diff --git a/src/lib.rs b/src/lib.rs
index 1df735f..27c6cac 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -14,9 +14,10 @@ pub mod calendar;
 pub use calendar::Calendar;
 mod task;
 pub use task::Task;
+pub mod provider;
+pub use provider::Provider;
 
 pub mod client;
-pub mod provider;
 pub mod cache;
 
 pub mod settings;
diff --git a/src/provider.rs b/src/provider.rs
index 926514d..e2663b0 100644
--- a/src/provider.rs
+++ b/src/provider.rs
@@ -1,3 +1,101 @@
 //! This modules abstracts data sources and merges them in a single virtual one
 
-pub struct Provider {}
+use std::error::Error;
+
+use chrono::{DateTime, Utc};
+
+use crate::traits::CalDavSource;
+use crate::Calendar;
+use crate::Task;
+
+
+pub struct Provider<S, L>
+where
+    S: CalDavSource,
+    L: CalDavSource,
+{
+    /// The remote server
+    server: S,
+    /// The local cache
+    local: L,
+
+    /// The last time the provider successfully synchronized both sources
+    last_sync: DateTime<Utc>,
+}
+
+impl<S,L> Provider<S, L>
+where
+    S: CalDavSource,
+    L: CalDavSource,
+{
+    /// Create a provider that will merge both sources
+    pub fn new(server: S, local: L, last_sync: DateTime<Utc>) -> Self {
+        Self { server, local, last_sync }
+    }
+
+    pub fn server(&self) -> &S { &self.server }
+    pub fn local(&self)  -> &L { &self.local }
+
+    pub async fn sync(&mut self) -> Result<(), Box<dyn Error>> {
+        self.pull_items_from_server().await?;
+        self.resolve_conflicts().await;
+        self.push_items_to_server().await;
+
+        // what to do with errors? Return Err directly? Go ahead as far as we can?
+        Ok(())
+    }
+
+    pub async fn pull_items_from_server(&mut self) -> Result<(), Box<dyn Error>> {
+        let cals_server = self.server.get_calendars_mut().await?;
+
+        for cal_server in cals_server {
+            let cal_local = match self.local.get_calendar_mut(cal_server.url().clone()).await {
+                None => {
+                    log::error!("TODO: implement here");
+                    continue;
+                },
+                Some(cal) => cal,
+            };
+
+            let server_mod = cal_server.get_tasks_modified_since(Some(self.last_sync), None);
+            let local_mod = cal_local.get_tasks_modified_since(Some(self.last_sync), None);
+
+            let mut tasks_to_add_to_local = Vec::new();
+            for (new_id, new_item) in &server_mod {
+                tasks_to_add_to_local.push((*new_item).clone());
+            }
+
+            let mut tasks_to_add_to_server = Vec::new();
+            for (new_id, new_item) in &local_mod {
+                if server_mod.contains_key(new_id) {
+                    log::warn!("Conflict for task {} ({}). Using the server version.", new_item.name(), new_id);
+                    continue;
+                }
+                tasks_to_add_to_server.push((*new_item).clone());
+            }
+
+            move_to_calendar(&mut tasks_to_add_to_local, cal_local);
+            move_to_calendar(&mut tasks_to_add_to_server, cal_server);
+        }
+
+        Ok(())
+    }
+
+    pub async fn resolve_conflicts(&mut self) {
+        log::error!("We should do something here");
+    }
+
+    pub async fn push_items_to_server(&mut self) {
+
+    }
+
+}
+
+
+fn move_to_calendar(tasks: &mut Vec<Task>, calendar: &mut Calendar) {
+    while tasks.len() > 0 {
+        let task = tasks.remove(0);
+        calendar.add_task(task);
+    }
+}
+
diff --git a/src/task.rs b/src/task.rs
index a3d95ab..c80037d 100644
--- a/src/task.rs
+++ b/src/task.rs
@@ -1,10 +1,12 @@
+use std::fmt::{Display, Formatter};
+
 use chrono::{Utc, DateTime};
 
 use serde::{Deserialize, Serialize};
 
 // TODO: turn into this one day
 //      pub type TaskId = String; // This is an HTML "etag"
-#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
+#[derive(Clone, Debug, PartialEq, Hash, Serialize, Deserialize)]
 pub struct TaskId {
     content: String,
 }
@@ -14,6 +16,12 @@ impl TaskId{
         Self { content:u }
     }
 }
+impl Eq for TaskId {}
+impl Display for TaskId {
+    fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), std::fmt::Error> {
+        write!(f, "{}", self.content)
+    }
+}
 
 /// A to-do task
 #[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
diff --git a/tests/sync.rs b/tests/sync.rs
index 32a144a..36fc893 100644
--- a/tests/sync.rs
+++ b/tests/sync.rs
@@ -18,18 +18,35 @@ async fn test_sync() {
     let mut provider = populate_test_provider().await;
     provider.sync().await.unwrap();
 
-    let cal_server = provider.server().get_calendars().await.unwrap();
-    let cal_local = provider.local().get_calendars().await.unwrap();
-    assert_eq!(cal_server, cal_local, "{:#?}\n{:#?}", cal_server, cal_local);
+    let cals_server = provider.server().get_calendars().await.unwrap();
+    let cals_local = provider.local().get_calendars().await.unwrap();
+    print_calendar_list(cals_local);
+    panic!();
+
+    //assert_eq!(cal_server, cal_local, "{:#?}\n{:#?}", cal_server, cal_local);
 
     panic!("TODO: also check that the contents are expected!");
 }
 
+/// A debug utility that pretty-prints calendars
+fn print_calendar_list(cals: &Vec<Calendar>) {
+    for cal in cals {
+        println!("CAL {}", cal.url());
+        for (_, item) in cal.get_tasks(None) {
+            let completion = if item.completed() {"✓"} else {" "};
+            println!("    {} {}", completion, item.name());
+        }
+    }
+}
+
 /// Populate sources with the following:
 /// * At the last sync: both sources had A, B, C, D, E, F, G, H at last sync
 /// * Before the newer sync, this will be the content of the sources:
 ///     * server: A,    C, D,  E', F',  G~, H , I
-///     * cache:  A, B,    D', E,  F'', G , H~,   J
+///     * cache:  A, B,    D', E,  F'', G , H~,    J
+///
+/// Hence, here is the expected result after the sync:
+///     * both:   A,       D', E', F',  G~, H~, I, J
 ///
 /// Notes:
 /// * X': name has been modified since the last sync