dsmiley commented on code in PR #1868:
URL: https://github.com/apache/solr/pull/1868#discussion_r1310608641


##########
solr/core/src/java/org/apache/solr/util/tracing/SimplePropagator.java:
##########
@@ -0,0 +1,304 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.solr.util.tracing;
+
+import io.opentracing.Scope;
+import io.opentracing.ScopeManager;
+import io.opentracing.Span;
+import io.opentracing.SpanContext;
+import io.opentracing.Tracer;
+import io.opentracing.Tracer.SpanBuilder;
+import io.opentracing.propagation.Format;
+import io.opentracing.propagation.TextMap;
+import io.opentracing.tag.Tag;
+import io.opentracing.util.GlobalTracer;
+import io.opentracing.util.ThreadLocalScopeManager;
+import java.lang.invoke.MethodHandles;
+import java.util.Collections;
+import java.util.Iterator;
+import java.util.Map;
+import java.util.Map.Entry;
+import java.util.concurrent.atomic.AtomicLong;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+public class SimplePropagator {
+  private static final Logger log = 
LoggerFactory.getLogger(MethodHandles.lookup().lookupClass());
+
+  private static final String TRACE_HOST_NAME =
+      System.getProperty("trace_host_name", System.getProperty("host"));
+
+  static final String TRACE_ID = System.getProperty("TRACE_ID", "X-Trace-Id");
+
+  private static final AtomicLong traceCounter = new AtomicLong(0);
+
+  public static void load() {
+    GlobalTracer.registerIfAbsent(
+        () -> {
+          log.info("Always-on trace id generation enabled.");
+          return new SimplePropagatorTracer();
+        });
+  }
+
+  /**
+   * Tracer that only aims to do simple header propagation, tailored to how 
Solr works.
+   *
+   * <p>Heavily inspired from JaegerTracer, NoopTracer
+   */
+  static class SimplePropagatorTracer implements Tracer {
+
+    private final ScopeManager scopeManager = new ThreadLocalScopeManager();
+
+    @Override
+    public ScopeManager scopeManager() {
+      return scopeManager;
+    }
+
+    @Override
+    public Span activeSpan() {
+      return scopeManager.activeSpan();
+    }
+
+    @Override
+    public Scope activateSpan(Span span) {
+      return scopeManager.activate(span);
+    }
+
+    @Override
+    public SpanBuilder buildSpan(String operationName) {
+      return new SimplePropagatorSpanBuilder(scopeManager);
+    }
+
+    @Override
+    public void close() {}
+
+    @Override
+    public <C> void inject(SpanContext spanContext, Format<C> format, C 
carrier) {
+      if (!format.equals(Format.Builtin.HTTP_HEADERS)) {
+        throw new IllegalArgumentException("unsupported format " + format);
+      }
+      String traceId = spanContext.toTraceId();
+      if (traceId != null && !traceId.isEmpty()) {
+        TextMap tm = (TextMap) carrier;
+        tm.put(TRACE_ID, traceId);
+      }
+    }
+
+    @Override
+    public <C> SpanContext extract(Format<C> format, C carrier) {
+      if (!format.equals(Format.Builtin.HTTP_HEADERS)) {
+        throw new IllegalArgumentException("unsupported format " + format);
+      }
+      String traceId = null;
+      TextMap tm = (TextMap) carrier;
+      Iterator<Entry<String, String>> it = tm.iterator();
+      while (it.hasNext()) {
+        var e = it.next();
+        // 'equalsIgnoreCase' because header name might be lowercase
+        if (e.getKey().equalsIgnoreCase(TRACE_ID)) {
+          traceId = e.getValue();
+          break;
+        }
+      }
+      if (traceId == null) {
+        traceId = newTraceId();
+      }
+      return new SimplePropagatorSpanContext(traceId);
+    }
+
+    @Override
+    public String toString() {
+      return "SimplePropagator";
+    }
+  }
+
+  private static final class SimplePropagatorSpanBuilder implements 
SpanBuilder {
+
+    private final ScopeManager scopeManager;
+    // storing parent to support the parent being injected via the asChildOf 
method
+    private SpanContext parent;
+
+    public SimplePropagatorSpanBuilder(ScopeManager scopeManager) {
+      this.scopeManager = scopeManager;
+    }
+
+    @Override
+    public Span start() {
+      if (parent != null) {
+        return new SimplePropagatorSpan(parent);
+      } else if (scopeManager.activeSpan() != null) {
+        return new SimplePropagatorSpan(scopeManager.activeSpan().context());

Review Comment:
   This is going to be the common case as more spans show up _within_ Solr.  
Could SimplePropagatorSpan and SimplePropagatorSpanContext be combined into 
one?  Create no new object; the context could *be* the span if all we are doing 
is propagation.
   
   I even wonder if we need a distinct builder?



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