Hi everyone,
the docs contain the following warning, with the emphasizes taken from the
official docs:
> Lookups in the **children** of the Route component are not evaluated at
> configuration time. The substitution is delayed until the Route element is
> evaluated. This means that ${...} expression **should not** be escaped as
> $${...}.
https://logging.apache.org/log4j/2.x/manual/appenders/delegating.html#Route
I have the following two configs, which seem to behave the same:
```xml
<Routing name="[...]">
<Routes pattern="$${ctx:foo}">
<Route key="true" ref="RollingFileFoo" />
<Route key="$${ctx:foo}" ref="RollingFile" />
</Routes>
</Routing>
```
vs.
```xml
<Routing name="[...]">
<Routes pattern="$${ctx:foo}">
<Route key="true" ref="RollingFileFoo" />
<Route key="${ctx:foo}" ref="RollingFile" />
</Routes>
</Routing>
```
The behaviour from my understanding is that `pattern` gets an expression
configured and that expression is interpolated at config time, why `$$` is
needed for escaping reasons. So at runtime the pattern actually is `${ctx:foo}`
and evaluated for each and every log statement passing the appender.
Step 2: If the expression doesn't find a key of the given name `foo`, then the
result of the pattern is kept as the pattern itself. That's why there's an
additional `key` with the pattern itself as a value to match against. And this
is where things become interesting:
**Is that result now expected to be with `$$` like configured in the file or
`$` only like taken at runtime?**
I'm a bit confused which the correct value is because both seem to work exactly
the same. Which shouldn't happen if the `key` attribute implements an equals
comparison to it's value.
```java
public Route getRoute(final String key) {
for (final Route route : routes) {
if (Objects.equals(route.getKey(), key)) {
return route;
}
}
return null;
}
```
https://github.com/apache/logging-log4j2/blob/9176b44193b107b1a6c1ca9efda22f79fa20db3a/log4j-core/src/main/java/org/apache/logging/log4j/core/appender/routing/Routes.java#L198C1-L205C6
>From my understanding, the escaping using `$$` in `pattern` is necessary
>because that pattern otherwise gets resolved already at config time, but it
>needs to be forwarded into runtime. If the difference is that `key` is simply
>NEVER resolved at all at config time and really only taken as string as-is,
>then I would have expected `$$` to fail, because that value is not available
>at runtime?
Thanks for your help!
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