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DataStream SDK: Event Filtering Guide

Overview

The DataStream SDK provides a powerful event filtering system that lets you process only the events your application cares about. This guide covers filter creation, chaining, and advanced patterns.

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Creating a Basic Filter

Filters are created using the EventFilter class. Each filter takes a predicate function that receives an event and returns a boolean.

from datastream import EventFilter

# Filter for high-priority events only
high_priority = EventFilter(
    predicate=lambda event: event.priority >= Priority.HIGH,
    name="high-priority-filter"
)

# Apply the filter to a stream
filtered_stream = stream.filter(high_priority)

Chaining Filters

Multiple filters can be chained together. Events must pass all filters in the chain to be delivered to your handler.

# Chain multiple filters
chain = (
    EventFilter(lambda e: e.priority >= Priority.HIGH)
    .and_then(EventFilter(lambda e: e.source == "payments"))
    .and_then(EventFilter(lambda e: e.region in ["us-east", "us-west"]))
)

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Advanced Filter Patterns

Once you have basic filtering working, you can use more sophisticated patterns to handle complex event routing scenarios.

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Dynamic Filter Registration

You can register filters at runtime based on configuration changes. This is useful for multi-tenant applications where each tenant has different event requirements.

The FilterRegistry class manages a collection of named filters that can be added, removed, or updated without restarting the application:

registry = FilterRegistry()

# Register a filter for tenant-specific events
registry.register(
    "tenant-acme",
    EventFilter(lambda e: e.metadata.get("tenant") == "acme")
)

# Later, update the filter
registry.update(
    "tenant-acme",
    EventFilter(lambda e: e.metadata.get("tenant") == "acme" and e.priority >= Priority.MEDIUM)
)

Conditional Filter Bypass

In some cases you may want to bypass filters entirely for certain event types. The bypass decorator marks a handler as exempt from the filter chain:

@stream.handler(bypass_filters=True)
def handle_system_events(event):
    """System events always get processed regardless of filters."""
    log.info(f"System event: {event.type}")

Performance Considerations

Filters are evaluated in chain order. Place the most selective filter first to short-circuit evaluation early. Each filter evaluation adds approximately 0.1ms of latency per event.

For high-throughput streams (over 10,000 events per second), consider using compiled filters which pre-evaluate static conditions:

compiled = EventFilter.compile(
    priority_gte=Priority.HIGH,
    sources=["payments", "orders"],
    regions=["us-east"]
)

Compiled filters are 5-10x faster than lambda-based filters for equivalent conditions.