how can I store a record with 2 keys in a key value store?

Question

This is a semi theoretical question, Im studying system design and I havent worked with KV stores. The question is about designing a chat app.

I have schema like this:

Message
PK | channel_id   bigint
   | message_id   bigint
_______________
   | user_id
   | content 
   | created_at
...

how would I organize this information to store in a Key - Value Store like Hbase, or Cassandra ?

Answer 1

Cassandra supports composite keys
So you can do a key of both channel_id + message_id

Example structure for your schema:

CREATE TABLE message ( 
   channel_id bigint, 
   message_id bigint, 
   user_id bigint, 
   content text,
   created_at date, 
   PRIMARY KEY (channel_id, message_id) 
);

However, cassandra keys are a bit harded then this.
With PRIMARY KEY (channel_id, message_id) you only define how to partition your data,
but there's also CLUSTERING KEY option to define sorting

In this case it cound be
PRIMARY KEY ((channel_id, message_id), created_at)

Which would mean partition by channel_id + message_id AND sort a partition based on created_at.

Further read on keys in cassandra:
Difference between keys in cassandra