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# transfer::copy::queue(n) 0.1 tcllib "Data transfer facilities"

## Name

transfer::copy::queue - Queued transfers

## Synopsis

- package require Tcl 8.4
- package require snit ?1.0?
- package require struct::queue ?1.4?
- package require transfer::copy ?0.2?
- package require transfer::copy::queue ?0.1?

- transfer::copy::queue objectName outchannel ?options...?
- objectName method ?arg arg ...?
- objectName destroy
- objectName busy
- objectName pending
- objectName put request

## Description

This package provides objects which serialize transfer requests for a single channel by means of a fifo queue. Accumulated requests are executed in order of entrance, with the first request reaching an idle object starting the execution in general. New requests can be added while the object is active and are defered until all requests entered before them have been completed successfully.

When a request causes a transfer error execution stops and all requests coming after it are not served. Currently this means that their completion callbacks are never triggered at all.

NOTE: Not triggering the completion callbacks of the unserved requests after an error stops the queue object is something I am not fully sure that it makes sense. It forces the user of the queue to remember the callbacks as well and run them. Because otherwise everything in the system which depends on getting a notification about the status of a request will hang in the air. I am slowly convincing myself that it is more sensible to trigger the relevant completion callbacks with an error message about the queue abort, and 0 bytes transfered.

All transfer requests are of the form

    	{type data options...}

where type is in {chan, string}, and data specifies the information to transfer. For chan the data is the handle of the channel containing the actual information to transfer, whereas for string data contains directly the information to transfer. The options are a list of them and their values, and are the same as are accepted by the low-level copy operations of the package transfer::copy. Note how just prepending the request with transfer::copy::do and inserting a channel handle in between data and options easily transforms it from a pure data structure into a command whose evaluation will perform the request.

## API

### Package commands

transfer::copy::queue objectName outchannel ?options...?

  This command creates a new queue object for the management of the channel outchannel, with an associated Tcl command whose name is objectName. This object command is explained in full detail in the sections Object command and Object methods. The set of supported options is explained in section Options.

  The object command will be created under the current namespace if the objectName is not fully qualified, and in the specified namespace otherwise. The fully qualified name of the object command is returned as the result of the command.

### Object command

All objects created by the ::transfer::copy::queue command have the following general form:

objectName method ?arg arg ...?

  The method method and its arg'uments determine the exact behavior of the command. See section Object methods for the detailed specifications.

### Object methods

objectName destroy

  This method destroys the object. Doing so while the object is busy will cause errors later on, when the currently executed request completes and tries to access the now missing data structures of the destroyed object.

objectName busy

  This method returns a boolean value telling us if the object is currently serving a request (i.e. busy, value True), or not (i.e. idle, value False).

objectName pending

  This method returns the number of requests currently waiting in the queue for their execution. A request currently served is not counted as waiting.

objectName put request

  This method enters the transfer request into the object's queue of waiting requests. If the object is idle it will become busy, immediately servicing the request. Otherwise servicing the new request will be defered until all preceding requests have been served.

## Options

The only option known is -on-status-change. It is optional and defaults to the empty list, disabling the reporting of status changes. Otherwise its argument is a command prefix which is invoked whenever the internal status of the object changed. The callback is invoked with two additional arguments, the result of the methods pending and busy, in this order. This allows any user to easily know, for example, when the object has processed all outstanding requests.

## Use

A possible application of this package and class is within a HTTP 1.1 server, managing the results waiting for transfer to the client.

It should be noted that in this application the system also needs an additional data structure which keeps track of outstanding results as they may come back in a different order than the requests from the client, and releases them to the actual queue in the proper order.

## Bugs, Ideas, Feedback

This document, and the package it describes, will undoubtedly contain bugs and other problems. Please report such in the category transfer of the Tcllib Trackers. Please also report any ideas for enhancements you may have for either package and/or documentation.

## Keywords

channel, copy, queue, transfer

## Category

Transfer module

## Copyright

Copyright © 2006 Andreas Kupries <andreas_kupries@users.sourceforge.net>