---
title: Filter observers by appended event metadata
---

Chronicle can filter reducers and reactors before they handle an appended event. You do this by putting filter attributes on the observer and appending matching metadata with the event.

## How filters correlate to append operations

| Append metadata | Observer attribute | Matches when |
| --- | --- | --- |
| `tags:` | `[FilterEventsByTag("...")]` | Any appended or static event tag matches any filter tag |
| `eventSourceType:` | `[EventSourceType("...")]` | The appended event source type matches exactly |
| `eventStreamType:` | `[EventStreamType("...")]` | The appended event stream type matches exactly |

## Important distinctions

- `[Tag]` and `[Tags]` on a reactor, reducer, or projection **label the observer itself**. They do not filter incoming events.
- `[FilterEventsByTag]`, `[EventSourceType]`, and `[EventStreamType]` on a reducer or reactor **filter which appended events are dispatched**.
- If you combine filter types, Chronicle applies them together. The event must satisfy every configured filter category.
- Multiple `[FilterEventsByTag]` attributes are additive. An event is dispatched when it matches any configured filter tag.

## Scope

Reducers and reactors use these filters during subscription. Projections still describe what they observe through event types, joins, and event sequence selection.

## Guides

- [Filter reducers and reactors by tag](/chronicle/events/filtering/by-tag/)
- [Filter reducers and reactors by event source type](/chronicle/events/filtering/by-event-source-type/)
- [Filter reducers and reactors by event stream type](/chronicle/events/filtering/by-event-stream-type/)