1.2.2 Restartable Actors

A restartable actor is any actor which can be rapidly restarted without accessing stable storage, when it abnormally terminates. A restartable actor is restarted from an actor image which comprises the actor's text and initialized data regions. The actor image is stored in persistent memory (unless the actor is executed in place, in which case the actor image is the actor's executable file, stored in non-persistent, physical memory). Restartable actors can use additional blocks of persistent memory to store their own data.

Figure 1-1 shows the state of a typical restartable actor at its initialization, during execution, and after having been hot restarted as a result of an error. The actor uses persistent memory to store some state data. After hot restart, the actor is reconstructed from its actor image, also in persistent memory. It is then re-executed from its initial entry point, and can retrieve the persistent state data which has been stored.

Figure 1-1 Typical restartable actor

In the hot restart architecture, management of restartable actors is assured by a ChorusOS supervisor actor known as the Hot Restart Controller. Restartable actors are monitored by the Hot Restart Controller, in that the Hot Restart Controller will detect a restartable actor's abnormal termination and automatically take the appropriate restart action if an abnormal termination occurs. In the context of hot restart, abnormal termination cases include unrecoverable errors such as division by zero, a segmentation fault, unresolved page fault, or invalid op code, and so on.

Restartable actors, like traditional ChorusOS actors, can be run in either user or supervisor mode. In addition, they can be run from the sysadm.ini file or C_INIT console, or spawned dynamically during system execution. Indeed, the restartable nature of restartable actors remains transparent to system actors such as the AM actor, responsible for loading and starting restartable actors. This is because restartable actors do not declare themselves restartable, but are run as restartable actors. More specifically, the way a restartable actor is initially run determines how it will be restarted when a restart occurs:

• Restartable actors which are run from the sysadm.ini file, or which are run directly from the C_INIT console, are restarted directly by the system when a restart occurs. These actors are known as direct restartable actors.

• Restartable actors which are spawned dynamically during system execution will be restarted by the actor which initially spawned them. These actors are known as indirect restartable actors.

The distinction between direct and indirect restartable actors provides a useful framework for the construction of restartable groups of actors, as described in the next section.

C_INIT and the Hot Restart Controller provide an interface specifically for running and spawning restartable actors. This interface is described in detail in Chapter 4, Programming With Restartable Actors.