You can monitor many inbound and outbound API calls without editing application source by attaching automatic instrumentation or observing supported Linux workloads with eBPF. That can reveal useful service-boundary activity—such as HTTP or RPC requests and database calls—but it cannot guarantee visibility into every call, payload, or business event. Coverage depends on the tool, runtime, operating system, protocols, libraries, and configuration.
What “without changing code” means
Zero-code instrumentation attaches telemetry to an application through an agent or similar component rather than requiring edits to its source. OpenTelemetry describes it this way: “Zero-code instrumentation adds the OpenTelemetry API and SDK capabilities to your application typically as an agent or agent-like installation.” It also notes that this instrumentation typically covers the libraries an application uses. OpenTelemetry’s zero-code instrumentation documentation explains the approach.
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That distinction matters: avoiding source changes is not the same as seeing everything the application does. Automatic capture can expose activity at supported library and service boundaries, while application-specific meaning may remain invisible.
Two ways to capture calls without source edits
Language agents and automatic instrumentation
An agent can attach to a runtime and instrument supported libraries automatically. OpenTelemetry documents mechanisms that vary by language and can include bytecode manipulation, monkey patching, or eBPF. Its zero-code page lists automatic instrumentation for .NET, Go, Java, JavaScript, PHP, and Python; that list is not a guarantee for every version, library, or deployment.
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When the libraries and configuration are supported, this approach can record service requests and responses, database calls, and message-queue activity. See the OpenTelemetry zero-code overview for its stated scope.
eBPF observation at the operating-system level
eBPF can observe supported Linux workloads from outside the application’s source code. OpenTelemetry eBPF Instrumentation (OBI) says it observes application executables and the operating system networking layer, and can capture supported traces, RED metrics, runtime metrics, and relationships between applications and networks without code or configuration changes. Its documented protocol coverage includes HTTP/S, HTTP/2, gRPC, Kafka, NATS, MQTT, PostgreSQL, MySQL, MSSQL, Redis, and others. Coverage is specific to OBI’s documented support, not a promise that any eBPF tool can interpret every workload.
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OBI’s documentation also says eBPF observation cannot always recover application-specific details and does not replace language-level instrumentation in every scenario. Review its capabilities and requirements before assuming your kernel, platform, protocols, or drivers are covered.
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What you can expect to see—and what may remain hidden
- Inbound calls: supported server-side transactions such as HTTP, HTTP/2, or gRPC requests, depending on instrumentation support.
- Outbound dependencies: supported client calls to APIs, databases, message systems, or other services.
- Network facts: connection or traffic activity visible at the operating-system layer. This is not necessarily equivalent to seeing complete request bodies or business context.
- Application-specific meaning: custom spans, business events, and domain-specific attributes generally require application code or another explicit source of context.
Do not treat “every API” as a technical guarantee. An automatic tool can only capture what its supported instrumentation points can observe and interpret. For example, an observed connection may identify traffic between services without explaining which customer action or internal business event caused it.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Zero-code versus code-based instrumentation
OpenTelemetry describes zero-code methods and code-based APIs or SDKs as complementary approaches. Zero-code can help teams instrument existing applications or situations where source changes are impractical; code-based instrumentation can add detail from inside the application. These are operational trade-offs, not benchmark results.
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| Consideration | Zero-code or eBPF approach | Code-based instrumentation |
|---|---|---|
| Source changes | Can avoid application-source edits for supported automatic coverage. | Uses APIs or SDKs in application code. |
| Detail | Best suited to supported libraries, protocols, and runtime or network boundaries. | Can add custom spans, application-specific attributes, and business events. |
| Compatibility | Depends on language, runtime, operating system or kernel, protocol, libraries, and tool support. | Depends on SDK and library support, plus the instrumentation the team implements. |
| Operational fit | Useful for existing applications, broad rollout, or cases where source edits are impractical. | Useful when teams need domain-specific context and control. |
| Combined use | Can provide baseline visibility across supported activity. | Can add application context where automatic capture is insufficient. |
See OpenTelemetry’s instrumentation overview for its comparison of the two methods. The OpenTelemetry documentation stated that the project was supported by more than 90 observability vendors in 2025; that is a dated figure, not a live count. OpenTelemetry documentation carries the framework overview.
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Check these points before deploying
- Confirm the environment: check the exact language and runtime versions, operating system or kernel, container or orchestration setup, and any tool-specific requirements.
- Map the traffic you need: list inbound and outbound protocols, databases, message systems, and drivers, then compare them with the tool’s documented support.
- Choose the telemetry destination: verify where traces, metrics, or other collected data will be sent and whether the destination accepts the format your instrumentation exports.
- Review data handling: determine what metadata or traffic details are collected, who can access them, and how they are retained. Network observation does not by itself establish that complete encrypted payload content is visible.
- Decide whether automatic detail is enough: if you need custom spans, business events, or attributes that libraries cannot infer, plan to add code-based instrumentation alongside the automatic baseline.
- Assess overhead in your own setup: do not assume instrumentation has zero cost. OBI’s export documentation warns that collecting every TCP send and receive call can have higher overhead than other statistics features; the actual impact depends on configuration and workload. See OBI’s data export guidance.
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