Anatomy of a request

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Access Control Layer

THIS PAGE IS UNDER CONSTRUCTION! JLRS 2014-10-23

This is a description of how EPrints and Apache handles an incoming request. Understanding this flow helps understand how an Access Control layer can be added to the system.

I will assume that you know how to locate a perl module file from the module name (e.g. EPrints::Apache::Rewrite will probably be ~/perl_lib/EPrints/Apache/Rewrite.pm, although this is not always the case!).

Flow of a request

Below are relevant parts of config files and perl modules that are used with when processing a request. The request will generally be dealt with by the EPrints::Apache::Rewrite module, and farmed out from there. How the request reaches this module is also explained below.

Apache core config ~/cfg/apache.conf

PerlSwitches -I/home/eprints/eprints-3.3.12/perl_lib
PerlModule EPrints
PerlPostConfigHandler +EPrints::post_config_handler

The post_config_handler does some sanity checks on the EPrints setup (e.g. is Apache listening to the ports that the repositories are configured to work under) See: http://perl.apache.org/docs/2.0/user/handlers/server.html#C_PerlPostConfigHandler_ for more info about the post_config_handler

Apache repository config ~/cfg/apache/ARCHIVEID.conf

<VirtualHost *:80>
...
  PerlTransHandler +EPrints::Apache::Rewrite
 
</VirtualHost>

This leads us to the backbone of EPrints - the Rewrite module - where URL_REWRITE_* triggers are called; content negotiation can happen, as well as many other wonderous things!

EPrints::Apache::Rewrite module

This explanation, and line numbers are taken from a specific version of the file: https://github.com/eprints/eprints/blob/88a36fcf1f17c7a04e60455d374b617709f7461d/perl_lib/EPrints/Apache/Rewrite.pm Obviously this file will change over time, so it's worth comparing it with the version you are using, and possibly other versions on GitHub e.g. https://github.com/eprints/eprints/blob/3.3/perl_lib/EPrints/Apache/Rewrite.pm

It's worth taking a few minutes to look at this file - specifically the sub handler.

EP_TRIGGER_URL_REWRITE

Line 123 calls the 'EP_TRIGGER_URL_REWRITE' trigger:

$repository->run_trigger( EPrints::Const::EP_TRIGGER_URL_REWRITE,
	request => $r,
	   lang => $lang,    # en
	   args => $args,    # "" or "?foo=bar"
	urlpath => $urlpath, # "" or "/subdir"
	cgipath => $cgipath, # /cgi or /subdir/cgi
	    uri => $uri,     # /foo/bar
	 secure => $secure,  # boolean
    return_code => \$rc,     # set to trigger a return
);

If any of the EP_TRIGGER_URL_REWRITE's return a return_code, this is returned. Information on triggers

CGI scripts

Line 157 deals with CGI scripts - redirecting to HTTPS if necessary. It looks for the CGI scripts in three locations Lines 195-199:

  • ~/archives/ARCHIVEID/cgi/
  • ~/site_lib/cgi/
  • ~/cgi/

If the cgi script is a 'user' script, it also defines a PerlAccessHandler Lines 214-220

if( $uri =~ m! ^/users\b !x )
{
	$r->push_handlers(PerlAccessHandler => [
		\&EPrints::Apache::Auth::authen,
		\&EPrints::Apache::Auth::authz
	] );
}

SWORD servicedocument

Lines 233-258 deal with the 'Sword' service document, via the CRUD interface.

REST interface

Lines 281-290 handle the REST interface, via EPrints::Apache::REST

EPrints URIs

Lines 292-374: EPrint URIs are normally of the form http://repository.blah/id/.... There are three main if blocks in this section that use regex's to match the URI:

  • Line 293 $uri =~ m! ^$urlpath/id/(repository|dump)$ !x matches two cases.
  • Line 318 $uri =~ m! ^$urlpath/id/([^\/]+)/(ext-.*)$ !x matches ??? Some RDF type stuff!? 'event/ext-foo'..?
  • Lines 345-347 (shown on one line here) $uri =~ s! ^$urlpath/id/(?: contents | ([^/]+)(?:/([^/]+)(?:/([^/]+))?)? )$ !!x matches '/' seperated dataset, dataobjid and field - or 'contents'. Request is passed to CRUD handler, and uses it's authen/authz:
$r->push_handlers(PerlAccessHandler => [
	sub { $crud->authen },
	sub { $crud->authz },
] );

EPrint IDs, Documents and EP_TRIGGER_DOC_URL_REWRITE

Lines 377-493 This block of code looks for requests starting with e.g. http://repository.blah/123 - where '123' is an EPrintID.

There are some redirects in this block to account for older URL that may be requested that had EPrintIDs and/or document positions zero-padded http://repository.blah/00000123 or http://repository.blah/00000123/01/Document.txt.

Each subsequent match on the $uri consumes part of it - e.g.

Line 377

$uri =~ s! ^$urlpath/(0*)([1-9][0-9]*)\b !!x

will remove the EPrintID from the start of $uri.

Lines 398-467 deal with document requests.

Line 398

$uri =~ s! ^/(0*)([1-9][0-9]*)\b !!x

will match elements after the EPrintID in the original URL - matching '45' in

  • http://repository.blah/123/45/Document.txt or
  • http://repository.blah/123/45.hassmallThumbnailVersion/Document.txt

(the second example shows the use of a 'relationship' that is processed using a EP_TRIGGER_DOC_URL_REWRITE trigger).

Lines 418-419 may be a bit confusing at first glance.

$uri =~ s! ^([^/]*)/ !!x;
my @relations = grep { length($_) } split /\./, $1;

They deal with document relationships - that are of the form .../DocID.relationship1.relationship2.relationshipN/....

For documents, thumbnails are presented as related documents, the relationship is e.g. 'hassmallThumbnailVersion'.

The first line gets anything from the start of $uri (the DocID already having been removed by line 398), to the next '/'.

The second line (possibly the least readable line of code in EPrints?):

  • takes the captured match ($1): .relationship1.relationship2.relationshipN
  • splits on '.'s: "","relationship1","relationship2","relationshipN"
  • for each of the split values (referenced as $_): grep for the length of the value. This effectively strips out empty elements (length = 0, grep doesn't return the value).
  • @relationships = "relationship1","relationship2","relationshipN"

Line 421 then assigns whatever is left of the URL as the filename.

Lines 423-426 use 'pnotes' (http://perl.apache.org/docs/2.0/api/Apache2/RequestUtil.html#C_pnotes_) to pass objects to the request handler.

Lines 433-436 call document specific authen and authz methods. These deal with the 'security' flag on the document, and are part of the core EPrints that needs to be addressed with ACL.

$r->push_handlers(PerlAccessHandler => [
	\&EPrints::Apache::Auth::authen_doc,
	\&EPrints::Apache::Auth::authz_doc
] );

Line 440 add the document-download log handler to the request cleanup phase:

$r->pool->cleanup_register(\&EPrints::Apache::LogHandler::document, $r);

Lines 443-458 call the EP_TRIGGER_DOC_URL_REWRITE triggers. There is one default trigger for this - in ~/lib/cfg.d/doc_rewrite.pl that handles the relationship-based document requests.

Lines 470-490 If the URL has an EPrint, but doesn't have a document, we reach this block - which updates the abstract page if necessary, and uses the EPrints templating system to render the page. It also logs the page view using EPrints::Apache::LogHandler::eprint.

Views

Lines 504-518 handle 'view' requests - and regenerating view pages.

auto javascript and css

Lines 519-543 deal with generating the 'auto' javascript and css files

Other static pages

Lines 545-560 deals with anything else.

Other headers

Lines 562-582 set a few headers for specific cases, and also register EPrints::Apache::Template as the handler for any .html files.


Notes

  • There are two 'content negotiation' functions - one in EPrints::Apache::Rewrite, and another in EPrints::Apache::CRUD.
  • API:EPrints/Const#:trigger learn about triggers ;o)

TODO (some should be seperate pages)

    • triggers
    • CRUD
    • ???
  • permit on DataObj
    • can_request_view / can_user_view
  • summary pages (content neg/ URL rewrite)
  • DOI - 5 metadata elements
  • 40x handling