diff --git a/ethcore/src/state/backend.rs b/ethcore/src/state/backend.rs
new file mode 100644
index 000000000..83adc2e84
--- /dev/null
+++ b/ethcore/src/state/backend.rs
@@ -0,0 +1,58 @@
+// Copyright 2015-2017 Parity Technologies (UK) Ltd.
+// This file is part of Parity.
+
+// Parity is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Parity is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Parity.  If not, see <http://www.gnu.org/licenses/>.
+
+//! A minimal "state backend" trait: an abstraction over the sources of data
+//! a blockchain state may draw upon.
+//!
+//! Currently assumes a very specific DB + cache structure, but
+//! should become general over time to the point where not even a
+//! merkle trie is strictly necessary.
+
+use std::sync::Arc;
+
+use state::Account;
+use util::{Address, HashDB, H256};
+
+/// State backend. See module docs for more details.
+pub trait Backend {
+	/// Treat the backend as a read-only hashdb.
+	fn as_hashdb(&self) -> &HashDB;
+
+	/// Treat the backend as a writeable hashdb.
+	fn as_hashdb_mut(&mut self) -> &mut HashDB;
+
+	/// Add an account entry to the cache.
+	fn add_to_account_cache(&mut self, addr: Address, data: Option<Account>, modified: bool);
+
+	/// Add a global code cache entry. This doesn't need to worry about canonicality because
+	/// it simply maps hashes to raw code and will always be correct in the absence of
+	/// hash collisions.
+	fn cache_code(&self, hash: H256, code: Arc<Vec<u8>>);
+
+	/// Get basic copy of the cached account. Does not include storage.
+	/// Returns 'None' if cache is disabled or if the account is not cached.
+	fn get_cached_account(&self, addr: &Address) -> Option<Option<Account>>;
+
+	/// Get value from a cached account.
+	/// `None` is passed to the closure if the account entry cached
+	/// is known not to exist.
+	/// `None` is returned if the entry is not cached.
+	fn get_cached<F, U>(&self, a: &Address, f: F) -> Option<U>
+		where F: FnOnce(Option<&mut Account>) -> U;
+
+	/// Get cached code based on hash.
+	fn get_cached_code(&self, hash: &H256) -> Option<Arc<Vec<u8>>>;
+}
diff --git a/ethcore/src/state/mod.rs b/ethcore/src/state/mod.rs
index 1b899d935..45cca6b3c 100644
--- a/ethcore/src/state/mod.rs
+++ b/ethcore/src/state/mod.rs
@@ -37,6 +37,8 @@ use util::trie::recorder::Recorder;
 mod account;
 mod substate;
 
+pub mod backend;
+
 pub use self::account::Account;
 pub use self::substate::Substate;