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Add comments to explain magic numbers

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Raphael Michel
2021-03-31 11:38:14 +02:00
parent ab61a9b190
commit da8470682e
1 changed files with 5 additions and 1 deletions
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6
src/pretix/base/services/quotas.py
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@@ -116,6 +116,7 @@ class QuotaAvailability:
for redisval, q in zip(d, evquotas): for redisval, q in zip(d, evquotas):
if redisval is not None: if redisval is not None:
data = [rv for rv in redisval.decode().split(',')] data = [rv for rv in redisval.decode().split(',')]
# Except for some rare situations, we don't want to use cache entries older than 2 minutes
if time.time() - int(data[2]) < 120 or allow_cache_stale: if time.time() - int(data[2]) < 120 or allow_cache_stale:
quotas_original.remove(q) quotas_original.remove(q)
quotas.remove(q) quotas.remove(q)
@@ -153,7 +154,7 @@ class QuotaAvailability:
# high load *to a specific calendar or event*, lots of parallel web requests will receive an "expired" result # high load *to a specific calendar or event*, lots of parallel web requests will receive an "expired" result
# around the same time, recompute quotas and write back to the cache. To avoid overloading redis with lots of # around the same time, recompute quotas and write back to the cache. To avoid overloading redis with lots of
# simultaneous write queries for the same page, we place a very naive and simple "lock" on the write process for # simultaneous write queries for the same page, we place a very naive and simple "lock" on the write process for
# these quotas. # these quotas. We choose 10 seconds since that should be well above the duration of a write.
lock_name = '_'.join([str(p) for p in sorted([q.pk for q in quotas])]) lock_name = '_'.join([str(p) for p in sorted([q.pk for q in quotas])])
if rc.exists(f'quotas:availabilitycachewrite:{lock_name}'): if rc.exists(f'quotas:availabilitycachewrite:{lock_name}'):
@@ -171,6 +172,9 @@ class QuotaAvailability:
[str(int(time.time()))] [str(int(time.time()))]
) for q in quotas ) for q in quotas
}) })
# To make sure old events do not fill up our redis instance, we set an expiry on the cache. However, we set it
# on 7 days even though we mostly ignore values older than 2 monites. The reasoning is that we have some places
# where we set allow_cache_stale and use the old entries anyways to save on performance.
rc.expire(f'quotas:{eventid}:availabilitycache', 3600 * 24 * 7) rc.expire(f'quotas:{eventid}:availabilitycache', 3600 * 24 * 7)
# We used to also delete item_quota_cache:* from the event cache here, but as the cache # We used to also delete item_quota_cache:* from the event cache here, but as the cache