tie(n)                        Tcl Data Structures                       tie(n)



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NAME
       tie - Array persistence

SYNOPSIS
       package require Tcl  8.4

       package require tie  ?1.1?

       ::tie::tie arrayvarname options... dstype dsname...

       ::tie::untie arrayvarname ?token?

       ::tie::info ties arrayvarname

       ::tie::info types

       ::tie::info type dstype

       ::tie::register dsclasscmd as dstype

       dsclasscmd objname ?dsname...?

       ds destroy

       ds names

       ds size

       ds get

       ds set dict

       ds unset ?pattern?

       ds setv index value

       ds unsetv index

       ds getv index

_________________________________________________________________

DESCRIPTION
       The tie package provides a framework for the creation of persistent Tcl
       array variables. It should be noted that the provided mechanism is
       generic enough to also allow its usage for the distribution of the
       contents of Tcl arrays over multiple threads and processes, i.e.
       communication.

       This, persistence and communication, is accomplished by tying) a Tcl
       array variable to a data source. Examples of data sources are other Tcl
       arrays and files.

       It should be noted that a single Tcl array variable can be tied to more
       than one data source. It is this feature which allows the framework to
       be used for communication as well. Just tie several Tcl arrays in many
       client processes to a Tcl array in a server and all changes to any of
       them will be distributed to all. Less centralized variants of this are
       of course possible as well.

USING TIES
   TIE API
       This section describes the basic API used to establish and remove ties
       between Tcl array variables and data sources. This interface is the
       only one a casual user has to be concerned about. The following
       sections about the various internal interfaces can be safely skipped.

       ::tie::tie arrayvarname options... dstype dsname...
              This command establishes a tie between the Tcl array whose name
              is provided by the argument arrayvarname and the data source
              identified by the dstype and its series of dsname arguments. All
              changes made to the Tcl array after this command returns will be
              saved to the data source for safekeeping (or distribution).

              The result of the command is always a token which identifies the
              new tie. This token can be used later to destroy this specific
              tie.

              varname arrayvarname (in)
                     The name of the Tcl array variable to connect the new tie
                     to.

              name|command dstype (in)
                     This argument specifies the type of the data source we
                     wish to access.  The dstype can be one of log, array,
                     remotearray, file, growfile, or dsource; in addition, the
                     programmer can register additional data source types.
                     Each dstype is followed by one or more arguments that
                     identify the data source to which the array is to be
                     tied.

              string dsname (in)
                     The series of dsname arguments coming after the dstype
                     identifies the data source we wish to connect to, and has
                     to be appropriate for the chosen type.

       The command understands a number of additional options which guide the
       process of setting up the connection between Tcl array and data source.


              -open  The Tcl array for the new tie is loaded from the data
                     source, and the previously existing contents of the Tcl
                     array are erased. Care is taken to not erase the previous
                     contents should the creation of the tie fail.

                     This option and the option -save exclude each other. If
                     neither this nor option -save are specified then this
                     option is assumed as default.

              -save  The Tcl array for the new tie is saved to the data
                     source, and the previously existing contents of the data
                     source are erased.

                     This option and the option -open exclude each other. If
                     neither this nor option -open are specified then option
                     -open is assumed as default.

              -merge Using this option prevents the erasure of any previously
                     existing content and merges the data instead. It can be
                     specified in conjunction with either -open or -save. They
                     determine how data existing in both Tcl array and data
                     source, i.e duplicates, are dealt with.

                     When used with -open data in the data source has
                     precedence.  In other words, for duplicates the data in
                     the data source is loaded into the Tcl array.

                     When used with -save data in the Tcl array has
                     precedence. In other words, for duplicates the data in
                     the Tcl array is saved into the data source.


       ::tie::untie arrayvarname ?token?
              This command dissolves one or more ties associated with the Tcl
              array named by arrayvarname. If no token is specified then all
              ties to that Tcl array are dissolved. Otherwise only the tie the
              token stands for is removed, if it is actually connected to the
              array. Trying to remove a specific tie not belonging to the
              provided array will cause an error.

              It should be noted that while severing a tie will destroy
              management information internal to the package the data source
              which was handled by the tie will not be touched, only closed.

              After the command returns none of changes made to the array will
              be saved to the data source anymore.

              The result of the command is an empty string.

              varname arrayname (in)
                     The name of a Tcl array variable which may have ties.

              handle token (in)
                     A handle representing a specific tie. This argument is
                     optional.


       ::tie::info ties arrayvarname
              This command returns a list of ties associated with the Tcl
              array variable named by arrayvarname. The result list will be
              empty if the variable has no ties associated with it.

       ::tie::info types
              This command returns a dictionary of registered types, and the
              class commands they are associated with.

       ::tie::info type dstype
              This command returns the fully resolved class command for a type
              name. This means that the command will follow a chain of type
              definitions ot its end.

   STANDARD DATA SOURCE TYPES
       This package provides the six following types as examples and standard
       data sources.

       log    This data source does not maintain any actual data, nor
              persistence. It does not accept any identifying arguments. All
              changes are simply logged to stdout.

       array  This data source uses a regular Tcl array as the origin of the
              persistent data. It accepts a single identifying argument, the
              name of this Tcl array. All changes are mirrored to that array.

       remotearray
              This data source is similar to array. The difference is that the
              Tcl array to which we are mirroring is not directly accessible,
              but through a send-like command.

              It accepts three identifying arguments, the name of the other
              Tcl array, the command prefix for the send-like accessor
              command, and an identifier for the remote entity hosting the
              array, in this order. All changes are mirrored to that array,
              via the command prefix. All commands will be executed in the
              context of the global namespace.

              send-like means that the command prefix has to have send syntax
              and semantics. I.e. it is a channel over which we can send
              arbitrary commands to some other entity.  The remote array data
              source however uses only the commands set, unset, array exists,
              array names, array set, and array get to retrieve and set values
              in the remote array.

              The command prefix and the entity id are separate to allow the
              data source to use options like -async when assembling the
              actual commands.

              Examples of command prefixes, listed with the id of the remote
              entity, without options. In reality only the part before the id
              is the command prefix:

              send tkname
                     The Tcl array is in a remote interpreter and is accessed
                     via Tk's X communication.

              comm::comm send hostportid
                     The Tcl array is in a remote interpreter and is accessed
                     through a socket.

              thread::send threadid
                     The Tcl array is in a remote interpreter in a different
                     thread of this process.


       file   This data source uses a single file as origin of the persistent
              data. It accepts a single identifying argument, the path to this
              file. The file has to be both readable and writable. It may not
              exist, the data source will create it in that case. This (and
              only this) situation will require that the directory for the
              file exists and is writable as well.

              All changes are saved in the file, as proper Tcl commands, one
              command per operation. In other words, the file will always
              contain a proper Tcl script.

              If the file exists when the tie using it is set up, then it will
              be compacted, i.e. superfluous operations are removed, if the
              operations log stored in it contains either at least one
              operation clearing the whole array, or at least 1.5 times more
              operations than entries in the loaded array.

       growfile
              This data source is like file in terms of the storage medium for
              the array data, and how it is configured. In constrast to the
              former it however assumes and ensures that the tied array will
              never shrink. I.e. the creation of new array entries, and the
              modification of existing entries is allowed, but the deletion of
              entries is not, and causes the data source to throw errors.

              This restriction allows us to simplify both file format and
              access to the file radically. For one, the file is read only
              once and the internal cache cannot be invalidated. Second,
              writing data is reduced to a simple append, and no compaction
              step is necessary. The format of the contents is the string
              representation of a dictionary which can be incrementally
              extended forever at the end.

       dsource
              This data source uses an explicitly specified data source object
              as the source for the persistent data. It accepts a single
              identifying argument, the command prefix, i.e. object command.

              To use this type it is necessary to know how the framework
              manages ties and what data source objects are.

              All changes are delegated to the specified object.

CREATING NEW DATA SOURCES
       This section is of no interest to the casual user of ties. Only
       developers wishing to create new data sources have to know the
       information provided herein.

   DATA SOURCE OBJECTS
       All ties are represented internally by an in-memory object which
       mediates between the tie framework and the specific data source, like
       an array, file, etc.  This is the data source object.

       Its class, the data source class is not generic, but specific to the
       type of the data source. Writing a new data source requires us to write
       such a class, and then registering it with the framework as a new type.

       The following subsections describe the various APIs a data source class
       and the objects it generates will have to follow to be compatible with
       the tie framework.

       Data source objects are normally automatically created and destroyed by
       the framework when a tie is created, or removed. This management can be
       explicitly bypassed through the usage of the "dsource" type.  The data
       source for this type is a data source object itself, and this object is
       outside of the scope of the tie framework and not managed by it.  In
       other words, this type allows the creation of ties which talk to pre-
       existing data source objects, and these objects will survive the
       removal of the ties using them as well.

   REGISTERING A NEW DATA SOURCE CLASS
       After a data source class has been written it is necessary to register
       it as a new type with the framework.

       ::tie::register dsclasscmd as dstype
              Using this command causes the tie framework to remember the
              class command dsclasscmd of a data source class under the type
              name dstype.

              After the call the argument dstype of the basic user command
              ::tie::tie will accept dstype as a type name and translate it
              internally to the appropriate class command for the creation of
              data source objects for the new data source.

   DATA SOURCE CLASS
       Each data source class is represented by a single command, also called
       the class command, or object creation command. Its syntax is

       dsclasscmd objname ?dsname...?
              The first argument of the class command is the name of the data
              source object to create.  The framework itself will always
              supply the string %AUTO%, to signal that the class command has
              to generate not only the object, but the object name as well.

              This is followed by a series of arguments identifying the data
              source the new object is for. These are the same dsname
              arguments which are given to the basic user command ::tie::tie.
              Their actual meaning is dependent on the data source class.

              The result of the class command has to be the fully-qualified
              name of the new data source object, i.e. the name of the object
              command.  The interface this command has to follow is described
              in the section DATA SOURCE OBJECT API

   DATA SOURCE OBJECT API
       Please read the section DATA SOURCE CLASS first, to know how to
       generate new object commands.

       Each object command for a data source object has to provide at least
       the methods listed below for proper inter-operation with the tie
       framework. Note that the names of most of the methods match the
       subcommands of the builtin array command.

       ds destroy
              This method is called when the object ds is destroyed. It now
              has to release all its internal resources associated with the
              external data source.

       ds names
              This command has to return a list containing the names of all
              keys found in the data source the object talks to. This is
              equivalent to array names.

       ds size
              This command has to return an integer number specifying the
              number of keys found in the data source the object talks to.
              This is equivalent to array size.

       ds get This command has to return a dictionary containing the data
              found in the data source the object talks to. This is equivalent
              to array get.

       ds set dict
              This command takes a dictionary and adds its contents to the
              data source the object talks to. This is equivalent to array
              set.

       ds unset ?pattern?
              This command takes a pattern and removes all elements whose keys
              matching it from the data source. If no pattern is specified it
              defaults to *, causing the removal of all elements. This is
              nearly equivalent to array unset.

       ds setv index value
              This command has to save the value in the data source the object
              talks to, under the key index.

              The result of the command is ignored. If an error is thrown then
              this error will show up as error of the set operation which
              caused the method call.

       ds unsetv index
              This command has to remove the value under the key index from
              the data source the object talks to.

              The result of the command is ignored. If an error is thrown then
              this error will show up as error of the unset operation which
              caused the method call.

       ds getv index
              This command has to return the value for the key index in the
              data source the object talks to.

       And here a small table comparing the data source methods to the regular
       Tcl commands for accessing an array.

                      Regular Tcl             Data source
                      -----------             -----------
                      array names a           ds names
                      array size  a           ds size
                      array get   a           ds get
                      array set   a dict      ds set   dict
                      array unset a pattern   ds unset ?pattern?
                      -----------             -----------
                      set a($idx) $val        ds setv   idx val
                      unset a($idx)           ds unsetv idx
                      $a($idx)                ds getv   idx
                      -----------             -----------


BUGS, IDEAS, FEEDBACK
       This document, and the package it describes, will undoubtedly contain
       bugs and other problems.  Please report such in the category tie of the
       Tcllib SF Trackers [http://sourceforge.net/tracker/?group_id=12883].
       Please also report any ideas for enhancements you may have for either
       package and/or documentation.

KEYWORDS
       array, database, file, metakit, persistence, tie, untie

CATEGORY
       Programming tools

COPYRIGHT
       Copyright (c) 2004-2008 Andreas Kupries <andreas_kupries@users.sourceforge.net>




tie                                   1.1                               tie(n)
