Scylla Specific Features

The following features are specific to Scylla Drivers and are not found in a non-Scylla driver. To use these features, install the driver.



Scylla is built around the concept o a sharded architecture. What it means for clients is that each piece of data is bound to specific CPU(s) on specific node(s). The ability of the driver to query specific shard (CPU) is called “shard-awareness”.

One can think of shard-awareness as token-awareness brought to a higher level. Token-aware drivers execute the queries on specific node(s) - where the data of interest “belongs”. This eliminates the network traffic between the coordinator node and the “data owning node” and thus leads to performance improvements. That idea can be taken further: the driver can open a separate connection to every CPU on the target node and use the right connection to query the right CPU on the right node. This eliminates the cross-CPU traffic on that node and results in even greater speedups.

NOTE: Only prepared statements benefit from shard-awareness.

“Basic” shard-awareness

Through extensions to the CQL protocol, Scylla node informs the incoming CQL connection about:

  1. the total number of shards within the node;

  2. the ID of the specific shard that handles this connection.

Driver opens new connections until it reaches or exceeds the number specified by cass_cluster_set_core_connections_per_host() and connections are evenly distributed among shards of this node. No particular action is needed to achieve shard-awareness this way, as this is the default behavior of scylla-cpp-driver. Re-linking with our library is enough to bring basic shard-awareness capabilities to an existing client application.

“Advanced” shard-awareness

Since Scylla 4.3 however, drivers can use a new, more powerful method of establishing per-shard connection pools. This is the recommended usage pattern, commonly referred to as “advanced” shard-awareness. The idea behind it is that Scylla listens for CQL connections on an additional port, by default 19042. Connections incoming to that port, are being routed to the shard determined by client-side (ephemeral) port number. Precisely, if a client socket has local port number P then that connection lands on shard P % shard_count. The function of the usual port 9042 (native_transport_port) is unchanged and non-shard-aware drivers should continue using it.

Advanced shard-awareness is the preferred mode because it reduces load on the cluster while building connection pools. The reason is that with basic shard-awareness, driver keeps opening CQL connections until it randomly reaches each shard, often ending up with some excess connections being established and discarded soon after. In advanced mode, driver opens only as many connections as needed.

NOTE: It’s important to unblock native_shard_aware_transport_port and/or native_shard_aware_transport_port_ssl in the firewall rules, if applicable.

NOTE: If the client app runs behind a NAT (e.g. on a desktop in the office network) while the Scylla cluster is hosted somewhere else (e.g. on Azure or AWS) then, most likely, the router at the office alters the client-side port numbers. In this case port-based (“advanced”) shard selection will not work and will fall back to the “basic” mode.

To enable the advanced mode, client code needs to assign a local port range to the driver by invoking cass_cluster_set_local_port_range(). This change requires recompilation of the application.