Class: Sequence

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# File 'backend/app/model/sequence.rb', line 10

def self.get(sequence)
  DB.open(true) do |db|

    Thread.current[:initialised_sequences] ||= {}

    if !Thread.current[:initialised_sequences][sequence]
      Thread.current[:initialised_sequences][sequence] = true

      DB.attempt {
        init(sequence, 0)
        return 0
      }.and_if_constraint_fails {
        # Sequence is already defined, which is fine
      }
    end

    # If we make it to here, the sequence already exists and needs to be incremented
    1000.times do
      old_value = db[:sequence].filter(:sequence_name => sequence.to_s).get(:value)
      updated_count = db[:sequence].filter(:sequence_name => sequence.to_s, :value => old_value).
                                    update(:value => old_value + 1)

      if updated_count == 0
        # Need to retry
        sleep(0.01)
      elsif updated_count == 1
        return old_value + 1
      else
        raise SequenceError.new("Unrecognised response from SQL update when generating next element in sequence '#{sequence}': #{updated_count}")
      end
    end

    raise SequenceError.new("Gave up trying to generate a sequence number for: '#{sequence}'")
  end
end