重试失败的 sqlalchemy 查询

Posted 2023-02-24

【中文标题】重试失败的 sqlalchemy 查询

【英文标题】：Retry failed sqlalchemy queries

【发布时间】：2019-04-16 15:40:04

【问题描述】：

每次我重新启动 mysql 服务时，我的应用都会在任何查询中收到以下错误：

result = self._query(query)
  File "/usr/local/lib/python3.6/site-packages/pymysql/cursors.py", line 328, in _query
    conn.query(q)
  File "/usr/local/lib/python3.6/site-packages/pymysql/connections.py", line 516, in query
    self._affected_rows = self._read_query_result(unbuffered=unbuffered)
  File "/usr/local/lib/python3.6/site-packages/pymysql/connections.py", line 727, in _read_query_result
    result.read()
  File "/usr/local/lib/python3.6/site-packages/pymysql/connections.py", line 1066, in read
    first_packet = self.connection._read_packet()
  File "/usr/local/lib/python3.6/site-packages/pymysql/connections.py", line 656, in _read_packet
    packet_header = self._read_bytes(4)
  File "/usr/local/lib/python3.6/site-packages/pymysql/connections.py", line 702, in _read_bytes
    CR.CR_SERVER_LOST, "Lost connection to MySQL server during query")
sqlalchemy.exc.OperationalError: (pymysql.err.OperationalError) (2013, 'Lost connection to MySQL server during query') [SQL: ...] [parameters: ...] (Background on this error at: http://sqlalche.me/e/e3q8)

之后的任何查询都会照常成功。

这只是一个常见的用例，通常我可能想根据错误重试任何查询。

有什么方法可以在一些低级别的sqlalchemy api 中捕获并重试查询？在我的代码中使用 try-except 或自定义 query 方法是不合理的，因为我使用它的次数太多且不可维护。

【问题讨论】：

自定义查询或围绕它的包装有什么问题？在这种情况下，将其用作每个查询的 try... except... 块的基础似乎是最明智的解决方案。你有minimal reproducible example格式的sn-p代码来演示这个问题吗？

【参考方案1】：

显然sqlalchemy 有一个很好的选项来自定义查询类，这正是我所需要的。

类实现：

import logging
from flask_sqlalchemy import BaseQuery
from sqlalchemy.exc import OperationalError
from time import sleep

class RetryingQuery(BaseQuery):

    __retry_count__ = 3
    __retry_sleep_interval_sec__ = 0.5

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)

    def __iter__(self):
        attempts = 0
        while True:
            attempts += 1
            try:
                return super().__iter__()
            except OperationalError as ex:
                if "Lost connection to MySQL server during query" not in str(ex):
                    raise
                if attempts < self.__retry_count__:
                    logging.debug(
                        "MySQL connection lost - sleeping for %.2f sec and will retry (attempt #%d)",
                        self.__retry_sleep_interval_sec__, attempts
                    )
                    sleep(self.__retry_sleep_interval_sec__)
                    continue
                else:
                    raise

用法：

class BaseModel(Model):
    ...
    query_class = RetryingQuery
    ...

db = SQLAlchemy(model_class=BaseModel, query_class=RetryingQuery)

【讨论】：

你能解释一下它背后的逻辑吗？

@Deva 通过使用这种错误拦截，您可以处理所有查询，而无需包装所有sqlalchemy api

我是一个初学者，你能解释一下如何覆盖 iter 我们能够重试查询方法

@Deva 这需要深入挖掘sqlalchemy 代码。大多数查询执行器最终都调用__iter__ 作为最低级别的公共点。我从他们的源代码和反复试验中发现了这一点。它绝对不是失败证明，可能无法涵盖所有​​情况

您好-这里是 SQLAlchemy 作者。我刚刚被指出这个食谱。我强烈建议不要使用上述模式。连接的“重试”应仅在事务的顶层执行，这就是池 pre_ping 功能的用途。如果您在事务中间丢失连接，则需要重新运行整个操作。显式优于隐式。

【参考方案2】：

非常感谢这个 sn-p，我不得不对其进行一些调整以直接使用 sqlalchemy.orm：如果它可以对任何人有用..

from sqlalchemy.exc import OperationalError, StatementError
from sqlalchemy.orm.query import Query as _Query
from time import sleep

class RetryingQuery(_Query):
    __max_retry_count__ = 3

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)

    def __iter__(self):
        attempts = 0
        while True:
            attempts += 1
            try:
                return super().__iter__()
            except OperationalError as ex:
                if "server closed the connection unexpectedly" not in str(ex):
                    raise
                if attempts <= self.__max_retry_count__:
                    sleep_for = 2 ** (attempts - 1)
                    logging.error(
                        "/!\ Database connection error: retrying Strategy => sleeping for s"
                    " and will retry (attempt # of ) \n Detailed query impacted: ".format(
                        sleep_for, attempts, self.__max_retry_count__, ex)
                )
                    sleep(sleep_for)
                    continue
                else:
                    raise
            except StatementError as ex:
                if "reconnect until invalid transaction is rolled back" not in str(ex):
                    raise
                self.session.rollback()

对于用法：将选项传递给 sessionmaker：

sqlalchemy.orm.sessionmaker(bind=engine, query_cls=RetryingQuery)

【参考方案3】：

我不得不稍微调整一下以使其与 Postgres 一起工作，后者有不同的错误消息。我知道这个问题被标记为mysql，但通过搜索找到了这个问题（并且遇到了完全相同的问题），因此可能会对某人有所帮助。

我还必须赶上StatementError: (sqlalchemy.exc.InvalidRequestError) Can't reconnect until invalid transaction is rolled back，它在重试发生之前炸毁了 Flask。

最后我做了指数退避，因为为什么不?

import logging
from flask_sqlalchemy import BaseQuery
from sqlalchemy.exc import OperationalError, StatementError
from time import sleep

class RetryingQuery(BaseQuery):
    __retry_count__ = 3

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)

    def __iter__(self):
        attempts = 0
        while True:
            attempts += 1
            try:
                return super().__iter__()
            except OperationalError as ex:
                if "server closed the connection unexpectedly" not in str(ex):
                    raise
                if attempts < self.__retry_count__:
                    sleep_for = 2 ** (attempts - 1)
                    logging.error(
                        "Database connection error:  - sleeping for s"
                        " and will retry (attempt # of )".format(
                            ex, sleep_for, attempts, self.__retry_count__
                        )
                    )
                    sleep(sleep_for)
                    continue
                else:
                    raise
            except StatementError as ex:
                if "reconnect until invalid transaction is rolled back" not in str(ex):
                    raise
                self.session.rollback()

【参考方案4】：

SQLAlchemy 还允许您侦听在创建 connection 之前触发的 engine_connect 事件。这使得实现悲观断开处理的自定义逻辑成为可能

下面的 sn-p 实现了重试的指数退避。它取自 Apache Airflow 的 SQLAlchemy Utils： http://airflow.apache.org/docs/1.10.3/_modules/airflow/utils/sqlalchemy.html

@event.listens_for(engine, "engine_connect")
    def ping_connection(connection, branch):
        """
        Pessimistic SQLAlchemy disconnect handling. Ensures that each
        connection returned from the pool is properly connected to the database.

        http://docs.sqlalchemy.org/en/rel_1_1/core/pooling.html#disconnect-handling-pessimistic
        """
        if branch:
            # "branch" refers to a sub-connection of a connection,
            # we don't want to bother pinging on these.
            return

        start = time.time()
        backoff = initial_backoff_seconds

        # turn off "close with result".  This flag is only used with
        # "connectionless" execution, otherwise will be False in any case
        save_should_close_with_result = connection.should_close_with_result

        while True:
            connection.should_close_with_result = False

            try:
                connection.scalar(select([1]))
                # If we made it here then the connection appears to be healthy
                break
            except exc.DBAPIError as err:
                if time.time() - start >= reconnect_timeout_seconds:
                    log.error(
                        "Failed to re-establish DB connection within %s secs: %s",
                        reconnect_timeout_seconds,
                        err)
                    raise
                if err.connection_invalidated:
                    log.warning("DB connection invalidated. Reconnecting...")

                    # Use a truncated binary exponential backoff. Also includes
                    # a jitter to prevent the thundering herd problem of
                    # simultaneous client reconnects
                    backoff += backoff * random.random()
                    time.sleep(min(backoff, max_backoff_seconds))

                    # run the same SELECT again - the connection will re-validate
                    # itself and establish a new connection.  The disconnect detection
                    # here also causes the whole connection pool to be invalidated
                    # so that all stale connections are discarded.
                    continue
                else:
                    log.error(
                        "Unknown database connection error. Not retrying: %s",
                        err)
                    raise
            finally:
                # restore "close with result"
                connection.should_close_with_result = save_should_close_with_result