omitEmptyFields doesn't work with %property(ENV_VAR)

[ziomill]

Hello,

I'm trying to use the "omitEmptyFields" to print/omit an ENV variable, without success. If the ENV_VAR 'CONTAINER_NAMESPACE' is not populated, the logger prints "null" as the field's value rather than omit the field. Note that the "omitEmptyFields" is working when used with %mdc directive. This is my configuration:

trace_id

    <encoder class="net.logstash.logback.encoder.LoggingEventCompositeJsonEncoder">
        <providers>
            <!-- TIMESTAMP field (UTC) -->
            <timestamp>
                <fieldName>Timestamp</fieldName>
                <timeZone>UTC</timeZone>
            </timestamp>
            <!-- Include TRACE-ID field from MDC -->
            <pattern>
                <omitEmptyFields>true</omitEmptyFields>
                <pattern>
                    {"TraceId":"%mdc{trace_id}"}
                </pattern>
            </pattern>
            <!-- Include SPAN-ID field from MDC -->
            <pattern>
                <omitEmptyFields>true</omitEmptyFields>
                <pattern>
                    {"SpanId":"%mdc{span_id}"}
                </pattern>
            </pattern>
            <!-- LOG_LEVEL field -->
            <logLevel>
                <fieldName>SeverityText</fieldName>
            </logLevel>
            <!-- MESSAGE field -->
            <message>
                <fieldName>Body</fieldName>
            </message>
            <!-- RESOURCE nested object -->
            <nestedField>
                <fieldName>Resource</fieldName>
                <providers>
                    <!-- NAMESPACE field -->
                    <pattern>
                        <omitEmptyFields>true</omitEmptyFields>
                        <pattern>
                            {"container-namespace":"%property{CONTAINER_NAMESPACE}"}
                        </pattern>
                    </pattern>
                </providers>
            </nestedField>
        </providers>
    </encoder>
<appended>

And this is the printed log record:

{
   "Timestamp":"2022-02-16T16:20:14.530Z",
   "SeverityText":"INFO",
   "Body":"gRPC Server started, listening on address: *, port: 11001",
   "Resource":{
      "container-namespace":"null"
   }
}

Thank you

[brenuart]

The Pattern json provider is using a standard Logback PatternLayout under the hood to resolve placeholders and converters in the pattern string. The resulting string is then converted into JSON and added to the final result. The omitEmptyFields property works by telling Jackson to omit fields whose value is considered empty.

When a property does not exist, the %property{} converter returns null and unfortunately the PatternLayout appends this value "asis" to its internal StringBuilder. So in your case your pattern is converted into the string {"container-namespace":"null"} before it is transformed into a valid JSON structure by Jackson. As you case see, the property is not null but is assigned a String value... it is therefore not empty and will not be filtered out by the omitEmptyField feature.

A String property is considered empty if it is empty or null. A possible approach is to default the property to an empty string if it does not exist (see Default value for variables). Something like this:

<pattern>
    <omitEmptyFields>true</omitEmptyFields>
    <pattern>
        {
            "TraceId":"%mdc{trace_id}",
            "SpanId":"%mdc{span_id}",
            "Resource": {"container-namespace":"${CONTAINER_NAMESPACE:-}"}
        }
    </pattern>
</pattern>

Unfortunately this won't work as expected and the variable is resolved into CONTAINER_NAMESPACE_IS_UNDEFINED unless the default value is set to a non empty string (See this issue for more information). Logback configuration does not like empty strings I'm afraid :-(

An alternative when using SpringBoot is to leveraged the <springProperty> custom action as follows:

<springProperty scope="context" name="CONTAINER_NAMESPACE" source="container.namespace" defaultValue="" />

<appender ...>
    <encoder class="net.logstash.logback.encoder.LoggingEventCompositeJsonEncoder">
        <providers>
            <pattern>
                <omitEmptyFields>true</omitEmptyFields>
                <pattern>
                    {
                        "TraceId":"%mdc{trace_id}",
                        "SpanId":"%mdc{span_id}",
                        "Resource": {"container-namespace":"${CONTAINER_NAMESPACE}"}
                    }
                </pattern>
            </pattern>

Finally, if your CONTAINER_NAMESPACE property is set once and for all when the application is started (i.e. does not change at runtime), then you could guard part of thee Logback configuration elements with a Janino <if> conditional statement so it would be included only if the property is defined.

Hope this helps.
Don't hesitate to come back with additional feedback if needed.

PS: %mdc{} works as expected because this converter returns an empty string instead of null when the MDC key does not exist...
PS2: I agree this is rather confusing and not very obvious... I'll see if we can do something about it in future releases...

[ziomill]

Hello @brenuart and thank you for the very exhaustive explanation.

I was just looking at the conditional logback capabilities provided by Janino, and I can confirm that it works as expected.
In my use cases, the environments variables are populated only once during the application startup and they never change, so I can go with Janino.

[ceki]

As a stop gap measure, you can define a variable named EMPTY with the value "" and use it as the default value:

<configuration>

  <variable name="EMPTY" value="" />
  ...  
    <pattern>
      <omitEmptyFields>true</omitEmptyFields>
      <pattern>
          {
            "Resource": {"container-namespace":"${CONTAINER_NAMESPACE:-${EMPTY}}"}
         }
      </pattern>
   </pattern>
   ....
</configuration>