问题描述
使用APIM，在 Inbound 中对请求的Body内容进行解析。客户端请求所传递的Request Body为XML格式，需要从Request Body中解析出多个(Element)节点值，然后设置通过(set-variable)为参数在后续使用。
但是验证发现，当且只当使用一个set-variable 从 Request Body中读取数据时候，是可以成功的。如果要读取第二个，第三个时，始终会遇见一个诡异的错误 Expression evaluation failed. Object reference not set to an instance of an object。 关键问题是，为什么第一个可以成功，第二个的语句和第一个完全一样，却面临如此问题？真是诡异！
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，需要获取 ID, Name， Code 和 Desc 值，策略语句如下：
<!-- 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语句后，错误消息为：
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."
]
}
说明：
- 绿色高亮部分为Set-Variable的语句，两者语法完全一样。
- 但第二次就出现了 未将对象应用到实例的异常。
错误截图：
问题解决
经过反复实验，问题肯定出现在 context.Request.Body.As<XElement> 上，是不是这个内容只能使用一次呢？ 经 Google 搜寻，终于得出了官方解释和解决办法：
官方解释
context.Request.Body.As<T> 和context.Response.Body.As<T>方法用As<T>的方式指定读取 Request 和 Response的Body内容，默认情况下，这个方式读取的时原始消息的Body流，读取一次后就变为不可用，也就是说只能 As<T>的方式一次。这就解释了为什么第二个Set Variable语句出现 Object 异常。
解决办法
正如文档中解释，使用 preserveContent : true 后，可以多次转换 Body Stream。
修改后的Policy为：
<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，测试解析XML文件动画：
注意：
- 因为APIM实例的内存存在限制，内部的Memory限制为500MB，当缓存的Request/Response的内容大于500MB的时候，就会出现 MessagePayLoadTooLarge异常。
- 当使用 preserveContent:true 后，会把当前的Body内容缓存在APIM实例的内存中，如果Body内容大于500MB，则会出现 MessagePayLoadTooLarge问题，所以对于Body Size过大的请求，不能使用 Buffer 及读取整个Response/Request Body在Policy代码中。
参考资料
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
当在复杂的环境中面临问题，格物之道需：浊而静之徐清，安以动之徐生。 云中，恰是如此!
分类: 【Azure API 管理】
标签: APIM, APIM Policy 解析XML语句, preserveContent:true, Expression evaluation failed, set-variable
