asafm commented on PR #22327:
URL: https://github.com/apache/pulsar/pull/22327#issuecomment-2024934741
Ok guys, there was a lot of information thrown around here, and for me most
of it was new.
I'll try to summarize it all here:
1. Log4j2 supports plugins. allowing you to extend Log4j. In our example, it
allows anyone to add a new type of Appender.
2. Prometheus Simple client has a dependency which we included in Pulsar:
```
<dependency>
<groupId>io.prometheus</groupId>
<artifactId>simpleclient_log4j2</artifactId>
</dependency>
```
The dependency includes a Log4j2 plugin, which adds a new `Prometheus`
type Appender. As @lhotari noted it is:
```
@Plugin(name = "Prometheus", category = "Core", elementType = "appender")
public final class InstrumentedAppender extends AbstractAppender {
```
This Appender receives the logs (log events) and exposes metrics about
them - how many logs per type, etc. In effect, it instruments Log4j2.
3. The `Prometheus` type appender is used as you noted in the log4j2.yaml
under Appenders
```
Prometheus:
name: Prometheus
```
and then referenced to "forward" the logs to that appender so they can be
counted.
```
# Default root logger configuration
Root:
level: "${sys:pulsar.log.root.level}"
additivity: true
AppenderRef:
- ref: "${sys:pulsar.log.appender}"
level: "${sys:pulsar.log.level}"
- ref: Prometheus
level: info
```
4. Log4j previously allowed to configure which packages to scan for
existence of the classes you need to extend if you wish to add a plugin. As you
@nodece noted in the link you provided, you don't really need it. You just need
the plugin to have the `@Plugin` annotation, which it does. This mean we can
safely remove the `packages` line from log4j2.yaml.
You do need to verify the Prometheus log4j metrics are still exposed as
double check.
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