struct::pool(3tcl) Tcl Data Structures struct::pool(3tcl)______________________________________________________________________________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?
_________________________________________________________________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 inte‐
ger 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 configu‐
ration 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 visi‐
tors/travelers can make a reservation;
· the dynamic IP-addresses that an ISP can dole out to sub‐
scribers;
· a car rental's collection of cars, which can be rented by cus‐
tomers;
· the class rooms in a school building, which need to be sched‐
uled;
· 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 sup‐
posed 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 argu‐
ment. 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 program‐
matic 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 pre‐
viously. 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 out‐
comes 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 item‐
Var 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 interac‐
tive tclsh session, where each step is explained. The second example
shows the usage in a server application that talks to a back-end appli‐
cation.
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. How‐
ever, 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 $car]."
} else {
# request was denied
puts "No car available."
}
Note how the if command uses the value returned by the request subcom‐
mand.
# Suppose Mr. Wiggly has a preference for the Trabant:
if { [CarPool request car -allocID "Mr. Wiggly" -prefer Trabant] } {
# request was honoured, process the variable `car'
puts "$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://source‐
forge.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(3tcl)
