Link to file doesn't need for a file to exist, but the containing directory must exist and should be writable by a OS user (cacheusr in uyour case probably).

I'd try to write into a temp dir first, where you're sure you have access:

set file = ##class(%File).TempFilename("pdf")
set sc = stream2.LinkToFile(file)
quit:$$$ISERR(sc) sc

record value of file somewhere (output to display or store in global) and check if the file was created).

%Save method also returns status, you should return it instead of $$$OK:

set sc = stream2.%Save()
quit sc

Okay, even if the classes/globals are the same there's a solution. Let's say you have Sample.Person class in namespaces SAMPLES and USER, each with their own data:

Class Sample.Person Extends %Persistent
{

Property Name;

Storage Default
{
<Data name="PersonDefaultData">
<Value name="1">
<Value>%%CLASSNAME</Value>
</Value>
<Value name="2">
<Value>Name</Value>
</Value>
</Data>
<DataLocation>^Sample.PersonD</DataLocation>
<DefaultData>PersonDefaultData</DefaultData>
<IdLocation>^Sample.PersonD</IdLocation>
<IndexLocation>^Sample.PersonI</IndexLocation>
<StreamLocation>^Sample.PersonS</StreamLocation>
<Type>%Library.CacheStorage</Type>
}

}

And you want to query both from the USER namespace. In that case create a new class extending Sample.Person in the USER namespace and modify storage like this:

Class Utils.SamplesPerson Extends (%Persistent, Sample.Person)
{

Storage Default
{
<Data name="PersonDefaultData">
<Value name="1">
<Value>%%CLASSNAME</Value>
</Value>
<Value name="2">
<Value>Name</Value>
</Value>
</Data>
<DataLocation>^["SAMPLES"]Sample.PersonD</DataLocation>
<DefaultData>PersonDefaultData</DefaultData>
<ExtentSize>200</ExtentSize>
<IdLocation>^["SAMPLES"]Sample.PersonD</IdLocation>
<IndexLocation>^["SAMPLES"]Sample.PersonI</IndexLocation>
<StreamLocation>^["SAMPLES"]Sample.PersonS</StreamLocation>
<Type>%Library.CacheStorage</Type>
}

Note that DataLocation, IndexLocation, IdLocation and StreamLocation point to the SAMPLES namespace.

Now query:

SELECT *
FROM Utils.SamplePerson

Would fetch data from SAMPLES namespace.

You can check out this RabbitMQ adapter. I had the same issue with stream interpretation and it's fixed there.

Abstract class could have this method:

ClassMethod GetNew() As test.Order {
  set class = "test.Order"
  if ..GetCompileDate($classname())'=..GetCompileDate(class) {
    set sc = $system.OBJ.Compile($classname(),"cukb /display=none")
  }
  quit $classmethod(class, "%New")
}

ClassMethod GetCompileDate(class As %Dictionary.CacheClassname) [CodeMode = expression]
{
$$$comClassKeyGet(class,$$$cCLASStimechanged)
}

Eduard, that's a good suggestion, but one issue I see is that if you are processing millions of documents, that's a lot of database overhead. Using a simple global reference with locks would work better as you would only have n files persisted up to the Pool Size. 

The solution I outlined at all times contains exactly up to PoolSize records, so I don't think it's a very big overhead. You can lock filenames I suppose, why not?

My next challenge is to determine a better way for the file service to pick off files from the OS folder without loading all the files into a potentially massive result set which can blow out local memory and in extreme cases quickly blow out CacheTemp DB size.

Ensemble File inbound adapter uses FileSet from %File class. This query uses $zsearch to iterate over files in a directory and populates a ppg with results at once. Calling Next on that result set only moves the ppg key. You can rewrite FileSet query to advance $zsearch  on a Next call. Don't know how it would affect performance though.

I recommend locking as a solution to that problem.

Create a table/class/global which holds {filename, lock, ownerJobId}.

All service jobs execute the same file search, take first filename, check the lock table. If it's empty - write into it and start processing the file.

If lock table has the entry take the next file, till you find one without lock.

After file is processed delete/move it and remove the entry from lock table.

On job shutdown purge the table records associated with job id.

This way you can scale jobs easily.

Didn't know about that. Great find!

Yes, the production is running and service is enabled.

Small addition: objects are always passed by reference, so usually you don't need to pass them with dot.

What does this output in a terminal:

set respText="{""access_token"":""4SDFDSFDSF-aSDASDASD"",""expires_in"":""3300"",""refresh_token"":"""",""scope"":""sms_send"",""token_type"":""Bearer""}"  
set sc = ##class(%ZEN.Auxiliary.jsonProvider).%ParseJSON(respText,,.pObject,1)
zw pObject

I tried it on a later verison and got:

pObject=<OBJECT REFERENCE>[1@%ZEN.proxyObject]
+----------------- general information ---------------
|      oref value: 1
|      class name: %ZEN.proxyObject
| reference count: 2
+----------------- attribute values ------------------
|           %changed = 1
|%data("access_token") = "4SDFDSFDSF-aSDASDASD"
|%data("expires_in") = 3300
|%data("refresh_token") = ""
|     %data("scope") = "sms_send"
|%data("token_type") = "Bearer"
|             %index = ""
+-----------------------------------------------------