External plugins are standalone, executable programs, that can be written
in any language. When ikiwiki starts up, it runs the program, and
communicates with it using XML RPC. If you want to [[write]] an external
plugin, read on..
ikiwiki contains one sample external plugin, named externaldemo. This is
written in perl, but is intended to be an example of how to write an
external plugin in your favorite programming language. Wow us at how much
easier you can do the same thing in your favorite language. ;-)
How external plugins use XML RPC
While XML RPC is typically used over http, ikiwiki doesn't do that.
Instead, the external plugin reads XML RPC data from stdin, and writes it
to stdout. To ease parsing, each separate XML RPC request or response must
start at the beginning of a line, and end with a newline. When outputting
XML RPC to stdout, be sure to flush stdout. Failure to do so will result
in deadlock!
An external plugin should operate in a loop. First, read a command from
stdin, using XML RPC. Dispatch the command, and return its result to
stdout, also using XML RPC. After reading a command, and before returning
the result, the plugin can output XML RPC requests of its own, calling
functions in ikiwiki. Note: Never make an XML RPC request at any other
time. Ikiwiki won't be listening for it, and you will deadlock.
When ikiwiki starts up an external plugin, the first RPC it will make
is to call the plugin's import() function. That function typically makes
an RPC to ikiwiki's hook() function, registering a callback.
An external plugin can use XML RPC to call any of the exported functions
documented in the [[plugin_interface_documentation|write]]. It can also
actually call any non-exported IkiWiki function, but doing so is a good way
to break your plugin when ikiwiki changes. There is currently no versioned
interface like there is for perl plugins, but external plugins were first
supported in ikiwiki version 2.6.
Accessing data structures
Ikiwiki has a few global data structures such as %config, which holds
its configuration. External plugins can use the getvar and setvar RPCs
to access any such global hash. To get the "url" configuration value,
call getvar("config", "url"). To set it, call
setvar("config", "url", "http://example.com/).
Notes on function parameters
The [[plugin_interface_documentation|write]] talks about functions that take
"named parameters". When such a function is called over XML RPC, such named
parameters look like a list of keys and values:
page, foo, destpage, bar, magnify, 1
If a name is repeated in the list, the later value overrides the earlier
one:
name, Bob, age, 20, name, Sally, gender, female
In perl, boiling this down to an associative array of named parameters is
very easy:
sub foo {
my %params=@list;
Other languages might not find it so easy. If not, it might be a good idea
to convert these named parameters into something more natural for the
language as part of their XML RPC interface.
Function injection
Some parts of ikiwiki are extensible by adding functions. For example, the
RCS interface relies on plugins providing several IkiWiki::rcs_* functions.
It's actually possible to do this from an external plugin too.
To make your external plugin provide an IkiWiki::rcs_update function, for
example, make an RPC call to inject. Pass it named parameters "name" and
"call", where "name" is the name of the function to inject into perl (here
"Ikiwiki::rcs_update" and "call" is the RPC call ikiwiki will make whenever
that function is run.
Limitations of XML RPC
Since XML RPC can't pass around references to objects, it can't be used
with functions that take or return such references. That means you can't
use XML RPC for cgi or formbuilder hooks (which are passed CGI and
FormBuilder perl objects), or use it to call template() (which returns a
perl HTML::Template object).
