如何从 Npgsql 异常判断调用是否值得重试（瞬态故障策略）

Question

我正在编写一个将连接到远程 postgres 服务器的服务。我正在寻找一种好方法来确定哪些异常应该被视为暂时的（值得重试），以及如何定义连接到远程数据库的适当策略。

该服务正在使用 Npgsql 进行数据访问。文档说 Npgsql 将抛出 sql 错误的 PostgresException 和 "server related issues".

的 NpgsqlException

到目前为止，我能想到的最好的办法是假设所有不是 PostgresExceptions 的异常都应该被视为可能是暂时的，值得重试，但是 PostgresException 意味着查询有问题并且重试无济于事。我的这个假设是否正确？

我正在使用 Polly 创建重试和断路器策略。因此，我的政策是这样的：

Policy.Handle<Exception>( AllButPotgresExceptions()) // if its a postgres exception we know its not going to work even with a retry, so don't
                       .WaitAndRetryAsync(new[]
                       {
                           TimeSpan.FromSeconds(1),
                           TimeSpan.FromSeconds(2),
                           TimeSpan.FromSeconds(4)
                       }, onRetry: (exception, span) => Log.Warning(exception, "Postgres Retry Failure: "))
                    .WrapAsync(
                           Policy.Handle<Exception>( AllButPotgresExceptions())
                               .AdvancedCircuitBreakerAsync(
                                   failureThreshold:.7, 
                                   samplingDuration: TimeSpan.FromSeconds(30), 
                                   minimumThroughput: 20, 
                                   durationOfBreak: TimeSpan.FromSeconds(30), 
                                   onBreak: (ex, timeSpan, context) => Log.Warning(ex, "Postres Circuit Breaker Broken: "), 
                                   onReset: (context) => Log.Warning("Postres Circuit Breaker Reset: "), 
                                   onHalfOpen: () => Log.Warning("Postres Circuit Breaker Half Open: ")
                               )));
        }
    }

    private static Func<Exception, bool> AllButPotgresExceptions()
    {
        return ex => ex.GetType() != typeof(PostgresException);
    }

是否有更好的方法来确定哪些错误可能是暂时的？

更新：

根据 Shay 的建议，我在 Npgsql 中打开了一个新问题并将我的政策更新为如下所示：

public static Policy PostresTransientFaultPolicy
    {
        get
        {
            return postgresTransientPolicy ?? (postgresTransientPolicy = Policy.Handle<Exception>( PostgresDatabaseTransientErrorDetectionStrategy())
                       .WaitAndRetryAsync(
                            retryCount: 10, 
                            sleepDurationProvider: retryAttempt => ExponentialBackoff(retryAttempt, 1.4), 
                            onRetry: (exception, span) => Log.Warning(exception, "Postgres Retry Failure: "))
                    .WrapAsync(
                           Policy.Handle<Exception>( PostgresDatabaseTransientErrorDetectionStrategy())
                               .AdvancedCircuitBreakerAsync(
                                   failureThreshold:.4, 
                                   samplingDuration: TimeSpan.FromSeconds(30), 
                                   minimumThroughput: 20, 
                                   durationOfBreak: TimeSpan.FromSeconds(30), 
                                   onBreak: (ex, timeSpan, context) => Log.Warning(ex, "Postres Circuit Breaker Broken: "), 
                                   onReset: (context) => Log.Warning("Postres Circuit Breaker Reset: "), 
                                   onHalfOpen: () => Log.Warning("Postres Circuit Breaker Half Open: ")
                               )));
        }
    }

    private static TimeSpan ExponentialBackoff(int retryAttempt, double exponent)
    {
        //TODO add random %20 variance on the exponent
        return TimeSpan.FromSeconds(Math.Pow(retryAttempt, exponent));
    }

    private static Func<Exception, bool> PostgresDatabaseTransientErrorDetectionStrategy()
    {
        return (ex) =>
        {                
            //if it is not a postgres exception we must assume it will be transient
            if (ex.GetType() != typeof(PostgresException))
                return true;

            var pgex = ex as PostgresException;
            switch (pgex.SqlState)
            {
                case "53000":   //insufficient_resources
                case "53100":   //disk_full
                case "53200":   //out_of_memory
                case "53300":   //too_many_connections
                case "53400":   //configuration_limit_exceeded
                case "57P03":   //cannot_connect_now
                case "58000":   //system_error
                case "58030":   //io_error

                //These next few I am not sure whether they should be treated as transient or not, but I am guessing so

                case "55P03":   //lock_not_available
                case "55006":   //object_in_use
                case "55000":   //object_not_in_prerequisite_state
                case "08000":   //connection_exception
                case "08003":   //connection_does_not_exist
                case "08006":   //connection_failure
                case "08001":   //sqlclient_unable_to_establish_sqlconnection
                case "08004":   //sqlserver_rejected_establishment_of_sqlconnection
                case "08007":   //transaction_resolution_unknown
                    return true;
            }

            return false;
        };
    }

Answer 1

你的方法很好。 NpgsqlException 通常意味着 network/IO 错误，尽管您可以检查内部异常并检查 IOException 以确定。

PostgreSQL报错时抛出PostgresException，多数情况下是查询的问题。但是，可能存在一些暂时的服务器端问题（例如连接过多），您可以检查 SQL 错误代码 - 请参阅 the PG docs.

向这些异常添加 IsTransient 属性可能是个好主意，在 PostgreSQL 本身中编码这些检查 - 欢迎您为此打开一个问题在 Npgsql 仓库上。

如何从 Npgsql 异常判断调用是否值得重试（瞬态故障策略）

How to tell from Npgsql exception if the call is worth a retry (transient fault strategy)

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postgresql

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microservices

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