diff --git a/core/src/inmemory.rs b/core/src/inmemory.rs
index 0fa90f4..6dacdd1 100644
--- a/core/src/inmemory.rs
+++ b/core/src/inmemory.rs
@@ -38,14 +38,16 @@ impl InMemoryStorage {
 }
 
 struct InnerTxn<'a> {
+    client_id: Uuid,
     guard: MutexGuard<'a, Inner>,
     written: bool,
     committed: bool,
 }
 
 impl Storage for InMemoryStorage {
-    fn txn(&self) -> anyhow::Result<Box<dyn StorageTxn + '_>> {
+    fn txn(&self, client_id: Uuid) -> anyhow::Result<Box<dyn StorageTxn + '_>> {
         Ok(Box::new(InnerTxn {
+            client_id,
             guard: self.0.lock().expect("poisoned lock"),
             written: false,
             committed: false,
@@ -54,16 +56,16 @@ impl Storage for InMemoryStorage {
 }
 
 impl<'a> StorageTxn for InnerTxn<'a> {
-    fn get_client(&mut self, client_id: Uuid) -> anyhow::Result<Option<Client>> {
-        Ok(self.guard.clients.get(&client_id).cloned())
+    fn get_client(&mut self) -> anyhow::Result<Option<Client>> {
+        Ok(self.guard.clients.get(&self.client_id).cloned())
     }
 
-    fn new_client(&mut self, client_id: Uuid, latest_version_id: Uuid) -> anyhow::Result<()> {
-        if self.guard.clients.contains_key(&client_id) {
-            return Err(anyhow::anyhow!("Client {} already exists", client_id));
+    fn new_client(&mut self, latest_version_id: Uuid) -> anyhow::Result<()> {
+        if self.guard.clients.contains_key(&self.client_id) {
+            return Err(anyhow::anyhow!("Client {} already exists", self.client_id));
         }
         self.guard.clients.insert(
-            client_id,
+            self.client_id,
             Client {
                 latest_version_id,
                 snapshot: None,
@@ -73,64 +75,57 @@ impl<'a> StorageTxn for InnerTxn<'a> {
         Ok(())
     }
 
-    fn set_snapshot(
-        &mut self,
-        client_id: Uuid,
-        snapshot: Snapshot,
-        data: Vec<u8>,
-    ) -> anyhow::Result<()> {
+    fn set_snapshot(&mut self, snapshot: Snapshot, data: Vec<u8>) -> anyhow::Result<()> {
         let client = self
             .guard
             .clients
-            .get_mut(&client_id)
+            .get_mut(&self.client_id)
             .ok_or_else(|| anyhow::anyhow!("no such client"))?;
         client.snapshot = Some(snapshot);
-        self.guard.snapshots.insert(client_id, data);
+        self.guard.snapshots.insert(self.client_id, data);
         self.written = true;
         Ok(())
     }
 
-    fn get_snapshot_data(
-        &mut self,
-        client_id: Uuid,
-        version_id: Uuid,
-    ) -> anyhow::Result<Option<Vec<u8>>> {
+    fn get_snapshot_data(&mut self, version_id: Uuid) -> anyhow::Result<Option<Vec<u8>>> {
         // sanity check
-        let client = self.guard.clients.get(&client_id);
+        let client = self.guard.clients.get(&self.client_id);
         let client = client.ok_or_else(|| anyhow::anyhow!("no such client"))?;
         if Some(&version_id) != client.snapshot.as_ref().map(|snap| &snap.version_id) {
             return Err(anyhow::anyhow!("unexpected snapshot_version_id"));
         }
-        Ok(self.guard.snapshots.get(&client_id).cloned())
+        Ok(self.guard.snapshots.get(&self.client_id).cloned())
     }
 
     fn get_version_by_parent(
         &mut self,
-        client_id: Uuid,
         parent_version_id: Uuid,
     ) -> anyhow::Result<Option<Version>> {
-        if let Some(parent_version_id) = self.guard.children.get(&(client_id, parent_version_id)) {
+        if let Some(parent_version_id) = self
+            .guard
+            .children
+            .get(&(self.client_id, parent_version_id))
+        {
             Ok(self
                 .guard
                 .versions
-                .get(&(client_id, *parent_version_id))
+                .get(&(self.client_id, *parent_version_id))
                 .cloned())
         } else {
             Ok(None)
         }
     }
 
-    fn get_version(
-        &mut self,
-        client_id: Uuid,
-        version_id: Uuid,
-    ) -> anyhow::Result<Option<Version>> {
-        Ok(self.guard.versions.get(&(client_id, version_id)).cloned())
+    fn get_version(&mut self, version_id: Uuid) -> anyhow::Result<Option<Version>> {
+        Ok(self
+            .guard
+            .versions
+            .get(&(self.client_id, version_id))
+            .cloned())
     }
 
     fn add_version(
         &mut self,
-        client_id: Uuid,
         version_id: Uuid,
         parent_version_id: Uuid,
         history_segment: Vec<u8>,
@@ -142,19 +137,21 @@ impl<'a> StorageTxn for InnerTxn<'a> {
             history_segment,
         };
 
-        if let Some(client) = self.guard.clients.get_mut(&client_id) {
+        if let Some(client) = self.guard.clients.get_mut(&self.client_id) {
             client.latest_version_id = version_id;
             if let Some(ref mut snap) = client.snapshot {
                 snap.versions_since += 1;
             }
         } else {
-            return Err(anyhow::anyhow!("Client {} does not exist", client_id));
+            return Err(anyhow::anyhow!("Client {} does not exist", self.client_id));
         }
 
         self.guard
             .children
-            .insert((client_id, parent_version_id), version_id);
-        self.guard.versions.insert((client_id, version_id), version);
+            .insert((self.client_id, parent_version_id), version_id);
+        self.guard
+            .versions
+            .insert((self.client_id, version_id), version);
 
         self.written = true;
         Ok(())
@@ -182,8 +179,8 @@ mod test {
     #[test]
     fn test_get_client_empty() -> anyhow::Result<()> {
         let storage = InMemoryStorage::new();
-        let mut txn = storage.txn()?;
-        let maybe_client = txn.get_client(Uuid::new_v4())?;
+        let mut txn = storage.txn(Uuid::new_v4())?;
+        let maybe_client = txn.get_client()?;
         assert!(maybe_client.is_none());
         Ok(())
     }
@@ -191,20 +188,20 @@ mod test {
     #[test]
     fn test_client_storage() -> anyhow::Result<()> {
         let storage = InMemoryStorage::new();
-        let mut txn = storage.txn()?;
-
         let client_id = Uuid::new_v4();
-        let latest_version_id = Uuid::new_v4();
-        txn.new_client(client_id, latest_version_id)?;
+        let mut txn = storage.txn(client_id)?;
 
-        let client = txn.get_client(client_id)?.unwrap();
+        let latest_version_id = Uuid::new_v4();
+        txn.new_client(latest_version_id)?;
+
+        let client = txn.get_client()?.unwrap();
         assert_eq!(client.latest_version_id, latest_version_id);
         assert!(client.snapshot.is_none());
 
         let latest_version_id = Uuid::new_v4();
-        txn.add_version(client_id, latest_version_id, Uuid::new_v4(), vec![1, 1])?;
+        txn.add_version(latest_version_id, Uuid::new_v4(), vec![1, 1])?;
 
-        let client = txn.get_client(client_id)?.unwrap();
+        let client = txn.get_client()?.unwrap();
         assert_eq!(client.latest_version_id, latest_version_id);
         assert!(client.snapshot.is_none());
 
@@ -213,9 +210,9 @@ mod test {
             timestamp: Utc::now(),
             versions_since: 4,
         };
-        txn.set_snapshot(client_id, snap.clone(), vec![1, 2, 3])?;
+        txn.set_snapshot(snap.clone(), vec![1, 2, 3])?;
 
-        let client = txn.get_client(client_id)?.unwrap();
+        let client = txn.get_client()?.unwrap();
         assert_eq!(client.latest_version_id, latest_version_id);
         assert_eq!(client.snapshot.unwrap(), snap);
 
@@ -226,8 +223,9 @@ mod test {
     #[test]
     fn test_gvbp_empty() -> anyhow::Result<()> {
         let storage = InMemoryStorage::new();
-        let mut txn = storage.txn()?;
-        let maybe_version = txn.get_version_by_parent(Uuid::new_v4(), Uuid::new_v4())?;
+        let client_id = Uuid::new_v4();
+        let mut txn = storage.txn(client_id)?;
+        let maybe_version = txn.get_version_by_parent(Uuid::new_v4())?;
         assert!(maybe_version.is_none());
         Ok(())
     }
@@ -235,20 +233,15 @@ mod test {
     #[test]
     fn test_add_version_and_get_version() -> anyhow::Result<()> {
         let storage = InMemoryStorage::new();
-        let mut txn = storage.txn()?;
-
         let client_id = Uuid::new_v4();
+        let mut txn = storage.txn(client_id)?;
+
         let version_id = Uuid::new_v4();
         let parent_version_id = Uuid::new_v4();
         let history_segment = b"abc".to_vec();
 
-        txn.new_client(client_id, parent_version_id)?;
-        txn.add_version(
-            client_id,
-            version_id,
-            parent_version_id,
-            history_segment.clone(),
-        )?;
+        txn.new_client(parent_version_id)?;
+        txn.add_version(version_id, parent_version_id, history_segment.clone())?;
 
         let expected = Version {
             version_id,
@@ -256,12 +249,10 @@ mod test {
             history_segment,
         };
 
-        let version = txn
-            .get_version_by_parent(client_id, parent_version_id)?
-            .unwrap();
+        let version = txn.get_version_by_parent(parent_version_id)?.unwrap();
         assert_eq!(version, expected);
 
-        let version = txn.get_version(client_id, version_id)?.unwrap();
+        let version = txn.get_version(version_id)?.unwrap();
         assert_eq!(version, expected);
 
         txn.commit()?;
@@ -271,41 +262,40 @@ mod test {
     #[test]
     fn test_snapshots() -> anyhow::Result<()> {
         let storage = InMemoryStorage::new();
-        let mut txn = storage.txn()?;
-
         let client_id = Uuid::new_v4();
+        let mut txn = storage.txn(client_id)?;
 
-        txn.new_client(client_id, Uuid::new_v4())?;
-        assert!(txn.get_client(client_id)?.unwrap().snapshot.is_none());
+        txn.new_client(Uuid::new_v4())?;
+        assert!(txn.get_client()?.unwrap().snapshot.is_none());
 
         let snap = Snapshot {
             version_id: Uuid::new_v4(),
             timestamp: Utc::now(),
             versions_since: 3,
         };
-        txn.set_snapshot(client_id, snap.clone(), vec![9, 8, 9])?;
+        txn.set_snapshot(snap.clone(), vec![9, 8, 9])?;
 
         assert_eq!(
-            txn.get_snapshot_data(client_id, snap.version_id)?.unwrap(),
+            txn.get_snapshot_data(snap.version_id)?.unwrap(),
             vec![9, 8, 9]
         );
-        assert_eq!(txn.get_client(client_id)?.unwrap().snapshot, Some(snap));
+        assert_eq!(txn.get_client()?.unwrap().snapshot, Some(snap));
 
         let snap2 = Snapshot {
             version_id: Uuid::new_v4(),
             timestamp: Utc::now(),
             versions_since: 10,
         };
-        txn.set_snapshot(client_id, snap2.clone(), vec![0, 2, 4, 6])?;
+        txn.set_snapshot(snap2.clone(), vec![0, 2, 4, 6])?;
 
         assert_eq!(
-            txn.get_snapshot_data(client_id, snap2.version_id)?.unwrap(),
+            txn.get_snapshot_data(snap2.version_id)?.unwrap(),
             vec![0, 2, 4, 6]
         );
-        assert_eq!(txn.get_client(client_id)?.unwrap().snapshot, Some(snap2));
+        assert_eq!(txn.get_client()?.unwrap().snapshot, Some(snap2));
 
         // check that mismatched version is detected
-        assert!(txn.get_snapshot_data(client_id, Uuid::new_v4()).is_err());
+        assert!(txn.get_snapshot_data(Uuid::new_v4()).is_err());
 
         txn.commit()?;
         Ok(())
diff --git a/core/src/server.rs b/core/src/server.rs
index c22fa31..09222d5 100644
--- a/core/src/server.rs
+++ b/core/src/server.rs
@@ -111,14 +111,12 @@ impl Server {
         client_id: ClientId,
         parent_version_id: VersionId,
     ) -> Result<GetVersionResult, ServerError> {
-        let mut txn = self.storage.txn()?;
-        let client = txn
-            .get_client(client_id)?
-            .ok_or(ServerError::NoSuchClient)?;
+        let mut txn = self.storage.txn(client_id)?;
+        let client = txn.get_client()?.ok_or(ServerError::NoSuchClient)?;
 
         // If a version with parentVersionId equal to the requested parentVersionId exists, it is
         // returned.
-        if let Some(version) = txn.get_version_by_parent(client_id, parent_version_id)? {
+        if let Some(version) = txn.get_version_by_parent(parent_version_id)? {
             return Ok(GetVersionResult::Success {
                 version_id: version.version_id,
                 parent_version_id: version.parent_version_id,
@@ -152,10 +150,8 @@ impl Server {
     ) -> Result<(AddVersionResult, SnapshotUrgency), ServerError> {
         log::debug!("add_version(client_id: {client_id}, parent_version_id: {parent_version_id})");
 
-        let mut txn = self.storage.txn()?;
-        let client = txn
-            .get_client(client_id)?
-            .ok_or(ServerError::NoSuchClient)?;
+        let mut txn = self.storage.txn(client_id)?;
+        let client = txn.get_client()?.ok_or(ServerError::NoSuchClient)?;
 
         // check if this version is acceptable, under the protection of the transaction
         if client.latest_version_id != NIL_VERSION_ID
@@ -173,7 +169,7 @@ impl Server {
         log::debug!("add_version request accepted: new version_id: {version_id}");
 
         // update the DB
-        txn.add_version(client_id, version_id, parent_version_id, history_segment)?;
+        txn.add_version(version_id, parent_version_id, history_segment)?;
         txn.commit()?;
 
         // calculate the urgency
@@ -206,10 +202,8 @@ impl Server {
     ) -> Result<(), ServerError> {
         log::debug!("add_snapshot(client_id: {client_id}, version_id: {version_id})");
 
-        let mut txn = self.storage.txn()?;
-        let client = txn
-            .get_client(client_id)?
-            .ok_or(ServerError::NoSuchClient)?;
+        let mut txn = self.storage.txn(client_id)?;
+        let client = txn.get_client()?.ok_or(ServerError::NoSuchClient)?;
 
         // NOTE: if the snapshot is rejected, this function logs about it and returns
         // Ok(()), as there's no reason to report an errot to the client / user.
@@ -245,7 +239,7 @@ impl Server {
             }
 
             // get the parent version ID
-            if let Some(parent) = txn.get_version(client_id, vid)? {
+            if let Some(parent) = txn.get_version(vid)? {
                 vid = parent.parent_version_id;
             } else {
                 // this version does not exist; "this should not happen" but if it does,
@@ -257,7 +251,6 @@ impl Server {
 
         log::debug!("accepting snapshot for version {version_id}");
         txn.set_snapshot(
-            client_id,
             Snapshot {
                 version_id,
                 timestamp: Utc::now(),
@@ -274,13 +267,11 @@ impl Server {
         &self,
         client_id: ClientId,
     ) -> Result<Option<(Uuid, Vec<u8>)>, ServerError> {
-        let mut txn = self.storage.txn()?;
-        let client = txn
-            .get_client(client_id)?
-            .ok_or(ServerError::NoSuchClient)?;
+        let mut txn = self.storage.txn(client_id)?;
+        let client = txn.get_client()?.ok_or(ServerError::NoSuchClient)?;
 
         Ok(if let Some(snap) = client.snapshot {
-            txn.get_snapshot_data(client_id, snap.version_id)?
+            txn.get_snapshot_data(snap.version_id)?
                 .map(|data| (snap.version_id, data))
         } else {
             None
@@ -288,8 +279,8 @@ impl Server {
     }
 
     /// Convenience method to get a transaction for the embedded storage.
-    pub fn txn(&self) -> Result<Box<dyn StorageTxn + '_>, ServerError> {
-        Ok(self.storage.txn()?)
+    pub fn txn(&self, client_id: Uuid) -> Result<Box<dyn StorageTxn + '_>, ServerError> {
+        Ok(self.storage.txn(client_id)?)
     }
 }
 
@@ -303,14 +294,15 @@ mod test {
 
     fn setup<INIT, RES>(init: INIT) -> anyhow::Result<(Server, RES)>
     where
-        INIT: FnOnce(&mut dyn StorageTxn) -> anyhow::Result<RES>,
+        INIT: FnOnce(&mut dyn StorageTxn, Uuid) -> anyhow::Result<RES>,
     {
         let _ = env_logger::builder().is_test(true).try_init();
         let storage = InMemoryStorage::new();
+        let client_id = Uuid::new_v4();
         let res;
         {
-            let mut txn = storage.txn()?;
-            res = init(txn.as_mut())?;
+            let mut txn = storage.txn(client_id)?;
+            res = init(txn.as_mut(), client_id)?;
             txn.commit()?;
         }
         Ok((Server::new(ServerConfig::default(), storage), res))
@@ -322,19 +314,17 @@ mod test {
         snapshot_version: Option<u32>,
         snapshot_days_ago: Option<i64>,
     ) -> anyhow::Result<(Server, Uuid, Vec<Uuid>)> {
-        let (server, (client_id, versions)) = setup(|txn| {
-            let client_id = Uuid::new_v4();
+        let (server, (client_id, versions)) = setup(|txn, client_id| {
             let mut versions = vec![];
 
             let mut version_id = Uuid::nil();
-            txn.new_client(client_id, Uuid::nil())?;
+            txn.new_client(Uuid::nil())?;
             debug_assert!(num_versions < u8::MAX.into());
             for vnum in 0..num_versions {
                 let parent_version_id = version_id;
                 version_id = Uuid::new_v4();
                 versions.push(version_id);
                 txn.add_version(
-                    client_id,
                     version_id,
                     parent_version_id,
                     // Generate some unique data for this version.
@@ -342,7 +332,6 @@ mod test {
                 )?;
                 if Some(vnum) == snapshot_version {
                     txn.set_snapshot(
-                        client_id,
                         Snapshot {
                             version_id,
                             versions_since: 0,
@@ -375,12 +364,12 @@ mod test {
             }
 
             // verify that the storage was updated
-            let mut txn = server.txn()?;
-            let client = txn.get_client(client_id)?.unwrap();
+            let mut txn = server.txn(client_id)?;
+            let client = txn.get_client()?.unwrap();
             assert_eq!(client.latest_version_id, new_version_id);
 
             let parent_version_id = existing_versions.last().cloned().unwrap_or_else(Uuid::nil);
-            let version = txn.get_version(client_id, new_version_id)?.unwrap();
+            let version = txn.get_version(new_version_id)?.unwrap();
             assert_eq!(version.version_id, new_version_id);
             assert_eq!(version.parent_version_id, parent_version_id);
             assert_eq!(version.history_segment, expected_history);
@@ -439,9 +428,8 @@ mod test {
 
     #[test]
     fn get_child_version_not_found_initial_nil() -> anyhow::Result<()> {
-        let (server, client_id) = setup(|txn| {
-            let client_id = Uuid::new_v4();
-            txn.new_client(client_id, NIL_VERSION_ID)?;
+        let (server, client_id) = setup(|txn, client_id| {
+            txn.new_client(NIL_VERSION_ID)?;
 
             Ok(client_id)
         })?;
@@ -455,9 +443,8 @@ mod test {
 
     #[test]
     fn get_child_version_not_found_initial_continuing() -> anyhow::Result<()> {
-        let (server, client_id) = setup(|txn| {
-            let client_id = Uuid::new_v4();
-            txn.new_client(client_id, NIL_VERSION_ID)?;
+        let (server, client_id) = setup(|txn, client_id| {
+            txn.new_client(NIL_VERSION_ID)?;
 
             Ok(client_id)
         })?;
@@ -473,12 +460,11 @@ mod test {
 
     #[test]
     fn get_child_version_not_found_up_to_date() -> anyhow::Result<()> {
-        let (server, (client_id, parent_version_id)) = setup(|txn| {
+        let (server, (client_id, parent_version_id)) = setup(|txn, client_id| {
             // add a parent version, but not the requested child version
-            let client_id = Uuid::new_v4();
             let parent_version_id = Uuid::new_v4();
-            txn.new_client(client_id, parent_version_id)?;
-            txn.add_version(client_id, parent_version_id, NIL_VERSION_ID, vec![])?;
+            txn.new_client(parent_version_id)?;
+            txn.add_version(parent_version_id, NIL_VERSION_ID, vec![])?;
 
             Ok((client_id, parent_version_id))
         })?;
@@ -492,13 +478,11 @@ mod test {
 
     #[test]
     fn get_child_version_gone_not_latest() -> anyhow::Result<()> {
-        let (server, client_id) = setup(|txn| {
-            let client_id = Uuid::new_v4();
-
+        let (server, client_id) = setup(|txn, client_id| {
             // Add a parent version, but not the requested parent version
             let parent_version_id = Uuid::new_v4();
-            txn.new_client(client_id, parent_version_id)?;
-            txn.add_version(client_id, parent_version_id, NIL_VERSION_ID, vec![])?;
+            txn.new_client(parent_version_id)?;
+            txn.add_version(parent_version_id, NIL_VERSION_ID, vec![])?;
 
             Ok(client_id)
         })?;
@@ -512,22 +496,17 @@ mod test {
 
     #[test]
     fn get_child_version_found() -> anyhow::Result<()> {
-        let (server, (client_id, version_id, parent_version_id, history_segment)) = setup(|txn| {
-            let client_id = Uuid::new_v4();
-            let version_id = Uuid::new_v4();
-            let parent_version_id = Uuid::new_v4();
-            let history_segment = b"abcd".to_vec();
+        let (server, (client_id, version_id, parent_version_id, history_segment)) =
+            setup(|txn, client_id| {
+                let version_id = Uuid::new_v4();
+                let parent_version_id = Uuid::new_v4();
+                let history_segment = b"abcd".to_vec();
 
-            txn.new_client(client_id, version_id)?;
-            txn.add_version(
-                client_id,
-                version_id,
-                parent_version_id,
-                history_segment.clone(),
-            )?;
+                txn.new_client(version_id)?;
+                txn.add_version(version_id, parent_version_id, history_segment.clone())?;
 
-            Ok((client_id, version_id, parent_version_id, history_segment))
-        })?;
+                Ok((client_id, version_id, parent_version_id, history_segment))
+            })?;
         assert_eq!(
             server.get_child_version(client_id, parent_version_id)?,
             GetVersionResult::Success {
@@ -550,12 +529,9 @@ mod test {
         );
 
         // verify that the storage wasn't updated
-        let mut txn = server.txn()?;
-        assert_eq!(
-            txn.get_client(client_id)?.unwrap().latest_version_id,
-            versions[2]
-        );
-        assert_eq!(txn.get_version_by_parent(client_id, versions[2])?, None);
+        let mut txn = server.txn(client_id)?;
+        assert_eq!(txn.get_client()?.unwrap().latest_version_id, versions[2]);
+        assert_eq!(txn.get_version_by_parent(versions[2])?, None);
 
         Ok(())
     }
@@ -661,13 +637,12 @@ mod test {
 
     #[test]
     fn add_snapshot_success_latest() -> anyhow::Result<()> {
-        let (server, (client_id, version_id)) = setup(|txn| {
-            let client_id = Uuid::new_v4();
+        let (server, (client_id, version_id)) = setup(|txn, client_id| {
             let version_id = Uuid::new_v4();
 
             // set up a task DB with one version in it
-            txn.new_client(client_id, version_id)?;
-            txn.add_version(client_id, version_id, NIL_VERSION_ID, vec![])?;
+            txn.new_client(version_id)?;
+            txn.add_version(version_id, NIL_VERSION_ID, vec![])?;
 
             // add a snapshot for that version
             Ok((client_id, version_id))
@@ -675,13 +650,13 @@ mod test {
         server.add_snapshot(client_id, version_id, vec![1, 2, 3])?;
 
         // verify the snapshot
-        let mut txn = server.txn()?;
-        let client = txn.get_client(client_id)?.unwrap();
+        let mut txn = server.txn(client_id)?;
+        let client = txn.get_client()?.unwrap();
         let snapshot = client.snapshot.unwrap();
         assert_eq!(snapshot.version_id, version_id);
         assert_eq!(snapshot.versions_since, 0);
         assert_eq!(
-            txn.get_snapshot_data(client_id, version_id).unwrap(),
+            txn.get_snapshot_data(version_id).unwrap(),
             Some(vec![1, 2, 3])
         );
 
@@ -690,15 +665,14 @@ mod test {
 
     #[test]
     fn add_snapshot_success_older() -> anyhow::Result<()> {
-        let (server, (client_id, version_id_1)) = setup(|txn| {
-            let client_id = Uuid::new_v4();
+        let (server, (client_id, version_id_1)) = setup(|txn, client_id| {
             let version_id_1 = Uuid::new_v4();
             let version_id_2 = Uuid::new_v4();
 
             // set up a task DB with two versions in it
-            txn.new_client(client_id, version_id_2)?;
-            txn.add_version(client_id, version_id_1, NIL_VERSION_ID, vec![])?;
-            txn.add_version(client_id, version_id_2, version_id_1, vec![])?;
+            txn.new_client(version_id_2)?;
+            txn.add_version(version_id_1, NIL_VERSION_ID, vec![])?;
+            txn.add_version(version_id_2, version_id_1, vec![])?;
 
             Ok((client_id, version_id_1))
         })?;
@@ -706,13 +680,13 @@ mod test {
         server.add_snapshot(client_id, version_id_1, vec![1, 2, 3])?;
 
         // verify the snapshot
-        let mut txn = server.txn()?;
-        let client = txn.get_client(client_id)?.unwrap();
+        let mut txn = server.txn(client_id)?;
+        let client = txn.get_client()?.unwrap();
         let snapshot = client.snapshot.unwrap();
         assert_eq!(snapshot.version_id, version_id_1);
         assert_eq!(snapshot.versions_since, 0);
         assert_eq!(
-            txn.get_snapshot_data(client_id, version_id_1).unwrap(),
+            txn.get_snapshot_data(version_id_1).unwrap(),
             Some(vec![1, 2, 3])
         );
 
@@ -721,15 +695,14 @@ mod test {
 
     #[test]
     fn add_snapshot_fails_no_such() -> anyhow::Result<()> {
-        let (server, client_id) = setup(|txn| {
-            let client_id = Uuid::new_v4();
+        let (server, client_id) = setup(|txn, client_id| {
             let version_id_1 = Uuid::new_v4();
             let version_id_2 = Uuid::new_v4();
 
             // set up a task DB with two versions in it
-            txn.new_client(client_id, version_id_2)?;
-            txn.add_version(client_id, version_id_1, NIL_VERSION_ID, vec![])?;
-            txn.add_version(client_id, version_id_2, version_id_1, vec![])?;
+            txn.new_client(version_id_2)?;
+            txn.add_version(version_id_1, NIL_VERSION_ID, vec![])?;
+            txn.add_version(version_id_2, version_id_1, vec![])?;
 
             // add a snapshot for unknown version
             Ok(client_id)
@@ -739,8 +712,8 @@ mod test {
         server.add_snapshot(client_id, version_id_unk, vec![1, 2, 3])?;
 
         // verify the snapshot does not exist
-        let mut txn = server.txn()?;
-        let client = txn.get_client(client_id)?.unwrap();
+        let mut txn = server.txn(client_id)?;
+        let client = txn.get_client()?.unwrap();
         assert!(client.snapshot.is_none());
 
         Ok(())
@@ -748,16 +721,15 @@ mod test {
 
     #[test]
     fn add_snapshot_fails_too_old() -> anyhow::Result<()> {
-        let (server, (client_id, version_ids)) = setup(|txn| {
-            let client_id = Uuid::new_v4();
+        let (server, (client_id, version_ids)) = setup(|txn, client_id| {
             let mut version_id = Uuid::new_v4();
             let mut parent_version_id = Uuid::nil();
             let mut version_ids = vec![];
 
             // set up a task DB with 10 versions in it (oldest to newest)
-            txn.new_client(client_id, Uuid::nil())?;
+            txn.new_client(Uuid::nil())?;
             for _ in 0..10 {
-                txn.add_version(client_id, version_id, parent_version_id, vec![])?;
+                txn.add_version(version_id, parent_version_id, vec![])?;
                 version_ids.push(version_id);
                 parent_version_id = version_id;
                 version_id = Uuid::new_v4();
@@ -769,8 +741,8 @@ mod test {
         server.add_snapshot(client_id, version_ids[0], vec![1, 2, 3])?;
 
         // verify the snapshot does not exist
-        let mut txn = server.txn()?;
-        let client = txn.get_client(client_id)?.unwrap();
+        let mut txn = server.txn(client_id)?;
+        let client = txn.get_client()?.unwrap();
         assert!(client.snapshot.is_none());
 
         Ok(())
@@ -778,23 +750,21 @@ mod test {
 
     #[test]
     fn add_snapshot_fails_newer_exists() -> anyhow::Result<()> {
-        let (server, (client_id, version_ids)) = setup(|txn| {
-            let client_id = Uuid::new_v4();
+        let (server, (client_id, version_ids)) = setup(|txn, client_id| {
             let mut version_id = Uuid::new_v4();
             let mut parent_version_id = Uuid::nil();
             let mut version_ids = vec![];
 
             // set up a task DB with 5 versions in it (oldest to newest) and a snapshot of the
             // middle one
-            txn.new_client(client_id, Uuid::nil())?;
+            txn.new_client(Uuid::nil())?;
             for _ in 0..5 {
-                txn.add_version(client_id, version_id, parent_version_id, vec![])?;
+                txn.add_version(version_id, parent_version_id, vec![])?;
                 version_ids.push(version_id);
                 parent_version_id = version_id;
                 version_id = Uuid::new_v4();
             }
             txn.set_snapshot(
-                client_id,
                 Snapshot {
                     version_id: version_ids[2],
                     versions_since: 2,
@@ -810,13 +780,13 @@ mod test {
         server.add_snapshot(client_id, version_ids[0], vec![9, 9, 9])?;
 
         // verify the snapshot was not replaced
-        let mut txn = server.txn()?;
-        let client = txn.get_client(client_id)?.unwrap();
+        let mut txn = server.txn(client_id)?;
+        let client = txn.get_client()?.unwrap();
         let snapshot = client.snapshot.unwrap();
         assert_eq!(snapshot.version_id, version_ids[2]);
         assert_eq!(snapshot.versions_since, 2);
         assert_eq!(
-            txn.get_snapshot_data(client_id, version_ids[2]).unwrap(),
+            txn.get_snapshot_data(version_ids[2]).unwrap(),
             Some(vec![1, 2, 3])
         );
 
@@ -825,11 +795,9 @@ mod test {
 
     #[test]
     fn add_snapshot_fails_nil_version() -> anyhow::Result<()> {
-        let (server, client_id) = setup(|txn| {
-            let client_id = Uuid::new_v4();
-
+        let (server, client_id) = setup(|txn, client_id| {
             // just set up the client
-            txn.new_client(client_id, NIL_VERSION_ID)?;
+            txn.new_client(NIL_VERSION_ID)?;
 
             // add a snapshot for the nil version
             Ok(client_id)
@@ -838,8 +806,8 @@ mod test {
         server.add_snapshot(client_id, NIL_VERSION_ID, vec![9, 9, 9])?;
 
         // verify the snapshot does not exist
-        let mut txn = server.txn()?;
-        let client = txn.get_client(client_id)?.unwrap();
+        let mut txn = server.txn(client_id)?;
+        let client = txn.get_client()?.unwrap();
         assert!(client.snapshot.is_none());
 
         Ok(())
@@ -847,14 +815,12 @@ mod test {
 
     #[test]
     fn get_snapshot_found() -> anyhow::Result<()> {
-        let (server, (client_id, data, snapshot_version_id)) = setup(|txn| {
-            let client_id = Uuid::new_v4();
+        let (server, (client_id, data, snapshot_version_id)) = setup(|txn, client_id| {
             let data = vec![1, 2, 3];
             let snapshot_version_id = Uuid::new_v4();
 
-            txn.new_client(client_id, snapshot_version_id)?;
+            txn.new_client(snapshot_version_id)?;
             txn.set_snapshot(
-                client_id,
                 Snapshot {
                     version_id: snapshot_version_id,
                     versions_since: 3,
@@ -874,9 +840,8 @@ mod test {
 
     #[test]
     fn get_snapshot_not_found() -> anyhow::Result<()> {
-        let (server, client_id) = setup(|txn| {
-            let client_id = Uuid::new_v4();
-            txn.new_client(client_id, NIL_VERSION_ID)?;
+        let (server, client_id) = setup(|txn, client_id| {
+            txn.new_client(NIL_VERSION_ID)?;
             Ok(client_id)
         })?;
 
diff --git a/core/src/storage.rs b/core/src/storage.rs
index e0c9621..011f231 100644
--- a/core/src/storage.rs
+++ b/core/src/storage.rs
@@ -39,48 +39,38 @@ pub struct Version {
 /// in storage must be as if each were executed sequentially in some order. In particular,
 /// un-committed changes must not be read by another transaction.
 ///
+/// Transactions with different client IDs cannot share any data, so it is safe to handle them
+/// concurrently.
+///
 /// Changes in a transaction that is dropped without calling `commit` must not appear in any other
 /// transaction.
 pub trait StorageTxn {
-    /// Get information about the given client
-    fn get_client(&mut self, client_id: Uuid) -> anyhow::Result<Option<Client>>;
+    /// Get information about the client for this transaction
+    fn get_client(&mut self) -> anyhow::Result<Option<Client>>;
 
-    /// Create a new client with the given latest_version_id
-    fn new_client(&mut self, client_id: Uuid, latest_version_id: Uuid) -> anyhow::Result<()>;
+    /// Create the client for this transaction, with the given latest_version_id. The client must
+    /// not already exist.
+    fn new_client(&mut self, latest_version_id: Uuid) -> anyhow::Result<()>;
 
     /// Set the client's most recent snapshot.
-    fn set_snapshot(
-        &mut self,
-        client_id: Uuid,
-        snapshot: Snapshot,
-        data: Vec<u8>,
-    ) -> anyhow::Result<()>;
+    fn set_snapshot(&mut self, snapshot: Snapshot, data: Vec<u8>) -> anyhow::Result<()>;
 
     /// Get the data for the most recent snapshot.  The version_id
     /// is used to verify that the snapshot is for the correct version.
-    fn get_snapshot_data(
-        &mut self,
-        client_id: Uuid,
-        version_id: Uuid,
-    ) -> anyhow::Result<Option<Vec<u8>>>;
+    fn get_snapshot_data(&mut self, version_id: Uuid) -> anyhow::Result<Option<Vec<u8>>>;
 
     /// Get a version, indexed by parent version id
-    fn get_version_by_parent(
-        &mut self,
-        client_id: Uuid,
-        parent_version_id: Uuid,
-    ) -> anyhow::Result<Option<Version>>;
+    fn get_version_by_parent(&mut self, parent_version_id: Uuid)
+        -> anyhow::Result<Option<Version>>;
 
     /// Get a version, indexed by its own version id
-    fn get_version(&mut self, client_id: Uuid, version_id: Uuid)
-        -> anyhow::Result<Option<Version>>;
+    fn get_version(&mut self, version_id: Uuid) -> anyhow::Result<Option<Version>>;
 
     /// Add a version (that must not already exist), and
     ///  - update latest_version_id
     ///  - increment snapshot.versions_since
     fn add_version(
         &mut self,
-        client_id: Uuid,
         version_id: Uuid,
         parent_version_id: Uuid,
         history_segment: Vec<u8>,
@@ -94,6 +84,6 @@ pub trait StorageTxn {
 /// A trait for objects able to act as storage.  Most of the interesting behavior is in the
 /// [`crate::storage::StorageTxn`] trait.
 pub trait Storage: Send + Sync {
-    /// Begin a transaction
-    fn txn(&self) -> anyhow::Result<Box<dyn StorageTxn + '_>>;
+    /// Begin a transaction for the given client ID.
+    fn txn(&self, client_id: Uuid) -> anyhow::Result<Box<dyn StorageTxn + '_>>;
 }
diff --git a/server/src/api/add_snapshot.rs b/server/src/api/add_snapshot.rs
index 1769ca7..745a5a9 100644
--- a/server/src/api/add_snapshot.rs
+++ b/server/src/api/add_snapshot.rs
@@ -70,9 +70,9 @@ mod test {
 
         // set up the storage contents..
         {
-            let mut txn = storage.txn().unwrap();
-            txn.new_client(client_id, version_id).unwrap();
-            txn.add_version(client_id, version_id, NIL_VERSION_ID, vec![])?;
+            let mut txn = storage.txn(client_id).unwrap();
+            txn.new_client(version_id).unwrap();
+            txn.add_version(version_id, NIL_VERSION_ID, vec![])?;
             txn.commit()?;
         }
 
@@ -114,8 +114,8 @@ mod test {
 
         // set up the storage contents..
         {
-            let mut txn = storage.txn().unwrap();
-            txn.new_client(client_id, NIL_VERSION_ID).unwrap();
+            let mut txn = storage.txn(client_id).unwrap();
+            txn.new_client(NIL_VERSION_ID).unwrap();
             txn.commit().unwrap();
         }
 
diff --git a/server/src/api/add_version.rs b/server/src/api/add_version.rs
index d6ac245..32cfad3 100644
--- a/server/src/api/add_version.rs
+++ b/server/src/api/add_version.rs
@@ -82,9 +82,11 @@ pub(crate) async fn service(
             }
             Err(ServerError::NoSuchClient) => {
                 // Create a new client and repeat the `add_version` call.
-                let mut txn = server_state.server.txn().map_err(server_error_to_actix)?;
-                txn.new_client(client_id, NIL_VERSION_ID)
-                    .map_err(failure_to_ise)?;
+                let mut txn = server_state
+                    .server
+                    .txn(client_id)
+                    .map_err(server_error_to_actix)?;
+                txn.new_client(NIL_VERSION_ID).map_err(failure_to_ise)?;
                 txn.commit().map_err(failure_to_ise)?;
                 continue;
             }
@@ -111,8 +113,8 @@ mod test {
 
         // set up the storage contents..
         {
-            let mut txn = storage.txn().unwrap();
-            txn.new_client(client_id, Uuid::nil()).unwrap();
+            let mut txn = storage.txn(client_id).unwrap();
+            txn.new_client(Uuid::nil()).unwrap();
             txn.commit().unwrap();
         }
 
@@ -181,8 +183,8 @@ mod test {
 
         // Check that the client really was created
         {
-            let mut txn = server.server_state.server.txn().unwrap();
-            let client = txn.get_client(client_id).unwrap().unwrap();
+            let mut txn = server.server_state.server.txn(client_id).unwrap();
+            let client = txn.get_client().unwrap().unwrap();
             assert_eq!(client.latest_version_id, new_version_id);
             assert_eq!(client.snapshot, None);
         }
@@ -197,8 +199,8 @@ mod test {
 
         // set up the storage contents..
         {
-            let mut txn = storage.txn().unwrap();
-            txn.new_client(client_id, version_id).unwrap();
+            let mut txn = storage.txn(client_id).unwrap();
+            txn.new_client(version_id).unwrap();
             txn.commit().unwrap();
         }
 
diff --git a/server/src/api/get_child_version.rs b/server/src/api/get_child_version.rs
index 4cf53ef..fef19c1 100644
--- a/server/src/api/get_child_version.rs
+++ b/server/src/api/get_child_version.rs
@@ -64,9 +64,9 @@ mod test {
 
         // set up the storage contents..
         {
-            let mut txn = storage.txn().unwrap();
-            txn.new_client(client_id, Uuid::new_v4()).unwrap();
-            txn.add_version(client_id, version_id, parent_version_id, b"abcd".to_vec())
+            let mut txn = storage.txn(client_id).unwrap();
+            txn.new_client(Uuid::new_v4()).unwrap();
+            txn.add_version(version_id, parent_version_id, b"abcd".to_vec())
                 .unwrap();
             txn.commit().unwrap();
         }
@@ -128,9 +128,9 @@ mod test {
 
         // create the client and a single version.
         {
-            let mut txn = storage.txn().unwrap();
-            txn.new_client(client_id, Uuid::new_v4()).unwrap();
-            txn.add_version(client_id, test_version_id, NIL_VERSION_ID, b"vers".to_vec())
+            let mut txn = storage.txn(client_id).unwrap();
+            txn.new_client(Uuid::new_v4()).unwrap();
+            txn.add_version(test_version_id, NIL_VERSION_ID, b"vers".to_vec())
                 .unwrap();
             txn.commit().unwrap();
         }
diff --git a/server/src/api/get_snapshot.rs b/server/src/api/get_snapshot.rs
index 6eff71a..0a1f030 100644
--- a/server/src/api/get_snapshot.rs
+++ b/server/src/api/get_snapshot.rs
@@ -48,8 +48,8 @@ mod test {
 
         // set up the storage contents..
         {
-            let mut txn = storage.txn().unwrap();
-            txn.new_client(client_id, Uuid::new_v4()).unwrap();
+            let mut txn = storage.txn(client_id).unwrap();
+            txn.new_client(Uuid::new_v4()).unwrap();
             txn.commit().unwrap();
         }
 
@@ -75,10 +75,9 @@ mod test {
 
         // set up the storage contents..
         {
-            let mut txn = storage.txn().unwrap();
-            txn.new_client(client_id, Uuid::new_v4()).unwrap();
+            let mut txn = storage.txn(client_id).unwrap();
+            txn.new_client(Uuid::new_v4()).unwrap();
             txn.set_snapshot(
-                client_id,
                 Snapshot {
                     version_id,
                     versions_since: 3,
diff --git a/sqlite/src/lib.rs b/sqlite/src/lib.rs
index e64c0c5..7441b17 100644
--- a/sqlite/src/lib.rs
+++ b/sqlite/src/lib.rs
@@ -78,12 +78,12 @@ impl SqliteStorage {
 }
 
 impl Storage for SqliteStorage {
-    fn txn<'a>(&'a self) -> anyhow::Result<Box<dyn StorageTxn + 'a>> {
+    fn txn(&self, client_id: Uuid) -> anyhow::Result<Box<dyn StorageTxn + '_>> {
         let con = self.new_connection()?;
         // Begin the transaction on this new connection. An IMMEDIATE connection is in
         // write (exclusive) mode from the start.
         con.execute("BEGIN IMMEDIATE", [])?;
-        let txn = Txn { con };
+        let txn = Txn { con, client_id };
         Ok(Box::new(txn))
     }
 }
@@ -93,6 +93,7 @@ struct Txn {
     // transactions by running `BEGIN ...` and `COMMIT` at appropriate times. So we will do
     // the same.
     con: Connection,
+    client_id: Uuid,
 }
 
 impl Txn {
@@ -126,7 +127,7 @@ impl Txn {
 }
 
 impl StorageTxn for Txn {
-    fn get_client(&mut self, client_id: Uuid) -> anyhow::Result<Option<Client>> {
+    fn get_client(&mut self) -> anyhow::Result<Option<Client>> {
         let result: Option<Client> = self
             .con
             .query_row(
@@ -138,7 +139,7 @@ impl StorageTxn for Txn {
                  FROM clients
                  WHERE client_id = ?
                  LIMIT 1",
-                [&StoredUuid(client_id)],
+                [&StoredUuid(self.client_id)],
                 |r| {
                     let latest_version_id: StoredUuid = r.get(0)?;
                     let snapshot_timestamp: Option<i64> = r.get(1)?;
@@ -170,22 +171,17 @@ impl StorageTxn for Txn {
         Ok(result)
     }
 
-    fn new_client(&mut self, client_id: Uuid, latest_version_id: Uuid) -> anyhow::Result<()> {
+    fn new_client(&mut self, latest_version_id: Uuid) -> anyhow::Result<()> {
         self.con
             .execute(
                 "INSERT OR REPLACE INTO clients (client_id, latest_version_id) VALUES (?, ?)",
-                params![&StoredUuid(client_id), &StoredUuid(latest_version_id)],
+                params![&StoredUuid(self.client_id), &StoredUuid(latest_version_id)],
             )
             .context("Error creating/updating client")?;
         Ok(())
     }
 
-    fn set_snapshot(
-        &mut self,
-        client_id: Uuid,
-        snapshot: Snapshot,
-        data: Vec<u8>,
-    ) -> anyhow::Result<()> {
+    fn set_snapshot(&mut self, snapshot: Snapshot, data: Vec<u8>) -> anyhow::Result<()> {
         self.con
             .execute(
                 "UPDATE clients
@@ -200,23 +196,19 @@ impl StorageTxn for Txn {
                     snapshot.timestamp.timestamp(),
                     snapshot.versions_since,
                     data,
-                    &StoredUuid(client_id),
+                    &StoredUuid(self.client_id),
                 ],
             )
             .context("Error creating/updating snapshot")?;
         Ok(())
     }
 
-    fn get_snapshot_data(
-        &mut self,
-        client_id: Uuid,
-        version_id: Uuid,
-    ) -> anyhow::Result<Option<Vec<u8>>> {
+    fn get_snapshot_data(&mut self, version_id: Uuid) -> anyhow::Result<Option<Vec<u8>>> {
         let r = self
             .con
             .query_row(
                 "SELECT snapshot, snapshot_version_id FROM clients WHERE client_id = ?",
-                params![&StoredUuid(client_id)],
+                params![&StoredUuid(self.client_id)],
                 |r| {
                     let v: StoredUuid = r.get("snapshot_version_id")?;
                     let d: Vec<u8> = r.get("snapshot")?;
@@ -237,29 +229,25 @@ impl StorageTxn for Txn {
 
     fn get_version_by_parent(
         &mut self,
-        client_id: Uuid,
+
         parent_version_id: Uuid,
     ) -> anyhow::Result<Option<Version>> {
         self.get_version_impl(
             "SELECT version_id, parent_version_id, history_segment FROM versions WHERE parent_version_id = ? AND client_id = ?",
-            client_id,
+            self.client_id,
             parent_version_id)
     }
 
-    fn get_version(
-        &mut self,
-        client_id: Uuid,
-        version_id: Uuid,
-    ) -> anyhow::Result<Option<Version>> {
+    fn get_version(&mut self, version_id: Uuid) -> anyhow::Result<Option<Version>> {
         self.get_version_impl(
             "SELECT version_id, parent_version_id, history_segment FROM versions WHERE version_id = ? AND client_id = ?",
-            client_id,
+            self.client_id,
             version_id)
     }
 
     fn add_version(
         &mut self,
-        client_id: Uuid,
+
         version_id: Uuid,
         parent_version_id: Uuid,
         history_segment: Vec<u8>,
@@ -268,7 +256,7 @@ impl StorageTxn for Txn {
             "INSERT INTO versions (version_id, client_id, parent_version_id, history_segment) VALUES(?, ?, ?, ?)",
             params![
                 StoredUuid(version_id),
-                StoredUuid(client_id),
+                StoredUuid(self.client_id),
                 StoredUuid(parent_version_id),
                 history_segment
             ]
@@ -281,7 +269,7 @@ impl StorageTxn for Txn {
                latest_version_id = ?,
                versions_since_snapshot = versions_since_snapshot + 1
              WHERE client_id = ?",
-                params![StoredUuid(version_id), StoredUuid(client_id),],
+                params![StoredUuid(version_id), StoredUuid(self.client_id),],
             )
             .context("Error updating client for new version")?;
 
@@ -306,8 +294,9 @@ mod test {
         let tmp_dir = TempDir::new()?;
         let non_existant = tmp_dir.path().join("subdir");
         let storage = SqliteStorage::new(non_existant)?;
-        let mut txn = storage.txn()?;
-        let maybe_client = txn.get_client(Uuid::new_v4())?;
+        let client_id = Uuid::new_v4();
+        let mut txn = storage.txn(client_id)?;
+        let maybe_client = txn.get_client()?;
         assert!(maybe_client.is_none());
         Ok(())
     }
@@ -316,8 +305,9 @@ mod test {
     fn test_get_client_empty() -> anyhow::Result<()> {
         let tmp_dir = TempDir::new()?;
         let storage = SqliteStorage::new(tmp_dir.path())?;
-        let mut txn = storage.txn()?;
-        let maybe_client = txn.get_client(Uuid::new_v4())?;
+        let client_id = Uuid::new_v4();
+        let mut txn = storage.txn(client_id)?;
+        let maybe_client = txn.get_client()?;
         assert!(maybe_client.is_none());
         Ok(())
     }
@@ -326,20 +316,20 @@ mod test {
     fn test_client_storage() -> anyhow::Result<()> {
         let tmp_dir = TempDir::new()?;
         let storage = SqliteStorage::new(tmp_dir.path())?;
-        let mut txn = storage.txn()?;
-
         let client_id = Uuid::new_v4();
-        let latest_version_id = Uuid::new_v4();
-        txn.new_client(client_id, latest_version_id)?;
+        let mut txn = storage.txn(client_id)?;
 
-        let client = txn.get_client(client_id)?.unwrap();
+        let latest_version_id = Uuid::new_v4();
+        txn.new_client(latest_version_id)?;
+
+        let client = txn.get_client()?.unwrap();
         assert_eq!(client.latest_version_id, latest_version_id);
         assert!(client.snapshot.is_none());
 
         let latest_version_id = Uuid::new_v4();
-        txn.add_version(client_id, latest_version_id, Uuid::new_v4(), vec![1, 1])?;
+        txn.add_version(latest_version_id, Uuid::new_v4(), vec![1, 1])?;
 
-        let client = txn.get_client(client_id)?.unwrap();
+        let client = txn.get_client()?.unwrap();
         assert_eq!(client.latest_version_id, latest_version_id);
         assert!(client.snapshot.is_none());
 
@@ -348,9 +338,9 @@ mod test {
             timestamp: "2014-11-28T12:00:09Z".parse::<DateTime<Utc>>().unwrap(),
             versions_since: 4,
         };
-        txn.set_snapshot(client_id, snap.clone(), vec![1, 2, 3])?;
+        txn.set_snapshot(snap.clone(), vec![1, 2, 3])?;
 
-        let client = txn.get_client(client_id)?.unwrap();
+        let client = txn.get_client()?.unwrap();
         assert_eq!(client.latest_version_id, latest_version_id);
         assert_eq!(client.snapshot.unwrap(), snap);
 
@@ -361,8 +351,9 @@ mod test {
     fn test_gvbp_empty() -> anyhow::Result<()> {
         let tmp_dir = TempDir::new()?;
         let storage = SqliteStorage::new(tmp_dir.path())?;
-        let mut txn = storage.txn()?;
-        let maybe_version = txn.get_version_by_parent(Uuid::new_v4(), Uuid::new_v4())?;
+        let client_id = Uuid::new_v4();
+        let mut txn = storage.txn(client_id)?;
+        let maybe_version = txn.get_version_by_parent(Uuid::new_v4())?;
         assert!(maybe_version.is_none());
         Ok(())
     }
@@ -371,18 +362,13 @@ mod test {
     fn test_add_version_and_get_version() -> anyhow::Result<()> {
         let tmp_dir = TempDir::new()?;
         let storage = SqliteStorage::new(tmp_dir.path())?;
-        let mut txn = storage.txn()?;
-
         let client_id = Uuid::new_v4();
+        let mut txn = storage.txn(client_id)?;
+
         let version_id = Uuid::new_v4();
         let parent_version_id = Uuid::new_v4();
         let history_segment = b"abc".to_vec();
-        txn.add_version(
-            client_id,
-            version_id,
-            parent_version_id,
-            history_segment.clone(),
-        )?;
+        txn.add_version(version_id, parent_version_id, history_segment.clone())?;
 
         let expected = Version {
             version_id,
@@ -390,12 +376,10 @@ mod test {
             history_segment,
         };
 
-        let version = txn
-            .get_version_by_parent(client_id, parent_version_id)?
-            .unwrap();
+        let version = txn.get_version_by_parent(parent_version_id)?.unwrap();
         assert_eq!(version, expected);
 
-        let version = txn.get_version(client_id, version_id)?.unwrap();
+        let version = txn.get_version(version_id)?.unwrap();
         assert_eq!(version, expected);
 
         Ok(())
@@ -405,41 +389,40 @@ mod test {
     fn test_snapshots() -> anyhow::Result<()> {
         let tmp_dir = TempDir::new()?;
         let storage = SqliteStorage::new(tmp_dir.path())?;
-        let mut txn = storage.txn()?;
-
         let client_id = Uuid::new_v4();
+        let mut txn = storage.txn(client_id)?;
 
-        txn.new_client(client_id, Uuid::new_v4())?;
-        assert!(txn.get_client(client_id)?.unwrap().snapshot.is_none());
+        txn.new_client(Uuid::new_v4())?;
+        assert!(txn.get_client()?.unwrap().snapshot.is_none());
 
         let snap = Snapshot {
             version_id: Uuid::new_v4(),
             timestamp: "2013-10-08T12:00:09Z".parse::<DateTime<Utc>>().unwrap(),
             versions_since: 3,
         };
-        txn.set_snapshot(client_id, snap.clone(), vec![9, 8, 9])?;
+        txn.set_snapshot(snap.clone(), vec![9, 8, 9])?;
 
         assert_eq!(
-            txn.get_snapshot_data(client_id, snap.version_id)?.unwrap(),
+            txn.get_snapshot_data(snap.version_id)?.unwrap(),
             vec![9, 8, 9]
         );
-        assert_eq!(txn.get_client(client_id)?.unwrap().snapshot, Some(snap));
+        assert_eq!(txn.get_client()?.unwrap().snapshot, Some(snap));
 
         let snap2 = Snapshot {
             version_id: Uuid::new_v4(),
             timestamp: "2014-11-28T12:00:09Z".parse::<DateTime<Utc>>().unwrap(),
             versions_since: 10,
         };
-        txn.set_snapshot(client_id, snap2.clone(), vec![0, 2, 4, 6])?;
+        txn.set_snapshot(snap2.clone(), vec![0, 2, 4, 6])?;
 
         assert_eq!(
-            txn.get_snapshot_data(client_id, snap2.version_id)?.unwrap(),
+            txn.get_snapshot_data(snap2.version_id)?.unwrap(),
             vec![0, 2, 4, 6]
         );
-        assert_eq!(txn.get_client(client_id)?.unwrap().snapshot, Some(snap2));
+        assert_eq!(txn.get_client()?.unwrap().snapshot, Some(snap2));
 
         // check that mismatched version is detected
-        assert!(txn.get_snapshot_data(client_id, Uuid::new_v4()).is_err());
+        assert!(txn.get_snapshot_data(Uuid::new_v4()).is_err());
 
         Ok(())
     }
diff --git a/sqlite/tests/concurrency.rs b/sqlite/tests/concurrency.rs
index 3250426..5a3a37a 100644
--- a/sqlite/tests/concurrency.rs
+++ b/sqlite/tests/concurrency.rs
@@ -12,8 +12,8 @@ fn add_version_concurrency() -> anyhow::Result<()> {
 
     {
         let con = SqliteStorage::new(tmp_dir.path())?;
-        let mut txn = con.txn()?;
-        txn.new_client(client_id, NIL_VERSION_ID)?;
+        let mut txn = con.txn(client_id)?;
+        txn.new_client(NIL_VERSION_ID)?;
         txn.commit()?;
     }
 
@@ -25,12 +25,12 @@ fn add_version_concurrency() -> anyhow::Result<()> {
         let con = SqliteStorage::new(tmp_dir.path())?;
 
         for _ in 0..N {
-            let mut txn = con.txn()?;
-            let client = txn.get_client(client_id)?.unwrap();
+            let mut txn = con.txn(client_id)?;
+            let client = txn.get_client()?.unwrap();
             let version_id = Uuid::new_v4();
             let parent_version_id = client.latest_version_id;
             std::thread::yield_now(); // Make failure more likely.
-            txn.add_version(client_id, version_id, parent_version_id, b"data".to_vec())?;
+            txn.add_version(version_id, parent_version_id, b"data".to_vec())?;
             txn.commit()?;
         }
 
@@ -49,15 +49,13 @@ fn add_version_concurrency() -> anyhow::Result<()> {
     // same `parent_version_id`.
     {
         let con = SqliteStorage::new(tmp_dir.path())?;
-        let mut txn = con.txn()?;
-        let client = txn.get_client(client_id)?.unwrap();
+        let mut txn = con.txn(client_id)?;
+        let client = txn.get_client()?.unwrap();
 
         let mut n = 0;
         let mut version_id = client.latest_version_id;
         while version_id != NIL_VERSION_ID {
-            let version = txn
-                .get_version(client_id, version_id)?
-                .expect("version should exist");
+            let version = txn.get_version(version_id)?.expect("version should exist");
             n += 1;
             version_id = version.parent_version_id;
         }