Environment variables for config options

[ibnesayeed]

Is there a way to set certain config option using environment variables? There are cases when it can be handy to pass some environment variables instead of making changes to the config files such as:

• In testing or development environments we want to run the instance with different configs frequently.
• In production environments if we want to run multiple instances on different hosts/ports and load balance then we will have to duplicate config files with very little differences.
• When the application is deployed for the first time using .war file, there is no way to overwrite the defaults unless the config files are modified and the and the application is reloaded by restarting Tomcat.

Here is an example of some configs from the wayback.xml file:

wayback.basedir=/tmp/openwayback
wayback.url.scheme=http
wayback.url.host=localhost
wayback.url.port=8080

I understand that XML files cannot load environment variables, but when these config options are loaded in Java files, at that time respective environment variables can take precedence if present otherwise fallback to the values set in the config files. Here is a pseudo example to illustrate the usage:

wayback.url.port = env["wayback-url-port"] || conf["wayback.url.port"] || 8080

I was deploying Wayback suing Docker Container and I wished this functionality was there.

[kris-sigur]

This is not directly supported. However, if you create your own WAR overlay (highly recommended) you can use this trick to read configuration from a properties file outside the webapp.

[ibnesayeed]

Thanks for the link @kris-sigur, that was a good read. But I am trying to run Wayback in containers (and I have done successful attempts) and to make it easier, I was looking for a way to pass some environment variables at the time of spinning containers instead of having different config files for every possible combination of cases I might want to run it. The trick described in that WAR overlay trick just allows me to have overrides in yet another file.

I was looking for ways to do so and I found about Spring Expression Language that is supported in Spring 3 (and from the schema used in wayback.xml file, it looks like it is using Spring 3). Don't you think it should be possible to it in the XML file itself? I usually don't work on Java projects so I don't have an idea, how will it all work, but I do feel that it is possible.

[ibnesayeed]

Do you think something like this will work?

<bean class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
  <property name="properties">
    <value>
        wayback.basedir=#{systemEnvironment['BASE_DIR']?:'/tmp/openwayback'}
        ...
      </value>
  </property>
</bean>

The intention is to capture the value of environment variable BASE_DIR and if it does not exist then fallback to the default /tmp/openwayback path.

[ibnesayeed]

Above approach seems to be working in my tests. :)

[kris-sigur]

I'm happy to have set you on the right path :)
I'm a little worried, though, that this makes editing the wayback.xml a bit more confusing.

[ibnesayeed]

@kris-sigur, I understand that it will cause confusion when editing wayback.xml. To solve that problem, how about the following approach?

<bean class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
  <property name="properties">
    <value>
      <!-- Customize these basic placeholders. -->
      wayback.basedir=/tmp/openwayback
      wayback.url.scheme=http
      wayback.url.host=localhost
      wayback.url.port=8080

      <!-- Environment variable overrides (only if present). No need to customize these. -->
      wayback.basedir=#{systemEnvironment['WAYBACK_BASEDIR']?:${wayback.basedir}}
      wayback.url.scheme=#{systemEnvironment['WAYBACK_URL_SCHEME']?:${wayback.url.scheme}}
      wayback.url.host=#{systemEnvironment['WAYBACK_URL_HOST']?:${wayback.url.host}}
      wayback.url.port=#{systemEnvironment['WAYBACK_URL_PORT']?:${wayback.url.port}}

      <!-- Derived placeholders. -->
      wayback.archivedir.1=${wayback.basedir}/files1/
      wayback.archivedir.2=${wayback.basedir}/files2/
      wayback.url.prefix=${wayback.url.scheme}://${wayback.url.host}:${wayback.url.port}
    </value>
  </property>
</bean>

[machawk1]

This approach of setting OpenWayback paths via environment variables (Pull request #220) is extremely relevant to my interests in regard to https://github.com/machawk1/wail , which relies on these paths for all of the bundled tools to communicate and have a common data source.

[kris-sigur]

@ibnesayeed I like the revised approach.

[ibnesayeed]

@kris-sigur, the sample code in the later approach has some syntax problems. The Spring expression interpreter is possibly not happy with the $ sign there or nested { } are an issue. If one can point out the proper way of doing it, I will make the changes in the relevant PR.

[machawk1]

@ibnesayeed, I am unfamiliar with the Spring config syntax but shouldn't the ternary operator be

result = #{condition?${valueIfTrue}:${valueIfFalse}}

If this form applies to Spring, no value is set if the condition (e.g., systemEnvironment['WAYBACK_URL_SCHEME']) is true.

[ibnesayeed]

@machawk1 this is called Elvis Operator syntax used in Groovy (and supported in Spring 3 Expressions) and it is a shorthand that can be used if valueIfTrue is same as condition. To illustrate it consider the following statement:

displayName = fullName ?: userName

It is equivalent to:

displayName = fullName ? fullName : userName

In this example displayName variable will be set to fullName if it is present, otherwise if it is blank or has a falsy value, userName will be used instead. Ruby alternative to this syntax will be something like this:

display_name = full_name || user_name

This is good way for setting fallback or default value.