async function readAll() {
  try {
    const request = k8sModel.getClient('/api/v1/namespaces/');
    const response = await request.get(`${USER_NAMESPACE}/secrets`.toString(), {
      headers: {
        Accept: 'application/json',
      },
    });
    const allUserInstance = [];
    const userData = response.data;
    for (const item of userData.items) {
      try {
        const userInstance = User.createUser({
          username: Buffer.from(item.data.username, 'base64').toString(),
          password: Buffer.from(item.data.password, 'base64').toString(),
          grouplist: JSON.parse(
            Buffer.from(item.data.grouplist, 'base64').toString(),
          ),
          email: Buffer.from(item.data.email, 'base64').toString(),
          extension: JSON.parse(
            Buffer.from(item.data.extension, 'base64').toString(),
          ),
        });
        allUserInstance.push(userInstance);
      } catch (error) {
        logger.debug(
          `secret ${item.metadata.name} is filtered in ${USER_NAMESPACE} due to user schema`,
        );
      }
    }
    return allUserInstance;
  } catch (error) {
    if (error.response) {
      throw error.response;
    } else {
      throw error;
    }
  }
}
@function readAll - return all users' info.
@async
@return {Promise<User[]>} A promise to all User instance list.