Detect non-existing keys for CRDTs [JIRA: CLIENTS-552]

There is a semantic difference between non-existing key and a default value may me important for downstream consumers. However, CRDT queries return default values: 0 for a counter and empty set for a set. My following examples are in Scala:

val fetchOp = new FetchCounter.Builder(location).build()
val c = client.execute(fetchOp).getDatatype // if I get 0, does the key exist in store?

For counters, there is a workaround that involves an additional query:

val fetchOp = new FetchValue.Builder(location).withOption(FetchValue.Option.HEAD, java.lang.Boolean.TRUE).build()
val res = client.execute(fetchOp) // res.isNotFound should equal(true). 
// Why isNotFound not supported by CRDTs?

For sets, there seem to be a hack that a context object should be null if set did not exist before (not sure if it's a valid way though).

The concept of non-existing key is important enough to support it explicitly (like with regular objects in the snippet above).

So another way around this for now is to set the "NotFoundOK" option on the command/operation to false, and then check the status of the future when it returns.

Location location = new Location(new Namespace(mapBucketType, bucketName), "404");
DtFetchOperation fetch = new DtFetchOperation.Builder(location).withNotFoundOK(false).build();
final RiakFuture<DtFetchOperation.Response, Location> future = cluster.execute(fetch);

future.await();
assertFalse(future.isSuccess()); // isSuccess = false because notFound is not ok
assertNotNull(future.get()); // Still returns new object.

If you set .withNotFoundOK(false), the isSuccess() property should be false for a "new" dataType, but future.get() will still return the new datatype object.

Digging into the implementation a little more today:

https://github.com/basho/riak_pb/blob/develop/src/riak_dt.proto#L113-L132

When the value field is missing from the message, the client should interpret it as a "not found".

Right now we just grab the value without seeing if it's there or not, which gives us the default value that the PB lib provides:

riak-java-client/src/main/java/com/basho/riak/client/core/converters/CrdtResponseConverter.java

Lines 102 to 121 in 3699bc0

    
           public RiakDatatype convert(RiakDtPB.DtFetchResp response) 
        
           { 
        
               RiakDatatype element; 
        
               switch (response.getType()) 
        
               { 
        
                   case COUNTER: 
        
                       element = new RiakCounter(response.getValue().getCounterValue()); 
        
                       break; 
        
                   case MAP: 
        
                       element = parseMap(response.getValue().getMapValueList()); 
        
                       break; 
        
                   case SET: 
        
                       element = parseSet(response.getValue().getSetValueList()); 
        
                       break; 
        
                   default: 
        
                       throw new IllegalStateException("No known datatype returned"); 
        
               } 
        
               return element; 
        
           }

I'll add a "notFound" parameter to the response object and check that property.

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Detect non-existing keys for CRDTs [JIRA: CLIENTS-552] #536

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	public RiakDatatype convert(RiakDtPB.DtFetchResp response)
	{
	RiakDatatype element;
	switch (response.getType())
	{
	case COUNTER:
	element = new RiakCounter(response.getValue().getCounterValue());
	break;
	case MAP:
	element = parseMap(response.getValue().getMapValueList());
	break;
	case SET:
	element = parseSet(response.getValue().getSetValueList());
	break;
	default:
	throw new IllegalStateException("No known datatype returned");
	}

	return element;
	}

Detect non-existing keys for CRDTs [JIRA: CLIENTS-552] #536

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alexmoore commented on Aug 5, 2016

alexmoore commented on Aug 5, 2016

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