diff --git a/sync-server/src/api/add_version.rs b/sync-server/src/api/add_version.rs
index 75398ecd9..cdc96db2a 100644
--- a/sync-server/src/api/add_version.rs
+++ b/sync-server/src/api/add_version.rs
@@ -1,8 +1,7 @@
-use crate::server::SyncServer;
+use crate::api::ServerState;
 use crate::types::{ClientId, HistorySegment, VersionId};
 use actix_web::{error, http::StatusCode, post, web, HttpResponse, Responder, Result};
 use serde::{Deserialize, Serialize};
-use std::sync::Arc;
 
 /// Request body to add_version
 #[derive(Serialize, Deserialize)]
@@ -14,7 +13,7 @@ pub(crate) struct AddVersionRequest {
 
 #[post("/client/{client_id}/add-version/{parent_version_id}")]
 pub(crate) async fn service(
-    data: web::Data<Arc<SyncServer>>,
+    data: web::Data<ServerState>,
     web::Path((client_id, parent_version_id)): web::Path<(ClientId, VersionId)>,
     body: web::Json<AddVersionRequest>,
 ) -> Result<impl Responder> {
diff --git a/sync-server/src/api/get_child_version.rs b/sync-server/src/api/get_child_version.rs
index 8512e6f4a..4383a6fbc 100644
--- a/sync-server/src/api/get_child_version.rs
+++ b/sync-server/src/api/get_child_version.rs
@@ -1,11 +1,10 @@
-use crate::server::SyncServer;
+use crate::api::ServerState;
 use crate::types::{ClientId, VersionId};
 use actix_web::{error, get, http::StatusCode, web, HttpResponse, Result};
-use std::sync::Arc;
 
 #[get("/client/{client_id}/get-child-version/{parent_version_id}")]
 pub(crate) async fn service(
-    data: web::Data<Arc<SyncServer>>,
+    data: web::Data<ServerState>,
     web::Path((client_id, parent_version_id)): web::Path<(ClientId, VersionId)>,
 ) -> Result<HttpResponse> {
     let result = data
diff --git a/sync-server/src/api/mod.rs b/sync-server/src/api/mod.rs
index b955fcc25..cdab25fca 100644
--- a/sync-server/src/api/mod.rs
+++ b/sync-server/src/api/mod.rs
@@ -1,2 +1,8 @@
+use crate::server::SyncServer;
+use std::sync::Arc;
+
 pub(crate) mod add_version;
 pub(crate) mod get_child_version;
+
+/// The type containing a reference to the SyncServer object in the Actix state.
+pub(crate) type ServerState = Arc<Box<dyn SyncServer>>;
diff --git a/sync-server/src/main.rs b/sync-server/src/main.rs
index 20b7b118f..191908fb2 100644
--- a/sync-server/src/main.rs
+++ b/sync-server/src/main.rs
@@ -1,5 +1,4 @@
 use actix_web::{App, HttpServer};
-use server::SyncServer;
 use std::sync::Arc;
 
 mod api;
@@ -10,11 +9,11 @@ mod types;
 
 #[actix_web::main]
 async fn main() -> std::io::Result<()> {
-    let sync_server = Arc::new(SyncServer::new());
+    let server_state = Arc::new(Box::new(server::NullSyncServer::new()));
 
     HttpServer::new(move || {
         App::new()
-            .data(sync_server.clone())
+            .data(server_state.clone())
             .service(api::get_child_version::service)
             .service(api::add_version::service)
     })
diff --git a/sync-server/src/server.rs b/sync-server/src/server/mod.rs
similarity index 54%
rename from sync-server/src/server.rs
rename to sync-server/src/server/mod.rs
index 8c4bdfdf2..df4377366 100644
--- a/sync-server/src/server.rs
+++ b/sync-server/src/server/mod.rs
@@ -2,16 +2,34 @@ use crate::types::{AddVersionResult, ClientId, GetVersionResult, HistorySegment,
 use failure::Fallible;
 use taskchampion::Uuid;
 
-/// The sync server's implementation; HTTP API method call through to methods on a single
-/// instance of this type.
-pub(crate) struct SyncServer {}
+pub(crate) trait SyncServer {
+    fn get_child_version(
+        &self,
+        client_id: ClientId,
+        parent_version_id: VersionId,
+    ) -> Fallible<Option<GetVersionResult>>;
 
-impl SyncServer {
+    fn add_version(
+        &self,
+        client_id: ClientId,
+        parent_version_id: VersionId,
+        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 {}
     }
+}
 
-    pub(crate) fn get_child_version(
+impl SyncServer for NullSyncServer {
+    fn get_child_version(
         &self,
         _client_id: ClientId,
         parent_version_id: VersionId,
@@ -23,7 +41,7 @@ impl SyncServer {
         }))
     }
 
-    pub(crate) fn add_version(
+    fn add_version(
         &self,
         _client_id: ClientId,
         _parent_version_id: VersionId,