Introduction

The distributed system is a critical problem where the various nodes must ensure the data integrity. As a software engineer specialized in the design and implementation of distributed systems, one can come across so many cases that impose the need for a near-perfect solution to transaction subsystems issues. One of the most commonly used protocols to handle this issue is the Two-Phase Commit (2PC) protocol.

In this blog I will talk about various topics, including the Two-Phase Commit protocol, which I believe are the best based on my experience and the most recent industrial practices. We will also go through its architecture, basic concepts, and main principles. So that you get a good grasp to implement 2PC in your distributed systems.

Understanding the Two-Phase Commit Architecture

The Two-Phase Commit protocol is a distributed algorithm which is designed to synchronize the processes collaborating in a distributed atomic transaction. Its primary function is to guarantee that either all nodes of a distributed system commit a transaction or all nodes abort it.

The 2PC architecture is mainly composed of two components:

• Coordinator – A central node that runs the commit protocol and manages all the parallel processes.
• Participants – The distributed nodes where the transaction is being performed and that all must agree on its result.

The name of the protocol indicates that this is achieved in two parts:

• Prepare Phase – The coordinator poses the question to all participants whether they are ready to commit the transaction.
• Commit Phase – Depending on the status of the responses in the prepare phase, the coordinator makes the decision either to commit or abort the transaction and informs all participants of the decision.

Core Concepts and Key Principles of 2PC

To get a comprehensive understanding of this protocol, one needs to learn these important terms.

• Atomicity: The 2PC protocol is meant to get a transaction to be treated as a lone indivisible unit. Either all risks in a transaction are executed correctly or none of them are executed.
• Consistency: 2PC is the master of the distributed system consistency by making sure that all that nodes will agree on the final state of the transaction.
• Durability: Once a transaction is committed, the changes are permanent, and they prevail over system crashes.
• Isolation: The protocol prevents concurrent transactions from interfering with each other.
• Fault Tolerance: The protocol comprises strategies to override different failure scenarios, like node crashes or network partitions.

Ordinary Cases of Two-Phase Commit Usage

From time to time, I have had a situation where the usage of 2-Phase Commit has been invaluable to me. A few of the most COMMON uses are:

1. Distributed databases –

In this particular system of distributed databases, 2PC is applied to the execution of a transaction that includes different databases. For example, an application for banking purposes which implies inter-server money transfer, calls the use of 2PC in the verification of such posting movements as the logging and debiting of accounts. 2PC provides a guarantee to do this or to undo it.

2. Microservices Architecture – Concerning microservices-based architecture, where each service has its own data store, 2PC can be used for a transaction that requires the involvement of multiple services that are located on different nodes. This is a main impact whereby there exists data uniformity throughout the whole system.

3. Cloud-based Systems – In the cloud, where various resources are distributed over multiple data centers, the 2PC ensures data consistency while performing the operations that are affecting multiple cloud regions or zones.

4. E-commerce Platforms – Lots of web pages with their trading sections rely on 2PC to manage transactions. For example, web pages sell items and accept payments and use several e-commerce systems.

Key Characteristics of Two-Phase Commit

The Two-Phase Commit protocol possesses some striking characteristics that eventually distinguish it from other modes of communication.

Synchronous Communication : The system bases itself upon the interaction between the coordinator and the participants only, provided that the former orchestrates communication (means); however, this setup could cause performance issues if the network is very sluggish.

Blocking Protoco l: Following a commitment decision, coordinators may more easily remain, thereby engaging resources in a blocking manner for a long time.

Strong Consistency : When it comes to the aspect of recovery at the expense of availability, 2PC is a perfect consistency of data.

Coordinator-Dependent : This algorithm’s prosperity is primarily based on the existence of a solitary coordinator correctly determining availability and performing the operation.

Deterministic Outcome : The 2PC has a property in which all sites gain the same agreement deciding the result of the transaction [2].

Implementing Two-Phase Commit in .NET

A Step-by-Step Guide

As a result of having been a .NET developer, I got an opportunity to apply a Two-Phase Commit protocol in my various projects. The protocol involves the following steps to enable you to implement 2PC in your .NET applications.

Step 1: Define the Transaction Coordinator

The first thing you need to do is write a class to define the transaction coordinator:

public class TransactionCoordinator

{

    private List participants = new List();

    public void AddParticipant(IParticipant participant)

    {

        participants.Add(participant);

    }

    public bool ExecuteTransaction()

    {

        // Implement the two phases here

        return PreparePhase() && CommitPhase();

    }

    private bool PreparePhase()

    // Implement prepare phase logic

    }

    private bool CommitPhase() 

    // Implement commit phase logic

    }

}

Step 2: Define the Participant Interface

Next, create an interface that all remain participants can implement:

public interface IParticipant
{

    bool Prepare();

    void Commit();

    void Rollback();

}

Step 3: Implement the Prepare Phase

Through the PreparePhase method of the TransactionCoordinator class, you can do it;

private bool PreparePhase()

{

    foreach (var participant in participants)

    {

        if (!participant.Prepare())

        {

            // If any participant is not ready, abort the transaction

            foreach (var p in participants)

            {

                p.Rollback();

            }

            return false;

        }

    }

    return true;

}

Step 4: Implement the Commit Phase

This is quite easy. On the CommitPhase method of the TransactionCoordinator class, you can put;

private bool CommitPhase()

{

    foreach (var participant in participants)

    {

        participant.Commit();

    }

    return true;

}

Step 5: Create Participant Implementations

Lately, you can choose to deploy the IParticipant interface that each site must have in your distributed system, thus giving a guarantee for the transaction to the whole protocol.

public class DatabaseParticipant : IParticipant

{

    public bool Prepare()

    {

        // Implement prepare logic for database operations

    }

    public void Commit()

    {

        // Implement commit logic for database operations

    }

    public void Rollback()

    {

        // Implement rollback logic for database operations

    }

}

Step 6: Use the Two-Phase Commit in Your Application

Finally, it is very simple to run 2PC in your code:

var coordinator = new TransactionCoordinator();

coordinator.AddParticipant(new DatabaseParticipant());

coordinator.AddParticipant(new PaymentServiceParticipant());

bool transactionResult = coordinator.ExecuteTransaction();

Advantages and Weaknesses of the Two-Phase Commit Protocol

As a result of my practice dealing with 2PC, I can mention several rise and downgrade aspects of the protocol.

Advantages –

• Strong Consistency : Every participant of the 2PC protocol will altogether reach a consensus about the transaction outcome, thus the data integrity is safe and sound.
• Atomicity : The protocol guarantees that the execution of transactions is an all-or-nothing operation thus forbidding the possibility of our other updates.
• Simplicity : Instead of more participatory and more complicated consensus protocols with a high level of executives to implement, 2PC is a very mean and easy-to-use solution.
• Widely Supported : Most of the DBs and distributed transaction managers inherently provide 2PC capability.

Weaknesses –

• Performance Overhead: The 2PC method works in synchronous mode where the communication between nodes happens one after another. It can lead to high development times as the communication gets encumbered and in the case of poor networks, particularly at the peripheries of the internet.
• Blocking: Participants are susceptible to blocking if the coordinator makes a decision and the pi participant can’t communicate with the participants, leading to system outages.
• Single Point of Failure: The coordinator is an actor in the system that can cause a failure in the mechanism.
• Limited Scalability: As the number of participants rises, so does the number of transactions, and 2PC’s performance becomes less and less efficient.
• Vulnerability to Network Partitions: In case of the appearance of network partitions, 2PC may be the reason for an endless blocking of resources.

Comparison with Other Similar Architectures

In 2PC evaluation, it is a must to juxtapose 2PC with a few other distributed consensus protocols as there may be some differences in this line.In 2PC evaluation, it is a must to juxtapose 2PC with a few other distributed consensus protocols as there may be some differences in this line.

Three-Phase Commit (3PC)

3PC has one more stage added between the preparation and the actual commit of 2PC, which, however, makes it possible to escape some of the troubles that 2PC faces. In this setup, the Pre-Commit phase is made, during which the system gets prepared for the Commit stage.

Strengths:

It is less blocking as compared to 2PC

Successfully withstands coordinator faults

Limitations

It is more complex than 2PC and it is also more computationally expensive

The partitioning of the network may impose indefinite blocking

Paxos

It is more complex than 2PC and it is also more computationally expensive

The partitioning of the network may impose indefinite blocking

Strengths:

The heavy fault-tolerance of the method speaks for itself by the strength of the system under the minority failures. It is essential to note that it can reach a consensus even with the minority of failures.

Limitations:

The heavy fault-tolerance of the method speaks for itself by the strength of the system under the minority failures. It is essential to note that it can reach a consensus even with the minority of failures.

Raft

Raft is a consensus algorithm that was aimed at the user as a rafting guide to be more understood, yet providing the same functionality as Paxos.

Strengths:

This is the fact that it is clearer and less demanding than Paxos

Raft maintains the strong consistency and fault tolerance.

Limitations: 

There is a slight chance of a performance decline in situations where Paxos is well optimized,

It is less renowned than Paxos in high production of battle-tested environments.

The Future of Two-Phase Commit in Modern Distributed Systems

Looking to the future of the distributed systems the role of Two-Phase Commit yet still exists, however it is changed. While an essentially secure Two-Phase Commit remains the protocol of choice in misconvergence, the need for its implementation in a way that solves new scaling and performance issues have motivated the development of suitable alternatives. In my practice, I have detected the tendency of the application of models of flexible consistency like CRDT and partially relaxed  scenarios  of eventual consistency, especially in large-scale, globally distributed applications. Therefore, these emerging paradigms might be the cornerstone of the future distributed systems, as the aforementioned character limits us from going into details. These models can considerably improve performance and availability but offer looser consistency guarantees. Nevertheless, the era would still see 2PC and its variants maintaining their status as one of the key elements in the stock of technical solutions shortly. The improvement prospect of 2PC is mainly directed at the reduction of its blocking nature and the enhancement of its resilience to network partitions. In the same manner, that smart cars will not entirely replace the standard vehicles, two-phase commit will not wholly vanish, but on the contrary, it will prove a robust means to address the fluctuating needs of modern apps. In addition to 2PC that offers elevated level consistency, other emerging techniques provide more balanced solutions f; or different use-cases and further set of operational boundaries.

Conclusion

As the diversity of distributed systems increases and their nature becomes more intricate choosing the proper type of consistency model, and selecting the right approach for each particular application type is essential. The Two-Phase Commit stays the inventor of all the tag wantings to good customer`s service alongside other methods to obtain a similar possibility of driving automatic cars. There are also new technologies, such as CRDT and eventual consistency, which seem to be a cure for the aching headache of the entire breed of unceasingly growing applications.

It is undeniable that the principles underlying 2PC are a cornerstone in both the future consensus protocols and distributed transaction management systems.  Many of the issues surrounding 2PC such as its performance will still be-the case or at least the guiding light for the development of future consensus protocols and DTM systems.