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initial commit of silvermirror, from http://my-svn.assembla.com/svn/arbez/silvermirror
author k0s <k0scist@gmail.com>
date Mon, 07 Sep 2009 15:39:06 -0400
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abb358e2434c initial commit of silvermirror, from http://my-svn.assembla.com/svn/arbez/silvermirror
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1 SilverMirror Whitepaper
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2
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3 It is necessary to maintain parallel directory structures of various
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4 resources across an arbitrary number of computers. The traditional
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5 approach is the central server model, where files live in one
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6 canonical location and the network is used to give access to the data.
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7 However, this model has deficiencies, chiefly among them that if the
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8 server goes down or must be moved a considerable amount of effort must
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9 be extended to set up a new central server.
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10
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11 Distributed version control, often of nominal use (in the case where a
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12 canonical trunk exists) is ideally suited to provide mirroring of
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13 desired resources across computers.
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14
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15 Implementation
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16
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17 A front end to a DVCS - most likely mercurial but potentially bzr -
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18 will be written to keep resources in sync across an arbitrary number
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19 of computers. The front end, called SilverMirror, may be used to push
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20 or pull changes to resources. Optionally, a daemon will monitor
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21 changes to resources and push or pull changes at desired intervals.
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22
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23 The use should be as natural as possible and require no interaction
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24 for everday tasks. A resource consists of a directory and all
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25 subdirectories and their contents. Once a resource is denoted as
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26 versioned, any change to the resource's directory structure should be
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27 mirrored across machines without user intervention. Files matching a
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28 pattern may be ignored, either globally or on a per resource basis,
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29 for the purpose of versioning.
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30
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31 Configuration
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32
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33 SilverMirror is configured via an INI file containing a section for
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34 each resource and a section for application configuration.
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35
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36 The main section, denoted [::SilverMirror::], has the following options:
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37
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38 * directory: base directory for SilverMirror. The SilverMirror
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39 configuration is stored in ${directory}/.silvermirror . If omitted,
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40 the user's home directory is used.
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41
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42 * ignore: global patterns of files and directories to ignore. Paths
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43 matching these patterns will not be versioned.
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44
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45 Each section has the following configuration options:
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46
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47 * directory: path of the resource. If a relative path is used, it is
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48 joined with the directory setting from the main section. If this
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49 setting is not specified, the section name is used as a relative path.
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50
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51 * ignore: paths not to version on a per resource basis. This is in
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52 addition to the patterns specified by the ignore setting in the main
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53 section.
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54
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55 * conflict: handler to resolve conflict.
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56
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57 * hosts: hosts to push/pull from
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58
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59 In order to ensure coherency among resources, all relevant
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60 configuration options must be synced prior to push/pull transactions.
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61
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62 Default Configuration:
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63
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64 [::SilverMirror::]
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65 conflict = ClobberRemote
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66
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67 Example of a more complex configuration:
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68
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69 [::SilverMirror::]
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70 conflict.push = ClobberRemote
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71 conflict.pull = ClobberLocal
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72
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73 Push
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74
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75 Push changes to remote resources. When resources are pushed, first
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76 changes are pulled from each remote host in turn, conflicts between
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77 local and remote changes are resolved (see Behavior on Conflicts),
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78 then local modifications are pushed. This is done to keep the
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79 resources in sync.
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80
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81 When new files are added to the resource they are automatically added
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82 to the hg repository. When resource files are edited the changes are
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83 pushed to the repository. When a conflict occurs between local
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84 resources and remote resources, the conflict handler is used.
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85
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86 Pull
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87
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88 Get changes to the cloud filesystem resources. If no host is
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89 specified, pull changes from all known + accessible hosts.
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90
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91 Namespaced Resources
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92
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93 It is possible to maintain versioning of a subdirectory within a
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94 resource.
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95
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96 Example:
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97
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98 [docs]
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99 directory = /path/to/docs
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100
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101 [docs:private]
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102
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103 This configuration snippet describes a resource, [docs:private],
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104 namespaced within the [docs] resource. [docs:private] inherits
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105 configuration and behavior from [docs] but may be dealt with
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106 separately. For example, some computers in the cloud may not have
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107 [docs:private] specified in their configuration and so will not get a
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108 copy of it upon pulling. A common use case is specifying a
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109 subdirectory to be omitted with the ignore option in the configuration
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110 file, then, when this subdirectory needs to be shared between multiple
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111 computers, removing it from the ignore values and including as a
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112 namespaced resource.
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113
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114 In the above example, because the directory option was not specified
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115 in the [docs:private] section, the path to [docs:private] is taken
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116 from its namespace (private) and the directory of its parent resource.
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117 So its base directory is /path/to/docs/private . If a relative path
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118 was specified in the directory option of the [docs:private] section,
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119 it would be joined with the base directory of [docs].
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120
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121 Behavior on Conflicts
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122
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123 Conflict handlers are set via setuptools entry points. Several
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124 conflict handlers are provided with SilverMirror:
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125
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126 * ClobberLocal: replace local changes with changes from remote files
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127
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128 * ClobberRemote: replace remote file changes with changes from local
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129 files
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130
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131 * Edit: invoke an editor (default: $EDITOR) to interactively resolve
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132 the conflicts
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133
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134 The conflict handler may also be specified from the command line:
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135
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136 silvermirror push -d ClobberRemote
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137
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138 Command Line Usage
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139
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140 silvermirror [push|pull] [resource] [options]
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141
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142 In the simplest invocation, SilverMirror is used with no command line
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143 arguments:
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144
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145 silvermirror
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146
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147 This pushes changes of the resource as determined by the current
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148 working directory after pulling outstanding changes from all
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149 applicable remote computers and invoking the conflict handler for
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150 push. If the current working directory is not within a resource, all
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151 resources will be pushed.
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152
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153 Finer control is obtained by specifying command line arguments:
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154
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155 [push|pull] : whether to use the push method (which includes pulling
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156 for changes; see above) or the pull method. If not specified, the
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157 resource is pushed.
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158
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159 [resource] : which resource to act upon. This can be the resource
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160 name, as specified in the .ini file, or the path to the base directory
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161 of the resource. Note that if a path is specified, it must be to the
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162 base directory of the resource as SilverMirror has no notion of
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163 disparate versioning within a resource. If the resource is not
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164 specified, the resource that the current working directory is within
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165 is used. If the current working directory is not in a resource path,
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166 all resources are acted upon in turn. If the key word "--all" is used,
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167 all resources will also be acted upon.
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168
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169 [options] : several command line switches are available to the
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170 silvermirror program:
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171
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172 * -d <handler> : specify which conflict handler to use. <handler>
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173 should be the name of the desired conflict handler. A list of all
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174 conflict handlers is available with the "--conflict-handlers" option.
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175
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176 * -H <host>, --host=<host> : pull and/or push only to specified
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177 hosts. If this option is used more than once, the hosts specified will
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178 be acted upon.
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179
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180 * --conflict-handlers : list the name and description (if
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181 specified) for all available conflict handlers.
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182
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183 * --help : displays help and usage information
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184
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185 Behavior Respecting Versioned Directories
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186
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187 SilverMirror does not desire to duplicate versioning on directories
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188 already under version control (svn, bzr, hg). These resources are
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189 automatically ignore. In a future implementation, these resources
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190 would optionally be checked out or updated upon a pull.
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191
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192 Automatic Syncronization
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193
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194 SilverMirror includes a script that will automatically invoke
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195 syncronizing the resources in a specified period of time. This daemon,
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196 called silvermirrord, is invoked from the command line with options
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197 parallel to the silvermirror program. One additional option, -s, tells
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198 how many seconds between syncs. Upon invocation, this program puts
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199 itself in the background and performs the desired sync every number of
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200 seconds specified. It is important that the conflict handler specified
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201 is noninteractive, otherwise the daemon will hang forever.
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202
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203 As an alternative, the silvermirror program may be invoked from a cron
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204 job.
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205
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206 Future Work
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207
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208 SilverMirror implements a cloud filesystem which may be accessed
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209 nearly transparently by an arbitrary number of computers. Several
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210 improvements could extend SilverMirror to solve several deficiencies
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211 of modern filesystem.
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212
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213 * tagging: in most filesystems, a file has a canonical location.
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214 However, it may be desirable to have the file accesible via multiple
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215 paths. In practice, this is achieved via symbolic links. However, this
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216 requires manual maintaince of links vs the canonical location. Noting
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217 that this problem is identical to tagging, a solution minimizing
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218 manual intervention could be added to SilverMirror.
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219
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220 * update of web documents: modern computers deal heavily with
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221 documents via URLs. It is noted that this includes files, the URL of a
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222 file with path ${PATH} being file://${PATH} noted implicitly from
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223 contexts. However, existing shells and operating systems have no
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224 mechanism for indicating that a "file" is a web resource. Such
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225 functionality could be added to SilverMirror so that up-to-date
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226 versions of web resources could be maintained. This infrastructure
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227 could also include notions for updating versioned resources (see
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228 Behavior Respecting Versioned Directories) with parallel notation.