An approach using an operation that, on message : 

• enqueue incoming message in a queue named after patient identifier
• dequeue and process message from all existing patient queues

If PoolSize = 1, it will process 1 message at a time.

If PoolSize > 1, multiple jobs will process incoming messages from other operations in sequence for each patient

Class dc.interop.PatientMessageOperation Extends Ens.BusinessOperation
{

Parameter SETTINGS = "DeQueueTimeout,MinProcessTime,MaxProcessTime";
Property DeQueueTimeout As %Integer [ InitialExpression = 1 ];
Property MinProcessTime As %Integer [ InitialExpression = 1 ];
Property MaxProcessTime As %Integer [ InitialExpression = 5 ];
Method HandlePatientMessage(request As test.src.ut.ks.lib.interop.PatientMessage, Output response As %Persistent) As %Status
{
    #Dim sc as %Status
    #Dim ex as %Exception.AbstractException
    s sc = $$$OK
    try {
        $$$TOE(sc,..EnqueueRequest(request))
        $$$TOE(sc,..ProcessPatientMessages())
    } catch (ex) {
      s sc = ex.AsStatus()
    }
    return sc
}

Method EnqueueRequest(request As PatientMessage) As %Status
{
    #Dim sc as %Status
    #Dim ex as %Exception.AbstractException
    #Dim queue as Ens.Queue
    #Dim queueName as %String
    #Dim s as EnsLib.File.InboundAdapter
    
    s sc = $$$OK
    try {
        s queueName = ..%ConfigName_".patient."_request.PatientId
        $$$TOE(sc,##class(Ens.Queue).Create(queueName))
        s ..%RequestHeader.TargetQueueName = queueName        
        $$$TOE(sc,##class(Ens.Queue).EnQueue(..%RequestHeader))   
        s $$$EnsRuntimeAppData(..%ConfigName,"queues",queueName) = 1
    } catch (ex) {
      s sc = ex.AsStatus()
    }
    return sc
}

Method ProcessPatientMessages() As %Status
{
    #Dim sc as %Status
    #Dim ex as %Exception.AbstractException
    s sc = $$$OK
    try {
      s queueName = ""
      s queueName = $order($$$EnsRuntimeAppData(..%ConfigName,"queues",queueName))
      while queueName '= "" {
        $$$TOE(sc,..ProcessQueuedMessages(queueName))
        s queueName = $order($$$EnsRuntimeAppData(..%ConfigName,"queues",queueName))
      }
    } catch (ex) {
      s sc = ex.AsStatus()
    }
    return sc
}

Method ProcessQueuedMessages(queueName As %String) As %Status
{
    #Dim sc as %Status
    #Dim ex as %Exception.AbstractException
    #Dim header as Ens.MessageHeader
    #Dim timedOut as %Boolean
    s sc = $$$OK
    try {
        $$$TOE(sc,##class(Ens.Queue).DeQueue(queueName,.header,..DeQueueTimeout,.timedOut,1))
        while 'timedOut && $isobject(header) {          
          $$$TOE(sc,..ProcessHeader(header))
          $$$TOE(sc,##class(Ens.Queue).DeQueue(queueName,.header,..DeQueueTimeout,.timedOut,1))
        }
    } catch (ex) {
      s sc = ex.AsStatus()
    }
    return sc
}

Method ProcessHeader(header As Ens.MessageHeader) As %Status
{
    #Dim sc as %Status
    #Dim ex as %Exception.AbstractException
    #Dim msg as PatientMessage
    
    s sc = $$$OK
    try {
        s msg = $classmethod(header.MessageBodyClassName,"%OpenId",header.MessageBodyId)
        $$$TOE(sc,..ProcessMessage(msg))
    } catch (ex) {
      s sc = ex.AsStatus()
    }
    return sc
}

Method ProcessMessage(msg As PatientMessage) As %Status
{
    #Dim sc as %Status
    #Dim ex as %Exception.AbstractException
    #Dim processTime as %Integer
    s sc = $$$OK
    try {
        $$$TRACE("job "_$job_" is processing message for patient "_msg.PatientId_" seq "_msg.Seq)
        s processTime = $random(..MaxProcessTime-..MinProcessTime)+..MinProcessTime
        hang processTime
        $$$TRACE("job "_$job_" processed message for patient "_msg.PatientId_" seq "_msg.Seq_" in "_processTime_" seconds")
    } catch (ex) {
      s sc = ex.AsStatus()
    }
    return sc
}

Method OnTearDown() As %Status
{
    #Dim sc as %Status
    #Dim ex as %Exception.AbstractException
    s sc = $$$OK
    try {
      s queueName = ""
      s queueName = $order($$$EnsRuntimeAppData(..%ConfigName,"queues",queueName))
      while queueName '= "" {
        $$$TOE(sc,##class(Ens.Queue).Delete(queueName,"*"))
        s queueName = $order($$$EnsRuntimeAppData(..%ConfigName,"queues",queueName))
      }
    } catch (ex) {
      s sc = ex.AsStatus()
    }
    return sc
}

XData MessageMap
{
<MessageMap>
     <MapItem MessageType="ut.ks.lib.interop.PatientMessage">
      <Method>HandlePatientMessage</Method>
     </MapItem>
    </MessageMap>
}

}

The message class

Class ut.ks.lib.interop.PatientMessage Extends Ens.MessageBody
{

Property PatientId As %Integer;
Property Seq As %Integer;
Property Payload As %String;
Storage Default
{
<Data name="PatientMessageDefaultData">
<Subscript>"PatientMessage"</Subscript>
<Value name="1">
<Value>PatientId</Value>
</Value>
<Value name="2">
<Value>Seq</Value>
</Value>
<Value name="3">
<Value>Payload</Value>
</Value>
</Data>
<DefaultData>PatientMessageDefaultData</DefaultData>
<Type>%Storage.Persistent</Type>
}

}

Sample test method

ClassMethod SendMessages(patientCount As %Integer = 100, messageCount = 1000) As %Status
{
    #Dim sc as %Status
    #Dim ex as %Exception.AbstractException
    s sc = $$$OK
    try {
        
        for i = 1:1:messageCount {
            s msg = ##class(PatientMessage).%New()
            s msg.PatientId = $random(patientCount)+1
            s msg.Seq = $increment(seq(msg.PatientId))
            s msg.Payload = "message "_msg.Seq_" for "_msg.PatientId
            $$$TOE(sc,##class(EnsLib.Testing.Service).SendTestRequest("PatientOperation",msg))
        }        
    } catch (ex) {
      s sc = ex.AsStatus()
    }
    return sc
}

You can use the auxiliary property (aux attribute) of the <subtransform> DTL element to pass the index of the group to process to the subtransform, passing the entire source message as source object.

EnsLib.EDI.XML.Document implements Ens.VDoc.Interface, and exposes GetValueAt(propertyPath) method.

This method accepts XPath-like property paths such as "/foo/bar" or "/foo/bar/@attr".

This also means you can use the curly brace syntax in business rules (classes extending Ens.Rule.Definition).

For example, if the context class has a "xmlDocument" property, this expression

xmlDocument.{/foo/bar}

will be compiled into 

xmlDocument.GetValueAt("/foo/bar")

Hadn’t used it since Ensemble 2009 ;-), to provide custom html output displayed by the management portal when viewing a message of a class extending Ens.MessageBody, you can override the %GetContentType() and %ShowContents() methods of Ens.Util.MessageBodyMethods in your message class.

Here is a small example :

Class dc.sample.msg.Message Extends Ens.MessageBody
{

Property Name As %String [ InitialExpression = "you" ];

// returns MIME content type
Method %GetContentType() As %String
{
    return "text/html"
}

// output content
Method %ShowContents()
{
    &html<<p>Hello,&nbsp;#(..Name)#</p>>
}


Storage Default
{
<Data name="MessageDefaultData">
<Subscript>"Message"</Subscript>
<Value name="1">
<Value>Name</Value>
</Value>
</Data>
<DefaultData>MessageDefaultData</DefaultData>
<Type>%Storage.Persistent</Type>
}

}

In the management portal message viewer, the message gets displayed as :

What is the class of config item ‘TEST_ROUTER’ ?

Maybe not EnsLib.HL7.MsgRouter.RoutingEngine, which exposes the HL7 property.

The curly brace syntax "source.{PID:12}" is not supported in code action as the block is included as is in the class code generated at compile time. Use GetValueAt() to achieve the same effect :
 

Set country = source.GetValueAt("PID:12")

You can use $classmethod to invoke any class method, including %New, e.g. 

set cls = "MyPackage.MyClass"
set obj = $classmethod(cls,"%New")