I think what you're looking for might be setting the isolation level to read committed. This will make the process wait for the in-progress changes have been committed, though you'll still want to make sure you handle SQLCODE -114 somehow, too. That's the code you get back if there's a timeout waiting for the lock. You should be able to set that using:

%SYSTEM.SQL.Util.SetOption("IsolationMode",1,.oldval)

If you do that before your query, the rest of the process will run at that isolation level.

You can use that same method to set the LockTimeout too, by the way. Default is 10 seconds.

Your tResult will have a property called %SQLCODE that gets set when the query is run. If %SQLCODE = 100, that means that the query ran successfully but returned no results or that you've reached the end of the result set. If %SQLCODE = 0, you have some results. If %SQLCODE is less than zero, that's an error code.

if tResult.%SQLCODE = 100{

//whatever you want to do for no results here

}

If you're encoding your data before sending it, you have to specify how it was encoded in a content encoding header so that the server you're sending data to knows how to decode it.

I think it's more likely, though, that it's an issue with your content type. Where you're setting it to "text/plain", if it's supposed to be json, you might try setting it to "application/json" instead.

If the file isn't accessible to link to directly, you may want to look into extending the %CSP.StreamServer class and linking to that. At a bare minimum, you'll want to override the OnPage and OnPreHTTP methods:

ClassMethod OnPage() As %Status
{
set myfile = ##class(%File).%New("/path/to/file")
do myfile.Open("S")
do myfile.OutputToDevice()
quit $$$OK
}

ClassMethod OnPreHTTP() As %Boolean [ Language = cache ]
{
do %response.SetHeader("Content-Disposition","attachment;filename=""filename""")
quit 1
}

Of course using your own file name and the path to the file. That's the local computer file path, not a URL. You also should set the content type appropriately using set %response.ContentType = "text/csv" or whatever the MIME type of the file is so that the browser can identify it correctly.

Unless you want to have to write another %CSP.StreamServer for every file, you'll have to pass the name of the filepath as an argument. So that would look more like:

ClassMethod OnPage() As %Status
{
set myfile = ##class(%File).%New(%request.Get("filepath"))
do myfile.Open("S")
do myfile.OutputToDevice()
quit $$$OK
}

ClassMethod OnPreHTTP() As %Boolean [ Language = cache ]
{
set filepath = %request.Get("filepath")
set %response.ContentType = "text/csv" //or whatever the appropriate MIME type is
do %response.SetHeader("Content-Disposition","attachment;filename="""_$P(filepath,"/",*)_"""")
quit 1
}

Then you could link to it as whatever the path to your stream server is with ?filepath=/path/to/file on the end.

If you take that approach, though, do some validation on the filepath and make sure it can ONLY go to the folder you want! Or, only pass the filename as a parameter to the page, and hard-code the folder in those methods.

Are you just trying to get the json contained in a character stream into a string a vice versa? If so, just read and write to and from the stream:

set json = ""
while 'stream.AtEnd
{
    set json = json_stream.Read()
}

That should get you the contents of the stream into a string.

do stream.Write(json)

That should write the json to a stream.

Or is that not what you're trying to do?

Here is some useful documentation.

You're going to want to make a class that extends %CSP.REST and set up an application that uses that class as its dispatch class. You'll have a URL map in that class to tell IRIS or Cache what to do with the request. Depending on your specific application, you might also want to get familiar with using %request and %response in that process.