# pool(n) 1.2.1 struct "Tcl Data Structures"

## NAME

pool - Create and manipulate pool objects (of discrete items)

## SYNOPSIS

package require Tcl 8.2
package require struct ?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 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/travellers 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 allocID type 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 labelled 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.
         }

## KEYWORDS

struct, pool, finite, discrete items