code: function addRevalidationFlags (revalidation, entryKey, revalidateTimeout) {
    debug(
      'Add revalidation control flags - timeout: %sms - %s',
      revalidateTimeout,
      entryKey
    )
    // local (node instance) flag is set to immediately prevent concurrent calls
    revalidation.local.add(entryKey, revalidateTimeout)
    // external (redis) flag is set to prevent multiple instances from duplicating
    // revalidation efforts
    return revalidation.external.add(entryKey, revalidateTimeout)
  }
docs: Adds revalidation flags locally (node instance) and external (redis) to
  prevent duplicated revalidation of same key.
  @param {Object} revalidation revalidation API
  @param {String} entryKey cache entry key
  @param {Number} revalidateTimeout time to timeout revalidation in milliseconds
  @returns {Promise}