Add structured log messages to CMAP.

driver-core/src/main/com/mongodb/internal/connection/InternalStreamConnection.java

@@ -87,7 +87,6 @@
 import static com.mongodb.internal.connection.ProtocolHelper.getSnapshotTimestamp;
 import static com.mongodb.internal.connection.ProtocolHelper.isCommandOk;
 import static com.mongodb.internal.logging.StructuredLogMessage.Level.DEBUG;
-import static java.lang.String.format;
 import static java.util.Arrays.asList;
 
 /**
@@ -289,9 +288,6 @@ private void initAfterHandshakeFinish(final InternalConnectionInitializationDesc
         initialServerDescription = initializationDescription.getServerDescription();
         opened.set(true);
         sendCompressor = findSendCompressor(description);
-        if (LOGGER.isDebugEnabled()) {
-            LOGGER.debug(format("Opened connection [%s] to %s", getId(), serverId.getAddress()));
-        }
     }
 
     private Map<Byte, Compressor> createCompressorMap(final List<MongoCompressor> compressorList) {
@@ -337,13 +333,8 @@ private Compressor createCompressor(final MongoCompressor mongoCompressor) {
     @Override
     public void close() {
         // All but the first call is a no-op
-        if (!isClosed.getAndSet(true)) {
-            if (LOGGER.isDebugEnabled()) {
-                LOGGER.debug(format("Closing connection %s", getId()));
-            }
-            if (stream != null) {
+        if (!isClosed.getAndSet(true) && (stream != null)) {
                 stream.close();
-            }
         }
     }
 
@@ -655,9 +646,6 @@ public void receiveMessageAsync(final int responseTo, final SingleResultCallback
             return;
         }
 
-        if (LOGGER.isTraceEnabled()) {
-            LOGGER.trace(format("Start receiving response on %s", getId()));
-        }
         readAsync(MESSAGE_HEADER_LENGTH, new MessageHeaderCallback((result, t) -> {
             if (t != null) {
                 close();

driver-core/src/main/com/mongodb/internal/event/EventReasonMessageResolver.java

@@ -0,0 +1,59 @@
+/*
+ * Copyright 2008-present MongoDB, Inc.
+ *
+ * Licensed 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 com.mongodb.internal.event;
+
+import com.mongodb.event.ConnectionCheckOutFailedEvent;
+import com.mongodb.event.ConnectionClosedEvent;
+
+/**
+ * <p>This class is not part of the public API and may be removed or changed at any time</p>
+ */
+public final class EventReasonMessageResolver {
+    private static final String MESSAGE_CONNECTION_POOL_WAS_CLOSED = "Connection pool was closed";
+
+    public static String getMessage(final ConnectionClosedEvent.Reason reason) {
+        switch (reason) {
+            case STALE:
+                return "Connection became stale because the pool was cleared";
+            case IDLE:
+                return "Connection has been available but unused for longer than the configured max idle time";
+            case ERROR:
+                return "An error occurred while using the connection";
+            case POOL_CLOSED:
+                return MESSAGE_CONNECTION_POOL_WAS_CLOSED;
+            default:
+                return "";
+        }
+    }
+
+    public static String getMessage(final ConnectionCheckOutFailedEvent.Reason reason) {
+        switch (reason) {
+            case TIMEOUT:
+                return "Wait queue timeout elapsed without a connection becoming available";
+            case CONNECTION_ERROR:
+                return "An error occurred while trying to establish a new connection";
+            case POOL_CLOSED:
+                return MESSAGE_CONNECTION_POOL_WAS_CLOSED;
+            default:
+                return "";
+        }
+    }
+
+    private EventReasonMessageResolver() {
+        //NOP
+    }
+}

driver-core/src/main/com/mongodb/internal/logging/StructuredLogMessage.java

@@ -34,14 +34,17 @@ public final class StructuredLogMessage {
     private final List<Entry> entries;
 
     public enum Component {
-        COMMAND
+        COMMAND,
+        CONNECTION
     }
 
     public enum Level {
         DEBUG
     }
 
     public static final class Entry {
+        public static final String NAME_SERVER_HOST = "serverHost";
+        public static final String NAME_SERVER_PORT = "serverPort";
         private final String name;
         private final Object value;