How to refer connection string in a patch config

Question

We are using a marketplace module that has a config file like this:

<appender name="DBLogAppender" type="Feature.DBLogAppender.SitecoreDatabaseLogAppender, Feature.DBLogAppender">
....
<param name="ConnectionString" value="<connectionstring>"/> //core db conn string
....
</appender>

All the non local environments are containers and the database names vary for each of them.

I would like to dynamically refer the connection string of the core database here - <connectionstring>

Is it possible.

Patch config XML:

<appender patch:after="*[@name=LogFileAppender]" name="SCBasicsAuditTrailDBAppender" type="SCBasics.AuditTrail.Appender.SitecoreDatabaseLogAppender,
              SCBasics.AuditTrail" >
              
              
      <!-- Filter items where message field start with 'AUDIT....' -->
      <filter type="log4net.Filter.StringMatchFilter">
        <regexToMatch value="^AUDIT" />
      </filter>

 

      <!-- Deny all other items -->
      <filter type="log4net.Filter.DenyAllFilter" />
      <!--his is because logs are buffered until a given buffersize is reached.
      The default value is 512, but you can lower this by modifying the following line of code-->
      <bufferSize value="1" />
      <param name="ConnectionType" value="System.Data.SqlClient.SqlConnection, System.Data, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" />
      <!--Double SLASH is Very Important-->
      <param name="ConnectionString" value="Data Source=ABC100\\SQLEXPRESS;Database=SCDemo_Core;user id=sa;password=abc@123"/>
      
      <param name="CommandText" value="INSERT INTO tbl_AuditTrail_Logs ([Date],[Thread],[Level],[Logger],[Message],[Exception], 
 

[SCUser], [SCAction], [SCItemPath], [SCLanguage] , [SCVersion] , [SCItemId], [SiteName],[SCMisc]) VALUES (@log_date, 

 

@thread, @log_level, @logger, @message, @exception, @scuser,  @scaction, @scitempath, @sclanguage, @scversion, 

 

@scitemid, @sitename, @scmisc)" />
      <param name="Parameter">
        <param name="ParameterName" value="@log_date" />
        <param name="DbType" value="DateTime" />
        <param name="Layout" type="log4net.Layout.PatternLayout">
          <param name="ConversionPattern" value="%d{yyyy&apos;-&apos;MM&apos;-&apos;dd 

 

HH&apos;:&apos;mm&apos;:&apos;ss&apos;.&apos;fff}" />
        </param>
      </param>
      <param name="Parameter">
        <param name="ParameterName" value="@thread" />
        <param name="DbType" value="String" />
        <param name="Size" value="255" />
        <param name="Layout" type="log4net.Layout.PatternLayout">
          <param name="ConversionPattern" value="%t" />
        </param>
      </param>
      <param name="Parameter">
        <param name="ParameterName" value="@log_level" />
        <param name="DbType" value="String" />
        <param name="Size" value="50" />
        <param name="Layout" type="log4net.Layout.PatternLayout">
          <param name="ConversionPattern" value="%p" />
        </param>
      </param>
      <param name="Parameter">
        <param name="ParameterName" value="@logger" />
        <param name="DbType" value="String" />
        <param name="Size" value="255" />
        <param name="Layout" type="log4net.Layout.PatternLayout">
          <param name="ConversionPattern" value="%c" />
        </param>
      </param>
      <param name="Parameter">
        <param name="ParameterName" value="@message" />
        <param name="DbType" value="String" />
        <param name="Size" value="4000" />
        <param name="Layout" type="log4net.Layout.PatternLayout">
          <param name="ConversionPattern" value="%m" />
        </param>
      </param>
      <param name="Parameter">
        <param name="ParameterName" value="@exception" />
        <dbType value="String" />
        <size value="2000" />
        <layout type="log4net.Layout.ExceptionLayout" />
      </param>
      <param name="Parameter">
        <param name="ParameterName" value="@scuser" />
        <dbType value="String" />
        <size value="255" />
        <layout type="SCBasics.AuditTrail.Log4NetLayout.PropertyLayout,SCBasics.AuditTrail">
          <param name="PropertyName" value="scuser" />
        </layout>
      </param>
      <param name="Parameter">
        <param name="ParameterName" value="@scaction" />
        <dbType value="String" />
        <size value="255" />
        <layout type="SCBasics.AuditTrail.Log4NetLayout.PropertyLayout,SCBasics.AuditTrail">
          <param name="PropertyName" value="scaction" />
        </layout>
      </param>
      <param name="Parameter">
        <param name="ParameterName" value="@scitempath" />
        <dbType value="String" />
        <size value="255" />
        <layout type="SCBasics.AuditTrail.Log4NetLayout.PropertyLayout,SCBasics.AuditTrail">
          <param name="PropertyName" value="scitempath" />
        </layout>
      </param>
      <param name="Parameter">
        <param name="ParameterName" value="@sclanguage" />
        <dbType value="String" />
        <size value="255" />
        <layout type="SCBasics.AuditTrail.Log4NetLayout.PropertyLayout,SCBasics.AuditTrail">
          <param name="PropertyName" value="sclanguage" />
        </layout>
      </param>
      <param name="Parameter">
        <param name="ParameterName" value="@scversion" />
        <dbType value="String" />
        <size value="255" />
        <layout type="SCBasics.AuditTrail.Log4NetLayout.PropertyLayout,SCBasics.AuditTrail">
          <param name="PropertyName" value="scversion" />
        </layout>
      </param>      
      <param name="Parameter">
        <param name="ParameterName" value="@scitemid" />
        <dbType value="String" />
        <size value="38" />
        <layout type="SCBasics.AuditTrail.Log4NetLayout.PropertyLayout,SCBasics.AuditTrail">
          <param name="PropertyName" value="scitemid" />
        </layout>
      </param>
      <param name="Parameter">
        <param name="ParameterName" value="@sitename" />
        <dbType value="String" />
        <size value="255" />
        <layout type="SCBasics.AuditTrail.Log4NetLayout.PropertyLayout,SCBasics.AuditTrail">
          <param name="PropertyName" value="sitename" />
        </layout>
      </param>
      <param name="Parameter">
        <param name="ParameterName" value="@scmisc" />
        <dbType value="String" />
        <size value="255" />
        <layout type="SCBasics.AuditTrail.Log4NetLayout.PropertyLayout,SCBasics.AuditTrail">
          <param name="PropertyName" value="scmisc" />
        </layout>
      </param>
    </appender>

Answer 1

You cannot.

You can only patch settings in the app_config/sitecore.config file. Settings in other files such as the web.config and app_config/layers.config files cannot be patched. If you need to change the settings in these files, you must edit them directly.

Source: https://doc.sitecore.com/developers/90/platform-administration-and-architecture/en/use-a-patch-file-to-customize-the-sitecore-configuration.html

Because of the above, you cannot reach the connectionStrings part of the configuration and therefore you will not be able to patch in a value from there (e.g. the value for "core") in another location in the configuration.

The Sitecore Module you are using is doing it wrong, basically. Instead of requiring a connection string to be set up, it should be asking for a connection string name.