Provided by: tcllib_1.17-dfsg-1_all 
NAME
struct::pool - Create and manipulate pool objects (of discrete items)
SYNOPSIS
package require Tcl 8.2
package require struct::pool ?1.2.3?
::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?
_________________________________________________________________________________________________
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:
• the seats in a cinema, theatre, train etc.. for which visitors/travelers can make
a reservation;
• the dynamic IP-addresses that an ISP can dole out to subscribers;
• a car rental's collection of cars, which can be rented by customers;
• the class rooms in a school building, which need to be scheduled;
• the database connections available to client-threads in a web-server application;
• the books in a library that customers can borrow;
• 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 Trackers
[http://core.tcl.tk/tcllib/reportlist]. 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>