Each and every piece of data that is sent to CloudWatch has to have a namespace attribute assigned to it. There is no default, and each service will usually be using a namespace in the form of AWS/servicename that corresponds to the name of the service. This allows us to easily distinguish between groups of metrics, whether accessing them from the console, the CLI, or SDK. 

When publishing custom metrics that have no AWS analog, we will also be required to specify a namespace. When creating metrics, we have complete freedom for choosing our namespaces as long as they do not exceed 256 alphanumeric characters (0–9, A–Z, and a–z). The following special characters are also allowed: period, hyphen, underscore, forward slash, hash, and colon.