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Question

I'm trying to implement distributed tracing in my kotlin app using spring cloud sleuth. I'm sending those data to the datadog. Now I'm able to trace my logs but I want to add some extra data to spans. Let's say I want to add info about user and be able to see it in datadog. Am I right that span tags are good for it? I'm sending the logs in json format to datadog but I cannot add tags here. (traceId and spanId are injected). Logback config:

        <encoder class="net.logstash.logback.encoder.LoggingEventCompositeJsonEncoder">
            <providers>
                <timestamp/>
                <version/>
                <message/>
                <loggerName/>
                <threadName/>
                <logLevel/>
                <logLevelValue/>
                <callerData/>
                <stackTrace/>
                <rootStackTraceElement/>
                <context/>
                <mdc/>
                <tags/>
                <logstashMarkers/>
                <arguments/>
            </providers>
        </encoder>

gradle:

implementation("org.jetbrains.kotlin:kotlin-reflect")
implementation("org.jetbrains.kotlin:kotlin-stdlib-jdk8")
implementation("org.springframework.cloud:spring-cloud-starter-netflix-eureka-server")
implementation("org.springframework.cloud:spring-cloud-starter-vault-config")
implementation("org.springframework.cloud:spring-cloud-starter-sleuth")
implementation("org.springframework.boot:spring-boot-starter-actuator")
implementation("ch.qos.logback:logback-classic:1.2.3")
implementation("net.logstash.logback:logstash-logback-encoder:6.6")
implementation("org.zalando:logbook-spring-boot-starter:2.4.2")
developmentOnly("org.springframework.boot:spring-boot-devtools")
runtimeOnly("io.micrometer:micrometer-registry-datadog")
testImplementation("org.springframework.boot:spring-boot-starter-test")

and to add the tag I'm trying

@NewSpan
fun newSpanTest() {
    tracer.currentSpan()!!.tag("user", "123")
    log.info("other span")
    otherTestService.sameSpanTest()
}

example log:

{"@timestamp":"2021-03-09T21:04:46.953+01:00","@version":"1","message":"other span","logger_name":"com.microservices.text.rpg.servicediscovery.TestService","thread_name":"http-nio-8761-exec-3","level":"INFO","level_value":20000,"caller_class_name":"com.microservices.text.rpg.servicediscovery.TestService","caller_method_name":"newSpanTest","caller_file_name":"TestService.kt","caller_line_number":25,"traceId":"e61fd165d7c84776","spanId":"5e61f9b51b51619b"}

shouldn't be that 'user' injected into MDC and then into logs?

Answer 1

TL;DR

spring.sleuth.baggage.correlation-fields automatically sets baggage values to Slf4j's MDC so you only need to set the baggage field.

Longer version with background

I suppose you use Sleuth out of the box (uses Brave):

1. Fist get familiar with tag , baggage and their differences ; you can also read about them in Brave docs
2. Check Brave's ScopeDecorator , CorrelationScopeDecorator and MDCScopeDecorator

The spring.sleuth.baggage.correlation-fields property automatically sets baggage values to Slf4j's MDC so you only need to set the baggage field.

Also, using MDCScopeDecorator , you can set the baggage values to Slf4j's MDC programmatically, you can see how to do it in Sleuth docs :

// configuration
@Bean
BaggageField countryCodeField() {
    return BaggageField.create("country-code");
}

@Bean
ScopeDecorator mdcScopeDecorator() {
    return MDCScopeDecorator.newBuilder()
            .clear()
            .add(SingleCorrelationField.newBuilder(countryCodeField())
                    .flushOnUpdate()
                    .build())
            .build();
}

// service
@Autowired
BaggageField countryCodeField;

countryCodeField.updateValue("new-value");