author | Denis Laxalde <denis.laxalde@logilab.fr> |
Wed, 29 Mar 2017 13:29:41 +0200 | |
branch | 3.25 |
changeset 12114 | 40446d4e1ee2 |
parent 10491 | c67bcee93248 |
permissions | -rw-r--r-- |
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[doc] Book entry for the undo functionnality.
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Undoing changes in CubicWeb |
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[doc] Book entry for the undo functionnality.
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--------------------------- |
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[doc] Book entry for the undo functionnality.
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3 |
|
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Many desktop applications offer the possibility for the user to |
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undo its last changes : this *undo feature* has now been |
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integrated into the CubicWeb framework. This document will |
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introduce you to the *undo feature* both from the end-user and the |
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application developer point of view. |
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|
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But because a semantic web application and a common desktop |
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application are not the same thing at all, especially as far as |
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undoing is concerned, we will first introduce *what* is the *undo |
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feature* for now. |
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|
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What's *undoing* in a CubicWeb application |
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
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|
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What is an *undo feature* is quite intuitive in the context of a |
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desktop application. But it is a bit subtler in the context of a |
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Semantic Web application. This section introduces some of the main |
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differences between a classical desktop and a Semantic Web |
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applications to keep in mind in order to state precisely *what we |
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want*. |
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|
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The notion transactions |
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``````````````````````` |
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|
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A CubicWeb application acts upon an *Entity-Relationship* model, |
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described by a schema. This allows to ensure some data integrity |
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properties. It also implies that changes are made by all-or-none |
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groups called *transactions*, such that the data integrity is |
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preserved whether the transaction is completely applied *or* none |
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of it is applied. |
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|
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A transaction can thus include more actions than just those |
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directly required by the main purpose of the user. For example, |
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when a user *just* writes a new blog entry, the underlying |
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*transaction* holds several *actions* as illustrated below : |
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|
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* By admin on 2012/02/17 15:18 - Created Blog entry : Torototo |
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|
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#. Created Blog entry : Torototo |
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#. Added relation : Torototo owned by admin |
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#. Added relation : Torototo blog entry of Undo Blog |
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#. Added relation : Torototo in state draft (draft) |
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#. Added relation : Torototo created by admin |
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|
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Because of the very nature (all-or-none) of the transactions, the |
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"undoable stuff" are the transactions and not the actions ! |
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|
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Public and private actions within a transaction |
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``````````````````````````````````````````````` |
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|
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Actually, within the *transaction* "Created Blog entry : |
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Torototo", two of those *actions* are said to be *public* and |
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the others are said to be *private*. *Public* here means that the |
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public actions (1 and 3) were directly requested by the end user ; |
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whereas *private* means that the other actions (2, 4, 5) were |
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triggered "under the hood" to fulfill various requirements for the |
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user operation (ensuring integrity, security, ... ). |
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|
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And because quite a lot of actions can be triggered by a "simple" |
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end-user request, most of which the end-user is not (and does not |
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need or wish to be) aware, only the so-called public actions will |
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appear [1]_ in the description of the an undoable transaction. |
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[doc] Book entry for the undo functionnality.
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|
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* By admin on 2012/02/17 15:18 - Created Blog entry : Torototo |
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[doc] Book entry for the undo functionnality.
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|
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#. Created Blog entry : Torototo |
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#. Added relation : Torototo blog entry of Undo Blog |
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[doc] Book entry for the undo functionnality.
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|
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But note that both public and private actions will be undone |
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together when the transaction is undone. |
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[doc] Book entry for the undo functionnality.
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|
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[doc] Book entry for the undo functionnality.
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(In)dependent transactions : the simple case |
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[doc] Book entry for the undo functionnality.
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76 |
```````````````````````````````````````````` |
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[doc] Book entry for the undo functionnality.
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|
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[doc] Book entry for the undo functionnality.
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A CubicWeb application can be used *simultaneously* by different users |
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(whereas a single user works on an given office document at a |
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[doc] Book entry for the undo functionnality.
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given time), so that there is not always a single history |
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time-line in the CubicWeb case. Moreover CubicWeb provides |
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[doc] Book entry for the undo functionnality.
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security through the mechanism of *permissions* granted to each |
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user. This can lead to some transactions *not* being undoable in |
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some contexts. |
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[doc] Book entry for the undo functionnality.
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85 |
|
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In the simple case two (unprivileged) users Alice and Bob make |
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relatively independent changes : then both Alice and Bob can undo |
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their changes. But in some case there is a clean dependency |
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between Alice's and Bob's actions or between actions of one of |
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them. For example let's suppose that : |
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|
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- Alice has created a blog, |
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- then has published a first post inside, |
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- then Bob has published a second post in the same blog, |
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- and finally Alice has updated its post contents. |
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96 |
|
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Then it is clear that Alice can undo her contents changes and Bob |
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98 |
can undo his post creation independently. But Alice can not undo |
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her post creation while she has not first undone her changes. |
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It is also clear that Bob should *not* have the |
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101 |
permissions to undo any of Alice's transactions. |
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102 |
|
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103 |
|
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More complex dependencies between transactions |
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105 |
`````````````````````````````````````````````` |
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106 |
|
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But more surprising things can quickly happen. Going back to the |
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108 |
previous example, Alice *can* undo the creation of the blog after |
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109 |
Bob has published its post in it ! But this is possible only |
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110 |
because the schema does not *require* for a post to be in a |
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111 |
blog. Would the *blog entry of* relation have been mandatory, then |
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112 |
Alice could not have undone the blog creation because it would |
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113 |
have broken integrity constraint for Bob's post. |
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114 |
|
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115 |
When a user attempts to undo a transaction the system will check |
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116 |
whether a later transaction has explicit dependency on the |
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117 |
would-be-undone transaction. In this case the system will not even |
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118 |
attempt the undo operation and inform the user. |
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119 |
|
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120 |
If no such dependency is detected the system will attempt the undo |
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121 |
operation but it can fail, typically because of integrity |
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122 |
constraint violations. In such a case the undo operation is |
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123 |
completely [3]_ rollbacked. |
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124 |
|
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125 |
|
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126 |
The *undo feature* for CubicWeb end-users |
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127 |
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
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128 |
|
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129 |
The exposition of the undo feature to the end-user through a Web |
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130 |
interface is still quite basic and will be improved toward a |
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131 |
greater usability. But it is already fully functional. For now |
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132 |
there are two ways to access the *undo feature* as long as the it |
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133 |
has been activated in the instance configuration file with the |
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134 |
option *undo-support=yes*. |
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135 |
|
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136 |
Immediately after having done the change to be canceled through |
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137 |
the **undo** link in the message. This allows to undo an |
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138 |
hastily action immediately. For example, just after having |
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139 |
validated the creation of the blog entry *A second blog entry* we |
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140 |
get the following message, allowing to undo the creation. |
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141 |
|
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142 |
.. image:: /images/undo_mesage_w600.png |
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143 |
:width: 600px |
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144 |
:alt: Screenshot of the undo link in the message |
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145 |
:align: center |
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146 |
|
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147 |
At any time we can access the **undo-history view** accessible from the |
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148 |
start-up page. |
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149 |
|
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150 |
.. image:: /images/undo_startup-link_w600.png |
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151 |
:width: 600px |
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152 |
:alt: Screenshot of the startup menu with access to the history view |
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153 |
:align: center |
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154 |
|
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155 |
This view will provide inspection of the transaction and their (public) |
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156 |
actions. Each transaction provides its own **undo** link. Only the |
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157 |
transactions the user has permissions to see and undo will be shown. |
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158 |
|
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159 |
.. image:: /images/undo_history-view_w600.png |
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160 |
:width: 600px |
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161 |
:alt: Screenshot of the undo history main view |
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162 |
:align: center |
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163 |
|
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164 |
If the user attempts to undo a transaction which can't be undone or |
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165 |
whose undoing fails, then a message will explain the situation and |
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166 |
no partial undoing will be left behind. |
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167 |
|
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168 |
This is all for the end-user side of the undo mechanism : this is |
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169 |
quite simple indeed ! Now, in the following section, we are going |
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170 |
to introduce the developer side of the undo mechanism. |
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171 |
|
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172 |
The *undo feature* for CubicWeb application developers |
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173 |
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
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174 |
|
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175 |
A word of warning : this section is intended for developers, |
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176 |
already having some knowledge of what's under CubicWeb's hood. If |
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177 |
it is not *yet* the case, please refer to CubicWeb documentation |
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178 |
http://docs.cubicweb.org/ . |
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179 |
|
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180 |
Overview |
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181 |
```````` |
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182 |
|
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183 |
The core of the undo mechanisms is at work in the *native source*, |
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184 |
beyond the RQL. This does mean that *transactions* and *actions* |
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185 |
are *no entities*. Instead they are represented at the SQL level |
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186 |
and exposed through the *DB-API* supported by the repository |
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187 |
*Connection* objects. |
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188 |
|
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189 |
Once the *undo feature* has been activated in the instance |
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190 |
configuration file with the option *undo-support=yes*, each |
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191 |
mutating operation (cf. [2]_) will be recorded in some special SQL |
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192 |
table along with its associated transaction. Transaction are |
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193 |
identified by a *txuuid* through which the functions of the |
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194 |
*DB-API* handle them. |
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195 |
|
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196 |
On the web side the last commited transaction *txuuid* is |
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197 |
remembered in the request's data to allow for imediate undoing |
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198 |
whereas the *undo-history view* relies upon the *DB-API* to list |
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199 |
the accessible transactions. The actual undoing is performed by |
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200 |
the *UndoController* accessible at URL of the form |
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201 |
`www.my.host/my/instance/undo?txuuid=...` |
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202 |
|
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203 |
The repository side |
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204 |
``````````````````` |
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205 |
|
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206 |
Please refer to the file `cubicweb/server/sources/native.py` and |
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|
207 |
`cubicweb/transaction.py` for the details. |
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|
208 |
|
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209 |
The undoing information is mainly stored in three SQL tables: |
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210 |
|
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211 |
`transactions` |
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212 |
Stores the txuuid, the user eid and the date-and-time of |
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213 |
the transaction. This table is referenced by the two others. |
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214 |
|
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215 |
`tx_entity_actions` |
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216 |
Stores the undo information for actions on entities. |
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217 |
|
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218 |
`tx_relation_actions` |
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219 |
Stores the undo information for the actions on relations. |
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220 |
|
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221 |
When the undo support is activated, entries are added to those |
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222 |
tables for each mutating operation on the data repository, and are |
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223 |
deleted on each transaction undoing. |
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|
224 |
|
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225 |
Those table are accessible through the following methods of the |
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226 |
repository `Connection` object : |
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|
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`undoable_transactions` |
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Returns a list of `Transaction` objects accessible to the user |
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and according to the specified filter(s) if any. |
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|
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`tx_info` |
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Returns a `Transaction` object from a `txuuid` |
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|
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`undo_transaction` |
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Returns the list of `Action` object for the given `txuuid`. |
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|
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NB: By default it only return *public* actions. |
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239 |
|
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The web side |
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```````````` |
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242 |
|
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The exposure of the *undo feature* to the end-user through the Web |
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interface relies on the *DB-API* introduced above. This implies |
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that the *transactions* and *actions* are not *entities* linked by |
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*relations* on which the usual views can be applied directly. |
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247 |
|
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That's why the file `cubicweb/web/views/undohistory.py` defines |
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some dedicated views to access the undo information : |
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250 |
|
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`UndoHistoryView` |
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This is a *StartupView*, the one accessible from the home |
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page of the instance which list all transactions. |
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254 |
|
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`UndoableTransactionView` |
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This view handles the display of a single `Transaction` object. |
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257 |
|
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`UndoableActionBaseView` |
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This (abstract) base class provides private methods to build |
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the display of actions whatever their nature. |
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261 |
|
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`Undoable[Add|Remove|Create|Delete|Update]ActionView` |
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263 |
Those views all inherit from `UndoableActionBaseView` and |
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each handles a specific kind of action. |
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265 |
|
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`UndoableActionPredicate` |
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This predicate is used as a *selector* to pick the appropriate |
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view for actions. |
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269 |
|
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Apart from this main *undo-history view* a `txuuid` is stored in |
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the request's data `last_undoable_transaction` in order to allow |
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immediate undoing of a hastily validated operation. This is |
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handled in `cubicweb/web/application.py` in the `main_publish` and |
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`add_undo_link_to_msg` methods for the storing and displaying |
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respectively. |
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276 |
|
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Once the undo information is accessible, typically through a |
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`txuuid` in an *undo* URL, the actual undo operation can be |
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performed by the `UndoController` defined in |
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`cubicweb/web/views/basecontrollers.py`. This controller basically |
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extracts the `txuuid` and performs a call to `undo_transaction` and |
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in case of an undo-specific error, lets the top level publisher |
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handle it as a validation error. |
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284 |
|
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285 |
|
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Conclusion |
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~~~~~~~~~~ |
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288 |
|
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289 |
The undo mechanism relies upon a low level recording of the |
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290 |
mutating operation on the repository. Those records are accessible |
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through some method added to the *DB-API* and exposed to the |
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292 |
end-user either through a whole history view of through an |
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immediate undoing link in the message box. |
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294 |
|
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The undo feature is functional but the interface and configuration |
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296 |
options are still quite reduced. One major improvement would be to |
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297 |
be able to filter with a finer grain which transactions or actions |
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one wants to see in the *undo-history view*. Another critical |
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299 |
improvement would be to enable the undo feature on a part only of |
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300 |
the entity-relationship schema to avoid storing too much useless |
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301 |
data and reduce the underlying overhead. |
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302 |
|
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But both functionality are related to the strong design choice not |
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to represent transactions and actions as entities and |
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305 |
relations. This has huge benefits in terms of safety and conceptual |
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306 |
simplicity but prevents from using lots of convenient CubicWeb |
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307 |
features such as *facets* to access undo information. |
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308 |
|
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309 |
Before developing further the undo feature or eventually revising |
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310 |
this design choice, it appears that some return of experience is |
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311 |
strongly needed. So don't hesitate to try the undo feature in your |
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312 |
application and send us some feedback. |
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313 |
|
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314 |
|
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315 |
Notes |
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~~~~~ |
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317 |
|
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.. [1] The end-user Web interface could be improved to enable |
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319 |
user to choose whether he wishes to see private actions. |
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320 |
|
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321 |
.. [2] There is only five kind of elementary actions (beyond |
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322 |
merely accessing data for reading): |
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323 |
|
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* **C** : creating an entity |
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* **D** : deleting an entity |
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* **U** : updating an entity attributes |
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327 |
* **A** : adding a relation |
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328 |
* **R** : removing a relation |
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329 |
|
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.. [3] Meaning none of the actions in the transaction is |
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331 |
undone. Depending upon the application, it might make sense |
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332 |
to enable *partial* undo. That is to say undo in which some |
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333 |
actions could not be undo without preventing to undo the |
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334 |
others actions in the transaction (as long as it does not |
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335 |
break schema integrity). This is not forbidden by the |
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336 |
back-end but is deliberately not supported by the front-end |
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337 |
(for now at least). |