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Periodically-updating pymongo/MongoDB incremental MapReduce example
def incremental_map_reduce(
counter_table_name = "IncrementalMRCounters",
counter_key = None,
max_datetime = None,
reset = False,
force = False):
""" This method performs an incremental map-reduce on any new data in 'source_table_name'
into 'target_table_name'. It can be run in a cron job, for instance, and on each execution will
process only the new, unprocessed records.
The set of data to be processed incrementally is determined non-invasively (meaning the source table is not
written to) by using the queued_date field 'source_queued_date_field_name'. When a record is ready to be processed,
simply set its queued_date (which should be indexed for efficiency). When incremental_map_reduce() is run, any documents
with queued_dates between the counter in 'counter_key' and 'max_datetime' will be map/reduced.
If reset is True, it will clear 'target_table_name' and do a map reduce across all records older
than max_datetime.
If unspecified/None, counter_key defaults to counter_table_name:LastMaxDatetime.
now =
if max_datetime is None:
max_datetime = now
if reset:
logging.debug("Resetting, dropping table " + target_table_name)
time_limits = { "$lt" : max_datetime }
if counter_key is None:
counter_key = target_table_name + ":LastMaxDatetime"
# If we've run before, filter out anything that we've processed already.
last_max_datetime = None
last_max_datetime_record = db[counter_table_name].find_and_modify(
{'_id': counter_key},
{'$set': { 'inprogress': True}, '$push': { 'm': now } },
upsert = True
if force or last_max_datetime_record is None or not last_max_datetime_record.has_key('inprogress'):
# first time ever run, or forced to go ahead anyway
if last_max_datetime_record['inprogress']:
if last_max_datetime_record['m'][0] < now - datetime.timedelta(hours = 2):
# lock timed out, so go ahead...
logging.error(target_table_name + " lock is old. Ignoring it, but something was broken that caused it to not be unlocked...")
logging.warning(target_table_name + " mapreduce already in progress, skipping...")
raise RuntimeError(target_table_name + " locked since %s. Skipping..." % last_max_datetime_record['m'][0])
if not reset:
if last_max_datetime_record is not None:
last_max_datetime = last_max_datetime_record['value']
time_limits['$gt'] = last_max_datetime
logging.debug('~FR limit last_max_datetime = %s' % (last_max_datetime,))
except KeyError:
# This happened on staging. i guess it crashed somehow
# between the find_and_modify and the final update?
logging.error("~FR no value on message!")
query = { source_queued_date_field_name: time_limits }
ret = db[source_table_name].map_reduce(
out = { 'reduce' : target_table_name },
query = query,
full_response = True
num_processed = ret['counts']['input']
# Update our counter so we remember for the next pass.
already_processed_through = db[counter_table_name].update(
{'_id': counter_key},
{'$set': { 'inprogress': False, 'value': max_datetime }, '$unset': {'m': 1}},
upsert = False,
multi = False,
safe = True)
logging.debug("Processed %d completed surveys from %s through %s.\nmap_reduce details: %s" % (num_processed, last_max_datetime, max_datetime, ret))
return ret

Nice code. This looks generic except for the query referencing a 'cpl' field in the docs of the source table that contains the timestamp of the source documents. Or am I misunderstanding?


ses4j commented Jul 3, 2012

@jhasselkus: Thanks for the comment, you interpreted it right and I missed that. Replaced harcoded "cpl" with passed-in "source_queued_date_field_name". Added documentation to explain.

Thanks it helps a lot.
I had to change the 29th line to: now = datetime.datetime.utcnow() in order to be able process tweets by taking "created_at" field into account.

bersant commented Jan 10, 2014

Thanks! I rewrote a customized version of it for node.js and mongoose and it works flawlessly.

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