cassandra lookup by list of primary keys in java

Question

I am implementing a feature which requires looking up Cassandra by a list of primary keys.

Below is an example data where id is primary key

mytable
id          


        

Answer 1

If the id is the full primary key, then Cassandra supports this, although it's not recommended from performance point of view:

• request is sent to coordinator node
• coordinator node finds a replica for each of the id, and send individual request to them
• wait for results from every node, collect them to result set & send back

As result:

• all your sub-queries need to wait for slowest of the replicas
• you have an additional network hope from coordinator to replica
• you put more pressure to the coordinator node as it need to keep results in memory

If you do a lot of parallel, asynchronous requests for each of the id values from application, then you:

• avoid an additional hop - if you're using prepared statements with token-aware load balancing, then query is sent directly to replicas
• you may start to process results as you get them, not waiting for everything

So sending parallel asynchronous requests could be faster than sending one request with IN...