fix: snapshot entity metrics on main thread

Reviewed-on: #3

Dockerfile

@@ -1,4 +1,4 @@
-FROM gradle:8.6-jdk21
+FROM gradle:8.14-jdk21
 
 WORKDIR /workspace
 

README.md

@@ -51,6 +51,7 @@ http:
 metrics:
   enable_jvm: true
   enable_process: true
+  entity_snapshot_interval_seconds: 15
 movement:
   count_block_changes_only: true
 ```
@@ -77,7 +78,7 @@ The Gitea workflow expects these secrets/variables:
 
 ## Metrics
 
-Some metrics depend on server implementations (TPS/MSPT, player ping, tile entities). If a method is unavailable on your server build, those metrics are omitted.
+Some metrics depend on server implementations (TPS/MSPT, player ping, tile entities). If a method is unavailable on your server build, those metrics are omitted. Entity and tile counts are updated on a periodic main-thread snapshot.
 
 | Metric | Type | Labels | Description |
 | --- | --- | --- | --- |

src/main/java/com/prometheus/spigot/PrometheusSpigotPlugin.java

@@ -17,7 +17,9 @@ import java.net.InetSocketAddress;
 import java.util.Locale;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
+import org.bukkit.Bukkit;
 import org.bukkit.plugin.java.JavaPlugin;
+import org.bukkit.scheduler.BukkitTask;
 
 public final class PrometheusSpigotPlugin extends JavaPlugin {
     private HttpServer httpServer;
@@ -25,6 +27,7 @@ public final class PrometheusSpigotPlugin extends JavaPlugin {
     private MetricsRegistry metricsRegistry;
     private String bearerToken;
     private boolean countBlockMovementOnly;
+    private BukkitTask snapshotTask;
 
     @Override
     public void onEnable() {
@@ -43,6 +46,8 @@ public final class PrometheusSpigotPlugin extends JavaPlugin {
 
         metricsRegistry = new MetricsRegistry(getServer(), enableJvm, enableProcess);
 
+        scheduleSnapshotTask();
+
         registerListeners();
         startHttpServer();
     }
@@ -57,6 +62,10 @@ public final class PrometheusSpigotPlugin extends JavaPlugin {
             httpExecutor.shutdownNow();
             httpExecutor = null;
         }
+        if (snapshotTask != null) {
+            snapshotTask.cancel();
+            snapshotTask = null;
+        }
     }
 
     private void registerListeners() {
@@ -95,4 +104,12 @@ public final class PrometheusSpigotPlugin extends JavaPlugin {
         httpServer.start();
         getLogger().info(String.format(Locale.ROOT, "Prometheus endpoint listening on http://%s:%d%s", bindAddress, port, path));
     }
+
+    private void scheduleSnapshotTask() {
+        int intervalSeconds = getConfig().getInt("metrics.entity_snapshot_interval_seconds", 15);
+        long intervalTicks = Math.max(20L, intervalSeconds * 20L);
+
+        metricsRegistry.refreshEntitySnapshots();
+        snapshotTask = Bukkit.getScheduler().runTaskTimer(this, metricsRegistry::refreshEntitySnapshots, intervalTicks, intervalTicks);
+    }
 }

src/main/java/com/prometheus/spigot/metrics/MetricsRegistry.java

@@ -6,6 +6,7 @@ import java.lang.management.MemoryUsage;
 import java.lang.management.ThreadMXBean;
 import java.lang.reflect.Method;
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.Comparator;
 import java.util.HashMap;
 import java.util.List;
@@ -31,6 +32,8 @@ public final class MetricsRegistry {
     private final Method serverMsptMethod;
     private final Method playerPingMethod;
     private final Method chunkTileEntitiesMethod;
+    private volatile Map<String, Long> entityCountsSnapshot = Collections.emptyMap();
+    private volatile Map<String, Long> tileEntityCountsSnapshot = Collections.emptyMap();
 
     private final LongAdder chatMessages = new LongAdder();
     private final LongAdder playerMovements = new LongAdder();
@@ -120,6 +123,37 @@ public final class MetricsRegistry {
         pluginDisables.increment();
     }
 
+    public void refreshEntitySnapshots() {
+        Map<String, Long> entityCounts = new HashMap<>();
+        Map<String, Long> tileCounts = new HashMap<>();
+
+        for (var world : server.getWorlds()) {
+            for (Entity entity : world.getEntities()) {
+                EntityType type = entity.getType();
+                String label = normalizeLabel(type.name());
+                entityCounts.merge(label, 1L, Long::sum);
+            }
+
+            if (chunkTileEntitiesMethod != null) {
+                for (Chunk chunk : world.getLoadedChunks()) {
+                    Object result = invoke(chunkTileEntitiesMethod, chunk);
+                    if (!(result instanceof Object[] states)) {
+                        continue;
+                    }
+                    for (Object state : states) {
+                        if (state instanceof BlockState blockState) {
+                            String label = normalizeLabel(blockState.getType().name());
+                            tileCounts.merge(label, 1L, Long::sum);
+                        }
+                    }
+                }
+            }
+        }
+
+        entityCountsSnapshot = Collections.unmodifiableMap(entityCounts);
+        tileEntityCountsSnapshot = Collections.unmodifiableMap(tileCounts);
+    }
+
     public String render() {
         StringBuilder builder = new StringBuilder();
 
@@ -226,37 +260,14 @@ public final class MetricsRegistry {
     }
 
     private void appendEntityMetrics(StringBuilder builder) {
-        Map<String, Long> entityCounts = new HashMap<>();
+        appendLabeledGauge(builder, "spigot_entities_total", "Entities by type", "type", entityCountsSnapshot);
-        for (var world : server.getWorlds()) {
-            for (Entity entity : world.getEntities()) {
-                EntityType type = entity.getType();
-                String label = normalizeLabel(type.name());
-                entityCounts.merge(label, 1L, Long::sum);
-            }
-        }
-        appendLabeledGauge(builder, "spigot_entities_total", "Entities by type", "type", entityCounts);
     }
 
     private void appendTileEntityMetrics(StringBuilder builder) {
         if (chunkTileEntitiesMethod == null) {
             return;
         }
-        Map<String, Long> tileCounts = new HashMap<>();
+        appendLabeledGauge(builder, "spigot_tile_entities_total", "Tile entities by type", "type", tileEntityCountsSnapshot);
-        for (var world : server.getWorlds()) {
-            for (Chunk chunk : world.getLoadedChunks()) {
-                Object result = invoke(chunkTileEntitiesMethod, chunk);
-                if (!(result instanceof Object[] states)) {
-                    continue;
-                }
-                for (Object state : states) {
-                    if (state instanceof BlockState blockState) {
-                        String label = normalizeLabel(blockState.getType().name());
-                        tileCounts.merge(label, 1L, Long::sum);
-                    }
-                }
-            }
-        }
-        appendLabeledGauge(builder, "spigot_tile_entities_total", "Tile entities by type", "type", tileCounts);
     }
 
     private void appendPlayerPingMetrics(StringBuilder builder) {

src/main/resources/config.yml

@@ -6,5 +6,6 @@ http:
 metrics:
   enable_jvm: true
   enable_process: true
+  entity_snapshot_interval_seconds: 15
 movement:
   count_block_changes_only: true