require 'securerandom' module Gitlab # This class implements an 'exclusive lease'. We call it a 'lease' # because it has a set expiry time. We call it 'exclusive' because only # one caller may obtain a lease for a given key at a time. The # implementation is intended to work across GitLab processes and across # servers. It is a cheap alternative to using SQL queries and updates: # you do not need to change the SQL schema to start using # ExclusiveLease. # class ExclusiveLease LUA_CANCEL_SCRIPT = <<-EOS local key, uuid = KEYS[1], ARGV[1] if redis.call("get", key) == uuid then redis.call("del", key) end EOS def self.cancel(key, uuid) Gitlab::Redis.with do |redis| redis.eval(LUA_CANCEL_SCRIPT, keys: [redis_key(key)], argv: [uuid]) end end def self.redis_key(key) "gitlab:exclusive_lease:#{key}" end def initialize(key, timeout:) @redis_key = self.class.redis_key(key) @timeout = timeout @uuid = SecureRandom.uuid end # Try to obtain the lease. Return lease UUID on success, # false if the lease is already taken. def try_obtain # Performing a single SET is atomic Gitlab::Redis.with do |redis| redis.set(@redis_key, @uuid, nx: true, ex: @timeout) && @uuid end end # Returns true if the key for this lease is set. def exists? Gitlab::Redis.with do |redis| redis.exists(@redis_key) end end end end