diff --git a/aero-collections/src/calendar/mod.rs b/aero-collections/src/calendar/mod.rs
index 6537a4e..936f8c3 100644
--- a/aero-collections/src/calendar/mod.rs
+++ b/aero-collections/src/calendar/mod.rs
@@ -9,7 +9,7 @@ use aero_user::cryptoblob::{self, gen_key, open_deserialize, seal_serialize, Key
 use aero_user::storage::{self, BlobRef, BlobVal, RowRef, RowVal, Selector, Store};
 
 use crate::unique_ident::*;
-use crate::davdag::DavDag;
+use crate::davdag::{DavDag, IndexEntry, Token, BlobId, SyncChange};
 
 pub struct Calendar {
     pub(super) id: UniqueIdent,
@@ -20,8 +20,49 @@ impl Calendar {
     pub(crate) async fn open(
         creds: &Credentials,
         id: UniqueIdent,
-    ) -> Result<Self> {
-        todo!();
+        ) -> Result<Self> {
+        let bayou_path = format!("calendar/dag/{}", id);
+        let cal_path = format!("calendar/events/{}", id);
+
+        let mut davdag = Bayou::<DavDag>::new(creds, bayou_path).await?;
+        davdag.sync().await?;
+
+        let internal = RwLock::new(CalendarInternal {
+            id,
+            encryption_key: creds.keys.master.clone(),
+            storage: creds.storage.build().await?,
+            davdag,
+            cal_path,
+        });
+
+        Ok(Self { id, internal })
+    }
+
+    /// Sync data with backing store
+    pub async fn force_sync(&self) -> Result<()> {
+        self.internal.write().await.force_sync().await
+    }
+
+    /// Sync data with backing store only if changes are detected
+    /// or last sync is too old
+    pub async fn opportunistic_sync(&self) -> Result<()> {
+        self.internal.write().await.opportunistic_sync().await
+    }
+
+    pub async fn get(&self, blob_id: UniqueIdent, message_key: &Key) -> Result<Vec<u8>> {
+        self.internal.read().await.get(blob_id, message_key).await
+    }
+
+    pub async fn diff(&self, sync_token: Token) -> Result<(Token, Vec<SyncChange>)> {
+        self.internal.read().await.diff(sync_token).await
+    }
+
+    pub async fn put<'a>(&self, entry: IndexEntry, evt: &'a [u8]) -> Result<Token> {
+        self.internal.write().await.put(entry, evt).await
+    }
+
+    pub async fn delete(&self, blob_id: UniqueIdent) -> Result<Token> {
+        self.internal.write().await.delete(blob_id).await
     }
 }
 
@@ -30,5 +71,35 @@ struct CalendarInternal {
     cal_path: String,
     encryption_key: Key,
     storage: Store,
-    uid_index: Bayou<DavDag>,
+    davdag: Bayou<DavDag>,
+}
+
+impl CalendarInternal {
+    async fn force_sync(&mut self) -> Result<()> {
+        self.davdag.sync().await?;
+        Ok(())
+    }
+
+    async fn opportunistic_sync(&mut self) -> Result<()> {
+        self.davdag.opportunistic_sync().await?;
+        Ok(())
+    }
+
+    async fn get(&self, blob_id: BlobId, message_key: &Key) -> Result<Vec<u8>> {
+        todo!()
+    }
+
+    async fn put<'a>(&mut self, entry: IndexEntry, evt: &'a [u8]) -> Result<Token> {
+        //@TODO write event to S3
+        //@TODO add entry into Bayou
+        todo!();
+    }
+
+    async fn delete(&mut self, blob_id: BlobId) -> Result<Token> {
+        todo!();
+    }
+
+    async fn diff(&self, sync_token: Token) -> Result<(Token, Vec<SyncChange>)> {
+        todo!();
+    }
 }
diff --git a/aero-collections/src/davdag.rs b/aero-collections/src/davdag.rs
index 63a76a8..f668831 100644
--- a/aero-collections/src/davdag.rs
+++ b/aero-collections/src/davdag.rs
@@ -20,20 +20,31 @@ pub type IndexEntry = (BlobId, FileName, Etag);
 #[derive(Clone, Default)]
 pub struct DavDag {
     /// Source of trust
-    pub table: OrdMap<UniqueIdent, IndexEntry>,
+    pub table: OrdMap<BlobId, IndexEntry>,
 
     /// Indexes optimized for queries
-    pub idx_by_filename: OrdMap<FileName, UniqueIdent>,
+    pub idx_by_filename: OrdMap<FileName, BlobId>,
+
+    // ------------ Below this line, data is ephemeral, ie. not checkpointed
 
     /// Partial synchronization graph
-    pub ancestors: OrdMap<UniqueIdent, OrdSet<UniqueIdent>>,
+    pub ancestors: OrdMap<Token, OrdSet<Token>>,
 
     /// All nodes
-    pub all_nodes: OrdSet<UniqueIdent>,
+    pub all_nodes: OrdSet<Token>,
     /// Head nodes
-    pub heads: OrdSet<UniqueIdent>,
+    pub heads: OrdSet<Token>,
     /// Origin nodes
-    pub origins: OrdSet<UniqueIdent>,
+    pub origins: OrdSet<Token>,
+
+    /// File change token by token
+    pub change: OrdMap<Token, SyncChange>,
+}
+
+#[derive(Clone, Debug)]
+pub enum SyncChange {
+    Ok(FileName),
+    NotFound(FileName),
 }
 
 #[derive(Clone, Serialize, Deserialize, Debug)]
@@ -66,8 +77,8 @@ impl DavDag {
         DavDagOp::Put(self.sync_desc(), entry)
     }
 
-    pub fn op_delete(&self, ident: BlobId) -> DavDagOp {
-        DavDagOp::Delete(self.sync_desc(), ident)
+    pub fn op_delete(&self, blob_id: BlobId) -> DavDagOp {
+        DavDagOp::Delete(self.sync_desc(), blob_id)
     }
 
     // HELPER functions
@@ -129,33 +140,41 @@ impl DavDag {
     // INTERNAL functions
 
     /// Register a WebDAV item (put, copy, move)
-    fn register(&mut self, entry: IndexEntry) {
+    fn register(&mut self, sync_token: Option<Token>, entry: IndexEntry) {
         let (blob_id, filename, _etag) = entry.clone();
 
         // Insert item in the source of trust
         self.table.insert(blob_id, entry);
 
         // Update the cache
-        self.idx_by_filename.insert(filename, blob_id);
+        self.idx_by_filename.insert(filename.to_string(), blob_id);
+
+        // Record the change in the ephemeral synchronization map
+        if let Some(sync_token) = sync_token {
+            self.change.insert(sync_token, SyncChange::Ok(filename));
+        }
     }
 
     /// Unregister a WebDAV item (delete, move)
-    fn unregister(&mut self, ident: &UniqueIdent) {
+    fn unregister(&mut self, sync_token: Token, blob_id: &BlobId) {
         // Query the source of truth to get the information we
         // need to clean the indexes
-        let (_blob_id, filename, _etag) = match self.table.get(ident) {
+        let (_blob_id, filename, _etag) = match self.table.get(blob_id) {
             Some(v) => v,
             // Element does not exist, return early
             None => return,
         };
         self.idx_by_filename.remove(filename);
 
+        // Record the change in the ephemeral synchronization map
+        self.change.insert(sync_token, SyncChange::NotFound(filename.to_string()));
+
         // Finally clear item from the source of trust
-        self.table.remove(ident);
+        self.table.remove(blob_id);
     }
 
     /// When an event is processed, update the synchronization DAG
-    fn sync_dag(&mut self, sync_desc: &SyncDesc) -> bool {
+    fn sync_dag(&mut self, sync_desc: &SyncDesc) {
         let (parents, child) = sync_desc;
 
         // --- Update ANCESTORS
@@ -180,8 +199,6 @@ impl DavDag {
         
         // --- Update ALL NODES
         self.all_nodes.insert(*child);
-
-        true
     }
 }
 
@@ -193,14 +210,12 @@ impl BayouState for DavDag {
     
         match op {
             DavDagOp::Put(sync_desc, entry) => {
-                if new.sync_dag(sync_desc) {
-                    new.register(entry.clone());
-                }
+                new.sync_dag(sync_desc);
+                new.register(Some(sync_desc.1), entry.clone());
             },
             DavDagOp::Delete(sync_desc, blob_id) => {
-                if new.sync_dag(sync_desc) {
-                    new.unregister(blob_id);
-                }
+                new.sync_dag(sync_desc);
+                new.unregister(sync_desc.1, blob_id);
             },
             DavDagOp::Merge(sync_desc) => {
                 new.sync_dag(sync_desc);
@@ -227,7 +242,7 @@ impl<'de> Deserialize<'de> for DavDag {
         let mut davdag = DavDag::default();
 
         // Build the table + index
-        val.items.into_iter().for_each(|entry| davdag.register(entry));
+        val.items.into_iter().for_each(|entry| davdag.register(None, entry));
 
         // Initialize the synchronization DAG with its roots
         val.heads.into_iter().for_each(|ident| {