diff --git a/util/__init__.py b/util/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/util/_dask.py b/util/_dask.py
new file mode 100644
index 0000000..5ba6bea
--- /dev/null
+++ b/util/_dask.py
@@ -0,0 +1,400 @@
+"""
+Example usage:
+
+    `Map` and `Reduce` functions for Dask distributed client.
+
+    >>> from util._dask import dask_map, dask_reduce
+
+    # map and keep track of future <-> workitem mapping
+    >>> futures, futurekey2item = dask_map(
+    >>>     Processor().process,
+    >>>     workitems,
+    >>>     client=client,
+    >>>     NanoEventsFactory_kwargs={
+    >>>         "preload": lambda b: b.name in {'Jet_pt', 'Jet_eta'},
+    >>>         "schemaclass": NtupleSchema,
+    >>>     }
+    >>> )
+
+    # perform reduction and track failures
+    >>> final_future, failed_items = dask_reduce(
+    >>>     futures,
+    >>>     futurekey2item=futurekey2item,
+    >>>     client=client,
+    >>>     treereduction=16,
+    >>> )
+
+See also: `if __name__ == "__main__":` block for a complete example.
+"""
+
+from __future__ import annotations
+
+import dataclasses
+import typing as tp
+from collections import Counter, defaultdict
+from functools import partial
+
+import awkward as ak
+import uproot
+from coffea.nanoevents import NanoEventsFactory
+from coffea.processor import Accumulatable, accumulate
+from coffea.processor.executor import WorkItem
+from coffea.util import rich_bar
+from dask.distributed import Client
+from dask.tokenize import tokenize
+from rich.console import Group
+from rich.live import Live
+from rich.progress import Progress
+
+from util._futures import DynamicAsCompleted, FutureLike
+
+try:
+    from coffea.util import coffea_console
+except ImportError:
+    from rich.console import Console
+
+    coffea_console = Console()
+
+
+_processing_sentinel = object()
+_final_merge_sentinel = object()
+
+
+# group of progress bars for dask/future executor
+def pbar_group(datasets: list[str]) -> tuple[Live, dict[tp.Any, Progress]]:
+    pbars = {_processing_sentinel: rich_bar()}
+    pbars.update({ds: rich_bar() for ds in datasets})
+    pbars[_final_merge_sentinel] = rich_bar()
+    return Live(Group(*pbars.values()), console=coffea_console), pbars
+
+
+Result: tp.TypeAlias = Accumulatable | BaseException
+
+
+@dataclasses.dataclass(frozen=True, slots=True)
+class Failure:
+    item: WorkItem
+    reason: BaseException
+
+
+class ReduceSchedulingError(RuntimeError): ...
+
+
+acc = partial(accumulate, accum=None)
+
+
+def failed_future(future: FutureLike) -> bool:
+    # if we return an exception as a value, consider it failed (see wrapped_process)
+    # we catch any exception, but no RuntimeError. Maybe the user wants to raise that?
+    return issubclass(future.type, BaseException) or future.status == "error"
+
+
+def wrapped_process(
+    process_func: tp.Callable[[ak.Array], Result],
+    workitem: WorkItem,
+    /,
+    *,
+    NanoEventsFactory_kwargs: dict[str, tp.Any] | None = None,
+) -> Result:
+    f = uproot.open(workitem.filename)
+    if NanoEventsFactory_kwargs is None:
+        NanoEventsFactory_kwargs = {}
+    events = NanoEventsFactory.from_root(
+        f,
+        treepath=workitem.treename,
+        mode="virtual",
+        access_log=(access_log := []),
+        entry_start=workitem.entrystart,
+        entry_stop=workitem.entrystop,
+        **NanoEventsFactory_kwargs,
+    ).events()
+    events.metadata.update(workitem.usermeta)
+    try:
+        out = process_func(events)
+    except Exception as err:
+        # return err as value, no metrics
+        return err
+    bytesread = f.file.source.num_requested_bytes
+    report = {
+        "bytesread": bytesread,
+        "columns": access_log,
+        "bytesread_per_chunk": {
+            (workitem.filename, workitem.entrystart, workitem.entrystop): bytesread
+        },
+    }
+    return {"out": out, "report": report}
+
+
+def dask_map(
+    process_func: tp.Callable[[ak.Array], Result],
+    workitems: tp.Iterable[WorkItem],
+    /,
+    *,
+    client: Client,
+    NanoEventsFactory_kwargs: dict[str, tp.Any] | None = None,
+) -> tuple[list[FutureLike], dict[str, WorkItem]]:
+    futures = client.map(
+        partial(
+            wrapped_process,
+            process_func,
+            NanoEventsFactory_kwargs=NanoEventsFactory_kwargs,
+        ),
+        workitems,
+        pure=True,
+        key="process",
+        priority=0,
+    )
+    return futures, {f.key: wi for f, wi in zip(futures, workitems)}
+
+
+def dask_reduce(
+    futures: tp.Iterable[FutureLike],
+    *,
+    futurekey2item: tp.Mapping[str, WorkItem],
+    client: Client,
+    treereduction: int = 1 << 4,
+) -> tuple[FutureLike, defaultdict[list[Failure]]]:
+    items = list(futurekey2item.values())
+    datasets = [it.dataset for it in items]
+    unique_datasets = sorted(set(datasets))
+
+    live, pbars = pbar_group(unique_datasets)
+
+    with live:
+        # prepare some metadata for merging
+        # dataset -> number of items to do
+        ds2todo = Counter(datasets)
+        # create a buffer for each dataset (what we merge)
+        ds2buf = defaultdict(list)
+        # future.key -> dataset item
+        key2ds = {fk: wi.dataset for fk, wi in futurekey2item.items()}
+
+        # initialize progress bars
+        processing_task = pbars[_processing_sentinel].add_task(
+            "Processing", total=len(futures), unit="chunk"
+        )
+        dataset_merge_tasks = {}
+        for ds in unique_datasets:
+            total = ds2todo[ds]
+            dataset_merge_tasks[ds] = pbars[ds].add_task(
+                f"[cyan]Merging {total} [italic]{ds}[/italic] datasets into 1",
+                total=total,
+                unit="merge",
+            )
+
+        failed_items: defaultdict[list[Failure]] = defaultdict(list)
+        dynac = DynamicAsCompleted(futures)
+
+        # in-dataset merging loop, we merge first within datasets to avoid large accumulators in memory
+        # some reasonable value for the batch_size:
+        # yield in batches of treereduction, and we want at least 1 item per batch
+        batch_size = min(
+            treereduction, max(int(len(futures) / 100), 1)
+        )  # this is heuristic, can be tuned
+        for batch in dynac.iter_batches(batch_size=batch_size):
+            for future in batch:
+                ds = key2ds[future.key]
+
+                # subtract from todo
+                if not future.key.startswith("accumulate-"):
+                    ds2todo[ds] -= 1
+
+                # get buffer
+                buf = ds2buf[ds]
+
+                if failed_future(future):
+                    # let merge failures raise right away
+                    if future.key.startswith("accumulate-"):
+                        raise future.exception() from None
+
+                    # just collect bad futures coming from the processing step, do not merge them
+                    reason = future.result()
+                    item = futurekey2item[future.key]
+                    failure = Failure(item=item, reason=reason)
+
+                    # append to failed items
+                    failed_items[ds].append(failure)
+
+                    # all failed for this dataset
+                    if len(buf) == 0 and ds2todo[ds] == 0:
+                        del ds2todo[ds]
+                        del ds2buf[ds]
+
+                    # nothing to merge, skip
+                    continue
+
+                # update progress bars only for successful items
+                if future.key.startswith("accumulate-"):
+                    # merging task
+                    pbars[ds].update(dataset_merge_tasks[ds], advance=1)
+                else:
+                    pbars[_processing_sentinel].update(processing_task, advance=1)
+
+                # add future to buffer for merging
+                if future in buf:
+                    raise ReduceSchedulingError("Future already in buffer!")
+                buf.append(future)
+
+                # if this is the last item for this dataset, skip merging
+                # as we schedule it for final cross-dataset merging
+                if len(buf) == 1 and ds2todo[ds] == 0:
+                    continue
+
+                # submit treereduction merge if we have enough items
+                if len(buf) >= min(ds2todo[ds], treereduction) and len(buf) > 1:
+                    work = client.submit(
+                        acc,
+                        buf,
+                        key=f"accumulate-{tokenize(buf)}",
+                        priority=1,
+                    )
+
+                    # release explicit retention
+                    for f in buf:
+                        f.release()
+
+                    # add merged item to key2item, just use the first one of the
+                    # buffer in order to access the dataset later again
+                    key2ds[work.key] = ds
+
+                    # reset buffer
+                    buf.clear()
+
+                    # add back to the ac, recursively merge
+                    dynac.add(work)
+
+        del dynac
+
+        # make sure there's only 1 future per dataset in the buffer for the final merge
+        final_merge_futures = {}
+        for ds, todo in ds2todo.items():
+            buf = ds2buf[ds]
+            if todo != 0 or len(buf) != 1:
+                msg = f"dataset {ds} has {len(buf)} items in merge-buffer (should only be 1); chunks left to merge: {todo}"
+                raise ReduceSchedulingError(msg)
+            pbars[ds].update(dataset_merge_tasks[ds], advance=1)
+            final_merge_futures[ds] = buf[0]
+
+        final_total = 0
+        final_merge_task = pbars[_final_merge_sentinel].add_task(
+            f"[cyan]Merging {len(final_merge_futures)} merged datasets [italic](final)",
+            total=total,
+            unit="merge",
+        )
+
+        # not needed anymore
+        del ds2buf, ds2todo
+
+        # final merge across datasets
+        buf = []
+
+        dynac = DynamicAsCompleted(final_merge_futures.values())
+        for future in dynac:
+            if failed_future(future):
+                raise future.exception()
+
+            if future not in final_merge_futures.values():
+                # final merge progress
+                pbars[_final_merge_sentinel].update(
+                    final_merge_task,
+                    advance=1,
+                    total=final_total,
+                )
+
+            buf.append(future)
+            if len(buf) >= min(len(buf), treereduction) and len(buf) > 1:
+                future = client.submit(
+                    acc,
+                    buf,
+                    key=f"accumulate-{tokenize(buf)}",
+                    priority=2,
+                )
+
+                # release explicit retention
+                for f in buf:
+                    f.release()
+
+                # add merged item to key2item, just use the first one of the
+                # buffer in order to access the dataset later again
+                key2ds[future.key] = ds
+
+                # reset buffer
+                buf.clear()
+
+                # add back to the ac, recursively merge
+                dynac.add(future)
+
+                # add one to the pbar
+                final_total += 1
+
+        del dynac
+
+        # final result
+        assert len(buf) == 1
+        future = buf[0]
+        return future, failed_items
+
+
+if __name__ == "__main__":
+    # Run with: `python -m util._dask`
+    from coffea.nanoevents import NanoAODSchema
+    from dask.distributed import Client, LocalCluster
+
+    workitems = [
+        WorkItem(
+            filename="../coffea/tests/samples/nano_dy.root",
+            treename="Events",
+            entrystart=i * 10,
+            entrystop=(i + 1) * 10,
+            dataset="dy",
+            usermeta={"dataset": "dy"},
+            fileuuid="1234abcd",
+        )
+        for i in range(4)
+    ] + [
+        WorkItem(
+            filename="../coffea/tests/samples/nano_dimuon.root",
+            treename="Events",
+            entrystart=i * 10,
+            entrystop=(i + 1) * 10,
+            dataset="data",
+            usermeta={"dataset": "data"},
+            fileuuid="5678efgh",
+        )
+        for i in range(4)
+    ]
+
+    def process(events: ak.Array) -> ak.Array:
+        import random
+
+        if random.random() < 0.4:
+            raise ValueError("Random failure during processing!")
+        return ak.mean(events.Jet.pt)
+
+    with (
+        LocalCluster(n_workers=4, threads_per_worker=1) as cluster,
+        Client(cluster) as client,
+    ):
+        # map and keep track of future <-> workitem mapping
+        futures, futurekey2item = dask_map(
+            process,
+            workitems,
+            client=client,
+            NanoEventsFactory_kwargs={
+                "preload": lambda b: b.name in {"Jet_pt", "Jet_eta"},
+                "schemaclass": NanoAODSchema,
+            },
+        )
+
+        # perform reduction and track failures
+        final_future, failed_items = dask_reduce(
+            futures,
+            futurekey2item=futurekey2item,
+            client=client,
+            treereduction=3,
+        )
+
+        coffea_console.print("Failed items:", failed_items)
+        result = final_future.result()
+        coffea_console.print("Output:", result["out"])
+        coffea_console.print("Metrics:", result["report"])
diff --git a/util/_futures.py b/util/_futures.py
new file mode 100644
index 0000000..e214875
--- /dev/null
+++ b/util/_futures.py
@@ -0,0 +1,118 @@
+from __future__ import annotations
+
+import queue
+import typing as tp
+
+
+@tp.runtime_checkable
+class FutureLike(tp.Protocol):
+    def result(self) -> tp.Any: ...
+    def add_done_callback(self, fn: tp.Callable[[tp.Any], None]) -> None: ...
+
+
+class DynamicAsCompleted(tp.Iterator[FutureLike]):
+    """
+    An as_completed iterator that allows adding new futures dynamically.
+    Futures added via `add` will be yielded in the order they complete.
+
+    Example:
+
+        >>> dynac = DynamicAsCompleted(initial_futures)
+        >>> for future in dynac:
+        >>>     result = future.result()
+        >>>     if need_more_work(result):
+        >>>         dynac.add(submit_new_work())
+
+        # or using iter_batches
+        >>> for batch in dynac.iter_batches(batch_size=5):
+        >>>     for future in batch:
+        >>>         process(future)
+
+    """
+
+    def __init__(self, futures: tp.Iterable[FutureLike]) -> None:
+        self._queue: queue.Queue[FutureLike] = queue.Queue()
+        self._pending = 0
+        self._finished = 0
+
+        for future in futures:
+            self.add(future)
+
+    def __iter__(self) -> DynamicAsCompleted:
+        return self
+
+    def __next__(self) -> FutureLike | StopIteration:
+        # this means we are done
+        if self.pending == 0:
+            assert self._queue.empty()
+            raise StopIteration
+
+        # else wait for the next future to complete
+        future = self._queue.get()
+        self._pending -= 1
+        self._finished += 1
+        return future
+
+    @property
+    def pending(self) -> int:
+        return self._pending
+
+    @property
+    def finished(self) -> int:
+        return self._finished
+
+    def __repr__(self) -> str:
+        pending, finished = self.pending, self.finished
+        return f"<DynamicAsCompleted {pending=} {finished=}>"
+
+    def iter_batches(
+        self, batch_size: int, *, flush_partial: bool = True
+    ) -> tp.Iterator[list[FutureLike]]:
+        if batch_size <= 0:
+            raise ValueError("batch_size must be a positive integer")
+
+        batch: list[FutureLike] = []
+        while True:
+            try:
+                future = next(self)
+            except StopIteration:
+                break
+            batch.append(future)
+            if len(batch) == batch_size:
+                yield batch
+                batch = []
+
+        if batch and flush_partial:
+            yield batch
+
+    def add(self, future: FutureLike) -> FutureLike:
+        if not isinstance(future, FutureLike):
+            raise TypeError(
+                "add expects a future that implements 'add_done_callback' instance"
+            )
+
+        self._pending += 1
+        future.add_done_callback(self._queue.put)
+        return future
+
+
+if __name__ == "__main__":
+    import concurrent.futures
+    import time
+
+    def sleep_and_return(n: int) -> int:
+        time.sleep(n)
+        return n
+
+    with concurrent.futures.ThreadPoolExecutor() as executor:
+        initial_futures = [executor.submit(sleep_and_return, i) for i in [3, 1, 4]]
+        dynac = DynamicAsCompleted(initial_futures)
+
+        for future in dynac:
+            result = future.result()
+            print(f"Completed: {result}")
+            if result == 1:
+                new_future = executor.submit(sleep_and_return, 2)
+                dynac.add(new_future)
+
+        print("All done!")