OpenWhisk actions (equivalent to functions in other frameworks) have two life cycle events: initialization and execution. The very first activation of an action (a cold start), will initiate both events. Future activations of the action, if they're contemporaneous, will only generate execution events. This is the warm execution, which can take advantage of previous activations. To provide scaling, it is necessary to sometimes create new execution environments for the repeated actions, and so that means new initialization events.

While one cannot rely that all executions after a cold start are warm, it's possible to for the action code to check if it's warm and forgo internal initialization as needed. This is a convenient trick that's common to avoid expensive setup.

For Java actions running in OpenWhisk, this can be done with static fields.

A nice flow of the lifecycle can be seen on slide 24 here http://www.slideshare.net/psuter/openwhisk-deep-dive-the-act....