問題描述
使用APIM，在 Inbound 中對請求的Body内容進行解析。用戶端請求所傳遞的Request Body為XML格式，需要從Request Body中解析出多個(Element)節點值，然後設定通過(set-variable)為參數在後續使用。
但是驗證發現，當且隻當使用一個set-variable 從 Request Body中讀取資料時候，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，為什麼第一個可以成功，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，需要擷取 ID, Name， Code 和 Desc 值，政策語句如下：
<!--
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，檢視Trace語句後，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，兩者文法完全一樣。
- 但第二次就出現了 未将對象應用到執行個體的異常。
錯誤截圖：
問題解決
經過反複實驗，問題肯定出現在 context.Request.Body.As<XElement> 上，是不是這個内容隻能使用一次呢？ 經 Google 搜尋，終于得出了官方解釋和解決辦法：
官方解釋
文檔連結：https://docs.microsoft.com/en-us/azure/api-management/api-management-policy-expressions#ContextVariables
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内容，預設情況下，這個方式讀取的時原始消息的Body流，讀取一次後就變為不可用，也就是說隻能 As<T>的方式一次。這就解釋了為什麼第二個Set Variable語句出現 Object 異常。
解決辦法
正如文檔中解釋，使用 preserveContent : true 後，可以多次轉換 Body Stream。
修改後的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> 修改後，測試解析XML檔案動畫：
注意：
- 因為APIM執行個體的記憶體存在限制，内部的Memory限制為500MB，當緩存的Request/Response的内容大于500MB的時候，就會出現 MessagePayLoadTooLarge異常。
- 當使用 preserveContent:true 後，會把目前的Body内容緩存在APIM執行個體的記憶體中，如果Body内容大于500MB，則會出現 MessagePayLoadTooLarge問題，是以對于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。
參考資料
API Management policy expressions - Context variable - IMessageBody ： https://docs.microsoft.com/en-us/azure/api-management/api-management-policy-expressions#ContextVariables
Get an attribute value from XML Response in azure apim ： https://stackoverflow.com/questions/68618339/get-an-attribute-value-from-xml-response-in-azure-apim
XElement Class ： https://docs.microsoft.com/en-us/dotnet/api/system.xml.linq.xelement?view=net-6.0
當在複雜的環境中面臨問題，格物之道需：濁而靜之徐清，安以動之徐生。 雲中，恰是如此!