If
              duplicate item names occur on the command line, an error is
              raised. If one or more of the items already exist in the pool,
              this also is considered an error.

       poolName clear ?-force?
              Removes all items from the pool. If there are any allocated
              items at the time when the command is invoked, an error is
              raised. This behaviour may be modified through the -force
              argument. If it is supplied on the command line, the pool will
              be cleared regardless the allocation state of its items.

       poolName destroy ?-force?
              Destroys the pool data structure, all associated variables and
              the associated pool object command. By default, the command
              checks whether any items are still allocated and raises an error
              if such is the case. This behaviour may be modified through the
              argument -force. If it is supplied on the command line, the pool
              data structure will be destroyed regardless allocation state of
              its items.

       poolName info type ?arg?
              Returns various information about the pool for further
              programmatic use. The type argument indicates the type of
              information requested. Only the type allocID uses an additional
              argument.

              allocID itemName
                     returns the allocID of the item whose name is itemName.
                     Free items have an allocation id of -1.

              allitems
                     returns a list of all items in the pool.

              allocstate
                     Returns a list of key-value pairs, where the keys are the
                     items and the values are the corresponding allocation
                     id's. Free items have an allocation id of -1.

              cursize
                     returns the current pool size, i.e. the number of items
                     in the pool.

              freeitems
                     returns a list of items that currently are not allocated.

              maxsize
                     returns the maximum size of the pool.


       poolName maxsize ?maxsize?
              Sets or queries the maximum size of the pool, depending on
              whether the maxsize argument is supplied or not. If maxsize is
              supplied, the maximum size of the pool will be set to that
              value. If no argument is supplied, the current maximum size of
              the pool is returned. In this variant, the command is an alias
              for:

              poolName info maxsize.

              The maxsize argument has to be a positive integer.

       poolName release itemName
              Releases the item whose name is itemName that was allocated
              previously. An error is raised if the item was not allocated at
              the time when the command was issued.

       poolName remove itemName ?-force?
              Removes the item whose name is itemName from the pool. If the
              item was allocated at the time when the command was invoked, an
              error is raised. This behaviour may be modified through the
              optional argument -force. If it is supplied on the command line,
              the item will be removed regardless its allocation state.

       poolName request itemVar ?options?
              Handles a request for an item, taking into account a possible
              preference for a particular item. There are two possible
              outcomes depending on the availability of items:

              [1]    The request is honoured, an item is allocated and the
                     variable whose name is passed with the argument itemVar
                     will be set to the name of the item that was allocated.
                     The command returns 1.

              [2]    The request is denied. No item is allocated. The variable
                     whose name is itemVar is not set.  Attempts to read
                     itemVar may raise an error if the variable was not
                     defined before issuing the request. The command returns
                     0.

       The return values from this command are meant to be inspected. The
       examples below show how to do this. Failure to check the return value
       may result in erroneous behaviour. If no preference for a particular
       item is supplied through the option -prefer (see below), then all
       requests are honoured as long as items are available.

       The following options are supported:

              -allocID allocID
                     If the request is honoured, an item will be allocated to
                     the entity identified by allocID. If the allocation state
                     of an item is queried, it is this allocation ID that will
                     be returned. If the option -allocID is not supplied, the
                     item will be given to and owned by dummyID. Allocation
                     id's may be anything except the value -1, which is
                     reserved for free items.

              -prefer preferredItem
                     This option modifies the allocation strategy as follows:
                     If the item whose name is preferredItem is not allocated
                     at the time when the command is invoked, the request is
                     honoured (return value is 1). If the item was allocated
                     at the time when the command was invoked, the request is
                     denied (return value is 0).

EXAMPLES
       Two examples are provided. The first one mimics a step by step
       interactive tclsh session, where each step is explained. The second
       example shows the usage in a server application that talks to a back-
       end application.

       Example 1

       This example presents an interactive tclsh session which considers the
       case of a Car rental's collection of cars. Ten steps explain its usage
       in chronological order, from the creation of the pool, via the most
       important stages in the usage of a pool, to the final destruction.

       Note aside:

       In this example, brand names are used to label the various items.
       However, a brand name could be regarded as a property of an item.
       Because the pool command is not designed to manage properties of items,
       they need to be managed separately. In the latter case the items should
       be labeled with more neutral names such as: car1, car2, car3 , etc ...
       and a separate database or array should hold the brand names associated
       with the car labels.

                   1. Load the package into an interpreter
                   % package require pool
                   0.1

                   2. Create a pool object called `CarPool' with a maximum size of 55 items (cars):
                   % pool CarPool 55
                   CarPool

                   4. Add items to the pool:
                   % CarPool add Toyota Trabant Chrysler1 Chrysler2 Volkswagen

                   5. Somebody crashed the Toyota. Remove it from the pool as follows:
                   % CarPool remove Toyota

                   6. Acquired a new car for the pool. Add it as follows:
                   % CarPool add Nissan

                   7. Check whether the pool was adjusted correctly:
                   % CarPool info allitems
                   Trabant Chrysler1 Chrysler2 Volkswagen Nissan


       Suspend the interactive session temporarily, and show the programmatic
       use of the request subcommand:

                   # Mrs. Swift needs a car. She doesn't have a preference for a
                   # particular car. We'll issue a request on her behalf as follows:
                   if { [CarPool request car -allocID "Mrs. Swift"] }  {
                       # request was honoured, process the variable `car'
                       puts "$car has been allocated to [CarPool info allocID $car]."
                   } else {
                       # request was denied
                        puts "No car available."
                   }


       Note how the if command uses the value returned by the request
       subcommand.

                   # Suppose Mr. Wiggly has a preference for the Trabant:
                   if { [CarPool request car -allocID "Mr. Wiggly" -prefer Trabant] }  {
                       # request was honoured, process the variable `car'
                       puts "$car has been allocated to [CarPool info allocID $car]."
                   } else {
                       # request was denied
                        puts "The Trabant was not available."
                   }


       Resume the interactive session:

                   8. When the car is returned then you can render it available by:
                   % CarPool release Trabant

                   9. When done, you delete the pool.
                   % CarPool destroy
                   Couldn't destroy `CarPool' because some items are still allocated.

                   Oops, forgot that Mrs. Swift still occupies a car.

                   10. We force the destruction of the pool as follows:
                   % CarPool destroy -force


       Example 2

       This example describes the case from which the author's need for pool
       management originated. It is an example of a server application that
       receives requests from client applications. The client requests are
       dispatched onto a back-end application before being returned to the
       client application. In many cases there are a few equivalent instances
       of back-end applications to which a client request may be passed along.
       The file descriptors that identify the channels to these back-end
       instances make up a pool of connections. A particular connection may be
       allocated to just one client request at a time.

                   # Create the pool of connections (pipes)
                   set maxpipes 10
                   pool Pipes $maxpipes
                   for {set i 0} {$i < $maxpipes} {incr i} {
                       set fd [open "|backendApplication" w+]
                       Pipes add $fd
                   }

                   # A client request comes in. The request is identified as `clientX'.
                   # Dispatch it onto an instance of a back-end application
                   if { [Pipes request fd -allocID clientX] } {
                       # a connection was allocated
                       # communicate to the back-end application via the variable `fd'
                       puts $fd "someInstruction"
                       # ...... etc.
                   } else {
                       # all connections are currently occupied
                       # store the client request in a queue for later processing,
                       # or return a 'Server busy' message to the client.
                   }


BUGS, IDEAS, FEEDBACK
       This document, and the package it describes, will undoubtedly contain
       bugs and other problems.  Please report such in the category struct ::
       pool of the Tcllib SF Trackers
       [http://sourceforge.net/tracker/?group_id=12883].  Please also report
       any ideas for enhancements you may have for either package and/or
       documentation.

KEYWORDS
       discrete items, finite, pool, struct

CATEGORY
       Data structures

COPYRIGHT
       Copyright (c) 2002, Erik Leunissen <e.leunissen@hccnet.nl>




struct                               1.2.1                     struct::pool(n)
