BitLogger/docs/api/target-has-prefix.md

name, group, category, update-time, description, key-word

name	group	category	update-time	description	key-word
target-has-prefix	api	filtering	20260512	Create a reusable record predicate that matches records by target prefix.	target filter predicate public

Target-has-prefix

Create a RecordPredicate that returns true when a record target starts with a given prefix. This helper is commonly used with with_filter(...), filter_sink(...), all_of(...), and routing logic.

Interface

pub fn target_has_prefix(prefix : String) -> RecordPredicate {}

input

• prefix : String - Prefix expected at the start of rec.target.

output

• RecordPredicate - Predicate that matches records whose target starts with the given prefix.

Explanation

Detailed rules explaining key parameters and behaviors

• Matching is based on String::has_prefix(...) over the full target string.
• This helper does not modify records; it only returns a reusable predicate.
• Prefix-based matching is especially useful when using hierarchical targets such as app.worker.job.
• It composes cleanly with all_of(...), any_of(...), and not_(...).

How to Use

Here are some specific examples provided.

When Filter A Target Namespace

When a logger should keep one target subtree:

let logger = Logger::new(console_sink(), target="app")
  .with_filter(target_has_prefix("app.worker"))

In this example, only records under app.worker... remain visible.

And child targets continue to compose naturally.

When Combine With Level Rules

When both namespace and severity matter:

let predicate = all_of([
  target_has_prefix("service.api"),
  level_at_least(Level::Warn),
])

In this example, the predicate stays reusable across multiple loggers or sinks.

Error Case

e.g.:

• If prefix is empty, the predicate effectively matches every target because every string starts with an empty prefix.

• If no target matches the prefix, the predicate simply returns false for those records.

Notes

Notes are here.

1. This helper is most useful when targets are structured hierarchically.

2. Prefer this helper over ad hoc inline prefix logic for readability.

3. Use target_is(...) instead when exact equality is required.

4. Combine with message_contains(...) or field predicates for narrower routing rules.