What is the REST catalog, and why has it become important?

The REST catalog is a standardized HTTP API specification for Iceberg catalog operations — listing tables, resolving a table's current metadata location, committing new snapshots — letting any client speak one common protocol to interact with a catalog, regardless of what's actually implementing that catalog behind the API.

Before the REST catalog specification existed, different catalog implementations (Hive Metastore, AWS Glue, JDBC-based catalogs) each had their own specific client libraries and protocols, meaning an engine needed dedicated integration code for each catalog type it wanted to support; the REST catalog collapses this into one standard interface that any compliant catalog server can implement and any compliant engine can speak.

This has become significant enough that it's implemented by multiple major vendors under their own catalog products — Snowflake's Polaris, Databricks' Unity Catalog, and AWS's Glue-backed REST endpoint among them — which is what lets an Iceberg table effectively become portable across vendor ecosystems: as long as both the writing and reading tools speak the REST catalog protocol, the underlying catalog implementation becomes an interchangeable detail rather than a lock-in point.