diff --git a/aero-collections/src/calendar/mod.rs b/aero-collections/src/calendar/mod.rs
index d2217b8..6537a4e 100644
--- a/aero-collections/src/calendar/mod.rs
+++ b/aero-collections/src/calendar/mod.rs
@@ -1,13 +1,19 @@
 pub mod namespace;
 
 use anyhow::Result;
+use tokio::sync::RwLock;
 
+use aero_bayou::Bayou;
 use aero_user::login::Credentials;
+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;
 
 pub struct Calendar {
-    a: u64,
+    pub(super) id: UniqueIdent,
+    internal: RwLock<CalendarInternal>,
 }
 
 impl Calendar {
@@ -18,3 +24,11 @@ impl Calendar {
         todo!();
     }
 }
+
+struct CalendarInternal {
+    id: UniqueIdent,
+    cal_path: String,
+    encryption_key: Key,
+    storage: Store,
+    uid_index: Bayou<DavDag>,
+}
diff --git a/aero-collections/src/davdag.rs b/aero-collections/src/davdag.rs
new file mode 100644
index 0000000..696b985
--- /dev/null
+++ b/aero-collections/src/davdag.rs
@@ -0,0 +1,185 @@
+use serde::{Deserialize, Deserializer, Serialize, Serializer};
+use im::{OrdMap, OrdSet, ordset};
+
+use aero_bayou::*;
+
+use crate::unique_ident::UniqueIdent;
+
+/// Parents are only persisted in the event log,
+/// not in the checkpoints.
+pub type Parents = Vec<UniqueIdent>;
+pub type Etag = String;
+pub type FileName = String;
+pub type IndexEntry = (FileName, Etag);
+
+#[derive(Clone, Default)]
+pub struct DavDag {
+    /// Source of trust
+    pub table: OrdMap<UniqueIdent, IndexEntry>,
+
+    /// Indexes optimized for queries
+    pub idx_by_filename: OrdMap<FileName, UniqueIdent>,
+
+    /// Partial synchronization graph
+    /// parent -> direct children
+    pub successors: OrdMap<UniqueIdent, OrdSet<UniqueIdent>>,
+    
+    /// Head nodes
+    pub heads: OrdSet<UniqueIdent>,
+}
+
+#[derive(Clone, Serialize, Deserialize, Debug)]
+pub enum DavDagOp {
+    /// Merge is a virtual operation run when multiple heads are discovered
+    Merge(Parents, UniqueIdent),
+
+    /// Add an item to the collection
+    Put(Parents, UniqueIdent, IndexEntry),
+
+    /// Delete an item from the collection
+    Delete(Parents, UniqueIdent),
+}
+
+impl DavDag {
+    pub fn op_merge(&self, ident: UniqueIdent) -> DavDagOp {
+        DavDagOp::Merge(self.heads_vec(), ident)
+    }
+
+    pub fn op_put(&self, ident: UniqueIdent, entry: IndexEntry) -> DavDagOp {
+        DavDagOp::Put(self.heads_vec(), ident, entry)
+    }
+
+    pub fn op_delete(&self, ident: UniqueIdent) -> DavDagOp {
+        DavDagOp::Delete(self.heads_vec(), ident)
+    }
+
+    // HELPER functions
+    pub fn heads_vec(&self) -> Vec<UniqueIdent> {
+        self.heads.clone().into_iter().collect()
+    }
+
+    // INTERNAL functions
+    fn register(&mut self, ident: UniqueIdent, entry: IndexEntry) {
+        // Insert item in the source of trust
+        self.table.insert(ident, entry.clone());
+
+        // Update the cache
+        let (filename, _etag) = entry;
+        self.idx_by_filename.insert(filename, ident);
+    }
+
+    fn unregister(&mut self, ident: &UniqueIdent) {
+        // Query the source of truth to get the information we
+        // need to clean the indexes
+        let (filename, _etag) = match self.table.get(ident) {
+            Some(v) => v,
+            None => return,
+        };
+        self.idx_by_filename.remove(filename);
+
+        // Finally clear item from the source of trust
+        self.table.remove(ident);
+    }
+
+    // @FIXME: maybe in case of error we could simply disable the sync graph
+    // and ask the client to rely on manual sync. For now, we are skipping the event
+    // which is midly satisfying.
+    fn sync_dag(&mut self, child: &UniqueIdent, parents: &[UniqueIdent]) -> bool {
+        // All parents must exist in successors otherwise we can't accept item
+        // do the check + update successors
+        let mut try_successors = self.successors.clone();
+        for par in parents.iter() {
+            match try_successors.get_mut(par) {
+                None => {
+                    tracing::warn!("Unable to push a Dav DAG sync op into the graph, an event is missing, it's a bug");
+                    return false
+                },
+                Some(v) => v.insert(*child),
+            };
+        }
+        self.successors = try_successors;
+
+        // Remove from HEADS this event's parents
+        parents.iter().for_each(|par| { self.heads.remove(par); });
+
+        // This event becomes a new HEAD in turn
+        self.heads.insert(*child);
+
+        // This event is also a future successor
+        self.successors.insert(*child, ordset![]);
+
+        true
+    }
+}
+
+impl BayouState for DavDag {
+    type Op = DavDagOp;
+
+    fn apply(&self, op: &Self::Op) -> Self {
+        let mut new = self.clone();
+    
+        match op {
+            DavDagOp::Put(parents, ident, entry) => {
+                if new.sync_dag(ident, parents.as_slice()) {
+                    new.register(*ident, entry.clone());
+                }
+            },
+            DavDagOp::Delete(parents, ident) => {
+                if new.sync_dag(ident, parents.as_slice()) {
+                    new.unregister(ident);
+                }
+            },
+            DavDagOp::Merge(parents, ident) => {
+                new.sync_dag(ident, parents.as_slice());
+            }
+        }
+
+        new
+    }
+}
+
+// CUSTOM SERIALIZATION & DESERIALIZATION
+#[derive(Serialize, Deserialize)]
+struct DavDagSerializedRepr {
+    items: Vec<(UniqueIdent, IndexEntry)>,
+    heads: Vec<UniqueIdent>,
+}
+
+impl<'de> Deserialize<'de> for DavDag {
+    fn deserialize<D>(d: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        let val: DavDagSerializedRepr = DavDagSerializedRepr::deserialize(d)?;
+        let mut davdag = DavDag::default();
+
+        // Build the table + index
+        val.items.into_iter().for_each(|(ident, entry)| davdag.register(ident, entry));
+
+        // Initialize the synchronization DAG with its roots
+        val.heads.into_iter().for_each(|ident| {
+            davdag.successors.insert(ident, ordset![]);
+            davdag.heads.insert(ident);
+        });
+
+        Ok(davdag)
+    }
+}
+
+impl Serialize for DavDag {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        // Indexes are rebuilt on the fly, we serialize only the core database
+        let items = self.table.iter().map(|(ident, entry)| (*ident, entry.clone())).collect();
+
+        // We keep only the head entries from the sync graph,
+        // these entries will be used to initialize it back when deserializing
+        let heads = self.heads_vec();
+
+        // Finale serialization object
+        let val = DavDagSerializedRepr { items, heads };
+        val.serialize(serializer)
+    }
+}
diff --git a/aero-collections/src/lib.rs b/aero-collections/src/lib.rs
index 269cd13..ef8b8d8 100644
--- a/aero-collections/src/lib.rs
+++ b/aero-collections/src/lib.rs
@@ -1,4 +1,5 @@
 pub mod unique_ident;
+pub mod davdag;
 pub mod user;
 pub mod mail;
 pub mod calendar;