Tom's Webware Documentation

Scheduling with Python and Webware

Tom Schwaller


Since version 0.5 the web application framework Webware has a scheduling plug-in called TaskKit. This QuickStart Guide describes how to use it in your daily work with Webware and also with normal Python programs.

Scheduling periodic tasks is a very common activity for users of a modern operating system. System administrators for example know very well how to start new cron jobs or the corresponding Windows analogues. So, why does a web application server like Webware/WebKit need it's own scheduling framework. The answer is simple: Because it knows better how to react to a failed job, has access to internal data structures, which otherwise would have to be exposed to the outside world and last but not least it needs scheduling capabilities anyway (e.g. for session sweeping and other memory cleaning operations).

Webware is developped with the object oriented scripting language Python so it seemed natural to write a general purpose Python based scheduling framework. One could think that this problem is already solved (remember the Python slogan: batteries included), but strange enough there has not much work been done in this area. The two standard Python modules sched.py and bisect.py are way to simple, not really object oriented and also not multithreaded. This was the reason to develop a new scheduling framework, which can not only be used with Webware but also with general purpose Python programs. Unfortunately scheduling has an annoying side effect. The more you delve into the subject the more it becomes difficult.

After some test implementations I discovered the Java scheduling framework of the Ganymede network directory management system and took it as a model for the Python implementation. Like any other Webware Kit or plug-in the TaskKit is self contained and can be used in other Python projects. This modularity is one of the real strengths of Webware and in sharp contrast to Zope where people tend to think in Zope and not in Python terms. In a perfect world one should be able to use web wrappers (for Zope, Webware, Quixote,..) around clearly designed Python classes and not be forced to use one framework. Time will tell if this is just a dream or if people will reinvent the "python weels" over and over again.

Tasks

The TaskKit implements the three classes Scheduler, TaskHandler and Task. Let's begin with the simplest one, i.e. Task. It's an abstract base class, from which you have to derive you own task classes by overriding the run()-method like in the following example:

from TaskKit.Task import Task
from time import time, strftime, localtime

class SimpleTask(Task):
    def run(self):
      print self.name(), strftime("%H:%M:%S", localtime(time()))

self.name() returns the name under which the task was registered by the scheduler. It is unique among all tasks and scheduling tasks with the same name will delete the old task with that name (so beware of that feature!). Another simple example which is used by WebKit itself is found in WebKit/Tasks/SessionTask.py.

from TaskKit.Task import Task

class SessionTask(Task):
    def __init__(self, sessions):
        Task.__init__(self)
        self._sessionstore = sessions
        
    def run(self):
        if self.proceed():
            self._sessionstore.cleanStaleSessions(self)

Here you see the proceed() method in action. It can be used by long running tasks to check if they should terminate. This is the case when the scheduler or the task itself has been stoped. The latter is achieved with a stopTask() call which is not recommented though. It's generally better to let the task finish and use the unregister() and disable() methods. The first really deletes the task after termination while the second only disables it's rescheduling. You can still use it afterwards. Right now the implementation of proceed()

def proceed(self):
    """
    Should this task continue running? Should be called periodically 
    by long tasks to check if the system wants them to exit.
    Returns 1 if its OK to continue, 0 if it's time to quit.
    """
    return not( self._close.isSet() or (not self._handle._isRunning) )     

uses the _close Event variable, which was also available trough the close() method. Don't count on that in future versions, it will probably be removed. Just use proceed() instead (take a look at TaskKit/Tests/BasicTest.py). Another API change after version 0.5 of Webware was the removal of the close variable in run(). If you plan to make serious use of TaskKit it's better to take the newest CVS snapshot of Webware, otherwise you will have to delete all occurences of close afterwards. Another thing to remember about tasks is, that they know nothing about scheduling, how often they will run (periodically or just once) or if they are on hold. All this is managed by the task wrapper class TaskManager, which will be discussed shortly. Let's look at some more examples first.

Generating static pages

On a high trafic web site (a la slashdot) it's common practice to use semistatic page generation techniques. For example you can generate the entry page as a static page once per minute. During this time the content will not be completely accurate (e.g. the number of comments will certainly increase), but nobody really cares about that. The benefit is a dramatic reduction of database requests. For other pages (like older news with comments attached) it gives more sense to generate static versions on demand. This is the case when the discussion has come to an end, but somebody adds a comment afterwards and implicitely changes the page by this action. Generating a static version will happen very seldom after the "hot phase" when getting data directly out of the database is more appropriate. So you need a periodic task which checks if there are new "dead" stories (e.g. no comments for 2 days) and marks them with a flag for static generation on demand. It should be clear by now, that an integrated Webware scheduling mechnism is very useful for this kind of things and the better approach than external cron jobs. Let's look a litle bit closer at the static generation technique now. First of all we need a PageGenerator class. To keep the example simple we just write the actual date into a file. In real live you will assemble much more complex data into such static pages.

from TaskKit.Task import Task
from time import *

html = '''<html>
<head><title>%s</title></head>
<body bgcolor="white">
<h1>%s</h1>
</body>
</html>
'''

class PageGenerator(Task):
    def __init__(self, filename):
        Task.__init__(self)
        self._filename = filename
        
    def run(self):
        f = open(self._filename, 'w')
        now = asctime(localtime(time()))
        f.write( html % ('Static Page',  now) )
        f.close()

Scheduling

That was easy. Now it's time to schedule our task. In the following example you can see how this is accomplished with TaskKit. As a general recommendation you should put all your tasks in a separate folder (with an empty __init__.py file to make this folder a Python package). First of all we create a new Scheduler object, start it as a thread and add a periodic page generation object (of type PageGenerator) with the addPeriodicAction method (this will probably be changed in the near future to the more constitent name addPeriodicTask). The first parameter here is the first execution time (which can be in the future), the second is the period (in seconds), the third an instance of our task class and the last parameter is a unique task name which allows us to find the task later on (e.g. if we want to change the period or put the task on hold).

from TaskKit.Scheduler import Scheduler
from Tasks.PageGenerator import PageGenerator
from time import *

def main():
    scheduler = Scheduler()
    scheduler.start()
    scheduler.addPeriodicAction(time(), 5, PageGenerator('static.html'), 'PageGenerator')
    sleep(20)
    scheduler.stop()
    
if __name__=='__main__':
    main()      

When you fire up this example you will notice that the timing is not 100% accurate. The reason for this seems to be an imprecise wait() function in the Python threading module. Unfortunately this method in indispensible because we need to be able to wake up a sleeping scheduler when scheduling new tasks with first execution times smaller than scheduler.nextTime(). This is achieved through the notify() method, which sets the notifyEvent (scheduler._notifyEvent.set()). On Unix we could use sleep and a signal to interrupt this system call, but TaskKit has to be plattform independant to be of any use. But don't worry, this impreciseness is not important for normal usage, because we are talking about scheduling in the minute (not second) range here. Unix cron jobs have a granularity of one minute, which is a good value for TaskKit too. Of course nobody can stop you starting tasks with a period of one second (but you have been warned that this is not a good idea, except for testing purposes).

Generating static pages again

Let's refine our example a little bit and plug it into Webware. We will write a Python servlet which loks like this:

every seconds
Task List
Task NamePeriod
SessionSweeper360
PageGenerator for static3.html30
PageGenerator for static1.html60
PageGenerator for static2.html120

When you click on the Generate button a new periodic PageGenerator task will be add to the Webware scheduler. Remember that this will generate a static page static.html every 60 seconds (if you use the default values). The new task name is "PageGenerator for filename", so you can use this servlet to change the settings of already scheduled tasks (by rescheduling) or add new PageGenerator tasks with different filenames. This is quite useless here, but as soon as you begin to parametrize your Task classes this approach can become quite powerful (consider for example a mail reminder form or collecting news from different news channels as periodic tasks with user defined parameters). In any case, don't be shy and contribute other interesting examples (the sky's the limit!).

Finally we come to the servlet code, which should be more or less self explanatory, except for the _action_ construct which is very well explained in the Webware documentation though (just in case you forgot that). app.taskManager() gives you the WebKit scheduler, which can be used to add new tasks. In real live you will have to make the scheduling information persistent and reschedule all tasks after a WebKit restart because it would be quite annoying to enter this data again and again. PersistantScheduler is a class which is on the ToDo list for the next TaskKit version and will probably be implemented with the new MiddleKit from Chuck Esterbrook. MiddleKit is a new object relational mapping framework for Python and greatly simplyfies this kind of developments. You'll certainly read more about it in the future.

import os, string, time
from ExamplePage import ExamplePage
from Tasks.PageGenerator import PageGenerator

class Schedule(ExamplePage):

    def writeContent(self):
        wr = self.write
        wr('<center><form method="post">')
        wr('<input type=submit name=_action_ value=Generate> ')
        wr('<input type=text name=filename value="static.html" size=20> every ') 
        wr('<input type=text name=seconds value=60 size=5> seconds')
        wr('</form>')
        wr('<table width=50% border=1 cellspacing=0>')
        wr('<tr bgcolor=00008B><th colspan=2><font color=white>Task List</font></th></tr>')
        wr('<tr bgcolor=#dddddd><td><b>Task Name</b></td><td><b>Period</b></td></tr>')
        for taskname, handler in self.application().taskManager().scheduled().items():
            wr('<tr><td>%s</td><td>%s</td></tr>' % (taskname, handler.period()))
        wr('</table></center>')

    def generate(self, trans):
        app = self.application()
        tm = app.taskManager()
        req = self.request()
        if req.hasField('filename') and req.hasField('seconds'):
            self._filename = req.field('filename')
            self._seconds = string.atoi(req.field('seconds'))
            task = PageGenerator(app.serverSidePath('Examples/' + self._filename))
            taskname = 'PageGenerator for ' + self._filename
            tm.addPeriodicAction(time.time(), self._seconds, task, taskname)    
        self.writeBody()
                                    
    def methodNameForAction(self, name):
        return string.lower(name)

    def actions(self):
        return ExamplePage.actions(self) + ['generate']      

The Scheduler

Now it's time to take a closer look at the Scheduler class itself. As you have seen in the examples above, writing tasks is only a matter of overloading the run() method in a derived class and adding it to the scheduler with addTimedAction, addActionOnDemand, addDailyAction or addPeriodicAction. The scheduler will wrap the Task in a TaskHandler structure which knows all the scheduling details and add it to its _scheduled or _onDemand dictionaries. The latter is populated by addActionOnDemand and contains tasks which can be called any time by scheduler.runTaskNow('taskname') as you can see in the following example. After that the task has gone.

scheduler = Scheduler()
scheduler.start()
scheduler.addActionOnDemand(SimpleTask(), 'SimpleTask')
sleep(5)
print "Demanding SimpleTask"
scheduler.runTaskNow('SimpleTask')
sleep(5)
scheduler.stop()

If you need a task more than one time it's better to start it regularly with one of the add*Action methods first. It will be added to the _scheduled dictionary. If you do not need the task for a certain time disable it with scheduler.disableTask('taskname') and enable it later with scheduler.enableTask('taskname'). There are some more methods (e.g. demandTask(), stopTask(), ...) in the Scheduler class which are all documented by doc strings. Take a look at them and write your own examples to understand the methods (and maybe find bugs ;-)).

When a periodic task is scheduled it is added in a wrapped version to the _scheduled dictionary first. The (most of the time sleeping) scheduler thread always knows when to wake up and start the next task whose wrapper is moved to the _runnning dictionary. After completion of the task thread the handle reschedules the task by putting it back from _running to _scheduled), calculating the next execution time nextTime and possibly waking up the scheduler. It is important to know that you can manipulate the handle while the task is running, eg. change the period or call runOnCompletion to request that a task be re-run after its current completion. For normal use you will probably not need the handles at all, but the more you want to manipulate the task execution, the more you will appreciate the TaskHandler API. You get all the available handles from the scheduler with the running('taskname), scheduled('taskname') and onDemand('taskname') methods.

In our last example which was contributed by Jay Love, who debugged, stress tested and contributed a lot of refinements to TaskKit, you see how to write a period modifying Task. This is quite weird but shows the power of handle manipulations. The last thing to remember is, that the scheduler does not start a separate thread for each periodic task. It uses a thread for each task run instead and at any time keeps the number of threads as small as possible.

class SimpleTask(Task):

    def run(self):
        if self.proceed():
            print self.name(), time()
            print "Increasing period"
            self.handle().setPeriod(self.handle().period()+2)
        else:
            print "Should not proceed", self.name()

As you can see the TaskKit framework is quite sophisticated and will hopefully be used by many people from the Python community. If you have further question, please feel free to ask them on the Webware mailing list. (last changes: 2. March 2001)

Info

[1] Webware: http://webware.sourceforge.net/
[2] Ganymede: http://www.arlut.utexas.edu/gash2/

Published under the GNU Free Documentation License.