Consistency Models in Distributed Systems

Consistency Models in Distributed Systems

Factors and Approach to understanding Eventual and Strong Consistency in Distributed Systems

Sofwan A. Lawal's photo
Sofwan A. Lawal
·Oct 5, 2022·

5 min read

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As data volume and complexity increase, distributed systems are becoming more and more widespread. This has caused distributed systems, such as databases, streaming data, and service-oriented architectures, to increase. Distributed systems must gracefully handle failures while maintaining service availability or data access. Any system may occasionally experience failures, but these must be quickly identified and dealt with to ensure a minimally disruptive recovery. This blog post will explore few consistency models for distributed systems.

A note on Distributed System

The term "distributed system" refers to a system that is distributed in nature. This means it is split across multiple locations and is not run entirely on a single machine. A distributed system could be a group of computers all hosted at the same location or a group of servers running in different data centres. The main goal of a distributed system is to achieve greater scalability and reliability than a single system can provide.

Even though many distributed systems are capable of automatically coordinating their actions, sometimes it's just not possible to do so. It is also difficult to speculate what will take place in the event that one of the computers loses internet connectivity or experiences some other problem. One of the reasons why we have consistency models is because of things like this.

What is Consistency?

The degree to which a system or any copies of the system accurately reflect the state of the system at any point in time is referred to as consistency. It is frequently used to assess a system's functionality or effectiveness. When talking about distributed systems, the term "consistency" refers to the agreement between data replicas or the rules that govern how those replicas can be changed. Distributed systems have the potential to be either consistent or inconsistent, depending on the methods that are used to design and implement them.

We'll look at these two consistency models for distributed systems:

  • Eventual consistency
  • Strong consistency

Transactions in Distributed Systems

In a distributed system, a series of operations that are carried out all at once make up a transaction. When a transaction is considered consistent, it must finish successfully without causing any errors or affecting any other transactions that are still open. You have the option of either committing or rolling back a transaction. A transaction is said to have been committed once it has been successfully applied, and it is said to have been rolled back whenever an error occurs while the transaction is being executed. All of the replicas will be brought up to date with the most recent information once a transaction has been successfully committed by all the replicas.

Eventual Consistency in Distributed Systems

Distributed systems, such as cloud computing, are becoming more prevalent in today's technology landscape. As a result, there are many challenges associated with distributed systems. These challenges include eventual consistency, which is when distributed systems eventually reach a point of consistency. In other words, there will be agreement across all parts of a distributed system.

As the number of nodes increases and the network becomes more complex, it can become harder for eventual consistency in distributed systems. Eventual consistency can lead to unnecessary costs and inefficiencies for distributed systems that need to maintain consistency. Distributed system administrators and backend engineers must understand eventual consistency and how it can impact their distributed system.

The advantage of eventually consistent operations is that it does not require resource-intensive methods such as locking and can accommodate many concurrent operations. However, the disadvantage is that data may not be accurate and consistent until a certain amount of time has passed, which means that it can take a while until the system is fully functional.

Strong Consistency in Distributed Systems

In distributed systems, communication between components must be reliable and timely to function efficiently and produce quality output. Reliable communication means that data must be sent and received correctly; timely communication implies that components should always receive new data when it arrives rather than waiting until later to process it.

Strong consistency is a model where replicas of a system are consistent with the system's state at a given time. Strong consistency is a form of consistency where replicas of a system are consistent with the system's state at a given time. Replicas are updated with new information so that they are always consistent with one another.

The advantage of strong consistency is that it can accurately reflect the latest state of the data, but the disadvantage is that it can become an issue if multiple clients are accessing the data at the same time because the clients may end up getting different versions of the data.

Distributed Transaction

A distributed transaction is a transaction applied to a distributed database, and it is typically composed of two phases: Commit and Abort. In the commit phase, all updates made by the transaction are applied to the database. It is important to note that updates are applied atomically, which means that the database does not have a partially updated state. Only one transaction can be applied to the database at any given time, and updates from other transactions are queued until the current transaction is finished. In the abort phase, the transaction is rolled back into the database.

Factors that contribute to Consistency

Distributed systems are critical for modern applications. They rely on multiple nodes to function correctly and are common in web services, cloud computing and other distributed systems. Several factors contribute to consistency across distributed systems:

  1. Message routing: Messages should always go directly from one node to another. This minimises the number of hops a message has to make and reduces latency.
  2. Data replication: All nodes should have identical copies of all data, so they can all be accessed simultaneously. If any changes are made to the data, a single copy should be updated on all nodes.
  3. Time synchronisation: All nodes must agree on the time to set their clocks appropriately. This is typically handled via a network clock or a reliable time source.

Conclusion

Consistency is an essential aspect of distributed systems that helps us ensure that the system remains functional and reliable. Two consistency models for distributed systems are eventual consistency and strong consistency. Distributed transactions are applied to distributed databases and are composed of two phases: commit and abort.

 
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