<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <style type="text/css"> /* <![CDATA[ */ @import "branding/css/tigris.css"; @import "branding/css/inst.css"; /* ]]> */</style> <link rel="stylesheet" type="text/css" media="print" href="branding/css/print.css"/> <script type="text/javascript" src="branding/scripts/tigris.js"></script> <title>Use of WebDAV in Subversion</title> </head> <body> <div class="app"> <h2>Use of WebDAV in Subversion</h2> <p> This document details how WebDAV is used within the <a href="http://subversion.tigris.org/">Subversion product</a>. Specifically, how the client side interfaces with <a href="http://www.webdav.org/neon/">Neon</a> to generate WebDAV requests over the wire, and what the server must do to map incoming WebDAV requests into operations against the Subversion repository. Note that the server side is implemented as an <a href="http://httpd.apache.org/">Apache 2.0</a> module, operating as a back-end for its mod_dav functionality. </p> <p> This document heavily refers to the <a href="http://subversion.tigris.org/files/documents/15/17/svn-design.html">Subversion design document</a> and the <a href="http://www.webdav.org/deltav/">latest Delta-V protocol draft</a>. Details of those documents will <em>not</em> be replicated here. </p> <table width="70%" border="0" cellspacing="0" cellpadding="10" style="background-color: #ff0000; color: white; margin-left: auto; margin-right: auto"> <tr> <td> <strong>NOTE:</strong> Subversion uses DeltaV for its communication, but the Subversion client is <strong>not</strong> a general-purpose DeltaV client. In fact, it expects some custom features from the server. Further, the Subversion server is <strong>not</strong> a general-purpose DeltaV server. It implements a strict <strong>subset</strong> of the DeltaV specification. A WebDAV or DeltaV client may very well be able to interoperate with it, but only if that client operates within the narrow confines of those features the server has implemented. </td> </tr> <tr> <td> Version 2.0 of Subversion will address WebDAV interoperability (Class 1, Class 2, and DeltaV features). Each of the custom features expected by the client actually has an alternate mechanism available in DeltaV, but in a much less efficient form. </td> </tr> <tr> <td> It is expected that Version 1.0 will support read-only, Class 1 WebDAV clients. Any "low-hanging fruit" to increase DeltaV interoperability will be considered. </td> </tr> </table> <p></p> <h3>Basic Concepts</h3> <p> Subversion uses a tree-based format to describe a change set against the repository. This tree is constructed on the client side (by "walking" over the working copy) to describe the change. The tree is marshalled to the server as a linear sequence of changes to be applied to the repository. The repository can accept changes in a random-access manner, so the mapping from a tree to a set of changes works very well for the repository. </p> <p> Subversion provides properties on files, directories, and even the abstract concept of a revision. Each of the operations involving properties are mapped directly to WebDAV properties, which are manipulated with the <span class="method">PROPFIND</span> and <span class="method">PROPPATCH</span> HTTP methods. Revisions are modeled as DeltaV <span class="term">baselines</span>, so revision properties are available through a <span class="method">PROPFIND</span> on the baseline. </p> <p> The Subversion server can efficiently compute deltas between two revisions (these deltas are complete <span class="term">tree deltas</span>, not simple text deltas). DeltaV does not have a direct analogue for the tree delta concept. A client could discover changes by issuing a sequence of <span class="method">PROPFIND</span> requests on the various WebDAV resources, but this would be a time-consuming operation, involving many requests. Instead, Subversion marshals this concept as a custom WebDAV <span class="term">report</span>. Using this report, the client learns which items in the working copy are out of date and can issue <span class="method">GET</span> and <span class="method">PROPFIND</span> methods to fetch the new data. </p> <p> Tags and branches are simple copies in Subversion, which are handled with the WebDAV <span class="method">COPY</span>. </p> <blockquote class="comment"> <p>need to talk about copies somewhere. need to discuss how copy history is retained (svn does it automatically, but interop with other servers may require us to set a custom property on those servers.</p> </blockquote> <h3>DeltaV Concepts Used by Subversion</h3> <p> Subversion uses many of the DeltaV concepts, as listed below. Note that many of these concepts are not fully implemented by Subversion; we have implemented enough to meet our needs, but little more. </p> <dl> <dt>Baseline</dt> <dd> <span class="comment">further info to come...</span><p></p> </dd> <dt>Activity</dt> <dd> <span class="comment">further info to come...</span><p></p> </dd> <dt>Version Resource</dt> <dd> <span class="comment">further info to come...</span><p></p> </dd> <dt>Version-Controlled Configuration</dt> <dd> <span class="comment">further info to come...</span><p></p> </dd> <dt>Baseline Collection</dt> <dd> <span class="comment">further info to come...</span><p></p> </dd> <dt>Version-Controlled Resource</dt> <dd> <span class="comment">further info to come...</span><p></p> </dd> <dt>Working Resource (Feature)</dt> <dd> <span class="comment">further info to come...</span><p></p> </dd> <dt>Merge Feature</dt> <dd> <span class="comment">further info to come...</span><p></p> </dd> <dt>Label Feature</dt> <dd> <span class="comment">further info to come...</span><p></p> </dd> <dt>Version-Controlled-Collection Feature</dt> <dd> <span class="comment">further info to come...</span><p></p> </dd> </dl> <h3>Subversion Projects as URLs</h3> <p> The very first concept to define is how a project is exposed to the client. Subversion will expose all projects as URLs on a server. The files and subdirectories under this project will be exposed through the URL namespace. </p> <p> For example, let us assume that we have a project named "example". And let us say that this project will be exposed at the URL: <span class="url">http://subversion.tigris.org/repos/example/</span>. </p> <p> This mapping is set up through a set of configuration parameters for the Apache HTTP Server (which is hosting the Subversion code and the particular project in question). The configuration could look like: </p> <blockquote> <pre><Location /repos/example> DAV svn SVNPath /home/svn-projects/example </Location></pre> </blockquote> <p> Files and directories within the project will be directly mapped into the URL namespace. For example, if the project contains a file "file.c" in a subdirectory "sub", then the URL for that file will be <span class="url">http://subversion.tigris.org/repos/example/sub/file.c</span>. </p> <h3>Initial Checkout</h3> <p> When the user performs the initial checkout of a Subversion project, the client will issue a series of <span class="method">PROPFIND</span> and <span class="method">GET</span> requests. These requests will traverse the repository, pick up some necessary metadata, and then fetch the latest revision. </p> <p class="comment"> describe the OPTIONS request for fetching the activity collection set. describe the sequence of PROPFINDs to reach the baseline collection. </p> <p class="comment"> <i>(moved here from below; need to rewrite)</i><br/> When the initial checkout was performed, Subversion fetched the <span class="prop">DAV:activity-collection-set</span> value and stored it as a property on each directory in the working copy. property for each collection. This property lists all of the locations on the server where an activity may be created. The first of these locations will be stored on the client for use during the commit process. </p> <p class="comment"> Should probably describe the metadata we fetch, and how a checkout of "not the latest" (e.g. by date or revision) will work. </p> <h3>Committing a Change</h3> <p> Subversion commits are modeled using the "activity" concept from DeltaV. An activity can be viewed as a transaction for a set of resources. </p> <h3>Creating the activity</h3> <p> At commit time, the Subversion client will retrieve the stored <span class="prop">DAV:activity-collection-set</span> value to know where it should create the activity. Next, the client will generate a UUID (a unique value) to use for the activity's location. Finally, the client will issue a <span class="method">MKACTIVITY</span> method request, where the Request-URL is composed from the activity location and the UUID. This request will construct an activity to hold all of the changes for the commit. </p> <p>Abbreviated summary:</p> <dl> <dt>At checkout time:</dt> <dd> <div>Request: <span class="method">OPTIONS</span> for <span class="prop">DAV:activity-collection-set</span></div> <div>Response: <span class="url">http://www.example.com/repos/foo/$svn/act/</span></div> </dd> <dt>At commit time:</dt> <dd> <div>Request: <span class="method">MKACTIVITY</span> <span class="url">http://www.example.com/repos/foo/$svn/act/01234567-89ab-cdef-0123-456789abcdef</span></div> <div>Response: 201 (Created)</div> </dd> </dl> <p> The <span class="method">CHECKOUT</span> method can specify an activity to use upon checkout. This feature is used to associate all items with the newly-created activity. </p> <h3>Storing the commit message</h3> <p class="comment"> talk about checking out the baseline and applying a PROPPATCH to the working baseline. </p> <h3>Mapping changes to WebDAV</h3> <p> A change set in Subversion is specified with a "tree delta" (see the SVN design for more details on the changes that can be placed into a tree delta). The tree delta will be unravelled into a set of requests. These requests will be one of the following forms: </p> <dl> <dt>Delete file or directory</dt> <dd> These changes are mapped onto a <span class="method">DELETE</span> operation. The version resource of the target's parent collection is checked out using the <span class="method">CHECKOUT</span> method (into the current activity). The target (name) is then deleted from the resulting working collection using the <span class="method">DELETE</span> method. <p></p> </dd> <dt>Add file</dt> <dd> This is modeled by performing a <span class="method">CHECKOUT</span> of the version resource of the target's parent collection. The new file is created within the resulting working collection using a <span class="method">PUT</span> request. Properties are applied using <span class="method">PROPPATCH</span>. <p></p> </dd> <dt>Add directory</dt> <dd> This is modeled by performing a <span class="method">CHECKOUT</span> on the version resource of the target's parent collection. The new directory is created within the resulting working collection with a <span class="method">MKCOL</span> request. Properties are applied using <span class="method">PROPPATCH</span>. <p></p> </dd> <dt>Add file or directory, with previous ancestory (a copy)</dt> <dd> <span class="comment">need to fix this section</span> <p> A tree delta can specify that a file/directory originates as a copy of another file/dir. This copy may be further modified by additional elements the tree delta. </p> <p> This change will be modeled by performing a <code>CHECKOUT</code> on the version resource of the parent collection which will contain the new resource. The <code>VERSION-CONTROL</code> method will create a new version-controlled resource (VCR) within the working collection, with the VCR's <code>DAV:checked-in</code> property referring to the ancestor's version resource. </p> <blockquote> <p><strong>Note:</strong> it appears that we will use <code>COPY</code> to copy the appropriate resource into the working collection. This will create a new version history which is then placed into the working collection. The version history will use the <code>DAV:precursor-set</code> property to specify the version resource of the ancestor.</p> <p> Because a version resource does not specify the revision, it will not be possible to <code>COPY</code> a version resource into the working collection -- it will not tell us what revision was copied. Instead, we will most likely copy a version resource out of the appropriate baseline. This implies the client must be able to map from a URL/revision pair to a baselined version resource URL. </p> <p> The second issue is whether/how we set the <code>DAV:precursor-set</code> property of the version history. Or, more precisely, how we synthesize the value from information stored in the repository. This is still under investigation. </p> </blockquote> </dd> <dt>Replace file/dir by another file/dir</dt> <dd> This change does not have a WebDAV modeling because tree deltas model it as two, sequential operations: a <em>delete</em>, followed by an <em>add</em>. <p></p> </dd> <dt>Moving a file or directory</dt> <dd> This change does not have a WebDAV modeling because tree deltas model it as two, distinct operations: a <em>delete</em>, and an <em>add</em> with previous ancestry. <p></p> </dd> <dt>Replace file</dt> <dd> This is modeled with a <span class="method">CHECKOUT</span> on the target's version resource, followed by a <span class="method">PUT</span> to the resulting working resource. <p></p> </dd> <dt>Replace directory</dt> <dd> In Subversion terms, "replace directory" means that additions, deletions, and other changes will occur <em>within</em> the directory. Each of these changes are modeled individually, and the change to the directory is performed implicitly. Therefore, this "change" has no particular mapping into WebDAV. <p></p> </dd> <dt>Property delta</dt> <dd> A property delta (against a file or directory) maps directly to a <span class="method">PROPPATCH</span> in WebDAV terms. The target's version resource will be checked out using <span class="method">CHECKOUT</span> and the <span class="method">PROPPATCH</span> will be applied to the resulting working resource. </dd> </dl> <h3>Final Commit</h3> <p> The final action of the commit process is to issue a <span class="method">MERGE</span> request to the Subversion server, specifying that the activity (created earlier) be checked in and the corresponding version-controlled resources be updated to refer to the new version resources. </p> <p class="comment"> the comment below is not quite right. talk about the working baseline, and how that is used to create a new baseline (with the commit message on it) </p> <p> The version-controlled resources are also baseline-controlled, which means that updates to them will automatically create a new baseline. In essence, the commit will create a new baseline corresponding to the new Subversion revision. </p> <h3>Example</h3> <p class="comment"> <strong>Warning:</strong> this section has not been updated to reflect some recent changes to the SVN-to-DAV mapping. Consider it out of date until this warning is removed. </p> <p> Consider the following set of operations and its corresponding tree delta (taken from the SVN design document): </p> <ol> <li>rename <code>/dir1/dir2</code> to <code>/dir1/dir4</code>,</li> <li>rename <code>/dir1/dir3</code> to <code>/dir1/dir2</code>, and</li> <li>move <code>file3</code> from <var>/dir1/dir4</var> to <var>/dir1/dir2</var>.</li> </ol> <pre><tree-delta> <replace name='dir1'> <directory> <tree-delta> <replace name='dir2'> <directory ancestor='/dir1/dir3'> (1) <tree-delta> <new name='file3'> (2) <file ancestor='/dir1/dir2/file3'/> </new> </tree-delta> </directory> </replace> <delete name='dir3'/> (3) <new name='dir4'> (4) <directory ancestor='/dir1/dir2'> <tree-delta> <delete name='file3'/> (5) </tree-delta> </directory> </new> </tree-delta> </directory> </replace> </tree-delta> </pre> <p> Walking through this delta, we map out the WebDAV requests listed below. The numbers in the above delta roughly correspond to the numbered entries below. The correspondence is not exact because a specific, resulting behavior is typically based on a combination of a few elements in the delta. </p> <ol> <li> The <code><directory ancestor="/dir1/dir3"></code> specifies that we are overwriting <code>/dir1/dir2</code> with <code>/dir1/dir3</code>. <p> <code>CHECKOUT /dir1/dir2/</code><br/> <i>(returns a working resource URL for the directory)</i> </p> <p> <code>COPY /dir1/dir3/</code><br/> <code>Destination: http://www.example.com/$svn/wrk/.../</code><br/> <code>Overwrite: T</code> </p> </li> <li> <code>/dir1/dir2/file3</code> is new (since we just overwrote the original <code>dir2</code> directory), and originates from <code>/dir1/dir2/file3</code>. Thus, we simply <code>COPY</code> the file into the target directory's working resource: <p> <code>COPY /dir1/dir2/file3</code><br/> <code>Destination: http://www.example.com/$svn/wrk/.../file3</code> </p> </li> <li> <p> <code>CHECKOUT /dir1/dir3/</code><br/> <i>(returns a working resource URL for the directory)</i> </p> <p> <code>DELETE /$svn/wrk/.../</code> </p> </li> <li> We are going to creating a new subdirectory (<code>dir4</code>) in the <code>/dir1</code> directory. Since we don't have <code>/dir1</code> checked out yet, we do so: <p> <code>CHECKOUT /dir1/</code><br/> <i>(returns a working resource URL for the directory)</i> </p> <p> And now we copy the right directory into the new working resource: </p> <p> <code>COPY /dir1/dir2/</code><br/> <code>Destination: http://www.example.com/$svn/wrk/.../dir4/</code> </p> </li> <li> The <code>COPY</code> created a complete set of working resources on the server, so we simply delete the part that we don't want: <p> <code>DELETE: /$svn/wrk/.../dir4/file3</code> </p> </li> </ol> <h3>URL Layout</h3> <p> The Subversion server exposes repositories at user-defined URLs. For example, the "foo" repository might be located at <span class="url">http://www.example.com/repos/foo/</span>. However, the server also requires a number of other resources to be exposed for proper operation. These additional resources will be associated with each repository in a location under the main repository URL. By default, this location is "<span class="url">$svn</span>". It may be changed by using the <code>SVNSpecialURI</code> directive: </p> <blockquote> <pre><Location /repos/foo> DAV svn SVNPath /home/svn-projects/foo SVNSpecialURI .special </Location></pre> </blockquote> <p> Underneath the location specified by <code>SVNSpecialURI</code>, we will expose several collections. Assuming we use the default of "<span class="url">$svn</span>", the collections are: </p> <dl> <dt><span class="url">$svn/act/</span></dt> <dd> This area is where activity resources are created. The client will pick a unique name within this collection and issue a <span class="method">MKACTIVITY</span> for that URL. The client will then use the activity in further interactions. <p> No methods are allowed on the <span class="url">$svn/act/</span> resource. </p> <blockquote> <p>Note: actually, we may want to allow a <code>PROPFIND</code> with a <code>Depth: 1</code> header to allow clients to enumerate the current activities.</p> </blockquote> <p> Only a subset of methods are allowed on the activities within the collection. They are: <span class="method">PROPFIND</span>, <span class="method">MERGE</span> (commit the activity), and <span class="method">DELETE</span> (abort the activity). </p> <p> Per the Delta-V specification, all activity resources will have a <span class="prop">DAV:resourcetype</span> of <span class="prop">DAV:activity</span>. </p> </dd> <dt><span class="url">$svn/his/</span></dt> <dd> <span class="comment">do something with this section; we actually don't use version history resources. in the future, they might be modeled like this</span> <p> This collection contains the version history resources for files and directories in a project. Its internal layout is completely server-defined. Clients will receive URLs into this collection (or a subcollection) from various responses. </p> <p> No methods are allowed on the <span class="url">$svn/his/</span> resource. </p> <p> Internally, the URL namespace is laid out with URLs of the following form: </p> <blockquote class="url"> <p>$svn/his/<var>node-id</var></p> </blockquote> <p> The <var>node-id</var> is an internal value that Subversion uses to reference individual files and directories. This <var>node-id</var> is a single integer defined by the Subversion repository. Note that this is an undotted node id, which is the base for the entire history of a given node in the repository. </p> <p> The <span class="prop">DAV:resourcetype</span> of the <var>node-id</var> collection is <span class="prop">DAV:version-history</span>. </p> <blockquote class="comment"> <p><strong>Note:</strong> the above information is probably not quite correct. The issue of linking one version history to another is still open. Further, I think that node 73 and node 73.4.1 are each version histories (where the latter is linked to the former). 73.x and 73.4.1.x are the versions within the version history.</p> </blockquote> </dd> <dt><span class="url">$svn/ver/</span></dt> <dd> This collection contains the version resources for the project. <p> No methods are allowed on the <span class="url">$svn/ver/</span> resource. </p> <p> The layout of this collection is internal to the server. For reference purposes here (and to describe the implementation), it is laid out as: </p> <blockquote class="url"> <p>$svn/ver/<var>node-id</var>/<var>path</var></p> </blockquote> <p> Only read-only methods are allowed against these resources (e.g. <span class="method">GET</span>, <span class="method">PROPFIND</span>, <span class="method">REPORT</span>); all other methods are illegal. </p> <p> The <span class="prop">DAV:resourcetype</span> of a version resource is simply the value of the resource at checkin time (e.g. <code><D:resourcetype/></code> or <code><D:resourcetype><D:collection/></D:resourcetype></code>). </p> </dd> <dt><span class="url">$svn/wrk/</span></dt> <dd> This collection contains working resources for the resources that have been checked out with the <span class="method">CHECKOUT</span> method. The form and construction of this collection is server-defined, but is also well-defined so that clients may interact properly with collection versions that have been checked out. <p> No methods are allowed on the <span class="method">$svn/wrk/</span> resource. </p> <p> For reference purposes, the working resource URLs are constructed as: </p> <blockquote class="url"> <p>$svn/wrk/<var>activity</var>/<var>path</var></p> </blockquote> <p> Any method is allowed on the working resources, but no methods are allowed on any of its parents. </p> <p> The <span class="prop">DAV:resourcetype</span> of the working resources follows normal resource typing: <code><D:resourcetype/></code> for regular working resources, and <code><D:resourcetype><D:collection/></D:resourcetype></code> for working collections. </p> </dd> <dt><span class="url">$svn/vcc/</span></dt> <dd> <span class="comment">This section is not yet complete.</span> <p> version-controlled configuration... </p> <p> <code>$svn/vcc/root</code> as a singleton. </p> </dd> <dt><code>$svn/bln/</code></dt> <dd> <span class="comment">This section is not yet complete.</span> <p> baselines... </p> <p> <code>$svn/bln/<var>rev</var>/</code> </p> </dd> <dt><code>$svn/wbl/</code></dt> <dd> <span class="comment">This section is not yet complete.</span> <p> working baseline... </p> </dd> <dt><code>$svn/bc/</code></dt> <dd> <span class="comment">This section is not yet complete.</span> <p> baseline collection... </p> </dd> </dl> <h3>Property Management (and History/Log Reporting)</h3> <p class="comment"> this section needs to be reworked. the properties occur on the FS revisions (and exposed via baselines). </p> <p> As mentioned before, Subversion properties map onto WebDAV properties. For history/log reporting, the following WebDAV properties will be applied to each baseline (a Subversion revision) and to each version resource created by the revision. Since these resources are all version resources, the properties below are read-only. </p> <dl> <dt><code>DAV:comment</code></dt> <dd> This is the standard (dead) property for specifying a checkin comment. <p></p> </dd> <dt><code>DAV:creator-displayname</code></dt> <dd> This is a (dead) property that is generated from Subversion's concept of the "user" who made a particular change. <p></p> </dd> <dt><code>DAV:creationdate</code></dt> <dd> This is a read-only live property created by the server at commit time. </dd> </dl> <p> The history for a specified file will be generated using the <code>REPORT</code> method and a <code>DAV:property-report</code> report. A typical history will fetch the three properties mentioned above for each version of the file/directory. </p> <p> Based on the client design, it may be important to specify other read-only live properties for information about versions. For example, how many lines were added/removed in a particular checkin for a file? Creating these live properties will be quite straight-forward, and driven by the client design over time. </p> <blockquote> <p>Note: if we do this, however, then we'd end up tying the client to the server. Of course, if the client were run against another DeltaV server which didn't report these properties, then we'd simply not display them in the UI. (e.g. graceful degradation of functionality)</p> </blockquote> <h3>Fetching Status and Updates</h3> <p> After the initial checkout, the client can request a status report (what has been changed on the client, pending a commit; what has been changed on the server, pending an update). The update process is similar, except that we also fetch the changes from the server. </p> <p> The local changes can be handled entirely on the client side. The Working Copy library can easily handle the detection and reporting of these changes. We're concerned with efficiently detecting what has changed on the server. </p> <p> While it would be possible to traverse the repository, fetching the current state, and comparing that to the client state, it would not be efficient. The Subversion design enables the server to easily compute what has changed (relative to the client), if it is given a description of the client state. </p> <p> The core of the <em>status</em> and <em>update</em> commands is based on a custom Subversion-specific WebDAV report. This custom report will transmit the state of the working copy to the server, and the server response will specify which resources will need to be updated (fetched). </p> <p> The request is a standard <code>REPORT</code> request, with a custom XML body. The body will use the standard Subversion technique of reporting a top-level revision number, and then only reporting children that have different revisions. The result of the report will use the same technique of reporting only the resources where a change is found. If a change is found, the server will provide a URL to the version resource to fetch for the changed resource. The server will also report the current revision number. </p> <blockquote class="comment"> <p>The XML DTDs for the request and response are TBD.</p> </blockquote> <blockquote> <p><small> The custom report will tie the client to only those servers which support the report, but a future version of the software will contain a fallback codepath, a graceful degradation, to support other DeltaV servers. </small></p> </blockquote> <p> When an updated is performed, the client will fetch each of the URLs (using <code>GET</code> requests) provided in the server response. </p> <p> <code>GET</code> (and <code>PUT</code>) operations will transfer content in a "diff" format when possible. The mechanics of this will follow the Internet Draft, titled <a href="http://www.ietf.org/internet-drafts/draft-mogul-http-delta-10.txt">Delta Encoding in HTTP</a>. </p> <h3>Entity Tags (etags)</h3> <p> Etags are required to be unique across all versions of a resource. Luckily, this is very easy for a version control system. Each etag will be simply be the repository's <em>node-id</em> for the resource. </p> <p> Etags are used to generate diffs, following the guidelines in the aforementioned draft: <a href="http://www.ietf.org/internet-drafts/draft-mogul-http-delta-10.txt">Delta Encoding in HTTP</a>. The problem then becomes how to get the etag for each file stored on the client (we don't need etags for directories since we never fetch them). During a <em>checkout</em> or <em>update</em> process, this is easy: the etag is provided in the HTTP response headers for each file retrieved. </p> <p> The other part of the problem is getting the etag after a <em>commit</em> has occurred. The <code>MERGE</code> response provides a way to request properties from the version resources which are created as part of the checkin of the activity. The etag (and other properties) can be fetched using that mechanism. </p> <h3>Tags and Branches</h3> <p> Tags and branches within Subversion are performed by copying from one area to another. For example: </p> <blockquote> <pre> [.../src/my-project]$ svn cp trunk tags/1.0.3-rc4 [.../src/my-project]$ svn commit </pre> </blockquote> <p> In the above example, <code>tags/1.0.3-rc4</code> should now be considered readonly and will always reflect the status of <code>trunk</code>. </p> <p> These copies are handled just like a regular commit. An activity is created with <span class="method">MKACTIVITY</span>, a working resource is created via <span class="method">CHECKOUT</span> (for the target directory; <code>tags/</code> in our example above), and then a <span class="method">COPY</span> is performed. The activity is then merged back into the repository with a <span class="method">MERGE</span> request. </p> <h3>Server Requirements</h3> <p class="comment"> <strong>Warning:</strong> this section is out of date. The DeltaV draft has gone through a number of revisions, and our use of DeltaV has changed some. </p> <h3>DAV Methods</h3> <p> The server will need to implement the following WebDAV methods for proper operation: </p> <ul> <li><span class="method">OPTIONS</span></li> <li><span class="method">GET</span></li> <li><span class="method">DELETE</span></li> <li><span class="method">COPY</span></li> <li><span class="method">PROPPATCH</span></li> <li><span class="method">PROPFIND</span></li> <li><span class="method">MKACTIVITY</span></li> <li><span class="method">CHECKOUT</span></li> <li><span class="method">MERGE</span></li> <li><span class="method">REPORT</span></li> </ul> <p> The following methods are not required by Subversion at this time: </p> <ul> <li><span class="method">CHECKIN</span></li> <li><span class="method">UNCHECKOUT</span></li> <li><span class="method">UPDATE</span></li> <li><span class="method">LABEL</span></li> <li><span class="method">VERSION-CONTROL</span></li> <li><span class="method">BASELINE-CONTROL</span></li> <li><span class="method">MKWORKSPACE</span></li> </ul> <h3>DAV Properties</h3> <p> The following DeltaV properties will be implemented: </p> <ul> <!-- resource properties --> <li><span class="prop">DAV:comment</span></li> <li><span class="prop">DAV:creator-displayname</span></li> <li><span class="prop">DAV:supported-method-set</span></li> <li><span class="prop">DAV:supported-live-property-set</span></li> <li><span class="prop">DAV:supported-report-set</span></li> <li><span class="prop">DAV:version-controlled-configuration</span></li> <!-- version-controlled resource properties --> <li><span class="prop">DAV:checked-in</span></li> <li> <span class="prop">DAV:auto-version</span> is a readonly, empty element (auto versioning not supported). </li> <!-- checked-out resource properties --> <li><span class="prop">DAV:checked-out</span></li> <li> <span class="prop">DAV:predecessor-set</span> <div><i> Note: the Subversion design document is not clear on the mechanics of how multiple predecessors are merged to create a single, new revision. When this clarifies, then <code>DAV:predecessor-set</code> may end up containing more than zero or one predecessor URLs </i></div> </li> <!-- version resource properties --> <!-- predecessor-set --> <li> <span class="prop">DAV:version-name</span> is simply the (global) revision number. </li> <li><span class="prop">DAV:checkout-fork</span></li> <li><span class="prop">DAV:checkin-fork</span></li> <!-- checked-out (working resource) properties --> <!-- checkout-fork, checkin-fork --> <li><span class="prop">DAV:auto-update</span></li> <li> <span class="prop">DAV:subbaseline-set</span> is a readonly, empty property (sub-baselines not supported). </li> <li> <span class="prop">DAV:unreserved</span> is set to <code>F</code>. </li> <!-- vcc properties --> <li><span class="prop">DAV:baseline-controlled-collection</span></li> <!-- baseline properties --> <!-- subbaseline-set --> <li><span class="prop">DAV:baseline-collection</span></li> <!-- activity properties --> <li> <span class="prop">DAV:subactivity-set</span> is a readonly, empty property (sub-activities not supported). </li> <!-- version-controlled collection properties --> <li> <span class="prop">DAV:eclipsed-set</span> is always empty (internal members can never be eclipsed). </li> </ul> <p> Contrary to the DeltaV specification, the following required properties will not be implemented: </p> <ul> <li> <span class="prop">DAV:successor-set</span> - this may be an expensive operation to synthesize this value. </li> <li> <span class="prop">DAV:checkout-set</span> - we do not record what has actually been checked out, but use the working resource URL to provide the necessary information; thus we have no record of the data to populate this property. </li> <li> <span class="prop">DAV:merge-set</span> - our <span class="method">MERGE</span> method is solely to support a commit. It doesn't really support the arbitrary merging defined within the spec. </li> <li> <span class="prop">DAV:auto-merge-set</span> - same as for <span class="prop">DAV:merge-set</span>. </li> <li> <span class="prop">DAV:activity-version-set</span> - activities are only used for working resources, so versions cannot be part of an activity. <div><i>maybe this should be defined as the empty set?</i></div> </li> <li> <span class="prop">DAV:activity-checkout-set</span> activities are only used for working resources, and we do not record what working resources "exist". </li> <li> <span class="prop">DAV:activity-set</span> - activities are only used for working resources, so versions cannot be part of an activity. <div><i>maybe this should be defined as the empty set?</i></div> </li> <li> <span class="prop">DAV:version-controlled-binding-set</span> - we do not have version history resources to include in the property. </li> </ul> <h3>OPTIONS</h3> <p> The <span class="method">OPTIONS</span> request will signal that it supports the following DAV features: </p> <ul> <li><code>1</code></li> <li><code>2</code></li> <li><code>version-control</code></li> <li><code>checkout</code></li> <li><code>working-resource</code></li> <li><code>merge</code></li> <li><code>baseline</code></li> <li><code>activity</code></li> <li><code>version-controlled-collection</code></li> </ul> <h3>Reports</h3> <p> The <span class="prop">DAV:supported-report-set</span> property will signal support for the following reports: </p> <ul> <li>svn:update-report</li> <li>svn:log-report</li> </ul> <p> These reports are available only on the "public" resources (the VCRs). They are not available on the resources within the <span class="url">$svn/</span> area. </p> <h3>Notes, reminders</h3> <p> Discuss timeouts and auto-purge of activities (and the related working resources). <br/> Discuss the activity database maintained by mod_dav_svn. <br/> Discuss other implementation details of ra_dav and mod_dav_svn. </p> <h3><a name="rationale">Appendix A: Rationale</a></h3> <p> Several times, people have asked, "Why choose HTTP/WebDAV/DeltaV? That seems awfully bloated and ill-suited. Why didn't you design a custom, well-tuned protocol? Or maybe use the CVS protocol?" Listed below are a number of reasons for our choice of WebDAV as our network protocol. </p> <blockquote> <p><small> While this list could certainly be expanded with more reasons (and to be fair, with a list of reasons why WebDAV was a poor choice), it certainly demonstrates the basic reasons for our choice. </small></p> </blockquote> <blockquote> <p><small> Note: this list came from an email note, so the tone and point of view might be a bit off. Further word-smithing is welcome... </small></p> </blockquote> <h3>Builtin web browsing of the repository</h3> <blockquote> <p>For example, take a look at: <a href="http://svn.apache.org/repos/asf/subversion/trunk/README">http://svn.apache.org/repos/asf/subversion/trunk/README</a> (that's the HEAD right there; we also have URLs for every previous revision of every file)</p> </blockquote> <h3>DAV-based browsing</h3> <blockquote> <p>Use Web Folders or WebDrive or somesuch on your Windows box (or Windows XP's native DAV mounts) to browse the SVN repository with Windows Explorer. Mac OS X has builtin DAV server mounting. Nautilus has DAV capabilities. Then you have your Open Source tools such as cadaver, Goliath, etc.</p> </blockquote> <h3>People can use existing libraries</h3> <blockquote> <p>I couldn't even begin to count the number of HTTP tools and libraries available. If we had designed our own protocol, then we would have /none/ of those benefits. Heck, two HTTP library implementors (Joe Orton of Neon, and Daniel Stenberg of CURL) are regulars here. we wouldn't get that benefit. I've used Python's httplib (and a davlib of my own) to do a lot of testing of our server. No need to go and roll new protocol libraries.</p> </blockquote> <h3>Existing tools</h3> <blockquote> <p>One word: Ethereal :-) When we capture network traces, Ethereal already knows about HTTP. It's quite nice, but I know there are even better ones out there. But we also have other tools like squid and other (caching) proxies (see the next item).</p> </blockquote> <h3>Caching proxies</h3> <blockquote> <p>Subversion will work great with caching proxies. There is no longer a need for specialized tools like "cvsup". Just drop in a caching proxy, and you've already got your distributed read-only repository. That European dev team can just drop in the cache between them and the US server and their checkouts/updates will get cached for the benefit of the other team members. Commits will flow through, back to the US-based server.</p> </blockquote> <h3>Sophisticated and broad-choice authentication</h3> <blockquote> <p>We don't have to reimplement an authentication scheme for a new protocol. We can use all of the various schemes that have been defined for HTTP. Ever look at the CVS protocol? Ever see the "I Love You" or "I Hate You" lines? :-) That is all part of creating a new authentication scheme. But we get to use SSL and certificate-based auth if we want. Kerberos. NTLM. or even just simple Basic or Digest. And our users can come from text files, database, LDAP, or PAM. We don't have to reinvent the wheel cuz it is all available for Apache already.</p> </blockquote> <h3>Awesome network server</h3> <blockquote> <p>We don't have to worry about how to portably set TCP_CORK for optimal network packets. We don't have to worry about when sendfile() makes sense, or if it is available. We don't have to worry about dropped client connections, how to best use threads and processes to scale, request management, monitoring, logging, etc. Apache gives us all of that and a ton more. I *really* would not want to do that through xinetd. I mean... setting TCP_CORK on stdout? freaky :-)</p> </blockquote> <h3>Well-defined on-wire compression</h3> <blockquote> <p>We already have on-wire compression, similar to CVS's "-z#" switch. And we didn't do anything. The client library and server that we use just support it automatically for us, according to RFC 2616.</p> </blockquote> <h3>Future interoperability</h3> <blockquote> <p>In the future, we'll be able to interoperate with a multitude of IDEs and other WebDAV/DeltaV clients. As DeltaV becomes more prevalent, IDEs could very well use it for source code management, and we'll be right there without needing to write some MS/SCC library to interface to the tool.</p> </blockquote> <hr/> <address><a href="mailto:gstein@lyra.org">Greg Stein</a></address> <!-- Created: Thu Aug 10 19:14:20 PDT 2000 --> <!-- hhmts start --> Last modified: Fri Jan 25 12:54:20 PST 2002 <!-- hhmts end --> </div> </body> </html>