Code to accompany the blog post http://www.lukemelia.com/blog/archives/2010/01/17/redis-in-practice-whos-online/

redis-examples.rb

require 'redis'

# SADD key, member
# Adds the specified <i>member</i> to the set stored at <i>key</i>.

redis = Redis.new
redis.sadd 'my_set', 'foo' # => true
redis.sadd 'my_set', 'bar' # => true
redis.sadd 'my_set', 'bar' # => false
redis.smembers 'my_set'    # => ["foo", "bar"]

# SUNION key1 key2 ... keyN
# Returns the members of a set resulting from the union of all the sets stored at the specified keys.

# SUNIONSTORE <i>dstkey  key1 key2 ... keyN</i></b>
# Works like SUNION but instead of being returned the resulting set is stored as dstkey.

redis = Redis.new
redis.sadd 'set_a', 'foo'
redis.sadd 'set_a', 'bar'
redis.sadd 'set_b', 'bar'
redis.sadd 'set_b', 'baz'
redis.sunion 'set_a', 'set_b'                # => ["foo", "baz", "bar"]

redis.sunionstore 'set_ab', 'set_a', 'set_b'
redis.smembers 'set_ab'                      # => ["foo", "baz", "bar"]

# SINTER key1 key2 ... keyN
# Returns the members that are present in all sets stored at the specified keys.

redis = Redis.new
redis.sadd 'set_a', 'foo'
redis.sadd 'set_a', 'bar'
redis.sadd 'set_b', 'bar'
redis.sadd 'set_b', 'baz'
redis.sinter 'set_a', 'set_b'                # => ["bar"]

tracking-an-active-user.rb

# Defining the keys

def current_key
  key(Time.now.strftime("%M"))
end

def keys_in_last_5_minutes
  now = Time.now
  times = (0..5).collect {|n| now - n.minutes }
  times.collect{ |t| key(t.strftime("%M")) }
end

def key(minute)
  "online_users_minute_#{minute}"
end

# Tracking an Active User

def track_user_id(id)
  key = current_key
  redis.sadd(key, id)
end

# Who's online

def online_user_ids
  redis.sunion(*keys_in_last_5_minutes)
end

def online_friend_ids(interested_user_id)
  redis.sunionstore("online_users", *keys_in_last_5_minutes)
  redis.sinter("online_users", "user:#{interested_user_id}:friend_ids")
end