✨ Add split sink routing helpers

README.md

@@ -19,6 +19,7 @@ BitLogger 是一个基于 MoonBit 编写的结构化日志库。
 - 🧩 核心能力清晰：先把 logging core 做稳，再继续扩展 rotation/async 等能力。
 - 🏗️ 结构明确：按 `level / record / formatter / sinks / logger / global` 拆文件，便于继续维护。
 - 🔌 可扩展：支持 `fanout_sink(...)` 和 `callback_sink(...)`，方便后续桥接文件、指标或外部系统。
+- 🔀 可分流：支持 `split_sink(...)` / `split_by_level(...)`，可按谓词或 level 将日志路由到不同 sink。
 - 🧱 可组合：支持 `buffered_sink(...)` 和 `filter_sink(...)`，可以在不引入复杂 runtime 的前提下组合输出策略。
 - 🔎 可复用过滤：提供 `target_has_prefix(...)`、`message_contains(...)`、`level_at_least(...)`、`field_equals(...)` 等过滤辅助函数。
 - 🩹 可变换 Record：支持 `with_patch(...)`、`patch_sink(...)` 与脱敏/补字段/message 变换 helper。
@@ -127,6 +128,27 @@ ignore(logger.sink.flush())
 ```
 </details>
 
+<details><summary>按 level 分流 sink 示例</summary>
+
+```moonbit
+let logger = Logger::new(
+  split_by_level(
+    callback_sink(fn(rec) {
+      println("high priority: \{rec.level.label()} \{rec.message}")
+    }),
+    console_sink(),
+    min_level=Level::Warn,
+  ),
+  min_level=Level::Trace,
+  target="split",
+)
+
+logger.info("normal output")
+logger.warn("warning output")
+```
+
+</details>
+
 <details><summary>自定义文本 formatter 示例</summary>
 
 ```moonbit

bitlogger/BitLogger_wbtest.mbt

@@ -182,6 +182,58 @@ test "callback sink receives record" {
   inspect(captured_message.val, content="hello")
 }
 
+test "split sink routes records by predicate" {
+  let left_messages : Ref[Array[String]] = Ref::new([])
+  let right_messages : Ref[Array[String]] = Ref::new([])
+  let logger = Logger::new(
+    split_sink(
+      callback_sink(fn(rec) {
+        left_messages.val.push(rec.message)
+      }),
+      callback_sink(fn(rec) {
+        right_messages.val.push(rec.message)
+      }),
+      fn(rec) {
+        rec.target == "audit"
+      },
+    ),
+    min_level=Level::Info,
+    target="main",
+  )
+  logger.info("drop to right")
+  logger.log(Level::Info, "keep on left", target="audit")
+  inspect(left_messages.val.length(), content="1")
+  inspect(left_messages.val[0], content="keep on left")
+  inspect(right_messages.val.length(), content="1")
+  inspect(right_messages.val[0], content="drop to right")
+}
+
+test "split_by_level routes warn and error separately" {
+  let high_messages : Ref[Array[String]] = Ref::new([])
+  let low_messages : Ref[Array[String]] = Ref::new([])
+  let logger = Logger::new(
+    split_by_level(
+      callback_sink(fn(rec) {
+        high_messages.val.push(rec.level.label() + ":" + rec.message)
+      }),
+      callback_sink(fn(rec) {
+        low_messages.val.push(rec.level.label() + ":" + rec.message)
+      }),
+      min_level=Level::Warn,
+    ),
+    min_level=Level::Trace,
+    target="split",
+  )
+  logger.info("info")
+  logger.warn("warn")
+  logger.error("error")
+  inspect(high_messages.val.length(), content="2")
+  inspect(high_messages.val[0], content="WARN:warn")
+  inspect(high_messages.val[1], content="ERROR:error")
+  inspect(low_messages.val.length(), content="1")
+  inspect(low_messages.val[0], content="INFO:info")
+}
+
 test "callback sink sees child target and context logger shape" {
   let captured_target : Ref[String] = Ref::new("")
   let captured_message : Ref[String] = Ref::new("")

bitlogger/README.mbt.md

@@ -24,6 +24,8 @@ BitLogger 是一个基于 MoonBit 的结构化日志库。
 - `json_console_sink()` 提供 JSON 控制台输出
 - sink composition via `fanout_sink(...)`
 - `fanout_sink(...)` 支持多 sink 组合
+- sink routing via `split_sink(...)` and `split_by_level(...)`
+- `split_sink(...)`、`split_by_level(...)` 支持按谓词或 level 将日志路由到不同 sink
 - custom callback sink via `callback_sink(...)`
 - `callback_sink(...)` 支持自定义外部集成
 - buffered sink via `buffered_sink(...)`
@@ -142,6 +144,24 @@ test {
 }
 ```
 
+```mbt check
+test {
+  let logger = Logger::new(
+    split_by_level(
+      callback_sink(fn(rec) {
+        println("high priority: \{rec.level.label()} \{rec.message}")
+      }),
+      console_sink(),
+      min_level=Level::Warn,
+    ),
+    min_level=Level::Trace,
+    target="split",
+  )
+  logger.info("normal output")
+  logger.warn("warning output")
+}
+```
+
 ```mbt check
 test {
   let formatter = text_formatter(

bitlogger/sinks.mbt

@@ -247,6 +247,34 @@ pub impl[A : Sink, B : Sink] Sink for FanoutSink[A, B] with write(self, rec) {
   self.right.write({ ..rec })
 }
 
+pub struct SplitSink[A, B] {
+  left : A
+  right : B
+  predicate : (Record) -> Bool
+}
+
+pub fn[A, B] split_sink(left : A, right : B, predicate : (Record) -> Bool) -> SplitSink[A, B] {
+  { left, right, predicate }
+}
+
+pub fn[A, B] split_by_level(
+  left : A,
+  right : B,
+  min_level~ : Level = Level::Warn,
+) -> SplitSink[A, B] {
+  split_sink(left, right, fn(rec) {
+    rec.level.enabled(min_level)
+  })
+}
+
+pub impl[A : Sink, B : Sink] Sink for SplitSink[A, B] with write(self, rec) {
+  if (self.predicate)(rec) {
+    self.left.write(rec)
+  } else {
+    self.right.write(rec)
+  }
+}
+
 pub struct CallbackSink {
   callback : (Record) -> Unit
 }

docs/README-en.md

@@ -14,6 +14,7 @@ BitLogger currently provides:
 - context fields via `with_context_fields(...)`
 - optional timestamps via `with_timestamp()`
 - sink fanout via `fanout_sink(...)`
+- sink routing via `split_sink(...)` and `split_by_level(...)`
 - custom integration via `callback_sink(...)`
 - in-memory buffering via `buffered_sink(...)`
 - record filtering via `filter_sink(...)`
@@ -121,6 +122,25 @@ logger.info("three")
 ignore(logger.sink.flush())
 ```
 
+Level-based split sink:
+
+```moonbit
+let logger = Logger::new(
+  split_by_level(
+    callback_sink(fn(rec) {
+      println("high priority: \{rec.level.label()} \{rec.message}")
+    }),
+    console_sink(),
+    min_level=Level::Warn,
+  ),
+  min_level=Level::Trace,
+  target="split",
+)
+
+logger.info("normal output")
+logger.warn("warning output")
+```
+
 Custom text formatter:
 
 ```moonbit

docs/changes/0.3.0.md

@@ -29,6 +29,7 @@ version 0.3.0
 - feat: support formatter tokens `{timestamp}`, `{timestamp_ms}`, `{level}`, `{target}`, `{message}`, and `{fields}` in both runtime and JSON config paths
 - feat: add `FileRotation`, `file_rotation(...)`, and size-based file rotation with retained backups for `file_sink(...)`
 - feat: support `sink.rotation.max_bytes` and `sink.rotation.max_backups` in JSON logger config
+- feat: add `SplitSink`, `split_sink(...)`, and `split_by_level(...)` for routing records into different sinks by predicate or level
 
 ### Test
 
@@ -40,6 +41,7 @@ version 0.3.0
 - test: cover dropped-count reporting for bounded config-built queue
 - test: cover template-based formatter rendering and disabled token behavior
 - test: cover file rotation config parsing, config roundtrip, and native rotation behavior
+- test: cover split sink predicate routing and level-based routing behavior
 - test: add async logger lifecycle, config roundtrip, and batching/flush policy test seeds
 - build: verify `bitlogger_async --target native` and `examples/async_basic --target native` compile successfully
 
@@ -52,6 +54,7 @@ version 0.3.0
 - docs: update root README, English README, and Mooncake README with config usage notes
 - docs: update formatter examples to demonstrate template-based text rendering
 - docs: update file sink examples to demonstrate rotation and backup retention
+- docs: add split sink examples for level-based routing
 - docs: update root README and English README with async adapter notes and current scope
 - chore: ignore local `.mooncakes/` cache directory in git
 

examples/basic/main.mbt

@@ -17,6 +17,20 @@ fn main {
   )
   fanout_logger.info("dual output", fields=[@lib.field("kind", "fanout")])
 
+  let split_logger = @lib.Logger::new(
+    @lib.split_by_level(
+      @lib.callback_sink(fn(rec) {
+        println("high priority: \{rec.level.label()} \{rec.message}")
+      }),
+      @lib.console_sink(),
+      min_level=@lib.Level::Warn,
+    ),
+    min_level=@lib.Level::Trace,
+    target="split",
+  )
+  split_logger.info("normal output")
+  split_logger.warn("warning output")
+
   let timed_logger = @lib.Logger::new(@lib.console_sink(), min_level=@lib.Level::Info, target="timed")
     .with_timestamp()
   timed_logger.info("timestamp enabled", fields=[@lib.field("kind", "time")])