How to get ordered results from couchbase using bulk gets

Question

I am trying to improve performance of querying a couchbase view by using async gets. I have read their documentation about the proper way to do so, it goes something like:

Cluster cluster = CouchbaseCluster.create();
Bucket bucket = cluster.openBucket();


List<JsonDocument> foundDocs = Observable
.just("key1", "key2", "key3", "key4", "key5")
.flatMap(new Func1<String, Observable<JsonDocument>>() {
    @Override
    public Observable<JsonDocument> call(String id) {
        return bucket.async().get(id);
    }
})
.toList()
.toBlocking()
.single();

Which works great and fast, but since I rely on the order of the results, it seems that i need to do some extra work to keep the results ordered. In the example above, the JsonDocument list contains all 5 documents but the order changes randomly from call to call. Is there any ellegant way to order the result using JavaRx capabilities or couchbase Java SDK capabilities?

The only solution i can think of is saving the results in to a HashMap and then transform the original list of ids using this HashMap into an ordered list of JsonDocuments.

Answer 1

Instead of flatMap, you can either use:

• concatMap: will retain order, but actually wait for each inner GET to complete before firing the next one (could revert to sequential execution with less performance)
• concatMapEager: will immediately subscribe inner Observables (so trigger inner GET). Maintains the order by buffering responses that arrive out of order until they can be replayed at the correct index in the sequence. Best of both worlds in terms of ordering and performance.