Example usage of Redis - LUA script in Rails

This topic is about using Redis and LUA script in Rails.

Before proceeding, you may like to read [ Introduction to LUA script with Redis][1]

Redis doesn't have any command to calculate total number of keys

We'll use [zrangebyscore][2] to find number of members between index range.

Instal [redis-rb][3]

Start rails console

Prepare a dataset based on timestamp

 > t10 = 10.minutes.ago
=> 1386935151
> t20 = 20.minutes.ago
=> 1386934568
> t30 = 30.minutes.ago
=> 1386933971
> t40 = 40.minutes.ago
=> 1386933374

This is non-realistic data, may vary for you.

Populate redis with Sorted Set

> redis = Redis.new # initialize redis
=> 

# zadd - Add one or more members to a sorted set.
> redis.zadd 'T_KEY', t10, 'time-10-ago'
=> true
> redis.zadd 'T_KEY', t20, 'time-20-ago'
=> true
> redis.zadd 'T_KEY', t30, 'time-30-ago'
=> true
> redis.zadd 'T_KEY', t40, 'time-40-ago'
=> true

# zrangebyscore - Return a range of members in a sorted set, by score.
> redis.zrangebyscore('T_KEY', 1386933900, 1386936000)
=> ["time-30-ago", "time-20-ago", "time-10-ago"]

# with ruby
> redis.zrangebyscore('T_KEY', 1386933900, 1386936000).size
> 4

#Total count using LUA script
> redis.eval("return #redis.call('zrangebyscore', 'T_KEY', 1386933000, 1386936000)")
=> 4

Done. You have the count between a range of sorted set without using ruby to do the extra efforts.
With LUA script you can do the heavy calculation on redis server.
[1]: http://knowbuddy.kiprosh-app.com/kyu_entries/introduction-to-lua-script-with-redis
[2]: http://redis.io/commands/zrangebyscore
[3]: https://github.com/redis/redis-rb