PrometheusHttpServer prevent concurrent reads when reusable memory mode

exporters/prometheus/src/main/java/io/opentelemetry/exporter/prometheus/PrometheusHttpServer.java

@@ -23,6 +23,7 @@
 import java.io.UncheckedIOException;
 import java.net.InetSocketAddress;
 import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
 import java.util.concurrent.TimeUnit;
 import java.util.function.Predicate;
 import javax.annotation.Nullable;
@@ -70,6 +71,12 @@ public static PrometheusHttpServerBuilder builder() {
     this.memoryMode = memoryMode;
     this.prometheusRegistry = prometheusRegistry;
     prometheusRegistry.register(prometheusMetricReader);
+    // When memory mode is REUSABLE_DATA, concurrent reads lead to data corruption. To prevent this,
+    // we configure prometheus with a single thread executor such that requests are handled
+    // sequentially.
+    if (memoryMode == MemoryMode.REUSABLE_DATA) {
+      executor = Executors.newSingleThreadExecutor();
+    }
     try {
       this.httpServer =
           HTTPServer.builder()

exporters/prometheus/src/main/java/io/opentelemetry/exporter/prometheus/PrometheusHttpServerBuilder.java

@@ -11,6 +11,7 @@
 import io.opentelemetry.sdk.common.export.MemoryMode;
 import io.prometheus.metrics.model.registry.PrometheusRegistry;
 import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
 import java.util.function.Predicate;
 import javax.annotation.Nullable;
 
@@ -93,7 +94,13 @@ public PrometheusHttpServerBuilder setAllowedResourceAttributesFilter(
     return this;
   }
 
-  /** Set the {@link MemoryMode}. */
+  /**
+   * Set the {@link MemoryMode}.
+   *
+   * <p>If set to {@link MemoryMode#REUSABLE_DATA}, requests are served sequentially which is
+   * accomplished by overriding {@link #setExecutor(ExecutorService)} to {@link
+   * Executors#newSingleThreadExecutor()}.
+   */
   public PrometheusHttpServerBuilder setMemoryMode(MemoryMode memoryMode) {
     requireNonNull(memoryMode, "memoryMode");
     this.memoryMode = memoryMode;

exporters/prometheus/src/test/java/io/opentelemetry/exporter/prometheus/PrometheusHttpServerTest.java

@@ -24,6 +24,7 @@
 import io.opentelemetry.api.common.Attributes;
 import io.opentelemetry.internal.testing.slf4j.SuppressLogger;
 import io.opentelemetry.sdk.common.InstrumentationScopeInfo;
+import io.opentelemetry.sdk.common.export.MemoryMode;
 import io.opentelemetry.sdk.metrics.data.AggregationTemporality;
 import io.opentelemetry.sdk.metrics.data.MetricData;
 import io.opentelemetry.sdk.metrics.export.CollectionRegistration;
@@ -39,6 +40,7 @@
 import java.io.IOException;
 import java.net.ServerSocket;
 import java.nio.charset.StandardCharsets;
+import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Collections;
 import java.util.List;
@@ -47,6 +49,7 @@
 import java.util.concurrent.ScheduledExecutorService;
 import java.util.concurrent.ScheduledThreadPoolExecutor;
 import java.util.concurrent.ThreadPoolExecutor;
+import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicReference;
 import java.util.function.Predicate;
 import java.util.zip.GZIPInputStream;
@@ -129,6 +132,68 @@ void fetchPrometheus() {
                 + "target_info{kr=\"vr\"} 1\n");
   }
 
+  @Test
+  void fetch_ReusableMemoryMode() throws InterruptedException {
+    try (PrometheusHttpServer prometheusServer =
+        PrometheusHttpServer.builder()
+            .setHost("localhost")
+            .setPort(0)
+            .setMemoryMode(MemoryMode.REUSABLE_DATA)
+            .build()) {
+      AtomicBoolean collectInProgress = new AtomicBoolean();
+      AtomicBoolean concurrentRead = new AtomicBoolean();
+      prometheusServer.register(
+          new CollectionRegistration() {
+            @Override
+            public Collection<MetricData> collectAllMetrics() {
+              if (!collectInProgress.compareAndSet(false, true)) {
+                concurrentRead.set(true);
+              }
+              Collection<MetricData> response = metricData.get();
+              try {
+                Thread.sleep(1);
+              } catch (InterruptedException e) {
+                throw new RuntimeException(e);
+              }
+              if (!collectInProgress.compareAndSet(true, false)) {
+                concurrentRead.set(true);
+              }
+              return response;
+            }
+          });
+
+      WebClient client =
+          WebClient.builder("http://localhost:" + prometheusServer.getAddress().getPort())
+              .decorator(RetryingClient.newDecorator(RetryRule.failsafe()))
+              .build();
+
+      // Spin up 4 threads calling /metrics simultaneously. If concurrent read happens,
+      // collectAllMetrics will set concurrentRead to true and the test will fail.
+      List<Thread> threads = new ArrayList<>();
+      for (int i = 0; i < 4; i++) {
+        Thread thread =
+            new Thread(
+                () -> {
+                  for (int j = 0; j < 10; j++) {
+                    AggregatedHttpResponse response = client.get("/metrics").aggregate().join();
+                    assertThat(response.status()).isEqualTo(HttpStatus.OK);
+                  }
+                });
+        thread.setDaemon(true);
+        thread.start();
+        threads.add(thread);
+      }
+
+      // Wait for threads to complete
+      for (Thread thread : threads) {
+        thread.join();
+      }
+
+      // Confirm no concurrent reads took place
+      assertThat(concurrentRead.get()).isFalse();
+    }
+  }
+
   @Test
   void fetchOpenMetrics() {
     AggregatedHttpResponse response =