How should I model this in Redis? - redis

Question

FYI: Redis n00b.
I need to store search terms in my web app.
Each term will have two attributes: "search_count" (integer) and "last_searched_at" (time)
Example I've tried:
Redis.hset("search_terms", term, {count: 1, last_searched_at: Time.now})
I can think of a few different ways to store them, but no good ways to query on the data. The report I need to generate is a "top search terms in last 30 days". In SQL this would be a where clause and an order by.
How would I do that in Redis? Should I be using a different data type?
Thanks in advance!

Answer 1

I would consider two ordered sets.
When a search term is submitted, get the current timestamp and:
zadd timestamps timestamp term
zincrby counts 1 term
The above two operations should be atomic.
Then to find all terms in the given time interval timestamp_from, timestamp_to:
zrangebyscore timestamps timestamp_from timestamp_to
after you get these, loop over them and get the counts from counts.
Alternatively, I am curious whether you can use zunionstore. Here is my test in Ruby:
require 'redis'
KEYS = %w(counts timestamps results)
TERMS = %w(test0 keyword1 test0 test1 keyword1 test0 keyword0 keyword1 test0)
def redis
#redis ||= Redis.new
end
def timestamp
(Time.now.to_f * 1000).to_i
end
redis.del KEYS
TERMS.each {|term|
redis.multi {|r|
r.zadd 'timestamps', timestamp, term
r.zincrby 'counts', 1, term
}
sleep rand
}
redis.zunionstore 'results', ['timestamps', 'counts'], weights: [1, 1e15]
KEYS.each {|key|
p [key, redis.zrange(key, 0, -1, withscores: true)]
}
# top 2 terms
p redis.zrevrangebyscore 'results', '+inf', '-inf', limit: [0, 2]
EDIT: at some point you would need to clear the counts set. Something similar to what #Eli proposed (https://stackoverflow.com/a/16618932/410102).

Answer 2

Depends on what you want to optimize for. Assuming you want to be able to run that query very quickly and don't mind expending some memory, I'd do this as follows.
Keep a key for every second you see some search (you can go more or less granular if you like). The key should point to a hash of $search_term -> $count where $count is the number of times $search_term was seen in that second.
Keep another key for every time interval (we'll call this $time_int_key) over which you want data (in your case, this is just one key where your interval is the last 30 days). This should point to a sorted set where the items in the set are all of your search terms seen over the last 30 days, and the score they're sorted by is the number of times they were seen in the last 30 days.
Have a background worker that every second grabs the key for the second that occurred exactly 30 days ago and loops through the hash attached to it. For every $search_term in that key, it should subtract the $count from the score associated with that $search_term in $time_int_key
This way, you can just use ZRANGE $time_int_key 0 $m to grab the m top searches ([WITHSCORES] if you want the amounts they were searched) in O(log(N)+m) time. That's more than cheap enough to be able to run as frequently as you want in Redis for just about any reasonable m and to always have that data updated in real time.