Pyarrow integration
This section explains how to use beavers with pyarrow.
ETF value calculation example
In this example we want to calculate the value of ETFs. If you are not familiar with ETFs, think about them as just a basket of shares.
Starting with a table of individual share prices:
import pyarrow as pa
PRICE_SCHEMA = pa.schema(
[
pa.field("ticker", pa.string()),
pa.field("price", pa.float64()),
]
)
price_table = pa.Table.from_pylist(
[
{"ticker": "AAPL", "price": 174.79},
{"ticker": "GOOGL", "price": 130.25},
{"ticker": "MSFT", "price": 317.01},
{"ticker": "F", "price": 12.43},
{"ticker": "GM", "price": 35.28},
],
schema=PRICE_SCHEMA,
)
ticker | price |
---|---|
AAPL | 174.79 |
GOOGL | 130.25 |
MSFT | 317.01 |
F | 12.43 |
GM | 35.28 |
And another table containing the composition of each ETF:
ETF_COMPOSITION_SCHEMA = pa.schema(
[
pa.field("etf", pa.string()),
pa.field("ticker", pa.string()),
pa.field("quantity", pa.float64()),
]
)
etf_composition_table = pa.Table.from_pylist(
[
{"etf": "TECH", "ticker": "AAPL", "quantity": 2.0},
{"etf": "TECH", "ticker": "GOOGL", "quantity": 2.0},
{"etf": "TECH", "ticker": "MSFT", "quantity": 1.0},
{"etf": "CARS", "ticker": "F", "quantity": 3.0},
{"etf": "CARS", "ticker": "GM", "quantity": 2.0},
],
schema=ETF_COMPOSITION_SCHEMA,
)
etf | ticker | quantity |
---|---|---|
TECH | AAPL | 2.0 |
TECH | GOOGL | 2.0 |
TECH | MSFT | 1.0 |
CARS | F | 3.0 |
CARS | GM | 1.0 |
In a few line of pyarrow
we can derive the value of each ETF:
import pyarrow.compute as pc
ETF_VALUE_SCHEMA = pa.schema(
[
pa.field("etf", pa.string()),
pa.field("value", pa.float64()),
]
)
def calculate_etf_value(etf_composition: pa.Table, price: pa.Table) -> pa.Table:
positions_with_prices = etf_composition.join(price, keys=["ticker"])
values = pc.multiply(
positions_with_prices["price"], positions_with_prices["quantity"]
)
positions_with_prices = positions_with_prices.append_column("value", values)
return (
positions_with_prices.group_by("etf")
.aggregate([("value", "sum")])
.rename_columns(ETF_VALUE_SCHEMA.names)
)
etf_value_table = calculate_etf_value(
etf_composition=etf_composition_table, price=price_table
)
etf | value |
---|---|
TECH | 927.09 |
CARS | 72.57 |
ETF value calculation DAG
Once the business logic of the calculation is writen and tested it can be added into a Dag.
We'll be using the Dag pa
helper which makes it easier to deal with pyarrow
table in beavers.
First we define two source streams, made of pyarrow.Table
:
from beavers import Dag
dag = Dag()
price_source = dag.pa.source_table(schema=PRICE_SCHEMA, name="price")
etf_composition_source = dag.pa.source_table(
schema=ETF_COMPOSITION_SCHEMA, name="etf_composition"
)
Then we keep track of the latest value for each source stream:
price_state = dag.pa.latest_by_keys(price_source, ["ticker"])
etf_composition_state = dag.pa.latest_by_keys(
etf_composition_source,
["etf", "ticker"],
)
Lastly we put together the share prices and ETF composition:
etf_value_state = dag.state(calculate_etf_value).map(
etf_composition_state,
price_state,
)
And that's it:
price_source.set_stream(price_table)
etf_composition_source.set_stream(etf_composition_table)
dag.execute()
assert etf_value_state.get_value() == etf_value_table
Taming updates
This simple dag does the job of calculating the ETF value in real time.
But there is one issue.
The value of every ETF would update every time either price
or etf_composition
update.
Even if the updates comes on a ticker that is not relevant to the ETFs we are tracking.
In the example below, when the price of GameStop updates, we recalculate the value of every ETF. Even though their value hasn't changed:
new_price_updates = pa.Table.from_pylist(
[{"ticker": "GME", "price": 123.0}],
PRICE_SCHEMA,
)
price_source.set_stream(new_price_updates)
dag.execute()
assert len(etf_value_state.get_value()) == 2
assert etf_value_state.get_cycle_id() == dag.get_cycle_id()
To tame updates we need to identify which ETF needs updating.
ETF values can update because their composition has changed:
updated_because_of_composition = dag.pa.get_column(
etf_composition_source,
"etf",
)
Or because one of their component has updated:
def get_etf_to_update_because_of_price(
etf_composition_state: pa.Table, price_update: pa.Table
) -> pa.Array:
updated_tickers = pc.unique(price_update["ticker"])
return pc.unique(
etf_composition_state.filter(
pc.is_in(etf_composition_state["ticker"], updated_tickers)
)["etf"]
)
updated_because_of_price = dag.stream(
get_etf_to_update_because_of_price, pa.array([], pa.string())
).map(etf_composition_state, price_source)
We can then put it back together and only calculate updates for relevant ETFs:
stale_etfs = dag.pa.concat_arrays(
updated_because_of_price, updated_because_of_composition
)
def get_composition_for_etfs(
etf_composition_state: pa.Table, etfs: pa.Array
) -> pa.Table:
return etf_composition_state.filter(
pc.is_in(
etf_composition_state["etf"],
etfs,
)
)
stale_etf_compositions = dag.pa.table_stream(
get_composition_for_etfs, ETF_COMPOSITION_SCHEMA
).map(etf_composition_state, stale_etfs)
updated_etf = dag.pa.table_stream(calculate_etf_value, ETF_VALUE_SCHEMA).map(
stale_etf_compositions, price_state
)
And see that only the value "TECH" ETF updates when a tech stock update:
price_source.set_stream(
pa.Table.from_pylist(
[{"ticker": "MSFT", "price": 317.05}],
PRICE_SCHEMA,
)
)
dag.execute()
assert len(updated_etf.get_value()) == 1
etf | value |
---|---|
TECH | 927.13 |