Cassandra Python驱动自定义重试策略无法捕获ReadFailure异常

我来帮你排查这个问题——你遇到的ReadFailure没有触发自定义重试策略，核心原因是在你使用的cassandra-driver 3.25.0版本中，ReadFailure异常并不由on_request_error方法处理，它需要单独实现对应的重试回调方法。

问题根源 #

Cassandra Python驱动把不同类型的异常拆分到了不同的重试钩子中：

• on_read_timeout处理读取超时类异常
• on_read_failure专门处理ReadFailure这类副本读取失败的异常
• on_request_error则负责处理一些通用的请求错误（比如连接问题、协议错误等）

你的自定义策略里只覆盖了前几个方法，但漏掉了on_read_failure，所以遇到ReadFailure时驱动找不到对应的重试逻辑，直接抛出了异常。

解决方案 #

在你的KeyspacesRetryPolicy类中添加on_read_failure方法，逻辑和其他重试方法保持一致即可：

from cassandra.policies import RetryPolicy
import time  # 注意补充导入time，原代码中使用了time.sleep但未声明

class KeyspacesRetryPolicy(RetryPolicy):
    def __init__(self, RETRY_MAX_ATTEMPTS=5, base_delay=0.5, max_delay=10):
        # retry_num starts at 0
        self.max_retry_num = RETRY_MAX_ATTEMPTS-1
        self.base_delay = base_delay 
        self.max_delay = max_delay

    def __backoff(self, retry_num):
        # exponential backoff delay
        delay = min(self.base_delay * (2 ** (retry_num + 1)), self.max_delay)
        print(f"Backing off for {delay} seconds (retry number {retry_num})")
        time.sleep(delay)

    def on_read_timeout(self, query, consistency, required_responses, received_responses, data_retrieved, retry_num):
        if retry_num <= self.max_retry_num:
            self.__backoff(retry_num)
            print("retry on_read_timeout")
            return self.RETRY, consistency
        else:
            return self.RETHROW, None 

    def on_write_timeout(self, query, consistency, write_type, required_responses, received_responses, retry_num):
        if retry_num <= self.max_retry_num:
            self.__backoff(retry_num)
            print("retry on_write_timeout")
            return self.RETRY, consistency
        else:
            return self.RETHROW, None

    def on_unavailable(self, query, consistency, required_replicas, alive_replicas, retry_num):
        if retry_num <= self.max_retry_num:
            self.__backoff(retry_num)
            print("retry on_unavailable")
            return self.RETRY, consistency
        else:
            return self.RETHROW, None 

    def on_request_error(self, query, consistency, error, retry_num):
        if retry_num <= self.max_retry_num:
            self.__backoff(retry_num)
            print("retry on_request_error")
            return self.RETRY, consistency
        else:
            return self.RETHROW, None

    # 新增方法处理ReadFailure异常
    def on_read_failure(self, query, consistency, required_responses, received_responses, failures, retry_num):
        if retry_num <= self.max_retry_num:
            self.__backoff(retry_num)
            print("retry on_read_failure")
            return self.RETRY, consistency
        else:
            return self.RETHROW, None

额外提示 #

如果你之后遇到类似的WriteFailure异常，也需要添加对应的on_write_failure方法，逻辑和上面的on_read_failure类似。

修改后，当集群返回ReadFailure时，驱动就会调用你新增的on_read_failure方法，执行你定义的退避和重试逻辑了。

备注：内容来源于stack exchange，提问作者sechstein