Streams - Chalk

Chalk enables users to convert and aggregate streaming messages into Chalk features in realtime.

The first step towards building a streaming environment with Chalk is creating a streaming source from one of Chalk’s integrations. Chalk supports Kafka and Kinesis for streaming. Like other integration sources, source parameters and credentials may be specified in either the dashboard or the source instantiation in code.

from chalk.streams import KafkaSource, KinesisSource

kafka_source = KafkaSource(name="...")
kinesis_source = KinesisSource(name="...")

Now, let’s set up the Chalk features you’d like to materialize upon stream message ingestion.

from chalk import features, Features

@features(max_staleness="1d", etl_offline_to_online=True)
class StreamFeature:
    id: str
    value: str

Streaming resolvers produce feature values in a similar manner as online and offline resolvers. These features must be marked with a max staleness in order to store result values in offline cache. etl_offline_to_online should be set to True to access these values in online cache as well. If both of these parameters are set, once these features are created, they can accessed via online and offline query and used downstream by other online and offline resolvers.

Chalk supports three kinds of streaming resolvers: mapping, windowed, and continuous. On this page, we will focus on mapping, which creates one Chalk feature instance per message. Both windowed and continuous streaming will be covered in the aggregations page.

This is an example of a mapping resolver that maps from a bytes message to an instance of StreamFeature.

from chalk.streams import stream

@stream(source=source)
def stream_resolver(message: bytes) -> Features[StreamFeature.id, StreamFeature.value]:
    id, value = some_bytes_processing_step(message)
    return StreamFeature(id=id, value=value)

In this resolver, we pass in the raw bytes from the stream. If you’re using JSON as the encoding for messages on your topic, you can optionally specify a Pydantic Model as a wrapper for messages on the topic. Chalk will validate the encoding against the model.

from pydantic import BaseModel

class Message(BaseModel):
    id: str
    value: str
    ...

@stream(source=source)
def stream_resolver(message: Message) -> Features[StreamFeature.id, StreamFeature.value]:
    return StreamFeature(id=message.id, value=message.value)

Late arrivals

Chalk lets you configure whether resolvers should accept late-arriving stream messages. By default, Chalk attempts to consider any late arriving in stream resolvers. However, you can tune this behavior with the late_arrival_deadline argument to you stream source:

from chalk.streams import KafkaSource

# By default, the late_arrival_deadline is set to "infinity".
source = KafkaSource(late_arrival_deadline="30m")

If a message is older than the late_arrival_deadline when it is consumed, its resolver will not run.

Parsing

Sometimes, messages must be processed before resolver execution. Streaming resolvers can optionally support a parse function that preprocesses messages. Possible use cases include

Ingesting bytes and preprocessing before inputting data into a BaseModel.
Optionally skipping resolver execution for certain messages.
Re-keying for windowed stream aggregations.

The parse function runs before the resolver, and can transform the message into a format that the stream resolver understands. If the parse function returns None, the resolver will be skipped.

A simple parse function can ingest bytes into a BaseModel, which will be used as input for the streaming resolver.

from pydantic import BaseModel

class Message(BaseModel):
    id: str
    value: str

def parse_bytes(data: bytes) -> Message:
    id, value = some_bytes_processing_step(data)
    return Message(id=id, value=value)


@stream(source=source, parse=parse_bytes)
def stream_resolver(message: Message) -> Features[StreamFeature.id, StreamFeature.value]:
    return StreamFeature(id=message.id, value=message.value)

In the below example, only UserEventMessages which have a click_event property will be processed by resolve_clicks. Those where it is equal to None will not be processed.

# Child messages
class UserLoginEvent(BaseModel):
    ...

class UserClickEvent(BaseModel):
    ...

#Parent message contains one of the child message types
class UserEventMessage(BaseModel):
    login_event: Optional[UserLoginEvent] = None
    click_event: Optional[UserClickEvent] = None

def get_click_event(event: UserEventMessage) -> UserClickEvent:
    return event.click_event

@stream(source=str_source, parse=get_click_event)
def resolve_clicks(message: UserClickEvent) -> Features[...]:
    ...

Full example

In this example, each of the messages from our Kafka source will be converted to a StreamFeature instance. Our streaming message, embodied by KafkaMessage, contains a string representation of a naive datetime we would like to convert to a timezoned datetime. Refer to the aggregations page for why timezoned timestamps may be useful for streaming use cases.

Upon message arrival, the bytes are first parsed into KafkaMessage, then run through the parse function parse_message. The intermediate output ParsedMessage is fed to the stream_resolver, which produces Chalk features. Because we have specified max staleness and etl_offline_to_online, we can expect StreamFeature to be queryable in both online and offline contexts.

from datetime import datetime, timezone
from dateutil import parser
from pydantic import BaseModel
from chalk import features, Features
from chalk.streams import stream, KafkaSource


source = KafkaSource(name="...")

class KafkaMessage(BaseModel):
    id: str
    value: str
    naive_timestamp_str: str

class ParsedMessage(BaseModel):
    id: str
    value: str
    event_timestamp: datetime


@features(max_staleness="1d", etl_offline_to_online=True)
class StreamFeature:
    id: str
    value: str
    event_timestamp: datetime

def parse_message(kafka_message: KafkaMessage) -> ParsedMessage:
    parsed_timestamp: datetime = parser.parse(kafka_message.naive_timestamp_str)
    timezoned_timestamp = parsed_timestamp.replace(tzinfo=timezone.utc)
    return ParsedMessage(
        id=kafka_message.id,
        value=kafka_message.value,
        event_timestamp=timezoned_timestamp
    )

@stream(source=source)
def stream_resolver(message: ParsedMessage) -> Features[StreamFeature.id, StreamFeature.value]:
    return StreamFeature(id=message.id, value=message.value, event_timestamp=message.event_timestamp)

Testing Stream Resolvers

Testing stream resolvers can be difficult: streaming is a complex paradigm that involves multiple services that can be difficult to replicate.

If your streaming resolvers error in production, the errors will be logged and displayed on the dashboard for the resolver. But if you’re looking to unit test your stream resolvers, you have a few options.

One is to fetch data off your streaming source and feed it into the resolver directly. If your streaming resolver intakes bytes, you can do this without manipulation, since stream data is in bytes natively. but if it takes a BaseModel, you will have to perform a model.parse_raw() call on the raw bytes.

@stream(source=some_kinesis_source)
def kinesis_stream(
    message_raw: bytes,
) -> Features[KinesisPaymentFeatures.id, KinesisPaymentFeatures.amount]:
    ...

Assuming you have the above resolver, you can pull off messages from Kinesis and just insert them into your resolver. Like regular resolvers, stream resolvers should be unit-testable as regular Python functions.

import boto3

@stream(source=some_kinesis_source)
def kinesis_stream_resolver(
    message_raw: bytes,
) -> Features[KinesisPaymentFeatures.id, KinesisPaymentFeatures.amount]:
    ...

def fetch_messages():
    session = boto3.session.Session()
    stream_name=...
    client = session.client(
        "kinesis",
        region_name=...,
        aws_access_key_id=...,
        aws_secret_access_key=...
    )
    response = client.describe_stream(StreamName=stream_name)
    details = response['StreamDescription']
    response = client.get_shard_iterator(
        StreamName=stream_name, ShardId=details['Shards'][0]['ShardId'],
        ShardIteratorType='LATEST')
    shard_iter = response['ShardIterator']
    records = []
    while len(records) < 10:
        # polling Kinesis for records...
        response = client.get_records(ShardIterator=shard_iter, Limit=10)
        records.extend(response['Records'])
        shard_iter = response['NextShardIterator']
    messages = [record["Data"] for record in records]
    return messages

stream_messages = fetch_messages()
for message in stream_messages:
    result = kinesis_stream_resolver(message)
    print(result)

However, if you have a parse function, states, rekeying, custom timestamps, or other advanced functionality you want to test, you may have to send messages to the streaming server. Read more on how to use the ChalkClient to test streaming resolvers here)

from chalk.client import ChalkClient

stream_messages = fetch_messages()
keys = ["my_key"] * len(stream_messages)
client = ChalkClient()
resp = client.test_streaming_resolver(
        resolver="kinesis_stream_resolver",
        message_keys=keys,
        message_bodies=stream_messages,
    )
print(resp.features)

​Late arrivals

​Parsing

​Full example

​Testing Stream Resolvers

On this page

Late arrivals

Parsing

Full example

Testing Stream Resolvers