Citus users typically have a working set of a few hundred GB or more and run into memory and CPU limitations on a single machine on both the write and the read side.
Provided your data has a natural sharding dimension (e.g. tenant ID in a multi-tenant application), Citus can distribute tables across many servers to scale out memory, CPU and storage. It can also parallelise queries, index creation, aggregations, and deletion across the cluster, and it can horizontally scale out write throughput. This gives you the necessary tools to deal with very large data volumes.
Many of the underlying bits that make Citus MX possible will in time be rolled into the open source version. At the same time some of the ways we make it possible, such as by managing the streaming replication and high availability may take longer to productize and it's unclear in what form they'll evolve. For now Citus MX is available on Citus Cloud–due to the way we're able to take care of all of those things for you.
