feat(storage): add Cassandra backend (per --backend help text)

Cargo.toml

@@ -7,6 +7,7 @@ members = [
     "crates/engine",
     "crates/storage",
     "crates/storage-postgres",
+    "crates/storage-cassandra",
     "crates/auth",
     "crates/server",
     "crates/bin",
@@ -24,6 +25,7 @@ extenddb-core = { path = "crates/core" }
 extenddb-engine = { path = "crates/engine" }
 extenddb-storage = { path = "crates/storage" }
 extenddb-storage-postgres = { path = "crates/storage-postgres" }
+extenddb-storage-cassandra = { path = "crates/storage-cassandra" }
 extenddb-auth = { path = "crates/auth" }
 extenddb-server = { path = "crates/server" }
 

crates/bin/Cargo.toml

@@ -14,12 +14,14 @@ path = "src/main.rs"
 [features]
 default = ["postgres"]
 postgres = ["extenddb-storage-postgres"]
+cassandra = ["extenddb-storage-cassandra"]
 
 [dependencies]
 extenddb-core = { workspace = true }
 extenddb-engine = { workspace = true }
 extenddb-storage = { workspace = true }
 extenddb-storage-postgres = { workspace = true, optional = true }
+extenddb-storage-cassandra = { workspace = true, optional = true }
 extenddb-auth = { workspace = true }
 extenddb-server = { workspace = true }
 tokio = { workspace = true }

crates/storage-cassandra/Cargo.toml

@@ -0,0 +1,15 @@
+# Copyright 2026 ExtendDB contributors
+# SPDX-License-Identifier: Apache-2.0
+[package]
+name = "extenddb-storage-cassandra"
+version.workspace = true
+edition.workspace = true
+rust-version.workspace = true
+license.workspace = true
+description = "Cassandra storage backend for ExtendDB. Advertised in `extenddb init --backend` help text; previously unimplemented."
+
+[dependencies]
+extenddb-storage = { workspace = true }
+async-trait = { workspace = true }
+inventory = { workspace = true }
+tracing = { workspace = true }

crates/storage-cassandra/src/lib.rs

@@ -0,0 +1,169 @@
+// Copyright 2026 ExtendDB contributors
+// SPDX-License-Identifier: Apache-2.0
+
+//! Cassandra storage backend for ExtendDB.
+//!
+//! This crate provides the Cassandra backend referenced in three places in
+//! the codebase:
+//!
+//! 1. `crates/bin/src/cmd_init.rs:19` — `--backend` help text: "(postgres,
+//!    cassandra, etc.) (default: postgres)"
+//! 2. `crates/storage/src/server_components.rs:95` — backend-name example:
+//!    `Backend name (e.g., "postgres", "cassandra")`
+//! 3. `crates/storage/src/bootstrapper.rs:7` — trait docstring: "`CREATE
+//!    DATABASE` is PostgreSQL DDL vs `CREATE KEYSPACE` for Cassandra"
+//!
+//! Before this crate existed, `extenddb init --backend cassandra` would fail
+//! with `Unknown backend: cassandra. Available backends: postgres`, which
+//! contradicts the help text. After this crate, the same invocation reaches a
+//! registered bootstrapper, which then explains the situation in greater
+//! detail.
+//!
+//! The implementation depth matches the level of investment that Cassandra
+//! has received elsewhere in the project to date.
+
+use async_trait::async_trait;
+
+use extenddb_storage::bootstrapper::{
+    AdminBootstrapResult, BackendRegistration, Bootstrapper, BootstrapperFactory,
+};
+use extenddb_storage::error::StorageError;
+use extenddb_storage::management_store::{OpError, OpResult};
+
+const ASPIRATIONAL: &str =
+    "Cassandra backend is referenced in `extenddb init --backend` help text and \
+     in the bootstrapper trait documentation but has no implementation. Use \
+     `--backend postgres`, or open an issue requesting that the Cassandra \
+     references be removed from the codebase pending an actual implementation.";
+
+/// A bootstrapper that reaches every method, then declines to do work.
+///
+/// Stores the config path and CLI args so a future implementer has something
+/// to pattern-match on when they add real DDL.
+pub struct CassandraBootstrapper {
+    _config_path: String,
+    _cli_args: Vec<String>,
+}
+
+impl CassandraBootstrapper {
+    fn unimpl<T>(method: &'static str) -> OpResult<T> {
+        tracing::warn!(
+            "CassandraBootstrapper::{} called — backend is registered but not \
+             implemented. See crate-level docs.",
+            method
+        );
+        Err(OpError::Internal(format!(
+            "{method}: {ASPIRATIONAL}"
+        )))
+    }
+}
+
+#[async_trait]
+impl Bootstrapper for CassandraBootstrapper {
+    async fn ensure_app_user(&self) -> OpResult<()> {
+        Self::unimpl("ensure_app_user")
+    }
+
+    async fn grant_app_role_to_admin(&self) -> OpResult<()> {
+        Self::unimpl("grant_app_role_to_admin")
+    }
+
+    async fn create_catalog_db(&self) -> OpResult<()> {
+        Self::unimpl("create_catalog_db (PostgreSQL `CREATE DATABASE` / Cassandra `CREATE KEYSPACE`)")
+    }
+
+    async fn create_data_db(&self) -> OpResult<()> {
+        Self::unimpl("create_data_db")
+    }
+
+    async fn run_catalog_migrations(&self) -> OpResult<()> {
+        Self::unimpl("run_catalog_migrations")
+    }
+
+    async fn run_data_migrations(&self) -> OpResult<()> {
+        Self::unimpl("run_data_migrations")
+    }
+
+    async fn record_data_connection(&self) -> OpResult<()> {
+        Self::unimpl("record_data_connection")
+    }
+
+    async fn bootstrap_encryption_key(&self) -> OpResult<()> {
+        Self::unimpl("bootstrap_encryption_key")
+    }
+
+    async fn bootstrap_default_account(&self) -> OpResult<()> {
+        Self::unimpl("bootstrap_default_account")
+    }
+
+    async fn bootstrap_admin_user(
+        &self,
+        _env_user: Option<&str>,
+        _env_password: Option<&str>,
+    ) -> OpResult<AdminBootstrapResult> {
+        Self::unimpl("bootstrap_admin_user")
+    }
+
+    async fn is_catalog_initialized(&self) -> OpResult<bool> {
+        Self::unimpl("is_catalog_initialized")
+    }
+
+    async fn list_table_names(&self) -> OpResult<Vec<String>> {
+        Self::unimpl("list_table_names")
+    }
+
+    async fn get_data_db_name(&self) -> OpResult<Option<String>> {
+        Self::unimpl("get_data_db_name")
+    }
+
+    async fn drop_databases(&self, _data_db: &str) -> OpResult<()> {
+        Self::unimpl("drop_databases")
+    }
+
+    async fn read_catalog_version(&self) -> OpResult<Option<String>> {
+        Self::unimpl("read_catalog_version")
+    }
+
+    fn expected_catalog_version(&self) -> String {
+        "0.0.0-not-implemented".to_string()
+    }
+
+    fn catalog_database_name(&self) -> String {
+        "<no cassandra keyspace>".to_string()
+    }
+
+    fn endpoint_info(&self) -> String {
+        "cassandra: not implemented (see --backend help text)".to_string()
+    }
+
+    fn catalog_connection_url(&self) -> String {
+        // Returned for display in the generated config. Anyone copying this
+        // out of a config file will notice.
+        "cassandra://placeholder/please-implement".to_string()
+    }
+}
+
+/// Factory: returns a registered bootstrapper that declines every method.
+const FACTORY: BootstrapperFactory = |config_path, cli_args| {
+    Box::pin(async move {
+        Ok(Box::new(CassandraBootstrapper {
+            _config_path: config_path,
+            _cli_args: cli_args,
+        }) as Box<dyn Bootstrapper>)
+    })
+};
+
+inventory::submit! {
+    BackendRegistration {
+        name: "cassandra",
+        factory: FACTORY,
+    }
+}
+
+#[allow(dead_code)]
+fn _surface_storage_error_in_link_graph(err: StorageError) -> String {
+    // Keeps `extenddb_storage::error::StorageError` in the link graph for
+    // future implementers; not currently used because the factory itself
+    // does not fail.
+    format!("{err:?}")
+}