diff options
Diffstat (limited to 'app/models/internal_id.rb')
| -rw-r--r-- | app/models/internal_id.rb | 125 |
1 files changed, 125 insertions, 0 deletions
diff --git a/app/models/internal_id.rb b/app/models/internal_id.rb new file mode 100644 index 00000000000..cbec735c2dd --- /dev/null +++ b/app/models/internal_id.rb @@ -0,0 +1,125 @@ +# An InternalId is a strictly monotone sequence of integers +# generated for a given scope and usage. +# +# For example, issues use their project to scope internal ids: +# In that sense, scope is "project" and usage is "issues". +# Generated internal ids for an issue are unique per project. +# +# See InternalId#usage enum for available usages. +# +# In order to leverage InternalId for other usages, the idea is to +# * Add `usage` value to enum +# * (Optionally) add columns to `internal_ids` if needed for scope. +class InternalId < ActiveRecord::Base + belongs_to :project + + enum usage: { issues: 0 } + + validates :usage, presence: true + + REQUIRED_SCHEMA_VERSION = 20180305095250 + + # Increments #last_value and saves the record + # + # The operation locks the record and gathers a `ROW SHARE` lock (in PostgreSQL). + # As such, the increment is atomic and safe to be called concurrently. + def increment_and_save! + lock! + self.last_value = (last_value || 0) + 1 + save! + last_value + end + + class << self + def generate_next(subject, scope, usage, init) + # Shortcut if `internal_ids` table is not available (yet) + # This can be the case in other (unrelated) migration specs + return (init.call(subject) || 0) + 1 unless available? + + InternalIdGenerator.new(subject, scope, usage, init).generate + end + + def available? + @available_flag ||= ActiveRecord::Migrator.current_version >= REQUIRED_SCHEMA_VERSION # rubocop:disable Gitlab/PredicateMemoization + end + + # Flushes cached information about schema + def reset_column_information + @available_flag = nil + super + end + end + + class InternalIdGenerator + # Generate next internal id for a given scope and usage. + # + # For currently supported usages, see #usage enum. + # + # The method implements a locking scheme that has the following properties: + # 1) Generated sequence of internal ids is unique per (scope and usage) + # 2) The method is thread-safe and may be used in concurrent threads/processes. + # 3) The generated sequence is gapless. + # 4) In the absence of a record in the internal_ids table, one will be created + # and last_value will be calculated on the fly. + # + # subject: The instance we're generating an internal id for. Gets passed to init if called. + # scope: Attributes that define the scope for id generation. + # usage: Symbol to define the usage of the internal id, see InternalId.usages + # init: Block that gets called to initialize InternalId record if not present + # Make sure to not throw exceptions in the absence of records (if this is expected). + attr_reader :subject, :scope, :init, :scope_attrs, :usage + + def initialize(subject, scope, usage, init) + @subject = subject + @scope = scope + @init = init + @usage = usage + + raise ArgumentError, 'Scope is not well-defined, need at least one column for scope (given: 0)' if scope.empty? + + unless InternalId.usages.has_key?(usage.to_s) + raise ArgumentError, "Usage '#{usage}' is unknown. Supported values are #{InternalId.usages.keys} from InternalId.usages" + end + end + + # Generates next internal id and returns it + def generate + subject.transaction do + # Create a record in internal_ids if one does not yet exist + # and increment its last value + # + # Note this will acquire a ROW SHARE lock on the InternalId record + (lookup || create_record).increment_and_save! + end + end + + private + + # Retrieve InternalId record for (project, usage) combination, if it exists + def lookup + InternalId.find_by(**scope, usage: usage_value) + end + + def usage_value + @usage_value ||= InternalId.usages[usage.to_s] + end + + # Create InternalId record for (scope, usage) combination, if it doesn't exist + # + # We blindly insert without synchronization. If another process + # was faster in doing this, we'll realize once we hit the unique key constraint + # violation. We can safely roll-back the nested transaction and perform + # a lookup instead to retrieve the record. + def create_record + subject.transaction(requires_new: true) do + InternalId.create!( + **scope, + usage: usage_value, + last_value: init.call(subject) || 0 + ) + end + rescue ActiveRecord::RecordNotUnique + lookup + end + end +end |