Cosmos document update in Java, has no failure response although it indeed fail

Question

I use the Java outputItem.setValue(doc) to update existing doc, from Function APP.
I have two such functions in parallel.
Both update calls has no exception, and no failure return (it is void...), but only one of those calls
actually updated the document.

I wonder how can it be?
Do I miss something?
update of document can fail for several reasons (i.e old doc with old etag).

By the way same problem exist when using direct CosmosContainer object to update document. - container.upsertItem(doc)

Answer 1

I found something interesting:
It is wrong to call outputItem.setValue(collection) several times in Function (as I did, in order to update several documents) !
Only last call will be executed.

The proper way is to concat all items to one json list of items and call it once.
outputItem.setValue( [ collection1, collection2, ...]

Answer 2

I use Java in FunctionApp.
I use output binding to update documents.

I use about this code in the Function:
@FunctionName("name")
public HttpResponseMessage run(
@HttpTrigger(name = "req",
methods = {HttpMethod.POST, HttpMethod.GET},
authLevel = AuthorizationLevel.ANONYMOUS) HttpRequestMessage<Optional<String>> request,

        //get all session elements by the partitionKey
        @CosmosDBInput(name = "databasein",
        databaseName = "dbname",
        collectionName = "collectionname",

        sqlQuery = "select * from c where c.sessionID ... ", //WORKING !!!

        connectionStringSetting = "ConnectionStringSetting")       
        Object[] inputItems,  

        @CosmosDBOutput(name = "databaseout",
        databaseName = "dbnabe",
        collectionName = "collectionname",//sessions
        connectionStringSetting = "ConnectionStringSetting")            
        OutputBinding<String> outputItem, //outputItem holds reference to the BingoSessions container

        final ExecutionContext context) 
{
        . . .

and then I use
outputItem.setValue(documentString);
to create or update document.
}

It is partially working.
I DON'T GET ANY FEEDBACK (error, exception or even log), to know whether operation succedded or failed.
Only when I inspect the cosmosdb container, I can see if it was success or not.

When it fails, I have no idea why.