126 lines
4.4 KiB
Ruby
126 lines
4.4 KiB
Ruby
# An InternalId is a strictly monotone sequence of integers
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# generated for a given scope and usage.
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#
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# For example, issues use their project to scope internal ids:
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# In that sense, scope is "project" and usage is "issues".
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# Generated internal ids for an issue are unique per project.
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#
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# See InternalId#usage enum for available usages.
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#
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# In order to leverage InternalId for other usages, the idea is to
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# * Add `usage` value to enum
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# * (Optionally) add columns to `internal_ids` if needed for scope.
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class InternalId < ActiveRecord::Base
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belongs_to :project
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belongs_to :namespace
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enum usage: { issues: 0, merge_requests: 1, deployments: 2, milestones: 3, epics: 4 }
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validates :usage, presence: true
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REQUIRED_SCHEMA_VERSION = 20180305095250
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# Increments #last_value and saves the record
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#
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# The operation locks the record and gathers a `ROW SHARE` lock (in PostgreSQL).
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# As such, the increment is atomic and safe to be called concurrently.
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def increment_and_save!
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lock!
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self.last_value = (last_value || 0) + 1
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save!
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last_value
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end
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class << self
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def generate_next(subject, scope, usage, init)
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# Shortcut if `internal_ids` table is not available (yet)
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# This can be the case in other (unrelated) migration specs
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return (init.call(subject) || 0) + 1 unless available?
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InternalIdGenerator.new(subject, scope, usage, init).generate
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end
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def available?
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@available_flag ||= ActiveRecord::Migrator.current_version >= REQUIRED_SCHEMA_VERSION # rubocop:disable Gitlab/PredicateMemoization
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end
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# Flushes cached information about schema
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def reset_column_information
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@available_flag = nil
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super
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end
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end
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class InternalIdGenerator
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# Generate next internal id for a given scope and usage.
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#
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# For currently supported usages, see #usage enum.
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#
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# The method implements a locking scheme that has the following properties:
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# 1) Generated sequence of internal ids is unique per (scope and usage)
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# 2) The method is thread-safe and may be used in concurrent threads/processes.
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# 3) The generated sequence is gapless.
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# 4) In the absence of a record in the internal_ids table, one will be created
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# and last_value will be calculated on the fly.
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#
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# subject: The instance we're generating an internal id for. Gets passed to init if called.
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# scope: Attributes that define the scope for id generation.
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# usage: Symbol to define the usage of the internal id, see InternalId.usages
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# init: Block that gets called to initialize InternalId record if not present
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# Make sure to not throw exceptions in the absence of records (if this is expected).
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attr_reader :subject, :scope, :init, :scope_attrs, :usage
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def initialize(subject, scope, usage, init)
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@subject = subject
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@scope = scope
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@init = init
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@usage = usage
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raise ArgumentError, 'Scope is not well-defined, need at least one column for scope (given: 0)' if scope.empty?
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unless InternalId.usages.has_key?(usage.to_s)
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raise ArgumentError, "Usage '#{usage}' is unknown. Supported values are #{InternalId.usages.keys} from InternalId.usages"
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end
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end
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# Generates next internal id and returns it
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def generate
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subject.transaction do
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# Create a record in internal_ids if one does not yet exist
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# and increment its last value
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#
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# Note this will acquire a ROW SHARE lock on the InternalId record
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(lookup || create_record).increment_and_save!
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end
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end
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private
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# Retrieve InternalId record for (project, usage) combination, if it exists
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def lookup
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InternalId.find_by(**scope, usage: usage_value)
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end
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def usage_value
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@usage_value ||= InternalId.usages[usage.to_s]
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end
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# Create InternalId record for (scope, usage) combination, if it doesn't exist
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#
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# We blindly insert without synchronization. If another process
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# was faster in doing this, we'll realize once we hit the unique key constraint
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# violation. We can safely roll-back the nested transaction and perform
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# a lookup instead to retrieve the record.
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def create_record
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subject.transaction(requires_new: true) do
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InternalId.create!(
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**scope,
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usage: usage_value,
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last_value: init.call(subject) || 0
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)
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end
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rescue ActiveRecord::RecordNotUnique
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lookup
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end
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end
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end
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