【Patroni源码阅读】dcs初始化和导入

get_dcs.py

def get_dcs(config):
    # 获取所有DCS实现的名称，dcs_modules()通过pkgutil查询dcs目录下所有python文件的名称
    # 最终返回一个List，内容为：patroni.dcs.kubernetes 等等。
    modules = dcs_modules()

    for module_name in modules:
        name = module_name.split('.')[-1] # dcs名称：如etcd、kubernetes等等
        if name in config:  # 判断配置文件中是否有对应的名称的Section配置段
            try:
                module = importlib.import_module(module_name)
                for key, item in module.__dict__.items():  # iterate through the module content
                    # try to find implementation of AbstractDCS interface, class name must match with module_name
                    # 判断DCS文件中的Class是否是DCS的实现
                    if key.lower() == name and inspect.isclass(item) and issubclass(item, AbstractDCS):
                        # propagate some parameters
                        # 将namespace、name、scope等一些关键配置填充到dcs对应的配置
                        config[name].update({p: config[p] for p in ('namespace', 'name', 'scope', 'loop_wait',
                                             'patronictl', 'ttl', 'retry_timeout') if p in config})
                        return item(config[name])
            except ImportError:
                logger.debug('Failed to import %s', module_name)
    
    # 下面的逻辑知识为了打印错误日志，方便定位，没有实际意义
    available_implementations = []
    for module_name in modules:
        name = module_name.split('.')[-1]
        try:
            module = importlib.import_module(module_name)
            available_implementations.extend(name for key, item in module.__dict__.items() if key.lower() == name
                                             and inspect.isclass(item) and issubclass(item, AbstractDCS))
        except ImportError:
            logger.info('Failed to import %s', module_name)
    raise PatroniFatalException("""Can not find suitable configuration of distributed configuration store
Available implementations: """ + ', '.join(sorted(set(available_implementations))))