Running Scrapers

Once you've written your scraper scripts, you'll need to re-run them frequently to keep the news on your site up to date.

You can do this any way you like; cron would work fine.

There is also a daemon that comes with OpenBlock which is tailor-made for this purpose, although we're deprecating it as of the 1.1 release since there's no real reason for OpenBlock to reinvent this particular wheel.

Cron Configuration

Here's an example config file for running scrapers via cron.


You must set the DJANGO_SETTINGS_MODULE environment variable, and use the python interpreter that lives in your virtualenv. It's also crucial that the user who runs each cron job have permission to run those scripts, and permission to write to any log files, etc. that the scrapers write to. I recommend using the same (non-root) user account you used for installing openblock.

# Put this in

# Edit these as necessary
# Where do errors get emailed?

# Format:
# m     h dom mon dow user   command

# Retrieve flickr photos every 20 minutes.
0,20,40 *  *   *  *   $USER  $PYTHON $SCRAPERS/general/flickr/ -q

# Meetup can be slow due to hitting rate limits.
# Several times a day should be OK.
0 7,18,22 * * * $PYTHON $SCRAPERS/general/meetup/ -q

# Aggregates every 6 min.
*/6     0  0   0  0   $USER  $BINDIR/update_aggregates --quiet

A more extensive example is in the obdemo source code; look for sample_crontab.

As noted in Data Scraper Tutorial, it's a very good idea if scripts have a command-line option to discard all non-error output, since cron likes to email you with all output. When using cron, silence is golden.

Updaterdaemon Configuration


Since cron and similar tools work just fine, we're declaring Updaterdaemon deprecated; that is, we no longer recommend using it.

The daemon script is named and it lives in ebdata, more specifically at ebdata/retrieval/updaterdaemon/ To configure it, you need to write a (small) Python script that contains a list of TASKS.

There is an example config file at ebdata/retrieval/updaterdaemon/, and the one we use for obdemo is at obdemo/

What goes in the config file? Let's put together a (small) example based on the one for obdemo.

First, we need a function that imports and runs one of our scrapers, just once. Let's use the one from obdemo that creates "Events". Our function can look like:

def do_events():
    from obdemo.scrapers.add_events import main
    return main()

(Note that this function could do anything we want to run periodically; updaterdaemon actually doesn't know anything about scrapers per se. One other thing you probably want to do regularly is send out openblock's E-mail alerts.)

Next, we need a way to know when, or how often, that function should run. We'll use another function for that; let's call it a "time callback". The time callback takes one argument - a Python datetime - and returns True if we should run our scraper now, and False otherwise. Here's one that runs every ten minutes:

def every_ten_minutes(datetime):
    if datetime.minute % 10 == 0:
        return True
    return False

(Note that only wakes up and checks the time once per minute, so we don't need to be very careful here about the time check - we won't accidentally run this many times in one minute.)

(Note also that the example config file in ebdata/retrieval/updaterdaemon/ already contains factories to generate a number of useful time callbacks, such as multiple_daily, daily, and weekly. We could just import and call one of those. Read the source to see how they work.)

Finally, we need to wrap all this up in a list (or tuple) calles TASKS. This is what the script looks for in the config file. TASKS is a list of tuples, each in the form (time_callback, function_to_run, {keyword args}, {environ}).

We've already got the first two of those. What about the last two? keyword args is a dictionary of extra arguments to pass to our function. Ours doesn't actually need any, so we'll use an empty dictionary, like {}.

environ is a dictionary of environment variables to set before running our function. Generally this will need to set DJANGO_SETTINGS_MODULE. For the demo, we set it to obdemo.settings by default, unless there is already an environment variable by that name. This looks like:

env = {'DJANGO_SETTINGS_MODULE': os.environ.get('DJANGO_SETTINGS_MODULE', 'obdemo.settings')}

Putting it all together, we get this complete config file:

from ebdata.retrieval.updaterdaemon.config import multiple_hourly

def do_events():
    from obdemo.scrapers.add_events import main
    return main()

def every_ten_minutes(datetime):
    if datetime.minute % 10 == 0:
        return True
    return False

env = {'DJANGO_SETTINGS_MODULE': os.environ.get('DJANGO_SETTINGS_MODULE', 'obdemo.settings')}

    (every_ten_minutes, do_events, {}, env),

Testing the daemon

Give it a try:

$ python ebdata/ebdata/retrieval/updaterdaemon/ --config=/path/to/  start

If it works, nothing obvious should happen :) It's running in the background. You shouldn't expect anything to happen until the next multiple of 10 minutes. When it's time, check the log file to see if anything's happening:

$ tail -f /tmp/updaterdaemon.log

(Hit Ctrl-C to get out of that.)

If there's nothing in the main log, check the error log:

$ less /tmp/updaterdaemon.err

To stop the daemon, do this:

$ python ebdata/ebdata/retrieval/updaterdaemon/ stop

Installing the init script

UpdaterDaemon also comes with a script suitable for putting in /etc/init.d, so it will be restarted whenever the system is rebooted. To install this script, copy it from ebdata/retrieval/updaterdaemon/initscript into something like /etc/init.d/openblock-updaterdaemon. It is known to work on Ubuntu; let us know if you have trouble with it on other linux systems.

After copying, edit the script, setting a few crucial environment variables:

HERE should point to the virtualenv where you installed OpenBlock.

CONFIG should point to a config file as described in the previous sections.

SU_USER should be the name of the user account to use for running the daemon.

You might also want to set LOGFILE and ERRLOGFILE to control where the logs go.

Now try running the script as root:

$ sudo /etc/init.d/openblock-updaterdaemon start

Check the log files to make sure it's working.