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BitLogger/docs/api/fanout-sink.md
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---
name: fanout-sink
group: api
category: sink
update-time: 20260520
description: Create a sink that writes each record to two sinks.
key-word:
- sink
- fanout
- routing
- public
---
## Fanout-sink
Create a sink that writes every record to two underlying sinks. This helper is useful when the same log stream should be duplicated across multiple destinations.
### Interface
```moonbit
pub fn[A, B] fanout_sink(left : A, right : B) -> FanoutSink[A, B] {
```
#### input
- `left : A` - First sink destination.
- `right : B` - Second sink destination.
#### output
- `FanoutSink[A, B]` - Sink that duplicates records to both destinations.
### Explanation
Detailed rules explaining key parameters and behaviors
- Each record is written to both sinks.
- The right side receives a copied record so the two writes remain independent at the record value level.
- This helper is useful for combinations such as console plus JSON, or console plus callback capture.
### How to Use
Here are some specific examples provided.
#### When Need Dual Output Paths
When the same records should go to both human and machine-oriented outputs:
```moonbit
let logger = Logger::new(
fanout_sink(console_sink(), json_console_sink()),
target="dual",
)
```
In this example, each record is written to both console sinks.
### Error Case
e.g.:
- If only one path should receive a record conditionally, use `split_sink(...)` instead.
- Sink-specific failures still follow the behavior of the wrapped sinks.
### Notes
1. This helper is a duplication primitive, not a conditional router.
2. It is often useful in examples and test capture setups.