I’m building a website. It has groups that users can join.
The difference between this and “normal” groups is that membership is
temporary – when a user joins a group, he decides the length of membership: 5 days, a
week, 2 weeks, etc (choices are pre-defined). Or potentially all
memberships could be set to be of the same length – say a week – if
that makes thing simpler.
I’d like to have a running tally of the number of members of each
group. The number doesn’t need to be accurate up to the last second.
But it can’t be too out of date either – say, should be updated once
a day.
The “obvious” way to calculate the number of members seems to be
running a cron job, say daily, and go through every member of every
group one by one. If a membership has expired, remove that member
from the group and decrement the group’s membership count by 1.
That approach seems very inefficient and not very scalable. With a
large number of groups, it could take forever.
Can you think of a better way to do this? The membership counts do
not need to be accurate to the latest second. It can be approximate
and (slightly) out of date. Also if it makes a difference all
memberships can be set to be of the same length, say a week.
Store a list of how many people are currently in each group. Also store a list of days. Each day will contain a list of groups, and how many people to subtract from that group on that day.
When a person joins a group, add 1 to the group total, and add 1 to the people-to-subtract for that group on the day his/her membership will expire.
If a person’s expiration-date changes, remove 1 from the people-to-subtract from the old expiration-date (for that group), and add 1 to the new expiration-date.
Finally, of course, once a day subtract the correct amount from each group for that day.