Added instructions for duckdb setup

docs/client-connectivity.md

@@ -0,0 +1,95 @@
+# Client Connectivity
+
+How to connect various Iceberg clients to `ice-rest-catalog`.
+
+## DuckDB
+
+Connect DuckDB to an Iceberg REST catalog using a Bearer token.
+
+Official reference: [Iceberg REST Catalogs](https://duckdb.org/docs/current/core_extensions/iceberg/iceberg_rest_catalogs.html)
+
+### 1. Install extensions
+
+```sql
+INSTALL iceberg;
+INSTALL httpfs;   -- needed for S3/GCS-backed tables
+LOAD iceberg;
+LOAD httpfs;
+```
+
+### 2. Store the Bearer token in a secret
+
+If you already have a token (from your IdP, catalog UI, `curl`, etc.), put it in an Iceberg secret with `TOKEN`:
+
+```sql
+CREATE SECRET iceberg_secret (
+    TYPE iceberg,
+    TOKEN 'your_bearer_token_here'
+);
+```
+
+- Use the raw token only — do **not** include the `Bearer ` prefix.
+- DuckDB sends it as `Authorization: Bearer <token>` on REST catalog requests.
+
+Optional: add extra headers if your catalog requires them (e.g. GCP billing project):
+
+```sql
+CREATE SECRET iceberg_secret (
+    TYPE iceberg,
+    TOKEN 'your_bearer_token_here',
+    EXTRA_HTTP_HEADERS MAP {
+        'x-goog-user-project': 'your_gcp_project_id'
+    }
+);
+```
+
+### 3. Attach the REST catalog
+
+```sql
+ATTACH 'warehouse_name' AS my_catalog (
+    TYPE iceberg,
+    SECRET iceberg_secret,
+    ENDPOINT 'https://your-rest-catalog.example.com'
+);
+```
+
+| Parameter          | Meaning                                      |
+|--------------------|----------------------------------------------|
+| `'warehouse_name'` | Catalog warehouse name (from your provider)  |
+| `ENDPOINT`         | Base URL of the Iceberg REST catalog         |
+| `SECRET`           | Name of the secret from step 2               |
+
+### 4. Storage access
+
+The Bearer token authenticates to the REST catalog, not necessarily to S3/GCS where data lives.
+
+By default DuckDB uses vended credentials (`ACCESS_DELEGATION_MODE 'vended_credentials'`): the catalog returns temporary storage credentials when you load a table.
+
+If your catalog does not vend credentials, configure storage separately:
+
+```sql
+-- Example: direct S3 access
+CREATE SECRET s3_secret (
+    TYPE s3,
+    KEY_ID '...',
+    SECRET '...',
+    REGION 'us-east-1'
+);
+ATTACH 'warehouse' AS my_catalog (
+    TYPE iceberg,
+    SECRET iceberg_secret,
+    ENDPOINT 'https://catalog.example.com',
+    ACCESS_DELEGATION_MODE 'none'
+);
+```
+
+Supported storage backends: S3, S3 Tables, GCS (see [limitations](https://duckdb.org/docs/current/core_extensions/iceberg/iceberg_rest_catalogs.html#limitations)).
+
+### 5. Query tables
+
+```sql
+SHOW ALL TABLES;
+SELECT * FROM my_catalog.default.my_table LIMIT 10;
+```
+
+Tables are referenced as `catalog.schema.table`.

ice-rest-catalog/README.md

@@ -37,3 +37,4 @@ If `enabled` is true but the catalog backend is not etcd, the lock is ignored (w
 - [Catalog Import/Export](../docs/catalog-import-export.md) -- export and import catalog registry (namespaces and table metadata pointers) via CLI or REST API
 - [Catalog migration (1-node to 3-node etcd)](../docs/catalog-export-import-migration.md) -- migrate registry via `ice catalog-export` / `catalog-import`; optional recovery from an etcd snapshot backup first
 - [Rewriting table paths](../docs/rewrite-table-path.md) -- using Spark to rewrite paths in metadata files when copying a table to a new location
+- [Client Connectivity](../docs/client-connectivity.md) -- connecting to ice-rest-catalog from DuckDB (and other clients)