`patch:instead` removes an element with no attributes

Question

I want to patch the configuration for sharedSessionState to use MSSQL instead of memory. The default configuration from Sitecore.Analytics.Tracking.config renders as such:

Sitecore.Analytics.Tracking.config

<sharedSessionState defaultProvider="InProc">
  <providers>
    <clear/>
    <add name="InProc" type="System.Web.SessionState.InProcSessionStateStore"/>
  </providers>
  <manager type="Sitecore.Analytics.Tracking.SharedSessionState.SharedSessionStateManager, Sitecore.Analytics">
    <param desc="configuration" ref="tracking/sharedSessionState/config"/>
  </manager>
  <config type="Sitecore.Analytics.Tracking.SharedSessionState.SharedSessionStateConfig, Sitecore.Analytics">
    <param desc="maxLockAge">5000</param>
    <param desc="timeoutBetweenLockAttempts">10</param>
  </config>
</sharedSessionState>

The goal is to change the defaultProvider attribute and to replace the InProc provider. I attempt to do this with the following patch file:

z.Patch.config

<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/">
  <sitecore>
    <tracking>
      <sharedSessionState defaultProvider="InProc">
        <patch:attribute name="defaultProvider" value="mssql" />
        <providers patch:instead="providers">
          <clear />
          <add name="mssql" type="Sitecore.SessionProvider.Sql.SqlSessionStateProvider,Sitecore.SessionProvider.Sql" connectionStringName="sharedsession" pollingInterval="2" compression="true" sessionType="shared"/>
        </providers>
      </sharedSessionState>
    </tracking>
  </sitecore>
</configuration> 

This patch results in the sharedSessionState correctly changing its defaultProvider, but the providers element is completely missing. At first I thought the patching engine couldn't find the right place to add the element, given that I changed its parent in the same configuration, but fiddling around I discovered that the following patch does include the providers element correctly:

z.ModifiedPatch.config

<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/">
  <sitecore>
    <tracking>
      <sharedSessionState defaultProvider="InProc">
        <patch:attribute name="defaultProvider" value="mssql" />
        <providers customAttribute="whatever" patch:instead="providers">
          <clear />
          <add name="mssql" type="Sitecore.SessionProvider.Sql.SqlSessionStateProvider,Sitecore.SessionProvider.Sql" connectionStringName="sharedsession" pollingInterval="2" compression="true" sessionType="shared"/>
        </providers>
      </sharedSessionState>
    </tracking>
  </sitecore>
</configuration> 

The only change introduced in this modified patch is the addition of customAttribute="whatever" to the providers element.

Why is it seemingly not possible replace an element with an attribute-less element? Is there a better work-around in general than to add a bogus attribute?

Answer 1

Explanation

This is not an expected behavior. This is a defect in the Sitecore XML patching engine.

In the method Sitecore.Xml.Patch.XmlPatchHelper.InsertChild(), there's the following code:

case 'i': // this corresponds to patch:instead
  parent.InsertBefore(child, xmlNode);
  parent.RemoveChild(xmlNode);

So in your case, the new <providers> element is first inserted before the existing <providers> element, and then the old element is removed.

The tricky thing is the implementation of the method XmlNode.InsertBefore(newChild, refChild), which is a native XML method in .NET. This method has the following check:

if (newChild == refChild)
{
  return newChild;
}

So if the inserted node is the same in-memory object as the original node, nothing is going to happen. The new node will not be inserted, which means that the Sitecore code at parent.RemoveChild(xmlNode) is going to remove the <providers> node altogether.

I debugged through the Sitecore decompiled code and found that if both the original node and the new node have exactly the same data attributes (or no attributes at all), then object.ReferenceEquals(child, xmlNode) returns true. If the attributes differ, then it returns false, which means they are different objects and the replacement will succeed.

There's a defect somewhere in XmlPatchHelper that reuses the existing XML node object instead of creating a new one, which results in nothing being inserted, which in turn means the node being patched will be completely removed. This will only happen when using patch:instead.

I have registered this as a bug with Sitecore Support. If you want to track the status of the bug, use the reference number 94066.

Workarounds

It seems that currently you have the following options to work around this defect:

• Don't delete the existing providers—they won't be used anyway.
• First delete the whole node using <patch:delete /> and then add it from scratch.
• Add a bogus attribute to the patch node so that it differs from the original node.

Hopefully, this will be fixed by Sitecore at some point.

Answer 2

You don't need to worry about removing the existing providers, instead just add another one and change the default provider to the new one:

<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/" xmlns:set="http://www.sitecore.net/xmlconfig/set/">
  <sitecore>
    <tracking>
      <sharedSessionState set:defaultProvider="mssql">
        <providers>
          <add name="mssql" type="Sitecore.SessionProvider.Sql.SqlSessionStateProvider,Sitecore.SessionProvider.Sql" connectionStringName="sharedsession" pollingInterval="2" compression="true" sessionType="shared"/>
        </providers>
      </sharedSessionState>
    </tracking>
  </sitecore>
</configuration>

Note the above is using set to change the default provider value, it reads a little cleaner than the patch:attribute syntax but both do the same thing. You can read more about it in the Include File Patching Facilities document.

The merged config will then look like this, all other elements will remain unchanged and Sitecore will use whatever provider is defined as the default:

<tracking>
  <sharedSessionState defaultProvider="mssql">
    <providers>
      <clear/>
      <add name="InProc" type="System.Web.SessionState.InProcSessionStateStore"/>
      <add name="mssql" type="Sitecore.SessionProvider.Sql.SqlSessionStateProvider,Sitecore.SessionProvider.Sql" connectionStringName="sharedsession" pollingInterval="2" compression="true" sessionType="shared"/>
    </providers>
    ...
  </sharedSessionState>
</tracking>

Answer 3

This is happening because of the way you are trying to patch the providers section. If you want to delete the InProc provider and just add your mssql one you can do it like this:

<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/">
  <sitecore>
    <tracking>
      <sharedSessionState defaultProvider="InProc">
        <patch:attribute name="defaultProvider" value="mssql" />
        <providers>
          <add name="InProc" type="System.Web.SessionState.InProcSessionStateStore">
            <patch:delete />
          </add>
          <add name="mssql" type="Sitecore.SessionProvider.Sql.SqlSessionStateProvider,Sitecore.SessionProvider.Sql" connectionStringName="sharedsession" pollingInterval="2" compression="true" sessionType="shared"/>
        </providers>
      </sharedSessionState>
    </tracking>
  </sitecore>
</configuration> 

Notice that we are using patch:delete to remove the InProc provider, then just adding the mssql node into the list will add it to the merged config.

EDIT: Added a second option based on OP's comments

Another option would be to remove the providers element completely using patch:delete and then add it back in. I have used this many times with the <sites> list in the publish:end events. The config would look like this:

<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/">
  <sitecore>
    <tracking>
      <sharedSessionState defaultProvider="InProc">
        <patch:attribute name="defaultProvider" value="mssql" />
        <providers>
            <patch:delete />
        </providers>
        <providers>
          <add name="mssql" type="Sitecore.SessionProvider.Sql.SqlSessionStateProvider,Sitecore.SessionProvider.Sql" connectionStringName="sharedsession" pollingInterval="2" compression="true" sessionType="shared"/>
        </providers>
      </sharedSessionState>
    </tracking>
  </sitecore>
</configuration> 

As for which option you use will depend on how fine grained you want to be when removing child elements. If you want all gone and replaced with your updates, use option 2, if you want to only delete a few, option 1 is the best.