Dynamic DAGs/Parallel Execution

Hamilton now has pluggable execution, which allows for the following:

  1. Grouping of nodes into “tasks” (discrete execution unit between serialization boundaries)

  2. Executing the tasks in parallel, using any executor of your choice

You can run this executor using the Builder, a utility class that allows you to build a driver piece by piece. Note that you currently have to call enable_dynamic_execution(allow_experimental_mode=True) which will toggle it to use the V2 executor. Then, you can:

  1. Add task executors to specify how to run the tasks

  2. Add node gropuing strategies

  3. Add modules to crawl for functions

  4. Add a results builder to shape the results

Either constructing the driver, or using the builder and not calling enable_dynamic_execution will give you the standard executor. We highly recommend you use the builder pattern – while the constructor of the Driver will be fully backwards compatible according to the rules of semantic versioning, we may change it in the future (for 2.0).

Note that the new executor is required to handle dynamic creation of nodes (E.G. using Parallelizable[] and Collect[].

Let’s look at an example of the driver:

from my_code import foo_module, bar_module

from hamilton import driver
from hamilton.execution import executors

dr = (
    .with_config({"config_key": "config_value"})

dr.execute(["my_variable"], inputs={...}, overrides={...})

Note that we set a remote executor, and a local executor. While you can bypass this and instead set an execution_manager in the builder call (see Builder for documentation on the Builder),this goes along with the default grouping strategy, which is to place each node in its own group, except for dynamically generated (Parallelizable[]) blocks, which are each made into one group, and executed locally.

Thus, when you write a DAG like this (a simple map-reduce pattern):

from hamilton.htypes import Parallelizable

def url() -> Parallelizable[str]:
    for url_ in  _list_all_urls():
        yield url_

def url_loaded(url: str) -> str:
    return _load(urls)

def counts(url_loaded: str) -> str:
    return len(url_loaded.split(" "))

def total_words(counts: Collect[int]) -> int:
    return sum(counts)

The block containing counts and url_loaded will get marked as one task, repeated for each URL in url_loaded, and run on the remote executor (which in this case is the ThreadPoolExecutor).

Note that we currently have the following caveats:

  1. No nested Parallelizable[]/Collect[] blocks – we only allow one level of parallelization

  2. Serialization for Multiprocessing is suboptimal – we currently use the default pickle serializer, which breaks with certain cases. Ray, Dask, etc… all work well, and we plan to add support for joblib + cloudpickle serialization.

  3. Collect[] input types are limited to one per function – this is another caveat that we intend to get rid of, but for now you’ll want to concat/put into one function before collecting.

Known Caveats

If you’re familiar with multi-processing then these caveats will be familiar to you. If not, then you should be aware of the following:



  • Objects are by default pickled and sent to the remote executor, and then unpickled.

  • This can be slow, and can break with certain types of objects, e.g. OpenAI Client, DB Client, etc.


  • Make sure that your objects are serializable.

  • If you’re using a library that doesn’t support serialization, then one option is to have Hamilton instantiate the object in each parallel block. You can do this by making the code depend on something within the parallel block.

  • Another option is write a customer wrapper function that uses __set_state__ and __get_state__ to serialize and deserialize the object.

  • See [this issue](https://github.com/DAGWorks-Inc/hamilton/issues/743) for details and possible features to make this simpler to deal with.

Multiple Collects

Currently, by design (see all limitations here), you can only have one “collect” downstream of “parallel”.

So the following code WILL NOT WORK:

import logging

from hamilton import driver
from hamilton.execution.executors import SynchronousLocalTaskExecutor
from hamilton.htypes import Collect, Parallelizable
import pandas as pd

ANALYSIS_OB = tuple[tuple[str,...], pd.DataFrame]
ANALYSIS_RES = dict[str, str | float]

def split_by_cols(full_data: pd.DataFrame, columns: list[str]) -> Parallelizable[ANALYSIS_OB]:
    for idx, grp in full_data.groupby(columns):
        yield (idx, grp)

def sub_metric_1(split_by_cols: ANALYSIS_OB, number: float=1.0) -> ANALYSIS_RES:
    idx, grp = split_by_cols
    return {"key": idx, "mean": grp["spend"].mean() + number}

def sub_metric_2(split_by_cols: ANALYSIS_OB) -> ANALYSIS_RES:
    idx, grp = split_by_cols
    return {"key": idx, "mean": grp["signups"].mean()}

def metric_1(sub_metric_1: Collect[ANALYSIS_RES], columns: list[str]) -> pd.DataFrame:
    data = [[k for k in d["key"]] + [d["mean"], "spend"] for d in sub_metric_1]
    cols = list(columns) + ["mean", "metric"]
    return pd.DataFrame(data, columns=cols)

def metric_2(sub_metric_2: Collect[ANALYSIS_RES], columns: list[str]) -> pd.DataFrame:
    data = [[k for k in d["key"]] + [d["mean"], "signups"] for d in sub_metric_2]
    cols = list(columns) + ["mean", "metric"]
    return pd.DataFrame(data, columns=cols)

# this will not work because you can't have two Collect[] calls downstream from a Parallelizable[] call
def all_agg(metric_1: pd.DataFrame, metric_2: pd.DataFrame) -> pd.DataFrame:
    return pd.concat([metric_1, metric_2])

if __name__ == "__main__":
    from hamilton.execution import executors
    import __main__

    from hamilton.log_setup import setup_logging

    local_executor = executors.SynchronousLocalTaskExecutor()

    dr = (
    df = pd.DataFrame(
        index=pd.date_range('20230101', '20230110'),
            "signups": [1, 10, 50, 100, 200, 400, 700, 800, 1000, 1300],
            "spend": [10, 10, 20, 40, 40, 50, 100, 80, 90, 120],
            "region": ["A", "B", "C", "A", "B", "C", "A", "B", "C", "X"],
    ans = dr.execute(
            "full_data": df,
            "number": 3.1,
            "columns": ["region"],

To fix this, (this is documented in this issue) you can either create a new function that combines the two Collect[] calls that could be combined with @config.when.

def all_metrics(sub_metric_1: ANALYSIS_RES, sub_metric_2: ANALYSIS_RES) -> ANALYSIS_RES:
    return ... # join the two dicts in whatever way you want

def all_agg(all_metrics: Collect[ANALYSIS_RES]) -> pd.DataFrame:
    return ... # join them all into a dataframe

Or you use @resolve, with @group (scroll down a little), @inject, to set what should be determined to be collected at DAG construction time:

    decorate_with= lambda metric_names:
      inject( # this will annotate the function with @inject
         # it will then inject a group of values corresponding to the sources wanted
         sub_metrics=group(*[source(x) for x in metric_names])
def all_metrics(sub_metrics: list[ANALYSIS_RES], columns: list[str]) -> pd.DataFrame:
    frames = []
    for a in sub_metrics:
        frames.append(_to_frame(a, columns))
    return pd.concat(frames)

# then in your driver:
from hamilton import settings
_config = {settings.ENABLE_POWER_USER_MODE:True}
_config["metric_names"] = ["sub_metric_1", "sub_metric_2"]

# Then in the driver building pass in the configuration: