diff options
Diffstat (limited to 'lib/gitlab/exclusive_lease.rb')
-rw-r--r-- | lib/gitlab/exclusive_lease.rb | 66 |
1 files changed, 26 insertions, 40 deletions
diff --git a/lib/gitlab/exclusive_lease.rb b/lib/gitlab/exclusive_lease.rb index 7e8f35e9298..2dd42704396 100644 --- a/lib/gitlab/exclusive_lease.rb +++ b/lib/gitlab/exclusive_lease.rb @@ -1,66 +1,52 @@ +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: + # 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. # - # It is important to choose the timeout wisely. If the timeout is very - # high (1 hour) then the throughput of your operation gets very low (at - # most once an hour). If the timeout is lower than how long your - # operation may take then you cannot count on exclusivity. For example, - # if the timeout is 10 seconds and you do an operation which may take 20 - # seconds then two overlapping operations may hold a lease for the same - # key at the same time. - # - # This class has no 'cancel' method. I originally decided against adding - # it because it would add complexity and a false sense of security. The - # complexity: instead of setting '1' we would have to set a UUID, and to - # delete it we would have to execute Lua on the Redis server to only - # delete the key if the value was our own UUID. Otherwise there is a - # chance that when you intend to cancel your lease you actually delete - # someone else's. The false sense of security: you cannot design your - # system to rely too much on the lease being cancelled after use because - # the calling (Ruby) process may crash or be killed. You _cannot_ count - # on begin/ensure blocks to cancel a lease, because the 'ensure' does - # not always run. Think of 'kill -9' from the Unicorn master for - # instance. - # - # If you find that leases are getting in your way, ask yourself: would - # it be enough to lower the lease timeout? Another thing that might be - # appropriate is to only use a lease for bulk/automated operations, and - # to ignore the lease when you get a single 'manual' user request (a - # button click). - # 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:) - @key, @timeout = key, timeout + @redis_key = self.class.redis_key(key) + @timeout = timeout + @uuid = SecureRandom.uuid end - # Try to obtain the lease. Return true on success, + # 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, '1', nx: true, ex: @timeout) + 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) + redis.exists(@redis_key) end end - - # No #cancel method. See comments above! - - private - - def redis_key - "gitlab:exclusive_lease:#{@key}" - end end end |