feat(blog): post for declarative config

content/en/blog/2025/declarative-config.md

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+---
+title:
+  'The Declarative configuration journey: Why it took 5 years to ignore health
+  check endpoints in tracing'
+linkTitle: Declarative configuration journey
+date: 2025-10-17
+author: >-
+  [Gregor Zeitlinger](https://github.com/zeitlinger)(Grafana Labs), [Jay
+  DeLuca](https://github.com/jaydeluca) (Grafana Labs), [Marylia
+  Gutierrez](https://github.com/maryliag) (Grafana Labs)
+cSpell:ignore: Dotel marylia otelconf zeitlinger
+---
+
+One of the most persistent and popular feature requests for Java OpenTelemetry
+over the past couple of years has been the ability to efficiently [drop spans
+for health check endpoints][drop-spans-issue] – or any other low-value,
+cost-driving endpoints. This issue was first raised in August 2020, yet a
+comprehensive solution remained elusive for a surprisingly long time. Why did it
+take us five years to address this seemingly straightforward problem? The answer
+lies in the fundamental principles of OpenTelemetry's configuration system and
+the journey towards a more robust, flexible approach: declarative configuration.
+
+From the outset, OpenTelemetry relied on environment variables for
+configuration, a choice driven by their universal availability across languages
+and ease of parsing. However, as the need for more complex configuration use
+cases grew, the limitations of simple string-based environment variables became
+increasingly apparent, making advanced configurations cumbersome and difficult
+to manage.
+
+Enter declarative configuration, a powerful evolution that leverages YAML files
+to define OpenTelemetry settings. This shift allows for reading data from any
+tree-shaped source, fundamentally transforming how we approach complex
+configurations. Throughout this post, we'll explore how declarative
+configuration provides an elegant solution to the challenges of the past, and
+demonstrate its immediate impact with practical use cases like health check
+exclusion in Java.
+
+## Getting started
+
+The configuration file is language agnostic, so once you create one file, you
+can use it for all your SDKs. The only exceptions are the parameters with the
+specific language name that are only relevant to that language (for example,
+`java spring batch` parameter). Keep in mind that declarative configuration is
+**experimental**, so things might still change.
+
+The following example is a basic configuration file you can use to get started:
+
+```yaml
+file_format: '1.0-rc.1'
+
+resource:
+  attributes_list: ${OTEL_RESOURCE_ATTRIBUTES}
+  detection/development:
+    detectors:
+      - service: # will add "service.instance.id" and "service.name" from OTEL_SERVICE_NAME
+
+tracer_provider:
+  processors:
+    - batch:
+        exporter:
+          otlp_http:
+            endpoint: ${OTEL_EXPORTER_OTLP_ENDPOINT:-http://localhost:4318}/v1/traces
+
+meter_provider:
+  readers:
+    - periodic:
+        exporter:
+          otlp_http:
+            endpoint: ${OTEL_EXPORTER_OTLP_ENDPOINT:-http://localhost:4318}/v1/metrics
+
+logger_provider:
+  processors:
+    - batch:
+        exporter:
+          otlp_http:
+            endpoint: ${OTEL_EXPORTER_OTLP_ENDPOINT:-http://localhost:4318}/v1/logs
+```
+
+All you have to do is pass
+`OTEL_EXPERIMENTAL_CONFIG_FILE=/path/to/otel-config.yaml` to the application to
+activate the experimental declarative configuration option. This variable only
+works in the Java agent and JavaScript at the time of writing.
+
+## Declarative configuration in Java
+
+Let's now look at the broader implementation of declarative configuration within
+the Java ecosystem. As the pioneering language in this area, Java agent 2.21+
+now fully supports declarative configuration, with most instrumentations and
+features already functional. We are working to incorporate the remaining
+features throughout 2026, and you can track our progress on the [project
+board][java-project] and see the [list of features not yet
+supported][list-not-supported].
+
+Depending on whether you are starting fresh or migrating from using environment
+variables, there’s a few resources you can leverage:
+
+- The basic (language agnostic) configuration file example from the previous
+  section is the quickest way to get started when you don’t need any further
+  customizations.
+- The [migration configuration file][migration-file] maps the old environment
+  variables into the YAML schema, allowing for a drop in replacement for users
+  using workloads already configured with environment variables.
+- The [full configuration file][full-file] (“kitchen sink”) shows the entire
+  schema, annotated with documentation as comments. This is useful for users who
+  want to see all available options and their defaults.
+
+All of the above files work for any language that supports declarative
+configuration.
+
+In addition, there are many settings specific to Java agent that go into the
+instrumentation section of your configuration file. For example, if you have the
+system property `otel.instrumentation.spring-batch.experimental.chunk.new-trace`
+in your application, you can create the declarative configuration file by
+removing the `otel.instrumentation` prefix, splitting at . and converting - to
+\_.
+
+```yaml
+file_format: '1.0-rc.1'
+---
+instrumentation/development:
+  java:
+    spring_batch:
+      experimental:
+        chunk:
+          new_trace: true
+```
+
+With this configuration in place, developers can continue to use their Java
+instrumentation as they normally would, sending telemetry data to their chosen
+observability backend. Furthermore, the declarative configuration file provides
+the flexibility to expand and add more parameters as needed, allowing for highly
+customized and nuanced control over the observability setup.
+
+## Health check exclusion
+
+As mentioned in the introduction, one of the most popular feature requests in
+the Java community was to be able to exclude health checks (or other unimportant
+or noisy resources) from generating traces.
+
+To achieve this, you need to add a new `sampler` block within your
+`tracer_provider` configuration, as shown below:
+
+```yaml
+file_format: '1.0-rc.1'
+
+# ... the rest of the configuration ....
+
+tracer_provider:
+# Configure sampling to exclude health check endpoints.
+sampler:
+  rule_based_routing:
+    fallback_sampler:
+      always_on:
+    span_kind: SERVER
+    rules:
+      # Action to take when the rule matches. Must be DROP or RECORD_AND_SAMPLE.
+      - action: DROP
+        # The span attribute to match against.
+        attribute: url.path
+        # The pattern to compare the span attribute to.
+        pattern: /actuator.*
+# ... the rest of the tracer_provider configuration ...
+```
+
+See the [Java sampler documentation][java-sampler] for more details on the
+available options.
+
+Try it for yourself:
+
+1. Save [the complete configuration][complete-config]
+2. Run the Java agent with
+   `-Dotel.experimental.config.file=/path/to/otel-config.yaml`
+
+## Availability
+
+After reading about declarative configuration, you might be wondering where it
+is available and how you can start using it. You can find guidance on how to get
+started and which languages are supported in the
+[documentation][declarative-docs]. As of the time of writing of this post, Java
+is fully compliant and PHP, JavaScript and Go are partially compliant. To see
+the latest status, check the [compliance matrix][compliance-matrix].
+
+### Java
+
+As described previously, declarative configuration in
+[Java][java-declarative-config] is experimental but ready to use. Use the
+example we discussed earlier to set up your new configuration. If you have
+questions or feedback reach out in [`#otel-java`][slack-java] on the CNCF Slack.
+
+_Note to other language maintainers: It is useful to create a bridge module that
+adapts declarative config settings and environment variables to a common
+interface. For Java, this is the [Declarative Config Bridge][java-bridge]._
+
+### JavaScript
+
+The implementation in the JavaScript SDK is currently under development. A new
+package called [opentelemetry-configuration][js-package] has been created, and
+it handles both environment variables and declarative configuration. With this
+approach, the user doesn't need to change their instrumentation when they switch
+between environment variables and configuration file, since the new package
+handles it and returns the same configuration model for both cases. Currently,
+this configuration package is being added to other instrumentation packages, so
+they can take advantage of the declarative configuration. If you have questions,
+reach out in [`#otel-js`][slack-js] on the CNCF Slack.
+
+### PHP
+
+The PHP implementation is partially compliant, and you can start using it by
+[initializing from your config file][php-docs]. For help or feedback, reach out
+in [`#otel-php`][slack-php] on the CNCF Slack.
+
+### Go
+
+Go has a [partial implementation][go-package] of declarative configuration. Each
+supported schema version has its own corresponding package directory. For
+example, importing `go.opentelemetry.io/contrib/otelconf/v0.3.0` gives you the
+code that supports version 0.3.0 of the configuration schema. You can find all
+available versions in the [package index][go-package-index]. If you have
+questions on how to use it, reach out in [`#otel-go`][slack-go] on the CNCF
+Slack.
+
+## The journey
+
+So why did it actually take us five years to ignore health check endpoints in
+tracing?
+
+The journey to declarative configuration, and consecutively, the solution for
+health check exclusion, highlights a core tenet of OpenTelemetry: building
+sustainable solutions through rigorous specifications.
+
+From the outset, OpenTelemetry's reliance on environment variables, while
+universally available, proved increasingly complex for advanced configurations.
+New environment variables were eventually disallowed, creating a void that a
+more robust solution needed to fill.
+
+The replacement, as we’ve presented in this blog post, is declarative
+configuration. Crafting and agreeing upon the precise syntax and semantics was a
+time-consuming, and sometimes exhausting, process. For example, we discussed
+several proposals on how environment variables could be embedded until we came
+up with the current solution of using
+`${OTEL_EXPORTER_OTLP_ENDPOINT:-http://localhost:4318}`.
+
+This process serves as a powerful case study for how the OpenTelemetry community
+operates. It's a testament to establishing consensus, fostering collaboration,
+and the collective effort required to introduce significant new features and
+drive their implementation across diverse projects.
+
+## What's next for declarative configuration?
+
+The journey of declarative configuration is far from over. Our current focus
+involves a substantial effort to expand language support, which is crucial for
+ensuring that developers, regardless of their preferred tools, can leverage the
+benefits of a declarative approach.
+
+We are keenly interested in user feedback as we continue to develop and refine
+these features. We encourage you to begin experimenting with the current
+implementations and to actively communicate any missing functionalities, pain
+points, or areas for improvement. This collaborative approach will help us
+prioritize development efforts and ensure that the solutions we build truly meet
+the needs of the community. You share your feedback or questions using the
+channel [`#otel-config-file`][slack-config] from CNCF Slack.
+
+Beyond providing feedback, there are other ways to get involved and contribute
+to the growth of declarative configuration. Each OpenTelemetry SDK has a
+[Special Interest Groups (SIGs)][sigs] dedicated to its implementation. Joining
+these SIGs offers a direct avenue to understand the current status of
+development, participate in discussions, and identify opportunities to
+contribute. Whether it's through code contributions, documentation enhancements,
+or simply sharing your experiences, every contribution helps to advance the
+declarative configuration ecosystem. Your active participation is key to
+fostering a robust and versatile set of tools for modern application
+development.
+
+We hope to hear from you!
+
+## Additional resources
+
+To learn more about the work going on for declarative configuration, here are
+some additional resources to explore:
+
+- [Simplifying OpenTelemetry with Configuration - Alex Boten, Honeycomb & Jack
+  Berg, New Relic][yt-config]
+- [Declarative configuration documentation](/docs/languages/sdk-configuration/declarative-configuration/)
+- [Declarative configuration repository][declarative-repo]
+
+[drop-spans-issue]:
+  https://github.com/open-telemetry/opentelemetry-java-instrumentation/issues/1060
+[java-project]: https://github.com/orgs/open-telemetry/projects/151
+[migration-file]:
+  https://github.com/open-telemetry/opentelemetry-configuration/blob/main/examples/sdk-migration-config.yaml
+[full-file]:
+  https://github.com/open-telemetry/opentelemetry-configuration/blob/main/examples/kitchen-sink.yaml
+[java-sampler]:
+  https://github.com/open-telemetry/opentelemetry-java-contrib/tree/main/samplers
+[complete-config]:
+  https://gist.github.com/zeitlinger/09585b1ab57c454f87e6dcb9a6f50a5c
+[declarative-docs]: /docs/languages/sdk-configuration/declarative-configuration
+[compliance-matrix]:
+  https://github.com/open-telemetry/opentelemetry-specification/blob/main/spec-compliance-matrix.md#declarative-configuration
+[java-declarative-config]: /docs/zero-code/java/agent/declarative-configuration/
+[slack-java]: https://cloud-native.slack.com/archives/C014L2KCTE3
+[slack-js]: https://cloud-native.slack.com/archives/C01NL1GRPQR
+[slack-php]: https://cloud-native.slack.com/archives/C01NFPCV44V
+[slack-go]: https://cloud-native.slack.com/archives/C01NPAXACKT
+[slack-config]: https://cloud-native.slack.com/archives/C0476L7UJT1
+[java-bridge]:
+  https://github.com/open-telemetry/opentelemetry-java-instrumentation/tree/main/declarative-config-bridge
+[js-package]:
+  https://github.com/open-telemetry/opentelemetry-js/tree/main/experimental/packages/opentelemetry-configuration
+[php-docs]:
+  https://github.com/open-telemetry/opentelemetry-php/tree/main/src/Config/SDK#initialization-from-configuration-file
+[go-package]:
+  https://github.com/open-telemetry/opentelemetry-go-contrib/tree/main/otelconf
+[go-package-index]: https://pkg.go.dev/go.opentelemetry.io/contrib/otelconf
+[sigs]:
+  https://github.com/open-telemetry/community?tab=readme-ov-file#implementation-sigs
+[yt-config]: https://www.youtube.com/watch?v=u6svjtGpXO4
+[declarative-repo]:
+  https://github.com/open-telemetry/opentelemetry-configuration
+[list-not-supported]:
+  /docs/zero-code/java/agent/declarative-configuration/#not-yet-supported-features