diff --git a/src/taskstorage/inmemory.rs b/src/taskstorage/inmemory.rs
index ef654b0f3..e89d31bc7 100644
--- a/src/taskstorage/inmemory.rs
+++ b/src/taskstorage/inmemory.rs
@@ -10,6 +10,7 @@ struct Data {
     tasks: HashMap<Uuid, TaskMap>,
     base_version: u64,
     operations: Vec<Operation>,
+    working_set: Vec<Option<Uuid>>,
 }
 
 struct Txn<'t> {
@@ -104,6 +105,31 @@ impl<'t> TaskStorageTxn for Txn<'t> {
         Ok(())
     }
 
+    fn get_working_set(&mut self) -> Fallible<Vec<Option<Uuid>>> {
+        Ok(self.data_ref().working_set.clone())
+    }
+
+    fn add_to_working_set(&mut self, uuid: Uuid) -> Fallible<u64> {
+        let working_set = &mut self.mut_data_ref().working_set;
+        working_set.push(Some(uuid));
+        Ok(working_set.len() as u64)
+    }
+
+    fn remove_from_working_set(&mut self, index: u64) -> Fallible<()> {
+        let index = index as usize;
+        let working_set = &mut self.mut_data_ref().working_set;
+        if index >= working_set.len() || working_set[index].is_none() {
+            return Err(format_err!("No task found with index {}", index));
+        }
+        working_set[index] = None;
+        Ok(())
+    }
+
+    fn clear_working_set(&mut self) -> Fallible<()> {
+        self.mut_data_ref().working_set = vec![None];
+        Ok(())
+    }
+
     fn commit(&mut self) -> Fallible<()> {
         // copy the new_data back into storage to commit the transaction
         if let Some(data) = self.new_data.take() {
@@ -125,6 +151,7 @@ impl InMemoryStorage {
                 tasks: HashMap::new(),
                 base_version: 0,
                 operations: vec![],
+                working_set: vec![None],
             },
         }
     }
@@ -138,3 +165,127 @@ impl TaskStorage for InMemoryStorage {
         }))
     }
 }
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    // (note: this module is heavily used in tests so most of its functionality is well-tested
+    // elsewhere and not tested here)
+
+    #[test]
+    fn get_working_set_empty() -> Fallible<()> {
+        let mut storage = InMemoryStorage::new();
+
+        {
+            let mut txn = storage.txn()?;
+            let ws = txn.get_working_set()?;
+            assert_eq!(ws, vec![None]);
+        }
+
+        Ok(())
+    }
+
+    #[test]
+    fn add_to_working_set() -> Fallible<()> {
+        let mut storage = InMemoryStorage::new();
+        let uuid1 = Uuid::new_v4();
+        let uuid2 = Uuid::new_v4();
+
+        {
+            let mut txn = storage.txn()?;
+            txn.add_to_working_set(uuid1.clone())?;
+            txn.add_to_working_set(uuid2.clone())?;
+            txn.commit()?;
+        }
+
+        {
+            let mut txn = storage.txn()?;
+            let ws = txn.get_working_set()?;
+            assert_eq!(ws, vec![None, Some(uuid1), Some(uuid2)]);
+        }
+
+        Ok(())
+    }
+
+    #[test]
+    fn add_and_remove_from_working_set_holes() -> Fallible<()> {
+        let mut storage = InMemoryStorage::new();
+        let uuid1 = Uuid::new_v4();
+        let uuid2 = Uuid::new_v4();
+
+        {
+            let mut txn = storage.txn()?;
+            txn.add_to_working_set(uuid1.clone())?;
+            txn.add_to_working_set(uuid2.clone())?;
+            txn.commit()?;
+        }
+
+        {
+            let mut txn = storage.txn()?;
+            txn.remove_from_working_set(1)?;
+            txn.add_to_working_set(uuid1.clone())?;
+            txn.commit()?;
+        }
+
+        {
+            let mut txn = storage.txn()?;
+            let ws = txn.get_working_set()?;
+            assert_eq!(ws, vec![None, None, Some(uuid2), Some(uuid1)]);
+        }
+
+        Ok(())
+    }
+
+    #[test]
+    fn remove_working_set_doesnt_exist() -> Fallible<()> {
+        let mut storage = InMemoryStorage::new();
+        let uuid1 = Uuid::new_v4();
+
+        {
+            let mut txn = storage.txn()?;
+            txn.add_to_working_set(uuid1.clone())?;
+            txn.commit()?;
+        }
+
+        {
+            let mut txn = storage.txn()?;
+            let res = txn.remove_from_working_set(0);
+            assert!(res.is_err());
+            let res = txn.remove_from_working_set(2);
+            assert!(res.is_err());
+        }
+
+        Ok(())
+    }
+
+    #[test]
+    fn clear_working_set() -> Fallible<()> {
+        let mut storage = InMemoryStorage::new();
+        let uuid1 = Uuid::new_v4();
+        let uuid2 = Uuid::new_v4();
+
+        {
+            let mut txn = storage.txn()?;
+            txn.add_to_working_set(uuid1.clone())?;
+            txn.add_to_working_set(uuid2.clone())?;
+            txn.commit()?;
+        }
+
+        {
+            let mut txn = storage.txn()?;
+            txn.clear_working_set()?;
+            txn.add_to_working_set(uuid2.clone())?;
+            txn.add_to_working_set(uuid1.clone())?;
+            txn.commit()?;
+        }
+
+        {
+            let mut txn = storage.txn()?;
+            let ws = txn.get_working_set()?;
+            assert_eq!(ws, vec![None, Some(uuid2), Some(uuid1)]);
+        }
+
+        Ok(())
+    }
+}
diff --git a/src/taskstorage/kv.rs b/src/taskstorage/kv.rs
index 20c4da301..c8a6d85d5 100644
--- a/src/taskstorage/kv.rs
+++ b/src/taskstorage/kv.rs
@@ -51,10 +51,12 @@ pub struct KVStorage<'t> {
     tasks_bucket: Bucket<'t, Key, ValueBuf<Msgpack<TaskMap>>>,
     numbers_bucket: Bucket<'t, Integer, ValueBuf<Msgpack<u64>>>,
     operations_bucket: Bucket<'t, Integer, ValueBuf<Msgpack<Operation>>>,
+    working_set_bucket: Bucket<'t, Integer, ValueBuf<Msgpack<Uuid>>>,
 }
 
 const BASE_VERSION: u64 = 1;
 const NEXT_OPERATION: u64 = 2;
+const NEXT_WORKING_SET_INDEX: u64 = 3;
 
 impl<'t> KVStorage<'t> {
     pub fn new(directory: &Path) -> Fallible<KVStorage> {
@@ -62,6 +64,7 @@ impl<'t> KVStorage<'t> {
         config.bucket("tasks", None);
         config.bucket("numbers", None);
         config.bucket("operations", None);
+        config.bucket("working_set", None);
         let store = Store::new(config)?;
 
         // tasks are stored indexed by uuid
@@ -75,11 +78,17 @@ impl<'t> KVStorage<'t> {
         let operations_bucket =
             store.int_bucket::<ValueBuf<Msgpack<Operation>>>(Some("operations"))?;
 
+        // this bucket contains operations, numbered consecutively; the NEXT_WORKING_SET_INDEX
+        // number gives the index of the next operation to insert
+        let working_set_bucket =
+            store.int_bucket::<ValueBuf<Msgpack<Uuid>>>(Some("working_set"))?;
+
         Ok(KVStorage {
             store,
             tasks_bucket,
             numbers_bucket,
             operations_bucket,
+            working_set_bucket,
         })
     }
 }
@@ -118,6 +127,9 @@ impl<'t> Txn<'t> {
     fn operations_bucket(&self) -> &'t Bucket<'t, Integer, ValueBuf<Msgpack<Operation>>> {
         &self.storage.operations_bucket
     }
+    fn working_set_bucket(&self) -> &'t Bucket<'t, Integer, ValueBuf<Msgpack<Uuid>>> {
+        &self.storage.working_set_bucket
+    }
 }
 
 impl<'t> TaskStorageTxn for Txn<'t> {
@@ -271,6 +283,90 @@ impl<'t> TaskStorageTxn for Txn<'t> {
         Ok(())
     }
 
+    fn get_working_set(&mut self) -> Fallible<Vec<Option<Uuid>>> {
+        let working_set_bucket = self.working_set_bucket();
+        let numbers_bucket = self.numbers_bucket();
+        let kvtxn = self.kvtxn();
+
+        let next_index = match kvtxn.get(numbers_bucket, NEXT_WORKING_SET_INDEX.into()) {
+            Ok(buf) => buf.inner()?.to_serde(),
+            Err(Error::NotFound) => 1,
+            Err(e) => return Err(e.into()),
+        };
+
+        let mut res = Vec::with_capacity(next_index as usize);
+        for _ in 0..next_index {
+            res.push(None)
+        }
+
+        let curs = kvtxn.read_cursor(working_set_bucket)?;
+        for (i, u) in kvtxn.read_cursor(working_set_bucket)?.iter() {
+            let i: u64 = i.into();
+            res[i as usize] = Some(u.inner()?.to_serde());
+        }
+        Ok(res)
+    }
+
+    fn add_to_working_set(&mut self, uuid: Uuid) -> Fallible<u64> {
+        let working_set_bucket = self.working_set_bucket();
+        let numbers_bucket = self.numbers_bucket();
+        let kvtxn = self.kvtxn();
+
+        let next_index = match kvtxn.get(numbers_bucket, NEXT_WORKING_SET_INDEX.into()) {
+            Ok(buf) => buf.inner()?.to_serde(),
+            Err(Error::NotFound) => 1,
+            Err(e) => return Err(e.into()),
+        };
+
+        kvtxn.set(
+            working_set_bucket,
+            next_index.into(),
+            Msgpack::to_value_buf(uuid)?,
+        )?;
+        kvtxn.set(
+            numbers_bucket,
+            NEXT_WORKING_SET_INDEX.into(),
+            Msgpack::to_value_buf(next_index + 1)?,
+        )?;
+        Ok(next_index)
+    }
+
+    fn remove_from_working_set(&mut self, index: u64) -> Fallible<()> {
+        let working_set_bucket = self.working_set_bucket();
+        let numbers_bucket = self.numbers_bucket();
+        let kvtxn = self.kvtxn();
+
+        let next_index = match kvtxn.get(numbers_bucket, NEXT_WORKING_SET_INDEX.into()) {
+            Ok(buf) => buf.inner()?.to_serde(),
+            Err(Error::NotFound) => 1,
+            Err(e) => return Err(e.into()),
+        };
+        if index == 0 || index >= next_index {
+            return Err(format_err!("No task found with index {}", index));
+        }
+
+        match kvtxn.del(working_set_bucket, index.into()) {
+            Err(Error::NotFound) => Err(format_err!("No task found with index {}", index)),
+            Err(e) => Err(e.into()),
+            Ok(_) => Ok(()),
+        }
+    }
+
+    fn clear_working_set(&mut self) -> Fallible<()> {
+        let working_set_bucket = self.working_set_bucket();
+        let numbers_bucket = self.numbers_bucket();
+        let kvtxn = self.kvtxn();
+
+        kvtxn.clear_db(working_set_bucket)?;
+        kvtxn.set(
+            numbers_bucket,
+            NEXT_WORKING_SET_INDEX.into(),
+            Msgpack::to_value_buf(1)?,
+        )?;
+
+        Ok(())
+    }
+
     fn commit(&mut self) -> Fallible<()> {
         if let Some(kvtxn) = self.txn.take() {
             kvtxn.commit()?;
@@ -546,4 +642,125 @@ mod test {
         }
         Ok(())
     }
+
+    #[test]
+    fn get_working_set_empty() -> Fallible<()> {
+        let tmp_dir = TempDir::new("test")?;
+        let mut storage = KVStorage::new(&tmp_dir.path())?;
+
+        {
+            let mut txn = storage.txn()?;
+            let ws = txn.get_working_set()?;
+            assert_eq!(ws, vec![None]);
+        }
+
+        Ok(())
+    }
+
+    #[test]
+    fn add_to_working_set() -> Fallible<()> {
+        let tmp_dir = TempDir::new("test")?;
+        let mut storage = KVStorage::new(&tmp_dir.path())?;
+        let uuid1 = Uuid::new_v4();
+        let uuid2 = Uuid::new_v4();
+
+        {
+            let mut txn = storage.txn()?;
+            txn.add_to_working_set(uuid1.clone())?;
+            txn.add_to_working_set(uuid2.clone())?;
+            txn.commit()?;
+        }
+
+        {
+            let mut txn = storage.txn()?;
+            let ws = txn.get_working_set()?;
+            assert_eq!(ws, vec![None, Some(uuid1), Some(uuid2)]);
+        }
+
+        Ok(())
+    }
+
+    #[test]
+    fn add_and_remove_from_working_set_holes() -> Fallible<()> {
+        let tmp_dir = TempDir::new("test")?;
+        let mut storage = KVStorage::new(&tmp_dir.path())?;
+        let uuid1 = Uuid::new_v4();
+        let uuid2 = Uuid::new_v4();
+
+        {
+            let mut txn = storage.txn()?;
+            txn.add_to_working_set(uuid1.clone())?;
+            txn.add_to_working_set(uuid2.clone())?;
+            txn.commit()?;
+        }
+
+        {
+            let mut txn = storage.txn()?;
+            txn.remove_from_working_set(1)?;
+            txn.add_to_working_set(uuid1.clone())?;
+            txn.commit()?;
+        }
+
+        {
+            let mut txn = storage.txn()?;
+            let ws = txn.get_working_set()?;
+            assert_eq!(ws, vec![None, None, Some(uuid2), Some(uuid1)]);
+        }
+
+        Ok(())
+    }
+
+    #[test]
+    fn remove_working_set_doesnt_exist() -> Fallible<()> {
+        let tmp_dir = TempDir::new("test")?;
+        let mut storage = KVStorage::new(&tmp_dir.path())?;
+        let uuid1 = Uuid::new_v4();
+
+        {
+            let mut txn = storage.txn()?;
+            txn.add_to_working_set(uuid1.clone())?;
+            txn.commit()?;
+        }
+
+        {
+            let mut txn = storage.txn()?;
+            let res = txn.remove_from_working_set(0);
+            assert!(res.is_err());
+            let res = txn.remove_from_working_set(2);
+            assert!(res.is_err());
+        }
+
+        Ok(())
+    }
+
+    #[test]
+    fn clear_working_set() -> Fallible<()> {
+        let tmp_dir = TempDir::new("test")?;
+        let mut storage = KVStorage::new(&tmp_dir.path())?;
+        let uuid1 = Uuid::new_v4();
+        let uuid2 = Uuid::new_v4();
+
+        {
+            let mut txn = storage.txn()?;
+            txn.add_to_working_set(uuid1.clone())?;
+            txn.add_to_working_set(uuid2.clone())?;
+            txn.commit()?;
+        }
+
+        {
+            let mut txn = storage.txn()?;
+            txn.clear_working_set()?;
+            txn.add_to_working_set(uuid2.clone())?;
+            txn.add_to_working_set(uuid1.clone())?;
+            txn.commit()?;
+        }
+
+        {
+            let mut txn = storage.txn()?;
+            let ws = txn.get_working_set()?;
+            assert_eq!(ws, vec![None, Some(uuid2), Some(uuid1)]);
+        }
+
+        Ok(())
+    }
 }
diff --git a/src/taskstorage/mod.rs b/src/taskstorage/mod.rs
index 092707225..5c2d879bf 100644
--- a/src/taskstorage/mod.rs
+++ b/src/taskstorage/mod.rs
@@ -64,6 +64,20 @@ pub trait TaskStorageTxn {
     /// Replace the current list of operations with a new list.
     fn set_operations(&mut self, ops: Vec<Operation>) -> Fallible<()>;
 
+    /// Get the entire working set, with each task UUID at its appropriate (1-based) index.
+    /// Element 0 is always None.
+    fn get_working_set(&mut self) -> Fallible<Vec<Option<Uuid>>>;
+
+    /// Add a task to the working set and return its (one-based) index.  This index will be one greater
+    /// than the highest used index.
+    fn add_to_working_set(&mut self, uuid: Uuid) -> Fallible<u64>;
+
+    /// Remove a task from the working set.  Other tasks' indexes are not affected.
+    fn remove_from_working_set(&mut self, index: u64) -> Fallible<()>;
+
+    /// Clear all tasks from the working set in preparation for a garbage-collection operation.
+    fn clear_working_set(&mut self) -> Fallible<()>;
+
     /// Commit any changes made in the transaction.  It is an error to call this more than
     /// once.
     fn commit(&mut self) -> Fallible<()>;
@@ -78,6 +92,8 @@ pub trait TaskStorageTxn {
 ///  - tasks: a set of tasks indexed by uuid
 ///  - base_version: the number of the last version sync'd from the server
 ///  - operations: all operations performed since base_version
+///  - working_set: a mapping from integer -> uuid, used to keep stable small-integer indexes
+///    into the tasks.  The replica maintains this list.  It is not covered by operations.
 ///
 ///  The `operations` are already reflected in `tasks`, so the following invariant holds:
 ///  > Applying `operations` to the set of tasks at `base_version` gives a set of tasks identical