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
+communicates with it using [XML RPC][xmlrpc]. If you want to [[write]] an external
plugin, read on..
+[xmlrpc]: http://www.xmlrpc.com/
+
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. ;-)
+There's now a second external plugin, the [[rst]] plugin, written in
+python. It uses a `proxy.py`, a helper library for ikiwiki python plugins.
+
+[[!toc ]]
+
## How external plugins use XML RPC
While XML RPC is typically used over http, ikiwiki doesn't do that.
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.
+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
## Accessing data structures
-Ikiwiki has a few global data structures such as `%config`, which holds
+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/)`.
+The `%pagestate` is a special hash with a more complex format. To access
+it, external plugins can use the `getstate` and `setstate` RPCs. To access
+stored state, call `getstate("page", "id", "key")`, and to store state,
+call `setstate("page", "id", "key", "value")`.
+
+To access ikiwiki's ARGV array, call `getargv()`. To change its ARGV, call
+`setargv(array)`.
+
## Notes on function parameters
The [[plugin_interface_documentation|write]] talks about functions that take
to convert these named parameters into something more natural for the
language as part of their XML RPC interface.
+## undef
+
+XML RPC has a limitation that it does not have a way to pass
+undef/NULL/None. There is an extension to the protocol that supports this,
+but it is not yet available in the [[!cpan XML::RPC]] library used by
+ikiwiki.
+
+Until the extension is available, ikiwiki allows undef to be communicated
+over XML RPC by passing a sentinal value, a hash with a single key "null"
+with a value of an empty string. External plugins that need to communicate
+null values to or from ikiwiki will have to translate between undef and
+the sentinal.
+
## Function injection
-Some parts of ikiwiki are extensible by adding functions. For example, the
-RCS interface relies on plugins providing several IkiWiki::rcs_* functions.
+Some parts of ikiwiki are extensible by adding or overriding 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
+To make your external plugin override the `IkiWiki::formattime` 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
+"Ikiwiki::formattime" and "call" is the RPC call ikiwiki will make whenever
that function is run.
+If the RPC call is memoizable, you can also pass a "memoize" parameter, set
+to 1.
+
## 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
+100% 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).
+## Performance issues
+
+Since each external plugin is a separate process, when ikiwiki is
+configured to use lots of external plugins, it will start up slower, and
+use more resources. One or two should not be a problem though.
+
+There is some overhead in using XML RPC for function calls. Most plugins
+should find it to be pretty minimal though. In one benchmark, ikiwiki was
+able to perform 10000 simple XML RPC calls in 11 seconds -- 900 per second.
+
+Using external plugins for hooks such as `sanitize` and `format`, which
+pass around entire pages, and are run for each page rendered, will cause
+more XML RPC overhead than usual, due to the larger number of calls, and the
+large quantity of data conversion going on. In contrast, `preprocess` hooks
+are called generally rarely, and pass around minimal data.
+
+External plugins should avoid making RPC calls unnecessarily (ie, in a loop).
+Memoizing the results of appropriate RPC calls is one good way to minimize the
+number of calls.
+
+Injecting a replacement for a commonly called ikiwiki function
+could result in a lot more RPC calls than expected and slow
+everything down. `pagetitle`, for instance, is called about 100 times
+per page build. Whenever possible, you should tell ikiwiki to memoize
+injected functions.
+
+In general, use common sense, and your external plugin will probably
+perform ok.