diff --git a/src/error.rs b/src/error.rs
index 9dc471f67..bb09e2e1b 100644
--- a/src/error.rs
+++ b/src/error.rs
@@ -1,9 +1,17 @@
+#![feature(concat_idents)]
+
 use rustc_serialize::hex::*;
 
+#[derive(Debug)]
+pub enum BaseDataError {
+	NegativelyReferencedHash,
+}
+
 #[derive(Debug)]
 pub enum EthcoreError {
 	FromHex(FromHexError),
-	BadSize
+	BaseData(BaseDataError),
+	BadSize,
 }
 
 impl From<FromHexError> for EthcoreError {
@@ -11,3 +19,22 @@ impl From<FromHexError> for EthcoreError {
 		EthcoreError::FromHex(err)
 	}
 }
+
+impl From<BaseDataError> for EthcoreError {
+	fn from(err: BaseDataError) -> EthcoreError {
+		EthcoreError::BaseData(err)
+	}
+}
+
+// TODO: uncomment below once https://github.com/rust-lang/rust/issues/27336 sorted.
+/*macro_rules! assimilate {
+    ($name:ident) => (
+		impl From<concat_idents!($name, Error)> for EthcoreError {
+			fn from(err: concat_idents!($name, Error)) -> EthcoreError {
+				EthcoreError:: $name (err)
+			}
+		}
+    )
+}
+assimilate!(FromHex);
+assimilate!(BaseData);*/
\ No newline at end of file
diff --git a/src/memorydb.rs b/src/memorydb.rs
index 2f54b4524..cbb9e426b 100644
--- a/src/memorydb.rs
+++ b/src/memorydb.rs
@@ -4,6 +4,7 @@ use hash::*;
 use bytes::*;
 use sha3::*;
 use hashdb::*;
+use std::mem;
 use std::collections::HashMap;
 
 #[derive(Debug,Clone)]
@@ -85,19 +86,19 @@ impl MemoryDB {
 		for empty in empties { self.data.remove(&empty); }
 	}
 
-	/// Grab the number of references a particular `key` has. Returns None if the key
-	/// doesn't exist.
-	fn refs(&self, key: &H256) -> Option<i32> {
-		self.data.get(key).map(|&(_, rc)| rc)
-	}
-
-	/// Grab the value associated with a particular `key`. Returns None if the key
+	/// Grab the raw information associated with a key. Returns None if the key
 	/// doesn't exist.
 	///
-	/// Even when Some is returned, this is only guaranteed to return something useful
-	/// when `refs(key) > 0`.
-	fn value(&self, key: &H256) -> Option<&Bytes> {
-		self.data.get(key).map(|&(ref d, _)| d)
+	/// Even when Some is returned, the data is only guaranteed to be useful
+	/// when the refs > 0.
+	pub fn raw(&self, key: &H256) -> Option<&(Bytes, i32)> {
+		self.data.get(key)
+	}
+
+	pub fn drain(&mut self) -> HashMap<H256, (Bytes, i32)> {
+		let mut data = HashMap::new();
+		mem::swap(&mut self.data, &mut data);
+		data
 	}
 }
 
diff --git a/src/overlaydb.rs b/src/overlaydb.rs
index 99d66cd5f..cb6d396c4 100644
--- a/src/overlaydb.rs
+++ b/src/overlaydb.rs
@@ -1,59 +1,115 @@
 //! Disk-backed HashDB implementation.
 
+use error::*;
 use hash::*;
 use bytes::*;
 use sha3::*;
 use hashdb::*;
 use memorydb::*;
 use std::ops::*;
+use std::sync::*;
 use rocksdb::{DB, Writable};
 
 #[derive(Clone)]
 pub struct OverlayDB {
 	overlay: MemoryDB,
-	backing: DB,
+	backing: Arc<DB>,
 }
 
 impl OverlayDB {
 	/// Create a new instance of OverlayDB given a `backing` database.
-	fn new(backing: DB) {
-		self.backing = backing;
-		overlay = MemoryDB::new();
+	fn new(backing: DB) -> OverlayDB {
+		OverlayDB{ overlay: MemoryDB::new(), backing: Arc::new(backing) }
 	}
 
-	/// Commit all memory operations to the backing database.
-	fn commit(&mut self) {
-		unimplemented!();
+	/// Commit all memory operations to the backing database. 
+	fn commit(&mut self) -> Result<u32, EthcoreError> {
+		let mut ret = 0u32;
+		for i in self.overlay.drain().into_iter() {
+			let (key, (value, rc)) = i;
+			if rc != 0 {
+				let new_entry = match self.payload(&key) {
+					Some(x) => {
+						let (back_value, back_rc) = x;
+						if back_rc + rc < 0 {
+							return Err(From::from(BaseDataError::NegativelyReferencedHash));
+						}
+						self.put_payload(&key, (&back_value, rc + back_rc));
+					}
+					None => {
+						self.put_payload(&key, (&value, rc));
+					}
+				};
+				ret += 1;
+			}
+		}
+		Ok(ret)
 	}
 
 	/// Get the refs and value of the given key.
 	fn payload(&self, key: &H256) -> Option<(Bytes, i32)> {
 		unimplemented!();
 	}
+
+	/// Get the refs and value of the given key.
+	fn put_payload(&self, key: &H256, payload: (&Bytes, i32)) {
+		unimplemented!();
+	}
 }
 
 impl HashDB for OverlayDB {
 	fn lookup(&self, key: &H256) -> Option<Bytes> {
-		// TODO: return ok if positive; if negative, check backing - might be enough references there to make
+		// return ok if positive; if negative, check backing - might be enough references there to make
 		// it positive again.
-		let k = self.overlay.data.get(key);
+		let k = self.overlay.raw(key);
 		match k {
 			Some(&(ref d, rc)) if rc > 0 => Some(d.clone()),
 			_ => {
 				let memrc = k.map(|&(_, rc)| rc).unwrap_or(0);
 				match self.payload(key) {
-					Some((d, rc)) if rc + memrc > 0 => Some(d),
+					Some(x) => {
+						let (d, rc) = x;
+						if rc + memrc > 0 {
+							Some(d)
+						}
+						else {
+							None
+						}
+					}
+					// Replace above match arm with this once https://github.com/rust-lang/rust/issues/15287 is done.
+					//Some((d, rc)) if rc + memrc > 0 => Some(d),
 					_ => None,
 				}
 			}
 		}
 	}
 	fn exists(&self, key: &H256) -> bool {
-		// TODO: copy and adapt code above.
-		m_overlay.exists(key)
+		// return ok if positive; if negative, check backing - might be enough references there to make
+		// it positive again.
+		let k = self.overlay.raw(key);
+		match k {
+			Some(&(ref d, rc)) if rc > 0 => true,
+			_ => {
+				let memrc = k.map(|&(_, rc)| rc).unwrap_or(0);
+				match self.payload(key) {
+					Some(x) => {
+						let (d, rc) = x;
+						if rc + memrc > 0 {
+							true
+						}
+						else {
+							false
+						}
+					}
+					// Replace above match arm with this once https://github.com/rust-lang/rust/issues/15287 is done.
+					//Some((d, rc)) if rc + memrc > 0 => true,
+					_ => false,
+				}
+			}
+		}
 	}
-	fn insert(&mut self, value: &[u8]) -> H256 { m_overlay.insert(value) }
-	fn kill(&mut self, key: &H256) { m_overlay.kill(key); }
+	fn insert(&mut self, value: &[u8]) -> H256 { self.overlay.insert(value) }
+	fn kill(&mut self, key: &H256) { self.overlay.kill(key); }
 }
 
 #[test]