caching.rb

# frozen_string_literal: true

module Gitlab
  module GithubImport
    module Caching
      # The default timeout of the cache keys.
      TIMEOUT = 24.hours.to_i

      WRITE_IF_GREATER_SCRIPT = <<-EOF.strip_heredoc.freeze
      local key, value, ttl = KEYS[1], tonumber(ARGV[1]), ARGV[2]
      local existing = tonumber(redis.call("get", key))

      if existing == nil or value > existing then
        redis.call("set", key, value)
        redis.call("expire", key, ttl)
        return true
      else
        return false
      end
      EOF

      # Reads a cache key.
      #
      # If the key exists and has a non-empty value its TTL is refreshed
      # automatically.
      #
      # raw_key - The cache key to read.
      # timeout - The new timeout of the key if the key is to be refreshed.
      def self.read(raw_key, timeout: TIMEOUT)
        key = cache_key_for(raw_key)
        value = Redis::Cache.with { |redis| redis.get(key) }

        if value.present?
          # We refresh the expiration time so frequently used keys stick
          # around, removing the need for querying the database as much as
          # possible.
          #
          # A key may be empty when we looked up a GitHub user (for example) but
          # did not find a matching GitLab user. In that case we _don't_ want to
          # refresh the TTL so we automatically pick up the right data when said
          # user were to register themselves on the GitLab instance.
          Redis::Cache.with { |redis| redis.expire(key, timeout) }
        end

        value
      end

      # Reads an integer from the cache, or returns nil if no value was found.
      #
      # See Caching.read for more information.
      def self.read_integer(raw_key, timeout: TIMEOUT)
        value = read(raw_key, timeout: timeout)

        value.to_i if value.present?
      end

      # Sets a cache key to the given value.
      #
      # key - The cache key to write.
      # value - The value to set.
      # timeout - The time after which the cache key should expire.
      def self.write(raw_key, value, timeout: TIMEOUT)
        key = cache_key_for(raw_key)

        Redis::Cache.with do |redis|
          redis.set(key, value, ex: timeout)
        end

        value
      end

      # Adds a value to a set.
      #
      # raw_key - The key of the set to add the value to.
      # value - The value to add to the set.
      # timeout - The new timeout of the key.
      def self.set_add(raw_key, value, timeout: TIMEOUT)
        key = cache_key_for(raw_key)

        Redis::Cache.with do |redis|
          redis.multi do |m|
            m.sadd(key, value)
            m.expire(key, timeout)
          end
        end
      end

      # Returns true if the given value is present in the set.
      #
      # raw_key - The key of the set to check.
      # value - The value to check for.
      def self.set_includes?(raw_key, value)
        key = cache_key_for(raw_key)

        Redis::Cache.with do |redis|
          redis.sismember(key, value)
        end
      end

      # Sets multiple keys to a given value.
      #
      # mapping - A Hash mapping the cache keys to their values.
      # timeout - The time after which the cache key should expire.
      def self.write_multiple(mapping, timeout: TIMEOUT)
        Redis::Cache.with do |redis|
          redis.multi do |multi|
            mapping.each do |raw_key, value|
              multi.set(cache_key_for(raw_key), value, ex: timeout)
            end
          end
        end
      end

      # Sets the expiration time of a key.
      #
      # raw_key - The key for which to change the timeout.
      # timeout - The new timeout.
      def self.expire(raw_key, timeout)
        key = cache_key_for(raw_key)

        Redis::Cache.with do |redis|
          redis.expire(key, timeout)
        end
      end

      # Sets a key to the given integer but only if the existing value is
      # smaller than the given value.
      #
      # This method uses a Lua script to ensure the read and write are atomic.
      #
      # raw_key - The key to set.
      # value - The new value for the key.
      # timeout - The key timeout in seconds.
      #
      # Returns true when the key was overwritten, false otherwise.
      def self.write_if_greater(raw_key, value, timeout: TIMEOUT)
        key = cache_key_for(raw_key)
        val = Redis::Cache.with do |redis|
          redis
            .eval(WRITE_IF_GREATER_SCRIPT, keys: [key], argv: [value, timeout])
        end

        val ? true : false
      end

      def self.cache_key_for(raw_key)
        "#{Redis::Cache::CACHE_NAMESPACE}:#{raw_key}"
      end
    end
  end
end