Everybody wants to be king, but only one can win (synchronized via a distributed cache).
Update something everybody depends on without doing it multiple times or using a cron.

Cache must support the interface write(key, value, :expires_in => xxx, :unless_exist => true),
supported in:

• ActiveSupport::Cache::MemCacheStore
• ActiveSupport::Cache::LibmemcachedStore
• ActiveSupport::Cache::MemoryStore @ rails edge
• ActiveSupport::Cache::RedisStore

gem install gem_of_thrones

aspirant = GemOfThrones.new(
  :cache => Rails.cache, # where to store the lock ?
  :timeout => 60 # if current king does not react the next aspirant will take its place
)

Thread.new do
  loop do
    # if I can be king (there is no king or the old king did not do anything)
    if aspirant.rise_to_power
      # do something that should only be done by one
    end
    sleep 30 # if you choose a timeout greater then the throne timeout the king will always change
  end
end

For a really critical master, memcache isn't traditionally that great. Memcache's best high-availability offering is a cluster of servers with the key hashing to a different server, and a server can drop out (due to timeouts or sporadic failures or what have you), and then you lose cache coherency and some servers can think a key belongs somewhere and some servers think it belongs elsewhere, and it's just not that great. What you're left with then is either going with a single memcache (single point of failure, oy), or sacrificing true locks.

Redis might be slightly better solution as it at least offers some failover via replication, and I think it has a check-and-set operator.

The big heavies in this space are of course zookeeper etc, but that can be overboard.

Start memcached then rake.

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michael@grosser.it
License: MIT