The Catalog Wars — Apache Polaris vs. Unity Catalog

The metadata catalog has replaced the table format as the critical vendor lock-in layer. While Iceberg, Delta, and Hudi compete at the file and metadata level, the catalog — which governs table discovery, schema management, access control, and credential vending — determines which engines can access which data. Apache Polaris provides engine-neutral Iceberg REST API compliance. Databricks Unity Catalog offers deeper governance integration but ties optimization paths to Spark. Engineers must choose a catalog control plane that enables multi-engine access without creating a new lock-in dependency.

• Topics: Metadata Management
• Technologies: Apache Polaris, Unity Catalog, Apache Gravitino, Hive Metastore, AWS Glue Catalog
• Standards: Iceberg REST Catalog Spec
• Pain Points: Vendor Lock-In

1. Assess your current catalog. Most existing lakehouses run on Hive Metastore (HMS) or AWS Glue Catalog. HMS is operationally heavy (requires a backing RDBMS, no built-in RBAC, schema is Hive-centric). Glue is AWS-managed but locks you to the AWS ecosystem and lacks fine-grained access control beyond IAM.

   • If you are starting fresh, skip HMS entirely and adopt a REST catalog.
   • If you are on Glue, evaluate whether AWS-native tooling (Athena, EMR) is sufficient or whether you need multi-engine access.

2. Map engine requirements. List every engine that needs to read or write your tables: Spark, Trino, Flink, DuckDB, StarRocks, Dremio. Each engine has different catalog integration maturity:

   • Iceberg REST catalog is supported by Spark, Trino, Flink, PyIceberg, and DuckDB (via Iceberg extension).
   • Unity Catalog's REST API is Iceberg REST-compatible but includes Databricks-specific extensions for governance.

3. Compare Polaris RBAC vs. Unity governance. Polaris implements role-based access control at the catalog level with namespace-scoped grants. Unity Catalog provides column-level security, row filters, data masking, and audit logging. Choose based on your governance requirements:

   • Polaris: sufficient for most multi-engine analytics use cases.
   • Unity: required if you need column-level masking, row filters, or Databricks-native lineage tracking.

4. Evaluate credential vending capabilities. Modern REST catalogs issue short-lived, scoped storage credentials to query engines instead of distributing static IAM keys. Both Polaris and Unity support credential vending, but the implementation differs:

   • Polaris uses the Iceberg REST spec's loadTable response to vend S3 credentials scoped to the table's S3 prefix.
   • Unity vends credentials through its own API, which may require Databricks-specific client libraries.

5. Test multi-engine query compatibility. Deploy your chosen catalog in a test environment and verify that all engines can discover tables, read schemas, and execute queries. Pay attention to:

   • Partition spec compatibility across engines.
   • Statistics availability (some catalogs do not propagate column stats to all engines).
   • Write conflict resolution (how the catalog handles concurrent writes from different engines).

6. Plan migration from HMS. If migrating from Hive Metastore, the migration involves registering existing Iceberg tables with the new catalog. Apache Gravitino can act as a meta-catalog, federating across multiple underlying catalogs during the transition.

   • Run both catalogs in parallel during migration. Cut over engine by engine.

• Hive Metastore was the de facto catalog for a decade (2012–2022), despite being designed for Hive partition-based tables, not Iceberg or Delta.
• AWS Glue Data Catalog (2017) provided a managed HMS-compatible alternative but locked users into the AWS ecosystem.
• The Iceberg REST Catalog Spec (2022) defined a vendor-neutral catalog API, enabling catalog interoperability for the first time.
• Snowflake open-sourced Apache Polaris (2024), and Databricks open-sourced Unity Catalog (2024), signaling that the catalog — not the format — is the new competitive battleground.
• Apache Gravitino emerged as a meta-catalog for federating across Polaris, Unity, HMS, and Glue during the transition period.