How to get CAS value while performing an update using couchbase subdocument api?

Question

I want to perform update on documents. I am using couchbase subdocument api to update the documents.

While performing the update,I only have the document id at hand. However to get the current Cas value I have to perform a get to the couchbase.

My update looks like:

PrimaryBucket.mutateIn(document_id).upsert("path1","updatevalue1").upsert("path2","updatevalue2")

To handle optimistic locking, i want to use "mutateIn(id).withCas(<currentcasvalue>)"

While performing the update,I only have the document id at hand. However to get the current Cas value I have to perform a get to the couchbase. Is there a way to avoid fetching the whole document in order to only get the cas value to perform the update.

Is this the correct approach?

Answer 1

Yes, there is a way. You can do a Sub-Document lookup to retrieve a single small path from the document, and the result includes the CAS:

long cas = PrimaryBucket.lookupIn(document_id).get("path1").execute().cas();

PrimaryBucket.mutateIn(document_id).upsert("path1","updatevalue1").upsert("path2","updatevalue2").withCas(cas).execute();

If you don't have a path that's guaranteed to exist you can use one of the $document virtual xattr fields like so:

long cas = PrimaryBucket.lookupIn(document_id).get("$document.exptime", new SubdocOptionsBuilder().xattr(true)).execute().cas();