42 lines
1.4 KiB
Ruby
42 lines
1.4 KiB
Ruby
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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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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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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::REDIS_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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