import logging
from contextlib import contextmanager
from celery import shared_task
from django.core.cache import cache
from .event_listener import EventListener
LOCK_KEY = '_django_ethereum_events_cache_lock'
LOCK_VALUE = 'LOCK'
logger = logging.getLogger(__name__)
@contextmanager
def cache_lock(lock_id, lock_value):
"""Cache based locking mechanism.
Cache backends `memcached` and `redis` are recommended.
"""
# cache.add fails if the key already exists
status = cache.add(lock_id, lock_value)
try:
yield status
finally:
cache.delete(lock_id)
@shared_task
def event_listener():
"""
Celery task that transverses the blockchain looking for event logs.
This task should be run periodically via celerybeat to monitor for
new blocks in the blockchain.
Examples:
CELERYBEAT_SCHEDULE = {
'ethereum_events': {
'task': 'django_ethereum_events.tasks.event_listener',
'schedule': crontab(minute='*/2') # run every 2 minutes
}
}
"""
with cache_lock(LOCK_KEY, LOCK_VALUE) as acquired:
if acquired:
listener = EventListener()
try:
listener.execute()
except Exception:
logger.exception('Exception while running event listener task', exc_info=True)
daemon = listener.daemon
last_processed_block = daemon.block_number
daemon.last_error_block_number = last_processed_block + 1
daemon.save()
else:
logger.info('Event listener is already running. Skipping execution.')
