# Logging and observability

If you need visibility into SDK calls, like request/response logging, latency metrics,
trace spans, you can wire up a wrapper around `RatepayClient` that adds that behavior without modifying the SDK itself.

## Wrapping the client for logging/metrics

A logging/observability wrapper is the same pattern applied to production code instead of tests:
wrap a real `RatepayClient` instance, delegate every call to it, and add your cross-cutting behavior
around the delegated call.

`RatepayClient` declares many operations (authorize, capture, refund, getters, OTP, ...), and a Java class that
`implements RatepayClient` must override all of them. The simplest Java approach is a
`java.lang.reflect.Proxy`, which logs every call without listing each method. Kotlin can use `by delegate`
and override only what it needs.


```java Java
import com.ratepay.sdk.*;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Proxy;

public final class LoggingRatepayClient {
    private LoggingRatepayClient() {}

    public static RatepayClient wrap(RatepayClient delegate) {
        return (RatepayClient) Proxy.newProxyInstance(
            RatepayClient.class.getClassLoader(),
            new Class<?>[] {RatepayClient.class},
            (proxy, method, args) -> {
                long start = System.nanoTime();
                try {
                    Object result = method.invoke(delegate, args);
                    log(method.getName(), "OK", start, null);
                    return result;
                } catch (InvocationTargetException e) {
                    log(method.getName(), "EXCEPTION", start, e.getCause());
                    throw e.getCause();
                }
            });
    }

    private static void log(String operation, String result, long startNanos, Throwable error) {
        long elapsedMs = (System.nanoTime() - startNanos) / 1_000_000;
        if (error != null) {
            System.err.printf("ratepay.%s failed after %dms: %s%n", operation, elapsedMs, error);
        } else {
            System.out.printf("ratepay.%s -> %s (%dms)%n", operation, result, elapsedMs);
        }
    }
}
```


```kotlin Kotlin
import com.ratepay.sdk.*
import com.ratepay.sdk.exception.RatepayException
import com.ratepay.sdk.model.api.payment.*

class LoggingRatepayClient(private val delegate: RatepayClient) : RatepayClient by delegate {

    override fun authorize(request: AuthorizationRequest, idempotencyKey: String?): AuthorizeOutcome {
        val start = System.nanoTime()
        try {
            val outcome = delegate.authorize(request, idempotencyKey)
            log("authorize", outcome.kind().toString(), start)
            return outcome
        } catch (e: RatepayException) {
            log("authorize", "EXCEPTION", start, e)
            throw e
        }
    }


    private fun log(operation: String, result: String, startNanos: Long, error: Throwable? = null) {
        val elapsedMs = (System.nanoTime() - startNanos) / 1_000_000
        if (error != null) {
            System.err.println("ratepay.$operation failed after ${elapsedMs}ms: $error")
        } else {
            println("ratepay.$operation -> $result (${elapsedMs}ms)")
        }
    }
}
```

Swap in a real logging framework (SLF4J) or a metrics client (Micrometer) in place of the `System.out`/`System.err` calls shown here.

## Using the wrapper

Build the wrapper once, in the same place you build the underlying client, and use the wrapped instance
everywhere downstream code would otherwise use `RatepayClient` directly:


```java Java
RatepayClient client = LoggingRatepayClient.wrap(
    RatepayClient.builder()
        .clientId(System.getenv("RATEPAY_CLIENT_ID"))
        .clientSecret(System.getenv("RATEPAY_CLIENT_SECRET"))
        .environment(Environment.PRODUCTION)
        .build()
);
```


```kotlin Kotlin
val client: RatepayClient = LoggingRatepayClient(
    RatepayClient {
        clientId(System.getenv("RATEPAY_CLIENT_ID"))
        clientSecret(System.getenv("RATEPAY_CLIENT_SECRET"))
        environment(Environment.PRODUCTION)
    },
)
```