diff options
Diffstat (limited to 'lib/gitlab/exclusive_lease.rb')
-rw-r--r-- | lib/gitlab/exclusive_lease.rb | 30 |
1 files changed, 24 insertions, 6 deletions
diff --git a/lib/gitlab/exclusive_lease.rb b/lib/gitlab/exclusive_lease.rb index 2ef50286b1d..ffe49364379 100644 --- a/lib/gitlab/exclusive_lease.rb +++ b/lib/gitlab/exclusive_lease.rb @@ -15,6 +15,25 @@ module Gitlab # 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 def initialize(key, timeout:) @key, @timeout = key, timeout @@ -24,15 +43,14 @@ module Gitlab # false if the lease is already taken. def try_obtain # Performing a single SET is atomic - !!redis.set(redis_key, '1', nx: true, ex: @timeout) + Gitlab::Redis.with do |redis| + !!redis.set(redis_key, '1', nx: true, ex: @timeout) + end end - private + # No #cancel method. See comments above! - def redis - # Maybe someday we want to use a connection pool... - @redis ||= Redis.new(url: Gitlab::RedisConfig.url) - end + private def redis_key "gitlab:exclusive_lease:#{@key}" |