pool - Create and manipulate pool objects (of discrete items)
package require Tcl 8.2
package require struct ?1.3?
This package provides pool objects which can be used to manage finite collections of discrete items.
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 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.
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.
The following subcommands and corresponding arguments are available to any pool object command.
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
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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 $car]."
} else {
# request was denied
puts "No car available."
}
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Note how the if command uses the value returned by the request subcommand.
# Suppose Mr. Wiggly has a preference for the Trabant:
if { [CarPool request car -allocID "Mr. Wiggly" -prefer Trabant] } {
# request was honoured, process the variable `car'
puts "$car has been allocated to [CarPool info allocID $car]."
} else {
# request was denied
puts "The Trabant was not available."
}
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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
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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.
}
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discrete items, finite, pool, struct
Copyright © 2002, Erik Leunissen <e.leunissen@hccnet.nl>