Glossary

Term	Definition
Core Node	A cluster member that stores data shards, executes distributed read queries via a Ballista executor, and applies write operations the coordinator sends to its gRPC write service.
Coordinator Node	A cluster member that accepts client queries, routes SELECTs through a Ballista scheduler, routes DML to core nodes through a dedicated gRPC write service, and manages the metadata catalog.
Shard	A horizontal subset of a table's data, stored entirely on one core node (plus replicas).
Fragment / Query Stage	A portion of a distributed query plan that executes on a single core node's Ballista executor.
Replication Factor	The number of copies of each shard maintained across the cluster (denoted N in quorum formulas).
Exchange Operator	A plan node inserted by the Ballista scheduler that transfers data between core node executors during distributed query execution via Arrow Flight.
Push-Down	Optimization technique where DataFusion moves computation (filters, projections, partial aggregations) closer to the data.
Ballista Scheduler	The Apache Ballista component running on the coordinator that decomposes SELECT queries into query stages and assigns them to executors, which pull work from the scheduler.
Ballista Executor	The Apache Ballista component running on each core node that executes query stages using DataFusion.
TableProvider	A custom DataFusion TableProvider on the coordinator that expands a logical table into per-shard remote scans; the shard-local DuckDB SQL is executed on the core node by a custom execution-plan node.
DataFusion	The Apache query engine used by Ballista for SQL parsing, optimization, and physical plan execution.
Arrow Flight	A gRPC-based protocol for high-performance transfer of Apache Arrow data between Ballista executors (and back to the coordinator) during distributed query execution.
gRPC Write Service	A dedicated gRPC service hosted on core nodes through which the coordinator sends shard-local DML statements for execution and replication, separate from the Ballista pipeline.
Quorum	The minimum number of nodes (floor(N / 2) + 1, including the primary) that must acknowledge a write before the coordinator responds to the client.
Tail Replication	The asynchronous background process by which the coordinator retries writes to replicas that did not acknowledge within the synchronous quorum.