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## Dynamic Extension and Re-configuration

An application built on our Middleware may be dynamically extended and re-configured in several ways while it is in execution (i.e., while there are pending un-delivered messages). First, an Agent can dynamically change the handlers it has registered for a given Event. Second, new Agents may be added and existing Agents removed for an existing MStream. Third, new MStreams may be added and removed. Fourth, new Sites may be added and removed, and finally, MStreams may be moved dynamically from Site to Site.

When an MStream moves from one Site to another, it (logically) moves the code of all of the Agents attached to it to the new Site along with whatever state they have placed in their briefcase structures. We say an Agent "visits" a Site when its MStream visits the Site. When an Agent first visits a Site, its initialization code executes there and when an Agent is killed, its (optional) Finalization Handler runs at each location that has been visited by it. Agent state (consisting of global state variables and code) is replicated at each site that it visits until the Agent is destroyed. On Agent destruction, the Handlers that it has registered are de-registered, and the interpreter and state variables are freed at each Site that it has visited. We assume that Sites may fail or disconnect during execution. Site failure does not imply destruction of the MStreams that reside there. Failure processing is described in Section 2.3.

The Agent's briefcase specifies a consistency requirement for moves. When an Agent moves from Site to Site only the elements in the briefcase are copied from the source execution environment to the target. The remainder of the global state remains unaffected (and cached) at the source site of the move. On successful completion of a move, the Arrival Handlers of the MStream are invoked at the new Site where the MStream has moved.

Handlers may move the MStream to which they are attached and also may move other MStreams around as well as create and destroy MStreams. Handlers may also exit - destroying the Agent in which they are housed and may also destroy other Agents. Such actions may also be initiated from an external Shell. Re-configuration may be contained by using appropriate policy handlers.

All changes in the configuration of an MStream such as MStream movement, new Agent addition and deletion, and MStream destruction are deferred until the time when no Handlers of the MStream are executing. We call this the Atomic Handler Execution Model . Message delivery order is preserved despite dynamic reconfiguration, allowing both the sender and receiver to be in motion while asynchronous messages are pending delivery.

Applications built using Mobile Streams can be extended from multiple points of control; any handler or Shell that has acquired an open MStream handle, can attempt to re-configure or extend the reactive part of the system and these actions can occur concurrently. While this adds great flexibility, it also raises several security and stability issues. We provide a means of restricting system reconfiguration and extension using control Events that can invoke policy Handlers. These policy Handlers may be registered only by privileged Agents as described below. We follow a discretionary control philosophy by providing just the mechanism and leaving the policy up to individual applications. Controls may be placed via policy Handlers at a session-wide level, site-wide level and at the level of individual MStreams for various security-relevant Events.

In summary, our security mechanisms are based on the following three principles:

Session-wide control: We have built mechanisms to place session-wide controls over extension and reconfiguration via a centralized Session Leader MStream.

Site-specific control: Each site may specify security policies via a Site Controller MStream. Site-specific policies may be used to grant or deny MStream entry to a site and to sand-box the incoming MStream handler's code by using safe-Tcl mechanisms.

Stream-specific control: The MStream itself is regarded as an extensible entity to which Agents can be attached and detached. It can carry its own policy Handlers to allow or disallow such actions, as determined by its Stream Controller Agent.

Our security implementation works as follows: Messages are classified as data messages and control messages. Data messages are delivered directly to the MStream. Control messages are messages that can change the configuration of the distributed system. These are routed first through the trusted intermediary Session Leader that can accept or deny these actions via its policy handlers, and then through the Site Controller which may again accept or deny the action and finally through the Stream Controller for MStream-specific actions. We invite the interested reader to look at [#!Ranga99Mobile!#] for more details.

The mechanisms described above permit us to build highly flexible and extensible distributed reactive systems that are able to extend and re-configure themselves, and also to place constraints on how the system can be re-configured and extended.

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