62 lines
2.6 KiB
Ruby
62 lines
2.6 KiB
Ruby
module Gitlab
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# This class implements an 'exclusive lease'. We call it a 'lease'
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# because it has a set expiry time. We call it 'exclusive' because only
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# one caller may obtain a lease for a given key at a time. The
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# implementation is intended to work across GitLab processes and across
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# servers. It is a 'cheap' alternative to using SQL queries and updates:
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# you do not need to change the SQL schema to start using
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# ExclusiveLease.
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#
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# It is important to choose the timeout wisely. If the timeout is very
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# high (1 hour) then the throughput of your operation gets very low (at
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# most once an hour). If the timeout is lower than how long your
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# operation may take then you cannot count on exclusivity. For example,
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# if the timeout is 10 seconds and you do an operation which may take 20
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# seconds then two overlapping operations may hold a lease for the same
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# key at the same time.
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#
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# This class has no 'cancel' method. I originally decided against adding
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# it because it would add complexity and a false sense of security. The
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# complexity: instead of setting '1' we would have to set a UUID, and to
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# delete it we would have to execute Lua on the Redis server to only
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# delete the key if the value was our own UUID. Otherwise there is a
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# chance that when you intend to cancel your lease you actually delete
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# someone else's. The false sense of security: you cannot design your
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# system to rely too much on the lease being cancelled after use because
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# the calling (Ruby) process may crash or be killed. You _cannot_ count
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# on begin/ensure blocks to cancel a lease, because the 'ensure' does
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# not always run. Think of 'kill -9' from the Unicorn master for
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# instance.
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#
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# If you find that leases are getting in your way, ask yourself: would
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# it be enough to lower the lease timeout? Another thing that might be
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# appropriate is to only use a lease for bulk/automated operations, and
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# to ignore the lease when you get a single 'manual' user request (a
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# button click).
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#
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class ExclusiveLease
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def initialize(key, timeout:)
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@key, @timeout = key, timeout
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end
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# Try to obtain the lease. Return true on success,
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# false if the lease is already taken.
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def try_obtain
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# Performing a single SET is atomic
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!!redis.set(redis_key, '1', nx: true, ex: @timeout)
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end
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# No #cancel method. See comments above!
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private
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def redis
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# Maybe someday we want to use a connection pool...
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@redis ||= Redis.new(url: Gitlab::RedisConfig.url)
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end
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def redis_key
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"gitlab:exclusive_lease:#{@key}"
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end
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end
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end
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