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我们正在使用 Azure 表存储，并且在执行 InsertOrMerge 操作时偶尔会出现 408 超时。在这种情况下，我们想重试，但似乎没有针对这些错误遵循重试策略。

这是我们用来处理表格交互的一个类。GetFooEntityAsync 方法尝试从表存储中检索实体。如果不能，它会创建一个新的 FooEntity 并将其添加到表中（映射到 FooTableEntity）。

public class FooTableStorageBase
{
    private readonly string tableName;

    protected readonly CloudStorageAccount storageAccount;

    protected TableRequestOptions DefaultTableRequestOptions { get; }

    protected OperationContext DefaultOperationContext { get; }

    public CloudTable Table
    {
        get
        {
            return storageAccount.CreateCloudTableClient().GetTableReference(tableName);
        }
    }

    public FooTableStorage(string tableName)
    {
        if (String.IsNullOrWhiteSpace(tableName))
        {
            throw new ArgumentNullException(nameof(tableName));
        }

        this.tableName = tableName;

        storageAccount = CloudStorageAccount.Parse(ConnectionString);

        ServicePoint tableServicePoint = ServicePointManager.FindServicePoint(storageAccount.TableEndpoint);
        tableServicePoint.UseNagleAlgorithm = false;
        tableServicePoint.ConnectionLimit = 100; // Increasing connection limit from default of 2.

        DefaultTableRequestOptions = new TableRequestOptions()
        {
            PayloadFormat = TablePayloadFormat.JsonNoMetadata,
            MaximumExecutionTime = TimeSpan.FromSeconds(1),
            RetryPolicy = new OnTimeoutRetry(TimeSpan.FromMilliseconds(250), 3),
            LocationMode = LocationMode.PrimaryOnly 
        };

        DefaultOperationContext = new OperationContext();

        DefaultOperationContext.Retrying += (sender, args) =>
        {
            // This is never executed.
            Debug.WriteLine($"Retry policy activated in {this.GetType().Name} due to HTTP code {args.RequestInformation.HttpStatusCode} with exception {args.RequestInformation.Exception.ToString()}");
        };

        DefaultOperationContext.RequestCompleted += (sender, args) =>
        {
            if (args.Response == null)
            {
                // This is occasionally executed - we want to retry in this case.
                Debug.WriteLine($"Request failed in {this.GetType().Name} due to HTTP code {args.RequestInformation.HttpStatusCode} with exception {args.RequestInformation.Exception.ToString()}");
            }
            else
            {
                Debug.WriteLine($"{this.GetType().Name} operation complete: Status code {args.Response.StatusCode} at {args.Response.ResponseUri}");
            }
        };

        Table.CreateIfNotExists(DefaultTableRequestOptions, DefaultOperationContext);
    }

    public async Task<FooEntity> GetFooEntityAsync()
    {
        var retrieveOperation = TableOperation.Retrieve<FooTableEntity>(FooTableEntity.GenerateKey());

        var tableEntity = (await Table.ExecuteAsync(retrieveOperation, DefaultTableRequestOptions, DefaultOperationContext)).Result as FooTableEntity;

        if (tableEntity != null)
        {
            return tableEntity.ToFooEntity();
        }

        var fooEntity = CalculateFooEntity();

        var insertOperation = TableOperation.InsertOrMerge(new FooTableEntity(fooEntity));
        var executeResult = await Table.ExecuteAsync(insertOperation);

        if (executeResult.HttpStatusCode == 408)
        {
            // This is never executed.
            Debug.WriteLine("Got a 408");
        }

        return fooEntity;
    }

    public class OnTimeoutRetry : IRetryPolicy
    {
        int maxRetryAttempts = 3;

        TimeSpan defaultRetryInterval = TimeSpan.FromMilliseconds(250);

        public OnTimeoutRetry(TimeSpan deltaBackoff, int retryAttempts)
        {
            maxRetryAttempts = retryAttempts;
            defaultRetryInterval = deltaBackoff;
        }

        public IRetryPolicy CreateInstance()
        {
            return new OnTimeoutRetry(TimeSpan.FromMilliseconds(250), 3);
        }

        public bool ShouldRetry(int currentRetryCount, int statusCode, Exception lastException, out TimeSpan retryInterval, OperationContext operationContext)
        {
            retryInterval = defaultRetryInterval;
            if (currentRetryCount >= maxRetryAttempts)
            {
                return false;
            }

            // Non-retryable exceptions are all 400 ( >=400 and <500) class exceptions (Bad gateway, Not Found, etc.) as well as 501 and 505. 
            // This custom retry policy also retries on a 408 timeout.
            if ((statusCode >= 400 && statusCode <= 500 && statusCode != 408) || statusCode == 501 || statusCode == 505)
            {
                return false;
            }

            return true;
        }
    }
}

调用 GetFooEntityAsync() 时，偶尔会执行“请求失败”行。检查值时args.RequestInformation.HttpStatusCode= 408。但是：

  • Debug.WriteLine("Got a 408");在 GetFooEntity 方法中永远不会执行。

  • Debug.WriteLine($"Retry policy activated...在DefaultOperationContext.Retrying委托中永远不会执行（我希望这会执行两次 - 这不是重试吗？）。

  • DefaultOperationContext.RequestResults包含一长串结果（大多数状态码为 404，有些为 204）。

根据这篇（相当旧的）博客文章，代码在 400 和 500 之间以及 501 和 505 之间的异常是不可重试的。然而，超时（408）正是我们想要重试的情况。也许我需要为这种情况编写自定义重试策略。

我不完全理解 408 的来源，因为除了调用 RequestCompleted 委托时我无法在代码中找到它。我一直在为我的重试策略尝试不同的设置，但没有运气。我在这里想念什么？如何从表存储中获得重试 408 的操作？

编辑：我已更新代码以显示我实施的自定义重试策略，以重试 408 错误。但是，似乎我的重试断点仍未被命中，因此似乎未触发重试。我的重试策略未激活的原因可能是什么？

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0 回答 0