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new a1036ff Blog: Camel Quarkus Configuration Tips
a1036ff is described below
commit a1036ff70e2af9ddf4636194fd4b90ee189f37db
Author: aldettinger <aldettin...@gmail.com>
AuthorDate: Fri Jan 29 19:24:20 2021 +0100
Blog: Camel Quarkus Configuration Tips
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+---
+title: "Camel Quarkus Configuration Tips"
+date: 2021-01-29
+authors: ["aldettinger"]
+categories: ["Camel Quarkus"]
+preview: "Camel Quarkus Configuration Tips"
+summary: "Some tips related to configuration in Camel Quarkus"
+---
+
+Lately, I have exchanged with a member of the Camel community around
configuration properties in Camel Quarkus.
+It's really interesting to hear about situations people are facing out there,
so please keep reaching out to the community.
+Thinking back about the case at hand, I think there is room for a refresher
about some configuration tips.
+
+## Configuration via application.properties file
+
+Among the multiple ways to define configurations in Camel Quarkus, the more
common must be the `application.properties` file, as illustrated below:
+```properties
+basic=a-basic-value
+```
+
+There are few possibilities for a developer to use the defined configurations.
+One could use the `@ConfigProperty` annotation as beneath:
+```
+@ConfigProperty(name = "basic")
+String basicPropertyFromAnnotation;
+```
+
+It's also possible to access the configuration values programmatically via the
following code:
+```
+String basicPropertyFromConfigProvider =
ConfigProvider.getConfig().getValue("basic", String.class);
+```
+
+From the Camel side, the `{{...}}` notation could be issued, for instance, in
a simple expression:
+```
+from(...).setBody(simple("{{basic}}"))
+```
+
+It's worth noting that in a first pass the `application.properties` file is
parsed thanks to `java.util.Properties`.
+And as such, the corresponding character escaping rules apply. For instance
one could define a configuration using the mathematical square root sign
`\u221A` as beneath:
+```properties
+unicode = a-value-with-unicode-character-(\u221A9=3)
+```
+
+## Property expressions
+
+But there is more processing happening in a second pass thanks to
[smallrye-common](https://github.com/smallrye/smallrye-common) and it leads to
more features.
+For instance, with property expressions one could define a configuration as
below:
+```properties
+embedded = resolved-via-a
+property-expression = a-value-${embedded}-property-expression
+```
+
+Notice how the property from the second line embeds the value from the first
line. The resulting value at the end would be
`a-value-resolved-via-a-property-expression`.
+
+## Environment variables
+
+Another topic of interest is environment variable expansion. Look at the
following property definition where the USERNAME environment variable is used:
+```properties
+environment-variable = a-value-with-environment-variable-${USERNAME}
+```
+
+So, it is possible to include references to environment variables. In case
Camel Quarkus runs in an environment where the variable is not defined,
+it is even feasible to define a default value after the `:` character like
this:
+```properties
+environment-variable-or-default = ${UNEXISTING_ENV_VAR:a-default-value}
+```
+
+Last but not least, it's also possible to mix property expressions with
environment variables as in the following lines:
+```properties
+default-value = a-default-value-resolved-via-a-property-expression
+envvar-or-default-via-property-expression =
${UNEXISTING_ENV_VAR:${default-value}}
+```
+The more meaningful part is `${UNEXISTING_ENV_VAR:${default-value}}`. If the
environment variable `UNEXISTING_ENV_VAR` is not defined, we end up embedding
the `default-value` configured one line above.
+
+## A tricky situation
+
+The previous syntax `${VAR:default}` may sound familiar for some Camel users.
Indeed, it reminds us of some parts of the [Camel simple
language](/components/latest/languages/simple-language.html).
+For instance, some developers could end up defining a property like below:
+```properties
+date-expression = ${date:now}
+```
+And then using it in a Camel simple expression like that:
+```
+from(...).setBody(simple("{{date-expression}}")
+```
+But when the simple expression `{{date-expression}}` is evaluated at runtime,
the resulting body is... `null`
+
+It could sound a bit strange at first, but let's review this situation with
what we have learned so far.
+On the first pass, the `application.properties` file is parsed with respect to
the [Properties file
format](https://docs.oracle.com/javase/7/docs/api/java/util/Properties.html).
No special characters are used, so the parsing outcome would be `${date:now}`
as expected.
+
+But in the second pass, it turns out that this property value is interpreted
as the expansion of an environment variable named `date` with a default value.
As such, Camel is wrongly provided with the value `now`. The expansion
algorithm is detailed in
[io.smallrye.common.expression.Expression.parseString(...)](https://github.com/smallrye/smallrye-common/blob/0b59733491ff936808cd26a4b300f11fe3f2a5f0/expression/src/main/java/io/smallrye/common/expression/Expression.java#L245).
+Paying close attention, it appears that the environment value expansion could
be avoided using `$$`:
+```properties
+date-expression = $${date:now}
+```
+Exactly what we needed, Camel is now provided with the simple expression
`${date:now}` and set the body to a value like `Fri Jan 29 17:07:44 CET 2021`
on execution.
+
+## Summary
+
+At the end of the day, we had a refresher about `application.properties`,
property expressions, and environment variables.
+We have seen how to deal with a tricky situation. There must be even more
corner cases out there but we hope this article offered some pointers to help.
+
+The source code for this example could be found
[here](https://github.com/aldettinger/camel-quarkus-properties). And, the
Quarkus configuration reference guide is located
[there](https://quarkus.io/guides/config-reference).
+
+Many thanks for reading.
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