問題描述

使用APIM，在 Inbound 中對請求的Body內容進行解析。客戶端請求所傳遞的Request Body為XML格式，需要從Request Body中解析出多個(Element)節點值，然后設置通過(set-variable)為參數在后續使用。

但是驗證發現，當且只當使用一個set-variable 從 Request Body中讀取數據時候，是可以成功的。如果要讀取第二個，第三個時，始終會遇見一個詭異的錯誤 Expression evaluation failed. Object reference not set to an instance of an object。關鍵問題是，為什么第一個可以成功，第二個的語句和第一個完全一樣，卻面臨如此問題？真是詭異！

需要解析的XML格式如下：

<?xml version="1.0" encoding="utf-8"?>
<CDHotel xmlns="http://schemas.xmlsoap.org/soap/cdhotel/">
<Body>
<GetHotel xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://tempuri.org/">
<input>
<ID xmlns="http://schemas.datacontract.org/2014/01/wcf">202203081007001</ID>
<Name xmlns="http://schemas.datacontract.org/2014/01/wcf">Cheng Du Junyi Hotel</Name>
<Code xmlns="http://schemas.datacontract.org/2014/01/wcf">ICP1009100</Code>
</input>
</GetHotel>
</Body>
</CDHotel>

在APIM Policies中，需要獲取 ID, Name， Code 和 Desc 值，策略語句如下：

<!-- IMPORTANT:
    - Policy elements can appear only within the <inbound>, <outbound>, <backend> section elements.
    - To apply a policy to the incoming request (before it is forwarded to the backend service), place a corresponding policy element within the <inbound> section element.
    - To apply a policy to the outgoing response (before it is sent back to the caller), place a corresponding policy element within the <outbound> section element.
    - To add a policy, place the cursor at the desired insertion point and select a policy from the sidebar.
    - To remove a policy, delete the corresponding policy statement from the policy document.
    - Position the <base> element within a section element to inherit all policies from the corresponding section element in the enclosing scope.
    - Remove the <base> element to prevent inheriting policies from the corresponding section element in the enclosing scope.
    - Policies are applied in the order of their appearance, from the top down.
    - Comments within policy elements are not supported and may disappear. Place your comments between policy elements or at a higher level scope. -->
<policies>
    <inbound>
        <base />
        <set-variable name="myID" value="@(
 context.Request.Body.As<XElement>().Descendants().FirstOrDefault(x => x.Name.LocalName == "ID")?.Value
        )" />
        <set-variable name="myName" value="@(
 context.Request.Body.As<XElement>().Descendants().FirstOrDefault(x => x.Name.LocalName == "Name")?.Value
        )" />
        <set-variable name="myCode" value="@(
 context.Request.Body.As<XElement>().Descendants().FirstOrDefault(x => x.Name.LocalName == "Code")?.Value
        )" />
        <set-variable name="myDesc" value="@(
 context.Request.Body.As<XElement>().Descendants().FirstOrDefault(x => x.Name.LocalName == "Desc")?.Value
        )" />
    </inbound>
    <backend>
        <base />
    </backend>
    <outbound>
        <set-header name="myID" exists-action="override">
            <value>@((string)context.Variables["myID"])</value>
        </set-header>
        <set-header name="myName" exists-action="override">
            <value>@((string)context.Variables["myName"])</value>
        </set-header>
        <set-header name="myCode" exists-action="override">
            <value>@((string)context.Variables["myCode"])</value>
        </set-header>
        <set-header name="myDesc" exists-action="override">
            <value>@((string)context.Variables["myDesc"])</value>
        </set-header>
        <base />
    </outbound>
    <on-error>
        <base />
    </on-error>
</policies>

在APIM的Test功能，查看Trace語句后，錯誤消息為：

set-variable (0.905 ms)
    {
    "message": "Expression was successfully evaluated.",
    "expression": "\n context.Request.Body.As<XElement>().Descendants().FirstOrDefault(x => x.Name.LocalName == \"ID\")?.Value\n        ",
    "value": "202203081007001"
}
set-variable (0.013 ms)
    {
    "message": "Context variable was successfully set.",
    "name": "myID",
    "value": "202203081007001"
}
set-variable (7.898 ms)
    {
    "messages": [
        {
            "message": "Expression evaluation failed.",
            "expression": "\n context.Request.Body.As<XElement>().Descendants().FirstOrDefault(x => x.Name.LocalName == \"Name\")?.Value\n        ",
            "details": "Object reference not set to an instance of an object."
        },
        "Expression evaluation failed. Object reference not set to an instance of an object.", "Object reference not set to an instance of an object."
    ]
}

說明：

綠色高亮部分為Set-Variable的語句，兩者語法完全一樣。
但第二次就出現了未將對象應用到實例的異常。

錯誤截圖：

image

問題解決

經過反復實驗，問題肯定出現在 context.Request.Body.As<XElement> 上，是不是這個內容只能使用一次呢？經 Google 搜尋，終于得出了官方解釋和解決辦法：

官方解釋

文檔鏈接：https://docs.microsoft.com/en-us/azure/api-management/api-management-policy-expressions#ContextVariables

image

context.Request.Body.As<T> 和context.Response.Body.As<T>方法用As<T>的方式指定讀取 Request 和 Response的Body內容，默認情況下，這個方式讀取的時原始消息的Body流，讀取一次后就變為不可用，也就是說只能 As<T>的方式一次。這就解釋了為什么第二個Set Variable語句出現 Object 異常。

解決辦法

正如文檔中解釋，使用 preserveContent : true 后，可以多次轉換 Body Stream。

修改后的Policy為：

   <inbound>
        <base />
        <set-variable name="myID" value="@(
 context.Request.Body.As<XElement>(preserveContent:true).Descendants().FirstOrDefault(x => x.Name.LocalName == "ID")?.Value
        )" />
        <set-variable name="myName" value="@(
 context.Request.Body.As<XElement>(preserveContent:true).Descendants().FirstOrDefault(x => x.Name.LocalName == "Name")?.Value
        )" />
        <set-variable name="myCode" value="@(
 context.Request.Body.As<XElement>(preserveContent:true).Descendants().FirstOrDefault(x => x.Name.LocalName == "Code")?.Value
        )" />
        <set-variable name="myDesc" value="@(
 context.Request.Body.As<XElement>(preserveContent:true).Descendants().FirstOrDefault(x => x.Name.LocalName == "Desc")?.Value
        )" />
    </inbound>

修改后，測試解析XML文件動畫：

image

注意：

因為APIM實例的內存存在限制，內部的Memory限制為500MB，當緩存的Request/Response的內容大于500MB的時候，就會出現 MessagePayLoadTooLarge異常。
當使用 preserveContent:true 后，會把當前的Body內容緩存在APIM實例的內存中，如果Body內容大于500MB，則會出現 MessagePayLoadTooLarge問題，所以對于Body Size過大的請求，不能使用 Buffer 及讀取整個Response/Request Body在Policy代碼中。