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Publishing Repositories with mod_dav

Publishing Repositories with mod_dav. Greg Stein CollabNet, Inc. gstein@lyra.org, http://www.lyra.org/greg/. Agenda. WebDAV introduction mod_dav introduction The mod_dav API for backends Example: mod_dav_fs Example: mod_dav_svn Future. WebDAV Introduction What is it? (1 of 2).

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Publishing Repositories with mod_dav

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  1. Publishing Repositories with mod_dav Greg Stein CollabNet, Inc. gstein@lyra.org, http://www.lyra.org/greg/

  2. Agenda • WebDAV introduction • mod_dav introduction • The mod_dav API for backends • Example: mod_dav_fs • Example: mod_dav_svn • Future

  3. WebDAV IntroductionWhat is it? (1 of 2) • Web-based Distributed Authoring and Versioning • “DAV” is the usual short form • Goal: enable interoperability of tools for distributed web authoring • Turns the Web into a writeable medium

  4. WebDAV Introduction What is it? (2 of 2) • Applies to all kinds of content - not just HTML and images • Based on extensions to HTTP • Uses XML for properties, control, status • RFC 2518 and RFC 3253

  5. WebDAV IntroductionScenarios • Collaborative authoring • Network file system • Remote software engineering • Unified repository-access protocol

  6. WebDAV Introduction Some Tools and Clients • Open source • Joe Orton’s sitecopy and cadaver • Nautilus (gnome-vfs), KDE (kio) • Subversion • Python, Perl, C, Java client APIs • Commercial • MacOS X, most Adobe and Microsoft products • Dreamweaver and other authoring tools • Lots of (commercial) DAV servers

  7. mod_dav Introduction • mod_dav is an Apache module • Separate module for Apache 1.3 • Integrated into Apache 2.0 • On June 1, securityspace.com reported 171505 sites using mod_dav (6th most popular Apache module)

  8. mod_dav Introduction • mod_dav has an API for a “pluggable back-end repository” • Over three years old, but still evolving • Suggestions welcome! • The default back-end repository uses the native filesystem • Oracle and ClearCase are shipping products using mod_dav with custom back-ends

  9. mod_dav Back-ends • Sometimes called “providers” • Implemented as Apache modules • Register themselves with mod_dav when they are first loaded • Each has a short name for reference • Used in the DAV directive (e.g. DAV svn) • Associated with a portion of the URL tree

  10. Apache 2.0 DSO loader mod_dav Back-ends mod_dav mod_dav_fs FS

  11. mod_dav Hooks • mod_dav does the protocol work and most of the interaction with Apache • Back-ends provide groups of functionality • Each functional group defines a set of “hooks” (a vtable) • Some entire groups, and some individual hooks, are optional • The provider fills in the groups and hooks that it understands and can respond to

  12. Groups of Functionality • Content (“repository”) required • “Dead” properties required • “Live” properties optional, recommended • Locks optional • Versioning optional, very complex • Binding optional, ill-defined • Searching optional, ill-defined

  13. How to Start • Look at mod_dav_fs • Set up your Apache module • Register your provider with mod_dav • Set up your dav_provider structure • Begin with the repository hooks • Stub out the dead property hooks • Leave the other groups as NULL

  14. Key Structures • dav_resource - center of the universe • The backend object being operated upon • The “info” field for your private data • dav_error - returning errors • dav_response - returning multistatus info • See mod_dav.h

  15. dav_hooks_repository • get_resource • open/close/seek/write_stream • set_headers, deliver • copy/move/remove_resource • create_collection • walk • miscellaneous

  16. get_resource • Identify a resource based on the user request • Four parameters • The Apache request_rec pointer • The root of this DAV-enabled space • Label header and DAV:checked-in property • Remember stuff in resource->info

  17. Dead Properties • Client-defined name/value pairs • XML fragments for values • Namespaces, xml:lang • Namespace management (dav_xmlns_info) • Share namespace definitions to shrink output • Coordinate among different types of fragments that occur in a response

  18. dav_hooks_propdb • open, close • define_namespaces, output_value • map_namespaces, store • remove • exists • first_name, next_name • get_rollback, apply_rollback

  19. Live Properties • Server-defined properties • Typically, values are generated (not stored) • Managed through Apache’s formal hook system and a mod_dav vtable • Via the hooks, standard properties are handled by mod_dav, but can be overridden • Modules can add their own properties

  20. Live Property Hooks • Apache hooks • gather_propsets • insert_all_liveprops • find_liveprop • mod_dav hooks • insert_prop • is_writable • patch_validate, patch_exec, patch_commit, patch_rollback (these are for PROPPATCH)

  21. mod_dav_fsOverview • Storage is in the filesystem • Implements basic function groups • Repository • Dead and live properties • Locking

  22. mod_dav_fsRepository • “Collections” are directories • “Member resources” are files • Straight-forward mapping • Apache’s Alias directive handles all of the URI to filesystem translation • Eek! Permissions and ownership

  23. mod_dav_fsDead Properties • Each directory has a .DAV subdirectory • DBM files for each file, plus one for the directory itself (all are lazily-created) • Simple key/value storage • Need to copy/move/delete DBM files

  24. mod_dav_fsLive Properties • Most are pulled from stat() • DAV:getcontentlength • DAV:creationdate • etc. • Some require Apache sub-requests • Custom property to tweak the “executable bit” on files – assists with uploading CGIs

  25. mod_dav_fsLocks • Oh, the pain… • “Locknull” resources complicate matters • Implementation • Server-wide lock database • Per-directory (text) file records the locknull resources

  26. Subversion • Subversion (“SVN”) is an Open Source project to replace CVS • Well, replace and improve • Network protocol is WebDAV • Apache 2.0 is the network server • mod_dav handles the frontend and protocol, SVN handles the backend • http://subversion.tigris.org/

  27. mod_dav_svnOverview • Storage is in Subversion’s repository • Implements these function groups • Repository • Dead and live properties • Versioning (!) • No locking

  28. mod_dav_svnRepository • All data is stored in Subversion • Subversion models a filesystem • Move, copy, delete are simple API calls • We need to generate the content for a GET

  29. mod_dav_svnDead Properties • The Subversion repository provides arbitrary properties for each file / directory • Oh. Gee. Simple.

  30. mod_dav_svnLive Properties • Live properties are generated by querying the Subversion API • Most of the live properties are for dealing with DeltaV • Some custom live properties that simplify the client’s life

  31. mod_dav_svnVersioning • Big topic (DeltaV has a lot of stuff) • SVN implements a subset of DeltaV • Subversion is not a true DeltaV server • Full compatibility is deferred for now • Look at the code for more info

  32. Your Repository • Define mappings from URLs to “resources” • Resources can be anything: people, documents, rows in a table • Define what the operations mean for your repository (“move a person? that is changing their name”) • Are dead properties applicable? • What live props are available? Writable?

  33. Future • Evolve the mod_dav API • More integration with Apache, apr(-util) • Performance and memory usage changes • Documentation(!) • New Open Source providers • Catacomb (a MySQL backend) • Shims to implement a backend in Perl, Python, or PHP

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