Essential Features of The Virtual Synchrony Model
Virtual synchrony is usually presented to programmers through a simple distributed programming library that supports at least three basic interfaces. First, a process (an executing program) can join a process group. Each group has a name (a bit like a file name, although these names are interpreted within a network, not relative to a disk), and each has a list of members. The join primitive returns some form of handle on the group. The process can then register a handler for incoming events, and can send multicasts to the group.
The basic guarantee associated with the model is that all processes belonging to a group see the same events, in the same order. The platform senses failures (using timeouts) but reports them in a consistent manner to all group members. Multicast messages may be initiated concurrently by multiple senders, but will be delivered in some fixed order selected by the protocols implementing the model.
Notice that the guarantee just described embodies what may seem to be a contradiction. We know that timeout cannot be used to detect failures accurately. Yet virtual synchrony lets the user treat failure notifications (view changes) as trustworthy, infallible events. The key insight is that virtual synchrony is implemented by a software system that creates an abstraction within which the user's code is executed. Thus, failure detection by the platform (using timeouts) triggers an internal agreement protocol (within the platform). Only when this protocol terminates is a fault event delivered to the application. The application is spared from needing to implement the agreement mechanism, and sees a simple and seemingly accurate fault notification event.
Events are of several types. First, each received multicast is delivered as an event. But membership changes in the group are also reported through events; these are called new views of the group. Moreover, when a process joins a group, some existing member is asked to create a checkpoint: a message describing the state of the group at the time the process joined. This is reported to the new member as a state transfer event, and is used to initialize the joining process.
For example, suppose that an air traffic control system maintains some group associated with the airplanes flying in sector XYZ over Paris. Each air traffic controller who monitors that sector would have a process running on his or her machine, and these processes would join the XYZ group as they start up. The members would replicate the list of air traffic control plans and tracks associated with sector XYZ. Upon joining, a process would obtain a copy of the state of the sector as of the moment it joined, delivered as a checkpoint through a state transfer event. Loading such a checkpoint is analogous to reading a file that lists the current state of the sector. Later, as events occur that impact the status of the sector, they would be multicast so that all members of the group can see those events. Since each member is in the same state, and receives the same updates, each air traffic controller sees the same sector status and they see it evolve in the same manner. If a failure occurs, the surviving systems can take over roles that were previously held by the crashed one.
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