diff --git a/src/flow/src/util/mod.rs b/src/flow/src/util/mod.rs
new file mode 100644
index 0000000000..2ac02b7630
--- /dev/null
+++ b/src/flow/src/util/mod.rs
@@ -0,0 +1,5 @@
+//! utilitys including extend differential dataflow to deal with errors and etc.
+mod operator;
+mod reduce;
+pub use operator::CollectionExt;
+pub use reduce::ReduceExt;
diff --git a/src/flow/src/util/operator.rs b/src/flow/src/util/operator.rs
new file mode 100644
index 0000000000..e506f72f94
--- /dev/null
+++ b/src/flow/src/util/operator.rs
@@ -0,0 +1,215 @@
+use differential_dataflow::difference::{Multiply, Semigroup};
+use differential_dataflow::lattice::Lattice;
+use differential_dataflow::operators::arrange::Arrange;
+use differential_dataflow::trace::{Batch, Trace, TraceReader};
+use differential_dataflow::{AsCollection, Collection};
+use timely::dataflow::channels::pact::{Exchange, ParallelizationContract, Pipeline};
+use timely::dataflow::channels::pushers::Tee;
+use timely::dataflow::operators::generic::builder_rc::OperatorBuilder as OperatorBuilderRc;
+use timely::dataflow::operators::generic::operator::{self, Operator};
+use timely::dataflow::operators::generic::{InputHandle, OperatorInfo, OutputHandle};
+use timely::dataflow::operators::Capability;
+use timely::dataflow::{Scope, Stream};
+use timely::{Data, ExchangeData};
+
+pub trait StreamExt<G, D1>
+where
+    D1: Data,
+    G: Scope,
+{
+    /// Like `timely::dataflow::operators::generic::operator::Operator::unary`,
+    /// but the logic function can handle failures.
+    ///
+    /// Creates a new dataflow operator that partitions its input stream by a
+    /// parallelization strategy `pact` and repeatedly invokes `logic`, the
+    /// function returned by the function passed as `constructor`. The `logic`
+    /// function can read to the input stream and write to either of two output
+    /// streams, where the first output stream represents successful
+    /// computations and the second output stream represents failed
+    /// computations.
+    fn unary_fallible<D2, E, B, P>(
+        &self,
+        pact: P,
+        name: &str,
+        constructor: B,
+    ) -> (Stream<G, D2>, Stream<G, E>)
+    where
+        D2: Data,
+        E: Data,
+        B: FnOnce(
+            Capability<G::Timestamp>,
+            OperatorInfo,
+        ) -> Box<
+            dyn FnMut(
+                    &mut InputHandle<G::Timestamp, D1, P::Puller>,
+                    &mut OutputHandle<G::Timestamp, D2, Tee<G::Timestamp, D2>>,
+                    &mut OutputHandle<G::Timestamp, E, Tee<G::Timestamp, E>>,
+                ) + 'static,
+        >,
+        P: ParallelizationContract<G::Timestamp, D1>;
+
+    /// Like [`timely::dataflow::operators::map::Map::flat_map`], but `logic`
+    /// is allowed to fail. The first returned stream will contain the
+    /// successful applications of `logic`, while the second returned stream
+    /// will contain the failed applications.
+    fn flat_map_fallible<D2, E, I, L>(&self, name: &str, logic: L) -> (Stream<G, D2>, Stream<G, E>)
+    where
+        D2: Data,
+        E: Data,
+        I: IntoIterator<Item = Result<D2, E>>,
+        L: FnMut(D1) -> I + 'static;
+}
+
+/// Extension methods for differential [`Collection`]s.
+pub trait CollectionExt<G, D1, R>
+where
+    G: Scope,
+    R: Semigroup,
+{
+    /// Creates a new empty collection in `scope`.
+    fn empty(scope: &G) -> Collection<G, D1, R>;
+
+    /// Like [`Collection::map`], but `logic` is allowed to fail. The first
+    /// returned collection will contain successful applications of `logic`,
+    /// while the second returned collection will contain the failed
+    /// applications.
+    fn map_fallible<D2, E, L>(
+        &self,
+        name: &str,
+        mut logic: L,
+    ) -> (Collection<G, D2, R>, Collection<G, E, R>)
+    where
+        D2: Data,
+        E: Data,
+        L: FnMut(D1) -> Result<D2, E> + 'static,
+    {
+        self.flat_map_fallible(name, move |record| Some(logic(record)))
+    }
+
+    /// Like [`Collection::flat_map`], but `logic` is allowed to fail. The first
+    /// returned collection will contain the successful applications of `logic`,
+    /// while the second returned collection will contain the failed
+    /// applications.
+    fn flat_map_fallible<D2, E, I, L>(
+        &self,
+        name: &str,
+        logic: L,
+    ) -> (Collection<G, D2, R>, Collection<G, E, R>)
+    where
+        D2: Data,
+        E: Data,
+        I: IntoIterator<Item = Result<D2, E>>,
+        L: FnMut(D1) -> I + 'static;
+}
+
+impl<G, D1> StreamExt<G, D1> for Stream<G, D1>
+where
+    D1: Data,
+    G: Scope,
+{
+    fn unary_fallible<D2, E, B, P>(
+        &self,
+        pact: P,
+        name: &str,
+        constructor: B,
+    ) -> (Stream<G, D2>, Stream<G, E>)
+    where
+        D2: Data,
+        E: Data,
+        B: FnOnce(
+            Capability<G::Timestamp>,
+            OperatorInfo,
+        ) -> Box<
+            dyn FnMut(
+                    &mut InputHandle<G::Timestamp, D1, P::Puller>,
+                    &mut OutputHandle<G::Timestamp, D2, Tee<G::Timestamp, D2>>,
+                    &mut OutputHandle<G::Timestamp, E, Tee<G::Timestamp, E>>,
+                ) + 'static,
+        >,
+        P: ParallelizationContract<G::Timestamp, D1>,
+    {
+        let mut builder = OperatorBuilderRc::new(name.into(), self.scope());
+        builder.set_notify(false);
+
+        let operator_info = builder.operator_info();
+
+        let mut input = builder.new_input(self, pact);
+        let (mut ok_output, ok_stream) = builder.new_output();
+        let (mut err_output, err_stream) = builder.new_output();
+
+        builder.build(move |mut capabilities| {
+            // `capabilities` should be a single-element vector.
+            let capability = capabilities.pop().unwrap();
+            let mut logic = constructor(capability, operator_info);
+            move |_frontiers| {
+                let mut ok_output_handle = ok_output.activate();
+                let mut err_output_handle = err_output.activate();
+                logic(&mut input, &mut ok_output_handle, &mut err_output_handle);
+            }
+        });
+
+        (ok_stream, err_stream)
+    }
+
+    #[allow(clippy::redundant_closure)]
+    fn flat_map_fallible<D2, E, I, L>(
+        &self,
+        name: &str,
+        mut logic: L,
+    ) -> (Stream<G, D2>, Stream<G, E>)
+    where
+        D2: Data,
+        E: Data,
+        I: IntoIterator<Item = Result<D2, E>>,
+        L: FnMut(D1) -> I + 'static,
+    {
+        let mut storage = Vec::new();
+        self.unary_fallible(Pipeline, name, move |_, _| {
+            Box::new(move |input, ok_output, err_output| {
+                input.for_each(|time, data| {
+                    let mut ok_session = ok_output.session(&time);
+                    let mut err_session = err_output.session(&time);
+                    data.swap(&mut storage);
+                    for r in storage.drain(..).flat_map(|d1| logic(d1)) {
+                        match r {
+                            Ok(d2) => ok_session.give(d2),
+                            Err(e) => err_session.give(e),
+                        }
+                    }
+                })
+            })
+        })
+    }
+}
+
+impl<G, D1, R> CollectionExt<G, D1, R> for Collection<G, D1, R>
+where
+    G: Scope,
+    G::Timestamp: Data,
+    D1: Data,
+    R: Semigroup,
+{
+    fn empty(scope: &G) -> Collection<G, D1, R> {
+        operator::empty(scope).as_collection()
+    }
+
+    fn flat_map_fallible<D2, E, I, L>(
+        &self,
+        name: &str,
+        mut logic: L,
+    ) -> (Collection<G, D2, R>, Collection<G, E, R>)
+    where
+        D2: Data,
+        E: Data,
+        I: IntoIterator<Item = Result<D2, E>>,
+        L: FnMut(D1) -> I + 'static,
+    {
+        let (ok_stream, err_stream) = self.inner.flat_map_fallible(name, move |(d1, t, r)| {
+            logic(d1).into_iter().map(move |res| match res {
+                Ok(d2) => Ok((d2, t.clone(), r.clone())),
+                Err(e) => Err((e, t.clone(), r.clone())),
+            })
+        });
+        (ok_stream.as_collection(), err_stream.as_collection())
+    }
+}
diff --git a/src/flow/src/util/reduce.rs b/src/flow/src/util/reduce.rs
new file mode 100644
index 0000000000..c9e981acce
--- /dev/null
+++ b/src/flow/src/util/reduce.rs
@@ -0,0 +1,68 @@
+use differential_dataflow::difference::{Abelian, Semigroup};
+use differential_dataflow::lattice::Lattice;
+use differential_dataflow::operators::arrange::{Arranged, TraceAgent};
+use differential_dataflow::operators::reduce::ReduceCore;
+use differential_dataflow::trace::{Batch, Trace, TraceReader};
+use differential_dataflow::Data;
+use timely::dataflow::Scope;
+
+/// Extension trait for `ReduceCore`, currently providing a reduction based
+/// on an operator-pair approach.
+pub trait ReduceExt<G: Scope, K: Data, V: Data, R: Semigroup>
+where
+    G::Timestamp: Lattice + Ord,
+{
+    /// This method produces a reduction pair based on the same input arrangement. Each reduction
+    /// in the pair operates with its own logic and the two output arrangements from the reductions
+    /// are produced as a result. The method is useful for reductions that need to present different
+    /// output views on the same input data. An example is producing an error-free reduction output
+    /// along with a separate error output indicating when the error-free output is valid.
+    fn reduce_pair<L1, T1, L2, T2>(
+        &self,
+        name1: &str,
+        name2: &str,
+        logic1: L1,
+        logic2: L2,
+    ) -> (Arranged<G, TraceAgent<T1>>, Arranged<G, TraceAgent<T2>>)
+    where
+        T1: Trace + TraceReader<Key = K, Time = G::Timestamp> + 'static,
+        T1::Val: Data,
+        T1::R: Abelian,
+        T1::Batch: Batch,
+        L1: FnMut(&K, &[(&V, R)], &mut Vec<(T1::Val, T1::R)>) + 'static,
+        T2: Trace + TraceReader<Key = K, Time = G::Timestamp> + 'static,
+        T2::Val: Data,
+        T2::R: Abelian,
+        T2::Batch: Batch,
+        L2: FnMut(&K, &[(&V, R)], &mut Vec<(T2::Val, T2::R)>) + 'static;
+}
+
+impl<G: Scope, K: Data, V: Data, Tr, R: Semigroup> ReduceExt<G, K, V, R> for Arranged<G, Tr>
+where
+    G::Timestamp: Lattice + Ord,
+    Tr: TraceReader<Key = K, Val = V, Time = G::Timestamp, R = R> + Clone + 'static,
+{
+    fn reduce_pair<L1, T1, L2, T2>(
+        &self,
+        name1: &str,
+        name2: &str,
+        logic1: L1,
+        logic2: L2,
+    ) -> (Arranged<G, TraceAgent<T1>>, Arranged<G, TraceAgent<T2>>)
+    where
+        T1: Trace + TraceReader<Key = K, Time = G::Timestamp> + 'static,
+        T1::Val: Data,
+        T1::R: Abelian,
+        T1::Batch: Batch,
+        L1: FnMut(&K, &[(&V, R)], &mut Vec<(T1::Val, T1::R)>) + 'static,
+        T2: Trace + TraceReader<Key = K, Time = G::Timestamp> + 'static,
+        T2::Val: Data,
+        T2::R: Abelian,
+        T2::Batch: Batch,
+        L2: FnMut(&K, &[(&V, R)], &mut Vec<(T2::Val, T2::R)>) + 'static,
+    {
+        let arranged1 = self.reduce_abelian::<L1, T1>(name1, logic1);
+        let arranged2 = self.reduce_abelian::<L2, T2>(name2, logic2);
+        (arranged1, arranged2)
+    }
+}