=======
Logging
=======

You can change the log level, filter on classes to allow only specific classes
to log, and provide your own logger implementation.

Setting Log level
-----------------

The driver allows logging at three different levels: ``debug``,
``info`` and ``error``. The default level is ``error``.
The following example demonstrates how to set the logger to ``debug``.

.. code-block:: js

   const { MongoClient, Logger } = require('mongodb');

   // Connection URL
   const url = 'mongodb://localhost:27017';
   // Database Name
   const dbName = 'myproject';

   const client = new MongoClient(url);

   async function main(client) {
     // Set debug level
     Logger.setLevel('debug');

     const db = client.db('myproject');

     // Execute command { isMaster: true } against db
     await db.command({ isMaster: true });
   }

   // Function to connect to the server and run your code
   async function run() {
     try {
       // Connect the client to the server
       await client.connect();
       console.log('Connected successfully to server');

       await main(client);
     } finally {
       // Ensures that the client will close when you finish/error
       await client.close();
     }
   }
   
   // Runs your code
   run();

Filtering On specific classes
-----------------------------

You can set the Logger to only log specific class names. The following example
demonstrates how to log only the ``Db`` class.

.. code-block:: js

   async function main(client) {
     // Set debug level
     Logger.setLevel('debug');
     // Only log statements on 'Db' class
     Logger.filter('class', ['Db']);

     const db = client.db('myproject');

     // Execute command { isMaster: true } against db
     await db.command({ isMaster: true });
   }

Driver classes available for filtering:


* ``Db``\ : The Db instance log statements
* ``Server``\ : A server instance (either standalone, a mongos or replica set member)
* ``ReplSet``\ : Replica set related log statements
* ``Mongos``\ : Mongos related log statements
* ``Cursor``\ : Cursor log statements
* ``Pool``\ : Connection Pool specific log statements
* ``Connection``\ : Singular connection specific log statements
* ``Ping``\ : Replica set ping inquiry log statements

You can add your own classes to the logger by creating your own logger instances. 

.. code-block:: js

   const { Logger } = require('mongodb');

   class A {
     constructor() {
       this.logger = new Logger('A');
     }

     doSomething() {
       if (this.logger.isInfo()) {
         this.logger.info('logging A', {});
       }
     }
   }

   // Execute A
   const a = new A();
   a.doSomething();

Custom logger
-------------

The following example demonstrates how to define a custom logger.

.. code-block:: js

   async function main(client) {
     // Set debug level
     Logger.setLevel('debug');

     // Set our own logger
     Logger.setCurrentLogger(function(msg, context) {
       console.log(msg, context);
     });

     const db = client.db('myproject');

     // Execute command { isMaster: true } against db
     await db.command({ isMaster: true });
   }