Synchronization in Distributed Systems

Distributed System is a collection of computers connected via a high-speed communication network. In the distributed system, the hardware and software components communicate and coordinate their actions by message passing. Each node in distributed systems can share its resources with other nodes. So, there is a need for proper allocation of resources to preserve the state of resources and help coordinate between the several processes. To resolve such conflicts, synchronization is used. Synchronization in distributed systems is achieved via clocks. The physical clocks are used to adjust the time of nodes. Each node in the system can share its local time with other nodes in the system. The time is set based on UTC (Universal Time Coordination). UTC is used as a reference time clock for the nodes in the system. Clock synchronization can be achieved by 2 ways: External and Internal Clock Synchronization.

1. External clock synchronization is the one in which an external reference clock is present. It is used as a reference and the nodes in the system can set and adjust their time accordingly.
2. Internal clock synchronization is the one in which each node shares its time with other nodes and all the nodes set and adjust their times accordingly.

There are 2 types of clock synchronization algorithms: Centralized and Distributed.

1. Centralized is the one in which a time server is used as a reference. The single time-server propagates it’s time to the nodes, and all the nodes adjust the time accordingly. It is dependent on a single time-server, so if that node fails, the whole system will lose synchronization. Examples of centralized are-Berkeley the Algorithm, Passive Time Server, Active Time Server etc.
2. Distributed is the one in which there is no centralized time-server present. Instead, the nodes adjust their time by using their local time and then, taking the average of the differences in time with other nodes. Distributed algorithms overcome the issue of centralized algorithms like scalability and single point failure. Examples of Distributed algorithms are – Global Averaging Algorithm, Localized Averaging Algorithm, NTP (Network time protocol), etc.

Centralized clock synchronization algorithms suffer from two major drawbacks:

1. They are subject to a single-point failure. If the time-server node fails, the clock synchronization operation cannot be performed. This makes the system unreliable. Ideally, a distributed system should be more reliable than its individual nodes. If one goes down, the rest should continue to function correctly.
2. From a scalability point of view, it is generally not acceptable to get all the time requests serviced by a single-time server. In a large system, such a solution puts a heavy burden on that one process.

Distributed algorithms overcome these drawbacks as there is no centralized time-server present. Instead, a simple method for clock synchronization may be to equip each node of the system with a real-time receiver so that each node’s clock can be independently synchronized in real-time. Multiple real-time clocks (one for each node) are normally used for this purpose.