[
https://issues.apache.org/jira/browse/KAFKA-6142?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16300193#comment-16300193
]
Randall Hauch commented on KAFKA-6142:
--------------------------------------
It should be possible to have an empty string as the default:
{{$\{foo1.prop,env.MY_ENV_VAR:\}}}
> Connect worker configurations and connector configurations should accept
> environment variables
> ----------------------------------------------------------------------------------------------
>
> Key: KAFKA-6142
> URL: https://issues.apache.org/jira/browse/KAFKA-6142
> Project: Kafka
> Issue Type: Improvement
> Components: KafkaConnect
> Affects Versions: 1.0.0
> Reporter: Randall Hauch
> Labels: needs-kip
>
> Currently, when a worker or connector configuration is parsed, the values are
> used as-is without any kind of pre-processing before the value is used. It
> should be possible to define configuration properties such that string
> literal values or default values can use *_configuration variables_* that
> reference environment variables and/or system properties, and that these
> configuration variables are resolved/replaced before the configuration value
> is used.
> I propose doing this enhancement in Kafka client's {{ConfigDef}} by adding a
> {{ConfigDef.Transformer}} interface:
> {code:java}
> /**
> * Transform the configuration value.
> */
> public interface Transformer {
> /**
> * Transform the configuration value.
> * @param name The name of the configuration
> * @param value The value of the configuration
> * @return the preprocessed value
> * @throws ConfigException if the value is invalid.
> */
> Object apply(String name, Object value);
> }
> {code}
> and then allowing {{Transformer}} implementations to be passed to
> {{ConfigDef.define(...)}} such all existing signatures are maintained for
> backward compatibility. By default, the definition would use an identity
> transform that simply returns the value. The transformers would be called in
> {{ConfigDef.parseValue(...)}} before the {{parseType(...)}} method is called,
> and would also be called on the default value if one is provided.
> Then, a {{ConfigDef.ReplaceSystemVariables}} implementation would be provided
> to look in {{String}} values for zero or more variables defined with this
> EBNF grammar:
> {noformat}
> '$' '{' varName { ',' varName } [ ':' defaultValue ] '}'
> {noformat}
> where:
> * {{varName}} is the name of a Java system property or {{env.}} followed by
> the name of an environment variable, and
> * {{defaultValue}} specifies the replacement value used when no environment
> variable or system property is found, and defaults to an empty string.
> The value of the first system property or environment variable resolved is
> then used to replace the variable expression. This implementation would have
> trace or debug level logging to describe what it is doing.
> For example, the variable {{$\{env.KAFKA_HOME\}}} would replace the variable
> expression with the value of the {{KAFKA_HOME}} environment variable or an
> blank string if that variable doesn't exist.
> Likewise, the variable {{$\{foo.prop1,foo.prop2,env.MY_ENV_VAR:value\}}}
> would be replaced with the value of the {{foo.prop1}} system property if it
> exists, or with the value of the {{foo.prop2}} system property if it exists,
> or with the value of the {{MY_ENV_VAR}} environment variable if it exists, or
> {{value}} if none of the system properties exist.
--
This message was sent by Atlassian JIRA
(v6.4.14#64029)
