PyBlosxom: A microkernel approach to bloggingTed Leung

IntroductionThis presentation gives an overview of the important aspects of PyBlosxom from a developer's point ofview. My goal is to make it easier for people to write plugins for PyBlosxom and perhaps to attract someadditional folks who are interested in working on the core system itself.

PyBlosxom is a weblogging/blogging system written in Python. It is based on a minimal core engine thathas a few key characteristics:

Blog posts are stored as individual files in a category-oriented hierarchy of directories on thefilesystem of the blog's web-server. This means that no database software is required. Also, youcan use the text editor of your choice to create and update your weblog postings.Lots of the functionality of PyBlosxom weblogs are provided via a plugin architecture. The corefunctionality of the PyBlosxom engine is around 3400 lines of Python code.

PyBlosxom got its name and inspiration from Rael Dornfest's Blosxom, which is written in Perl. In fact,there are a number of other Blosxom like packages written in various languages. All of the members ofthis family of blogging packages share the filesystem model and a plugin architecture of some sort.

PyBlosxom allows for several different permalink URI representations. The standard representationprovides permalinks that include the year, month, and day of the weblog entry posting date. Forexample:

http://www.sauria.com/blog/2005/03/09/a-post

You can also use the hierarchical categories to provide the path to a posting:

http://www.sauria.com/blog/computers/internet/weblogs/pyblosxom/a-post

On my blog, both of these URIs refer to the same weblog posting. You can also look at groups of entriesby removing information at the end of the URI:

http://www.sauria.com/blog/ shows the root of my blog

http://www.sauria.com/blog/2005/03/09/ shows all posts dated on the 9th of March in 2005http://www.sauria.com/blog/2005/03/ shows all posts in March in 2005http://www.sauria.com/blog/2005/ shows all posts in 2005

http://www.sauria.com/blog/computers/internet/weblogs/pyblosxom/ shows all posts in the computers/internet/weblogs/pyblosxom categoryhttp://www.sauria.com/blog/computers/internet/ shows all posts in the computers/internet category

Another characteristic of Blosxom derived systems is the use of flavors. By appending a ?flav=<label> tothe end of any PyBlosxom URI, you can specify what output format (aka flavor) you would like. The

weblog world is full of potential output formats: HTML, XHTML, RSS (several different varieties),Atom, etc. The flavor system allows a PyBlosxom user to define a new flavor in order to produce outputin one or more of these formats. Additionally, flavors can be theme-oriented: sunshine, rainy-day, purple-pie-man, etc.

A flavor consists of a series of templates which are assembled to form the completed output. Using thistemplate system allows PyBlosxom users to use flavor templates built by users of other *osxom systemswith only a few modifications.

The plugin system allows users to add functionality beyond what comes with PyBlosxom by default.PyBlosxom plugins have access to the information that the core engine has. They can read and introducevariables that appear in PyBlosxom templates, they can inject new entries into the rendering pipeline,they can render output, and they can even work with the HTTP headers that accompanied the request fora weblog page.

Here some examples of functionality that is provided via plugins:

archives - create an access to archives, one link per monthcalendar nav - use a calendar as a way to navigate through archivesxmlrpc - metaweblog - implement the Metaweblog XML-RPC API to allow fancy posting/editingUIhttp 304 handling - implement conditional HTTP handling to save bandwidthautoping - ping all referenced urls that have trackback or pingback URI'slogging - log HTTP information to a filelucene - use Lucene to provide full text searchhandle text formatting of blog posts formatted using text+html, textile, or ReST

ConfigurationThere are two major areas the need to be configured when installing PyBlosxom: the web serverconnection and the PyBlosxom configuration file, config.py

Web server configurationPyBlosxom can interface with a web server in a number of ways. Originally PyBlosxom was designed asa CGI script, but starting with PyBlosxom 1.2 PyBlosxom will also be usable from mod_python,Twisted, or any WSGI compliant server.

Occasionally, people need to use something like Apache's mod_rewrite in order to get the weblog toshow up in the desired portion of the URI space.

The only other web server side issue is making sure that all of the PyBlosxom modules are available tothe CGI or servlet.

The PyBlosxom Configuration File: config.py

PyBlosxom's configuration file is called config.py, and basically contains a dict named py, whoseentries are the configuration variables for PyBlosxom. This dict ends up getting passed all the waythrough the core processing engine. Some of the important values that you need to set are

py['datadir'] the directory of the root of the PyBlosxom data, which will contain all the blog entries.

py['plugin_dirs'] a list of directories containing the plugins that you wish to use.

py['load_plugins'] a list containing the names of the plugins that you wish to use.

In order to use plugins you need to correctly set py['plugin_dirs'] and place the corresponding entryin py['load_plugins']

Once you have finished editing config.py to your liking, you can use pyblosxom.cgi to verify yourinstallation from the command line:

pyblosxom.cgi

You will need to be careful about operating system permissions between the web server user and the userPyBlosxom is running as. This will affect comment files and logs especially.

PyBlosxom ArchitecturePyBlosxom is implemented as a pipeline of plugins. The input to the pipeline is an HTTP request and theoutput is an HTTP response (most commonly containing HTML or RSS/Atom). There are abstractions torepresent the HTTP request and response, the configuration information, the entries and other data storedin PyBlosxom.

In PyBlosxom, a plugin is a Python module. You define functions in the module, and PyBlosxom willcall functions with a specific name at a well-defined point in the execution of the PyBlosxom pipeline.For example, a callback function named cb_start is called before most PyBlosxom processing begins.If your plugin module provides a function called cb_start, then it will be called.

PyBlosxom allows you to install many plugins, so a natural question is how do all those plugins interact.The py['load_plugins'] list serve two purposes. It tells PyBlosxom which plugins in the variousplugin directories will actually get used. It also defines a sequential order in which plugins will be called.If you don't provide a value for py['load_plugins'] (you must still provide a value forpy['plugin_dirs'] if you want to use any plugins), then PyBlosxom will load all the modules in all theplugin directories specified in py['plugin_dirs']. The plugins will be sorted by alphanumeric sortingon their names, and that will be the order in which they are called.

At a given callback point, every module function which provides a correctly named function will becalled. So if you load five plugin modules and each module provides a cb_start function, then all fivefunctions will be called when PyBlosxom reaches the start callback point. The order in which those fivefunctions are called will be determined by the rules described in the previous paragraph. This allowsmultiple plugins to get access to data at the proper point during PyBlosxom's execution. It also allowsplugins to exchange information if they know their relative ordering (it's best to use

py['load_plugins'] in this case.)

For advanced usage, it is possible to change the way that chains of callback functions are processed. Soinstead of calling every callback function in the chain, it's possible to arrange for a particular callbackchain to work differently. The cb_handle callback chain gives each callback function in the chain achance to try processing data. If the callback is successful then none of the remaining callback functionsin the chain will execute.

When a callback function is called it is passed a dictionary that contains the information in the HTTPrequest, the information from the configuration file, and any data that may exist in PyBlosxom at thatpoint in time (usually a list of weblog entries). I'll be referring to this dictionary as the "args dictionary'in the rest of this document.

Here's a skeleton of a start callback function -- remember that all callbacks are prefixed with cb_.

def cb_start(args): request = args['request'] # get the argument dictionary config = request.getConfiguration() # config file information http = request.getHttp() # HTTP header information cgi_form = http['form'] # including any CGI form data = request.getData() # the rest of the data

Basic PipelineNow we're ready to look at the various stages of the PyBlosxom pipeline. We're going to drill downthrough the various layers.

At the simplest level the execution of PyBlosxom is broken up into an initialization section and a "main"section. We'll skip over the initialization for the moment, since it is really only needed for entryparserusage, which we'll cover later on.

pyblosxom.jpg

(In the diagrams I've omitted the cb_ prefix on the names of all the callbacks.)

The main section calls the cb_start callback, which is where you would perform any globalinitialization in your plugin if you needed that initialization to happen before any real work got done inthe pipeline.

Next PyBlosxom will call any cb_handle callbacks. If a cb_handle callback is successful, PyBlosxomwill jump to the end of its processing, calling any cb_end callbacks before it returns an HTTP response.(Note that the HTTP request is processed by the cb_handle callback in this case). If the cb_handlecallback fails or there are no cb_handle callbacks registered, then processing passes to the blosxomhandler, which does its job and then exits via cb_end.

The cb_handle callback provides a way to bypass the Blosxom handler in case you need to do adifferent kind of processing. Currently this is used to implement XML-RPC handling in PyBlosxom,since XML-RPC handling consumes and produces completely different input and output from regularPyBlosxom processing.

Blosxom handlingThe blosxom_handler is a combination of a series of setup callbacks followed by a renderer that isactually responsible for producing the output.

blosxom-handler.jpg

The first of the setup callbacks, cb_renderer, is used to select a renderer to be used. At the moment, thecore distribution contains a a Blosxom compatible renderer and a debugging renderer. People haveexpressed interest in building renderers based on other templating engines, such as Cheetah.

Now we come to the first callback function that actually does something with the HTTP request. Thecb_pathinfo callback is charged with looking at the information in the request and modifying the datapart of the plugin args dictionary with information about the type of entry being requested:

'bl_type' - the type ('dir' or 'file')'pi_bl' - the HTTP PATH_INFO header'pi_yr' - the yar'pi_mo' - the month, either as a 2 digit number or 3 letter string'pi_da' - the day as a 2 digit number'root_datadir' - the full path to the entry folder in the filesystem'flavour - the flavour of output requested

After cb_pathinfo has identified the entry being requested (and its type), the cb_filelist callback isused to generate a list of entries. The entries will all be instances ofpyblosxom.entries.base.EntryBase, and will be stored in the data dictionary under the key'entry_list'. In the default PyBlosxom configuration, these entries are actually instances ofpyblosxom.entries.fileentry.FileEntry that represent weblog entries stored in the filesystemaccording to the Blosxom family rules. In particular, instances of FileEntry have the mtime (modifiedtime) of the file for each entry, which is used to sort the entries in reverse chronological order for atypical weblog presentation. By providing your own cb_filelist callback, you could provide entriesusing some other storage mechanism, at least in theory. Currently PyBlosxom doesn't support storingentries in other storage mechanisms, though it's something we are looking into fixing in the future.

After the list of entries has been generated, it will be sent to the renderer to be formatted for output.Before this happens, there are two callbacks that can be used to alter that data that will be rendered.

The cb_prepare callback is a general-purpose callback that should be used to modify the data before therendering phase. This can include injecting or filtering out entries, or modifying the data in specificentries. Another common thing to do in cb_prepare is to create new variables that can be accessed fromthe renderer, in order to provide some additional information. You can even use cb_prepare to handleHTTP POST requests, which is what the comments plugin does.

Just before rendering, the cb_logrequest callback is called. This is mostly used to log information to afile. To that end, it expects an entry in the argument dictionary whose key is filename. It also expects anargument entry keyed by return_code. This is why there is a separate callback (you ought to be able todo lots of logging type functionality using cb_prepare). The internal pathway through the PyBlosxomcore is slightly different from that of cb_prepare, in order to obtain the return_code for

cb_logrequest.

RenderingBy default, PyBlosxom uses a renderer that is based on the Blosxom template language. The rendererallows a template writer to insert variables into their weblog templates. At rendering time, the rendererreplaces the variables with their actual values -- that's the meaning of 'render' in the context ofPyBlosxom.

The Blosxoms combine several distinct templates in order to create a page in a weblog:

head - the content of this template is inserted before any entries are rendered.date_head - the content of this templete in inserted at the start of a new day (the output is sortedchronologically).story - the content of this template is inserted once for each entry to be rendered.date_foot - the content of this templete in inserted at the end of a day (the output is sortedchronologically).foot - the content of this template is appended after all entries have been rendered.

render.jpg

The Blosxom renderer also has callback functions that you can provide in order to override behavior.The callbacks are named after the templates they are associated. They are all executed right before thetemplate is rendered. There is also a callback that isn't associated with a template, cb_story_end that iscalled after the store template is rendered. Also, these callbacks are passed different arguments. They arepassed a dictionary that contains all the variables that the renderer will use for template rendering.

cb_head - before the head template is rendered.cb_date_head - before the date_head template is rendered.cb_story - before the story template is rendered.cb_story_end - the story template is rendered.cb_date_foot - before the date_foot template is rendered.cb_foot - before the foot template is rendered.

Entry ParsersDuring PyBlosxom initialization there a call to the cb_entryparser callback. The cb_entryparsercallback returns a dict whose keys are filename extensions and whose entries are functions uses to parseentry files whose extension is the key. This allows the user to use different formatting languages to writeweblog posts. PyBlosxom includes entryparsers that can deal with ReStructuredText and Textile formats,as well as Python source code.

The blosxom_entry_parser (the default) parses entry files with extensions of '.txt'. The first line of a filewill be the title of the entry, and the rest of the file will be the body of the entry. The body will besubstituted directly in to the blosxom templates, and can contain HTML.

Entry parsers also support their own plugin mechanism. There are two callbacks, cb_preformat and

cb_postformat that can be called before and after the entry parser has done whatever parsing it does.cb_preformat and cb_postformat must return a dict that contains entries for the 'title' and storykeys. The dict can have more entries that just these two. PyBlosxom includes preformatters for allowingwiki words and replacing linebreaks with <br /> or <p>.

XML-RPC SubsystemThe XML-RPC subsystem is the last of the major PyBlosxom subsystems that we need to cover. It isinteresting because it is implemented entirely via the plugin system - in fact it's contained entirely in thecontributed plugins distribution. What we want is a way for PyBlosxom to handle requests made viaXML-RPC. Initially, this is because we want to support some of the popular XML-RPC based blogauthoring API's, such as the Blogger and Metaweblog API's. But we'd also like to make it possible forplugins (like the pingback handler for the comments plugin) to offer some of their services viaXML-RPC, which means that we cannot hardwire the set of functions that are processed by theXML-RPC system.

xmlrpc.jpg

The XML-RPC system consists of a PyBlosxom plugin that acts as a dispatcher for all XML-RPCservices. This plugin, named xmlrpc, is wired in via PyBlosxom's cb_handle callback, since we want itto process any HTTP request that contains an XML-RPC request. Recall that cb_handle bypasses theregular blosxom_handler flow through the PyBlosxom engine. xmlrpc also takes care of authenticatingXML-RPC requests. The plugin takes care of dispatching XML-RPC requests by using a dispatch tablestored in the plugin's args dictionary. The table is a Python dictionary that is accessed via the 'methods'key.

Every plugin that wants to offer its services via XML-RPC needs to provide a cb_xml_rpc_registerfunction. This function must update the 'methods' dictionary with entries the map the XML-RPCmethod names to functions in the plugin's module. XML-RPC plugins may use any of the otherPyBlosxom callback methods as well.

ExampleNow let's take a look at a small plugin. This is my w3cdate plugin that creates a variable that containsthe date in the 'time_tuple' variable in the ISO8601 format used by many W3C specifications, theDublin Core extensions for RSS, and Atom.

import sys, time

def verify_installation(request): # No configuration parameters return 1

def cb_story(args): entry = args['entry'] time_tuple = entry['timetuple'] try: import datetime import US

dt = datetime.datetime.fromtimestamp(entry['mtime'], US.Pacific) entry['w3cdate'] = dt.isoformat() except ImportError: pass

try: import xml.utils.iso8601 tzoffset = time.timezone # if is_dst flag set, adjust for daylight savings time if time_tuple[8] == 1: tzoffset = time.altzone entry['w3cdate'] = xml.utils.iso8601.tostring(time.mktime(time_tuple),tzoffset) except ImportError: pass

The verify_installation function is part of PyBlosxom's installation system. Your plugin shouldcheck to make sure that all the entries that it needs in the configuration dictionary are present.

The plugin uses the cb_story callback in the Blosxom renderer, so it has direct access to the dictionaryof variables used by the renderer. The only trick here is that it uses some modules that aren't part of allPython installs (US, and xml.utils.iso8601), so we have to try to import the modules. If we can't importthe modules then we do nothing. Otherwise the variable dictionary is augmented with the new entry,which can then be used by any flavor of the story template.

Future PlansThere are a number of areas where we are working to enhance PyBlosxom.

We have ongoing work to improve our support for environments besides CGI scripts. We have donesome work on both WSGI and Twisted, which will appear when PyBlosxom 1.2 is released.

There is some support for static rendering in PyBlosxom, but it could be improved. We are also lookingat various caching schemes in order to improve the performance of dynamically generate weblogs.

Over the years there has been a lot of interest expressed in trying an alternate templating engine as aPyBlosxom renderer. Cheetah is the engine that is most commonly cited, but any other would bewelcome as well. This is an area where an interested party could make a new contribution.

A number of users have asked us to improve our internationalization support, so we are looking at waysto do the right thing.

On the plugin front, there has been a lot of activity around the comments plugin, which also supportsTrackback and Pingback. There is quite a bit of interest in spam reduction techniques such as picturebased challenges and comment moderation.

I hope that one day the Atom Working Group will complete work on a weblog authoring API. When thathappens, we will need to build a PyBlosxom plugin to support that API. This is another area wheresomeone could make a great contribution. If they were willing to get involved early, there is theopportunity to participate in work on the Atom API itself.

ConclusionsI would like to thank all of my fellow PyBlosxom contributors for their work on PyBlosxom. Workingon PyBlosxom was my introduction to programming in Python, and I am using PyBlosxom to run myweblog (with over 1200 posts) today. Special thanks to Wari Wahab for kicking the whole thing off, andto Will Guaraldi and Bill Mill for their extensive comments on this paper.

You can find out more about PyBlosxom at the PyBlosxom web site.

The PyBlosxom community hangs out on the PyBlosxom Mailing Lists, and has a Planet style weblogaggregator at PlanetPyBlosxom

This work is licensed under a Creative Commons License.