diff --git a/sync-server/src/main.rs b/sync-server/src/main.rs
index 880ce7b27..a8e4cc9de 100644
--- a/sync-server/src/main.rs
+++ b/sync-server/src/main.rs
@@ -1,6 +1,6 @@
 use actix_web::{App, HttpServer};
 use api::ServerState;
-use server::{NullSyncServer, SyncServer};
+use server::{InMemorySyncServer, SyncServer};
 
 mod api;
 mod server;
@@ -9,7 +9,7 @@ mod server;
 
 #[actix_web::main]
 async fn main() -> std::io::Result<()> {
-    let server_box: Box<dyn SyncServer> = Box::new(NullSyncServer::new());
+    let server_box: Box<dyn SyncServer> = Box::new(InMemorySyncServer::new());
     let server_state = ServerState::new(server_box);
 
     HttpServer::new(move || {
diff --git a/sync-server/src/server/inmemory.rs b/sync-server/src/server/inmemory.rs
new file mode 100644
index 000000000..37302282e
--- /dev/null
+++ b/sync-server/src/server/inmemory.rs
@@ -0,0 +1,108 @@
+use super::{
+    AddVersionResult, ClientId, GetVersionResult, HistorySegment, SyncServer, VersionId,
+    NO_VERSION_ID,
+};
+use failure::Fallible;
+use std::collections::HashMap;
+use std::sync::{Mutex, RwLock};
+use taskchampion::Uuid;
+
+/// An in-memory server backend that can be useful for testing.
+pub(crate) struct InMemorySyncServer {
+    clients: RwLock<HashMap<ClientId, Mutex<Client>>>,
+}
+
+struct Version {
+    version_id: VersionId,
+    history_segment: HistorySegment,
+}
+
+struct Client {
+    latest_version_id: VersionId,
+    // NOTE: indexed by parent_version_id!
+    versions: HashMap<VersionId, Version>,
+}
+
+impl InMemorySyncServer {
+    pub(crate) fn new() -> Self {
+        Self {
+            clients: RwLock::new(HashMap::new()),
+        }
+    }
+}
+
+impl SyncServer for InMemorySyncServer {
+    fn get_child_version(
+        &self,
+        client_id: ClientId,
+        parent_version_id: VersionId,
+    ) -> Fallible<Option<GetVersionResult>> {
+        let clients = self.clients.read().expect("poisoned lock");
+        if let Some(client) = clients.get(&client_id) {
+            let client = client.lock().expect("poisoned lock");
+            if let Some(version) = client.versions.get(&parent_version_id) {
+                return Ok(Some(GetVersionResult {
+                    version_id: version.version_id,
+                    parent_version_id,
+                    history_segment: version.history_segment.clone(),
+                }));
+            }
+        }
+        Ok(None)
+    }
+
+    fn add_version(
+        &self,
+        client_id: ClientId,
+        parent_version_id: VersionId,
+        history_segment: HistorySegment,
+    ) -> Fallible<AddVersionResult> {
+        let mut clients = self.clients.write().expect("poisoned lock");
+        if let Some(client) = clients.get_mut(&client_id) {
+            let mut client = client.lock().expect("poisoned lock");
+            if client.latest_version_id != NO_VERSION_ID {
+                if parent_version_id != client.latest_version_id {
+                    return Ok(AddVersionResult::ExpectedParentVersion(
+                        client.latest_version_id,
+                    ));
+                }
+            }
+
+            // invent a new ID for this version
+            let version_id = Uuid::new_v4();
+
+            client.versions.insert(
+                parent_version_id,
+                Version {
+                    version_id,
+                    history_segment,
+                },
+            );
+            client.latest_version_id = version_id;
+
+            Ok(AddVersionResult::Ok(version_id))
+        } else {
+            // new client, so insert a client with just this new version
+
+            let latest_version_id = Uuid::new_v4();
+            let mut versions = HashMap::new();
+            versions.insert(
+                parent_version_id,
+                Version {
+                    version_id: latest_version_id,
+                    history_segment,
+                },
+            );
+
+            clients.insert(
+                client_id,
+                Mutex::new(Client {
+                    latest_version_id,
+                    versions,
+                }),
+            );
+
+            Ok(AddVersionResult::Ok(latest_version_id))
+        }
+    }
+}
diff --git a/sync-server/src/server/mod.rs b/sync-server/src/server/mod.rs
index 5bda024f0..6d8593c03 100644
--- a/sync-server/src/server/mod.rs
+++ b/sync-server/src/server/mod.rs
@@ -1,6 +1,13 @@
 use failure::Fallible;
 use taskchampion::Uuid;
 
+mod inmemory;
+
+pub(crate) use inmemory::InMemorySyncServer;
+
+/// The distinguished value for "no version"
+pub const NO_VERSION_ID: VersionId = Uuid::nil();
+
 pub(crate) type HistorySegment = Vec<u8>;
 pub(crate) type ClientId = Uuid;
 pub(crate) type VersionId = Uuid;
@@ -33,38 +40,3 @@ pub(crate) trait SyncServer: Sync + Send {
         history_segment: HistorySegment,
     ) -> Fallible<AddVersionResult>;
 }
-
-// TODO: temporary
-/// A "null" sync server's implementation; HTTP API methods call through to methods on a single
-/// instance of this type.
-pub(crate) struct NullSyncServer {}
-
-impl NullSyncServer {
-    pub(crate) fn new() -> Self {
-        Self {}
-    }
-}
-
-impl SyncServer for NullSyncServer {
-    fn get_child_version(
-        &self,
-        _client_id: ClientId,
-        parent_version_id: VersionId,
-    ) -> Fallible<Option<GetVersionResult>> {
-        Ok(Some(GetVersionResult {
-            version_id: Uuid::new_v4(),
-            parent_version_id,
-            history_segment: b"abcd".to_vec(),
-        }))
-    }
-
-    fn add_version(
-        &self,
-        _client_id: ClientId,
-        _parent_version_id: VersionId,
-        _history_segment: HistorySegment,
-    ) -> Fallible<AddVersionResult> {
-        //Ok(AddVersionResult::Ok(Uuid::new_v4()))
-        Ok(AddVersionResult::ExpectedParentVersion(Uuid::new_v4()))
-    }
-}