Virtual Synchrony - Introduction

Introduction

Distributed computer systems often need a way to replicate data for sharing between programs running on multiple machines, connected by a network. Virtual synchrony is one of three major technologies for solving this problem. The key idea is to create a form of distributed state machine associated with the replicated data item. Called a process group, these state machines share copies of the data, and updates are delivered as events that occur in the same order at all the copies. If a process fails or crashes, this is reported to the other processes in the group; if a process joins, this is similarly reported, and a state transfer is used to initialize the joining member. An application with lots of data items to replicate might do so by using a single group for the whole set, or it could create different groups for different items, with the former approach used when the items are replicated at the identical places, and the latter being used when the replication patterns differ.

Each process group has a name, visible within the network. A single application program can connect itself to many groups at the same time. In effect, a process group becomes an abstraction for sharing data, coordinating actions, and monitoring other processes.

The term virtual synchrony refers to the fact that applications see the shared data evolve in what seems to be a synchronous manner. This form of synchronization is virtual because the actual situation is more complex than seems to be the case from a programmer's perspective. Like a compiler that sometimes reorders the execution of instructions for higher performance, or an operating system that sometimes stores random access memory on disk, virtual synchrony sometimes reorders events in ways that improve performance, and yet won't be noticeable to applications.

Using the virtual synchrony model, it is relatively easy to maintain fault-tolerant replicated data in a consistent state. One can then build all sorts of higher level abstractions over the basic replication mechanisms.

Virtual synchrony replication is used mostly when applications are replicating information that evolves extremely rapidly. As discussed further below, the kinds of applications that would need this model include multiuser role-playing games, air traffic control systems, stock exchanges, and telecommunication switches. Of course, there are other ways to solve the same problems. For example, most of today's online multiuser role-playing games give users a sense that they are sharing replicated data, but in fact the data lives in a server on a data center, and any information passes through the data centers. Those games probably wouldn't use models like virtual synchrony, at present. However, as they push towards higher and higher data rates, taking the server out of the critical performance path becomes important, and with this step, models such as virtual synchrony become valuable.

The trend towards cloud computing has increased interest in consistent state replication. Cloud computing systems are large virtualized data centers, operated by internet search or commerce firms such as IBM, Microsoft, Google and Amazon. Inside a cloud computing platform one finds services such as lock management systems (Google's is called Chubby, and Yahoo! uses one called Zookeeper), and these are implemented using virtual synchrony or closely related models. Other services that might be implemented using virtual synchrony include the cluster management tools that relaunch failed applications when nodes in a cluster crash, event notification tools that inform applications when significant events occur, and logging tools that help an application save its state for replay during recovery.