Child loggers

You can create child loggers sharing the Logging Context of their parent logger:

app.get('/', function (req, res) {
    var logger = req.logger.createLogger(); // equivalent to req.createLogger();
    ...
    logger.logMessage("This message is request correlated, but logged with a child logger");
});

Each child logger can have its own set of custom fields, which will be added to each messages:

var logger = req.logger.createLogger(); 
logger.setCustomFields({"field-a" :"value"})
// OR
var logger = req.logger.createLogger({"field-a" :"value"}); 

Child loggers can have a different logging threshold, so you can customize your log output with them:

var logger = req.logger.createLogger(); 
logger.setLoggingLevel("info");

Child loggers with global context

You can also create child loggers inheriting from the global logging context like this:

var log = require("cf-nodejs-logging-support");
...
var logger = log.createLogger(); 
logger.setCustomFields({"field-a" :"value"})
// OR
var logger = log.createLogger({"field-a" :"value"}); 

Keep in mind that the global context is empty and immutable. As mentioned in the logging context docs, adding properties to it is not possible.

Child loggers with custom context

New child loggers can be equipped with a new extensible and inheritable context by providing true for the createNewContext parameter:

var log = require("cf-nodejs-logging-support");
...
var logger = log.createLogger(null, true); 
logger.setTenantId("my-tenant-id");

Similar to a request context, a newly created custom context includes a generated correlation_id (uuid v4).

A practical use case for this feature would be correlating logs on events that reach your application through channels other than HTTP(S). By creating a child logger and assigning it (or using the auto-generated) correlation_id, any kind of event handling could be made to log correlated messages.

Custom field inheritance

When using child loggers the custom fields of the parent logger will be inherited. In case you have set a field for a child logger, which is already set for the parent logger, the value set to the child logger will be used.

Example for custom field inheritance:

// Register all custom fields that can occur.
log.registerCustomFields(["field-a", "field-b", "field-c"]);

// Set some fields which should be added to all messages
log.setCustomFields({"field-a": "1"});
log.info("test");
// ... "custom_fields": {"field-a": "1"} ...


// Define a child logger which adds another field
var loggerA = log.createLogger({"field-b": "2"});
loggerA.info("test");
// ... "custom_fields": {"field-a": "1", "field-b": "2"} ...

// Define a child logger which overwrites one field
var loggerB = log.createLogger({"field-a": "3"});
loggerB.info("test");
// ... "custom_fields": {"field-a": "3", "field-b": "2"} ...

// Request correlated logger instances inherit global custom fields and can overwrite them as well.
app.get('/', function (req, res) {
    req.logger.setCustomFields({"field-b": "4", "field-c": "5"});
    req.logger.info("test");
    // ... "custom_fields": {"field-a": "1", "field-b": "4", "field-c": "5"} ...
});