feat: Health handling (#70)

Author: thced

docs/superpowers/plans/2026-05-20-health-handler.md

@@ -0,0 +1,186 @@
+# Health handler
+
+**Date:** 2026-05-20
+**Status:** Design — ready for implementation plan
+
+## Problem
+
+Services built on this library need a `/health` endpoint that reports
+overall health plus per-dependency status. The expected wire format is:
+
+```json
+{
+  "outcome": "Up",
+  "dependencies": [
+    { "id": "jdbc", "status": "Up" }
+  ]
+}
+```
+
+We want a ready-to-use `HttpHandler` in this repo that produces that exact
+shape. The handler must not depend on any specific health-check provider —
+callers supply the data through a `Supplier`, so they can plug in whatever
+mechanism (off-the-shelf or in-house) computes their dependency statuses.
+
+## Goals
+
+1. Add `Handlers.healthHandler(Supplier<HealthOutcome>)` that:
+    - Accepts GET and HEAD only (405 otherwise, with `Allow: GET, HEAD`).
+    - Returns `200 OK` with `Content-Type: application/json` when `outcome` is `Up`.
+    - Returns `503 Service Unavailable` with the same body shape when `outcome` is `Down`.
+    - Never propagates a probe failure as a 500 — a throwing `Supplier` yields
+      `Down` + empty dependency list + 503.
+2. Define small public records `HealthOutcome` and `Dependency` in
+    `com.retailsvc.http` that own the wire shape, so this library has no
+    runtime or compile-time dependency on any specific health-check provider.
+3. Reuse existing infrastructure (`MethodLimitedHandler`, hand-rolled
+    JSON rendering á la `ProblemDetailRenderer`). No new third-party deps.
+
+## Non-goals
+
+- Bundling or running health checks. Callers compute their own outcome
+  (typically by adapting whatever check-runner they use into a
+  `HealthOutcome`) and pass it in via the `Supplier`.
+- Caching of probe results. If a caller's checks are expensive, they
+  memoize on their side of the `Supplier`.
+- A configurable wire format — the field names `outcome`, `dependencies`,
+  `id`, `status` and the string values `Up` / `Down` are fixed.
+- Configurable Content-Type or status codes — fixed at `application/json` +
+  200/503.
+- An integration test — unit coverage is sufficient; `MethodLimitedHandler`
+  itself is already integration-tested elsewhere.
+
+## Design
+
+### Public types — `com.retailsvc.http`
+
+```java
+public record HealthOutcome(String outcome, List<Dependency> dependencies) {
+  public HealthOutcome {
+    Objects.requireNonNull(outcome, "outcome");
+    dependencies = List.copyOf(Objects.requireNonNullElse(dependencies, List.of()));
+  }
+
+  public boolean isUp() {
+    return "Up".equalsIgnoreCase(outcome);
+  }
+}
+
+public record Dependency(String id, String status) {
+  public Dependency {
+    Objects.requireNonNull(id, "id");
+    Objects.requireNonNull(status, "status");
+  }
+}
+```
+
+`HealthOutcome.isUp()` is case-insensitive so callers that pass `"Up"`,
+`"UP"`, or `"up"` all map to a healthy 200 response.
+
+### Public API — `Handlers.healthHandler`
+
+```java
+public static HttpHandler healthHandler(Supplier<HealthOutcome> probe) {
+  Objects.requireNonNull(probe, "probe");
+  return new MethodLimitedHandler(exchange -> {
+    try (exchange) {
+      HealthOutcome outcome;
+      try {
+        outcome = probe.get();
+      } catch (RuntimeException e) {
+        LOG.warn("Health probe threw", e);
+        outcome = new HealthOutcome("Down", List.of());
+      }
+      byte[] body = HealthRenderer.toJson(outcome).getBytes(UTF_8);
+      int status = outcome.isUp() ? HTTP_OK : HTTP_UNAVAILABLE;
+      exchange.getResponseHeaders().add("Content-Type", "application/json");
+      exchange.sendResponseHeaders(status, body.length);
+      exchange.getResponseBody().write(body);
+    }
+  });
+}
+```
+
+`HTTP_OK` and `HTTP_UNAVAILABLE` come from `java.net.HttpURLConnection` (per
+project convention — no magic numbers).
+
+### Internal — `com.retailsvc.http.internal.HealthRenderer`
+
+Package-private final class with a private constructor and a single
+`static String toJson(HealthOutcome)` method. Implementation mirrors
+`ProblemDetailRenderer`: hand-rolled `StringBuilder`, manual JSON-string
+escaping for `\\`, `\"`, `\n`, `\r`, `\t`, `\b`, `\f`, and `\uXXXX` for any
+remaining control characters below `0x20`.
+
+### Caller-side wiring (illustrative, not part of this repo)
+
+```java
+server = OpenApiServer.builder()
+    .spec(spec)
+    .jsonMapper(mapper)
+    .handlers(operationHandlers)
+    .addHandler("/health", Handlers.healthHandler(() -> {
+      // Caller computes per-dependency statuses however they choose
+      // and adapts the result into HealthOutcome / Dependency.
+      List<Dependency> deps = List.of(
+          new Dependency("jdbc", checkDatabase() ? "Up" : "Down"),
+          new Dependency("cache", checkCache() ? "Up" : "Down"));
+      String outcome = deps.stream().allMatch(d -> "Up".equalsIgnoreCase(d.status()))
+          ? "Up" : "Down";
+      return new HealthOutcome(outcome, deps);
+    }))
+    .build();
+```
+
+The `Supplier` is the only place that knows how to compute health, which is
+exactly where any third-party integration belongs.
+
+## Error handling
+
+Health endpoints should never 500 — load balancers and orchestrators interpret
+5xx-from-health-probe as "treat instance as unhealthy" only some of the time;
+a 503 with a `Down` body is the unambiguous signal. The handler therefore
+funnels every probe failure into the same `Down`+503 path:
+
+- `Supplier` throws `RuntimeException`: caught; logged at `warn`; rendered
+  as `Down` with empty dependency list and 503.
+- `Supplier` returns `null`: treated identically to a throwing probe —
+  logged at `warn` and rendered as `Down`+503. (The handler asserts
+  non-null via an explicit check inside the same `try` block.)
+- IOException while writing the response: not caught here; `ExceptionFilter`
+  handles it (this is a transport-level failure, not a probe failure).
+
+## Testing
+
+Unit tests only (Surefire). New file `HealthHandlerTest` (or extension of
+`HandlersTest`):
+
+- GET, `Up` outcome with dependencies → 200, `application/json`, body
+  equals expected JSON (parsed back via Jackson in test scope, asserted
+  field by field).
+- GET, `Up` outcome with empty dependency list → 200, body has empty
+  array.
+- GET, `Down` outcome → 503, body still rendered.
+- HEAD → status code only, no body bytes.
+- POST → 405 with `Allow: GET, HEAD` header.
+- Probe throws `RuntimeException` → 503, body `{"outcome":"Down","dependencies":[]}`.
+- Probe returns `null` → 503, body `{"outcome":"Down","dependencies":[]}`
+  (same behaviour as a throwing probe).
+
+New file `HealthRendererTest`:
+
+- Round-trip outcomes through Jackson to confirm valid JSON.
+- Strings containing `"`, `\`, newline, tab, control char `
` are
+  escaped correctly.
+
+Records `HealthOutcome` and `Dependency` get tiny tests for null/empty
+argument validation and (`HealthOutcome` only) `isUp()` case-insensitivity.
+
+## Out of scope / future
+
+- Wiring the handler into `ServerLauncher` (the example launcher) — not
+  needed; the launcher exists for local development of the OpenAPI flow.
+- A second `healthHandler` overload that takes a `Callable` or
+  `CompletionStage` — no concrete need yet.
+- An integration test that exercises the handler through `OpenApiServer`
+  end-to-end.

docs/superpowers/specs/2026-05-20-health-handler-design.md

@@ -0,0 +1,18 @@
+package com.retailsvc.http;
+
+import java.util.Objects;
+
+/**
+ * A single dependency entry within a {@link HealthOutcome}.
+ *
+ * <p>The library translates {@code up} into the wire value {@code "Up"} or {@code "Down"} for the
+ * {@code status} field.
+ *
+ * @param id stable identifier of the dependency (e.g. {@code "jdbc"})
+ * @param up whether the dependency is healthy
+ */
+public record Dependency(String id, boolean up) {
+  public Dependency {
+    Objects.requireNonNull(id, "id");
+  }
+}

src/main/java/com/retailsvc/http/Dependency.java

@@ -5,13 +5,18 @@
 import static java.net.HttpURLConnection.HTTP_INTERNAL_ERROR;
 import static java.net.HttpURLConnection.HTTP_NOT_FOUND;
 import static java.net.HttpURLConnection.HTTP_NO_CONTENT;
+import static java.net.HttpURLConnection.HTTP_OK;
+import static java.net.HttpURLConnection.HTTP_UNAVAILABLE;
 import static java.nio.charset.StandardCharsets.UTF_8;
 
 import com.retailsvc.http.internal.ClasspathResourceHandler;
 import com.retailsvc.http.internal.MethodLimitedHandler;
 import com.retailsvc.http.internal.ProblemDetailRenderer;
 import com.sun.net.httpserver.HttpHandler;
 import java.io.IOException;
+import java.util.List;
+import java.util.Objects;
+import java.util.function.Supplier;
 import java.util.stream.Collectors;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -67,6 +72,67 @@ public static HttpHandler aliveHandler() {
         });
   }
 
+  /**
+   * Health endpoint handler. Accepts GET and HEAD; returns 200 with {@code application/json} body
+   * when the supplied probe reports {@code up == true}, and 503 with the same body shape otherwise.
+   * A probe that throws a {@link RuntimeException} or returns {@code null} is mapped to a {@code
+   * Down} outcome with an empty dependency list (and 503); the failure is never propagated to the
+   * default exception handler.
+   *
+   * <p>The wire shape is
+   *
+   * <pre>{@code
+   * {"outcome":"Up","dependencies":[{"id":"jdbc","status":"Up"}]}
+   * }</pre>
+   *
+   * <p>Serialisation is delegated to the supplied {@code jsonMapper} — typically the same {@link
+   * TypeMapper} the caller registered for {@code application/json} on the server. The handler hands
+   * the mapper a record-shaped DTO with the components in the order shown above; any standard JSON
+   * library (Gson, Jackson, …) serialises it identically.
+   *
+   * @param jsonMapper used to encode the wire-shape DTO to bytes
+   * @param probe supplier of the current {@link HealthOutcome}
+   */
+  public static HttpHandler healthHandler(TypeMapper jsonMapper, Supplier<HealthOutcome> probe) {
+    Objects.requireNonNull(jsonMapper, "jsonMapper");
+    Objects.requireNonNull(probe, "probe");
+    return new MethodLimitedHandler(
+        exchange -> {
+          try (exchange) {
+            HealthOutcome outcome;
+            try {
+              outcome = Objects.requireNonNull(probe.get(), "Health probe returned null");
+            } catch (RuntimeException e) {
+              LOG.warn("Health probe failed", e);
+              outcome = new HealthOutcome(false, List.of());
+            }
+            byte[] body = jsonMapper.writeTo(toWireShape(outcome));
+            int status = outcome.up() ? HTTP_OK : HTTP_UNAVAILABLE;
+            exchange.getResponseHeaders().add("Content-Type", "application/json");
+            exchange.sendResponseHeaders(status, body.length);
+            exchange.getResponseBody().write(body);
+          }
+        });
+  }
+
+  private static HealthBody toWireShape(HealthOutcome outcome) {
+    return new HealthBody(
+        label(outcome.up()),
+        outcome.dependencies().stream()
+            .map(d -> new DependencyBody(d.id(), label(d.up())))
+            .toList());
+  }
+
+  private static String label(boolean up) {
+    return up ? "Up" : "Down";
+  }
+
+  /** Wire-shape DTO for the health endpoint. Component order defines JSON field order. */
+  private record HealthBody(String outcome, List<DependencyBody> dependencies) {}
+
+  /** Wire-shape DTO for a single dependency entry. */
+  private record DependencyBody(String id, String status) {}
+
   /**
    * Serves a classpath resource. Content-Type is inferred from the file extension. The resource is
    * loaded eagerly; a missing resource fails immediately with {@link IllegalArgumentException}.

src/main/java/com/retailsvc/http/Handlers.java

@@ -0,0 +1,22 @@
+package com.retailsvc.http;
+
+import java.util.List;
+import java.util.Objects;
+
+/**
+ * Carrier for the {@link Handlers#healthHandler health handler} response.
+ *
+ * <p>The library translates {@code up} into the wire value {@code "Up"} or {@code "Down"} on the
+ * way out; callers work in booleans. Construct the outcome from whatever check-running mechanism
+ * the caller prefers — this library has no opinion.
+ *
+ * @param up overall health — {@code true} renders as {@code "Up"} with HTTP 200; {@code false}
+ *     renders as {@code "Down"} with HTTP 503
+ * @param dependencies per-dependency statuses; {@code null} is normalised to an empty list
+ */
+public record HealthOutcome(boolean up, List<Dependency> dependencies) {
+
+  public HealthOutcome {
+    dependencies = List.copyOf(Objects.requireNonNullElse(dependencies, List.of()));
+  }
+}

src/main/java/com/retailsvc/http/HealthOutcome.java

@@ -0,0 +1,23 @@
+package com.retailsvc.http;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatNullPointerException;
+
+import org.junit.jupiter.api.Test;
+
+class DependencyTest {
+
+  @Test
+  void holdsIdAndUp() {
+    Dependency d = new Dependency("jdbc", true);
+    assertThat(d.id()).isEqualTo("jdbc");
+    assertThat(d.up()).isTrue();
+  }
+
+  @Test
+  void rejectsNullId() {
+    assertThatNullPointerException()
+        .isThrownBy(() -> new Dependency(null, true))
+        .withMessageContaining("id");
+  }
+}