Distributed Systems Commit Protocols

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Questions and Answers

What happens if a timeout occurs during the coordinator's voting phase in 2PC?

The coordinator transitions directly to the global-decision phase.

The coordinator waits for all votes indefinitely.

The coordinator writes global-commit to local log.

The coordinator writes global-abort to local log and multicasts it. (correct)

In the 2PC protocol, what is the consequence of a single-node failure?

The protocol will retry the voting phase.

Only participants need to recover to continue functioning.

The blocking nature of 2PC prevents any further action. (correct)

The system remains available despite the failure.

What does the Three Phase Commit (3PC) protocol aim to improve over 2PC?

It adds an additional phase to eliminate commit and abort states. (correct)

It allows for faster recovery of failed nodes.

It guarantees immediate commit decisions regardless of node status.

It simplifies the communication among participants.

During the vote process, what must a participant do if it has not received a prepare message in a timely manner?

Write vote-abort to local log and exit. Signup and view all the answers

In the scenario where all participants send vote-commit in 2PC, what is the coordinator's next step?

Write global-commit to local log and multicast it. Signup and view all the answers

What characteristic of 2PC contributes to its vulnerability during a failure?

It requires all nodes to reach an agreement. Signup and view all the answers

What is a key feature of the Three Phase Commit protocol (3PC)?

It ensures no state leads directly to a commit or abort. Signup and view all the answers

What should a participant do upon receiving a global-decision from the coordinator after voting to commit?

Write global-decision to local log. Signup and view all the answers

What is the primary design goal of Raft?

Understandability Signup and view all the answers

In Raft, what happens if followers do not hear from a leader for an extended period?

They become candidates and initiate an election Signup and view all the answers

Which state is not one of the three states that nodes can be in Raft?

Proxy Signup and view all the answers

What is the role of the leader during normal operation in Raft?

To collect commands from clients and manage the log Signup and view all the answers

Which statement about log replication in Raft is true?

The leader is solely responsible for appending commands to its log Signup and view all the answers

What ensures that only servers with up-to-date logs can become a leader in Raft?

Log consistency protocols Signup and view all the answers

What action does the leader take after a new log entry is committed?

It notifies followers and executes the command Signup and view all the answers

What is the purpose of the AppendEntries messages in Raft?

To replicate log entries from the leader to followers Signup and view all the answers

What is the role of the coordinator in the three-phase commit protocol?

To initiate the commit process and manage states Signup and view all the answers

If a participant fails during the pre-commit state, what can the coordinator do?

Commit and instruct the failed participant to commit upon recovery Signup and view all the answers

What happens to participants when the coordinator fails during the prepare phase?

They can decide to abort the operation Signup and view all the answers

In the three-phase commit protocol, what is the response if the majority of participants are in the Ready state and no one is in the Pre-commit state?

Abort the transaction Signup and view all the answers

During which phase can a coordinator assume an abort decision if a participant fails?

Init phase Signup and view all the answers

How does a participant behave when it is blocked in the Ready state and cannot get a global decision?

Contacts another participant for a decision Signup and view all the answers

Which of the following is NOT a possibility when a participant fails?

The coordinator can never commit Signup and view all the answers

What is the outcome if there is at least one participant in the Init phase when a decision to abort is made?

The abort decision holds Signup and view all the answers

What is a key feature of replicated state machines in distributed database management systems?

They ensure that all replicas process operations sequentially. Signup and view all the answers

In the context of distributed DBMS, what does 2PC (Two-Phase Commit) facilitate?

Atomic commitment across multiple partitions. Signup and view all the answers

How many participants are required in a Byzantine Fault Tolerant (BFT) system to tolerate f failing nodes?

3f + 1 participants Signup and view all the answers

What does the Bitcoin blockchain primarily serve as?

A public ledger that records transactions. Signup and view all the answers

What is required to add a block to the Bitcoin blockchain?

Solving a mathematical proof of work. Signup and view all the answers

What happens when a miner successfully solves the proof of work in Bitcoin?

The miner earns Bitcoins and broadcasts the new block. Signup and view all the answers

What assumption is made regarding the participating nodes in permissionless blockchain systems?

No single entity controls most of the compute resources. Signup and view all the answers

What is the primary reason behind the difficulty of finding a solution in proof of work?

To link the block to the existing chain securely. Signup and view all the answers

What does a misbehaving user attempt in a blockchain environment?

Double spend their money. Signup and view all the answers

What is the role of miners in the Bitcoin network?

To collect pending transactions into blocks. Signup and view all the answers

What does the 3PC protocol guarantee in a synchronous system?

It ensures liveness as long as a majority of nodes are alive. Signup and view all the answers

Which of the following is a key characteristic of the CAP theorem?

It limits distributed systems to two out of three desired properties. Signup and view all the answers

What is a disadvantage of the 3PC protocol compared to 2PC?

It requires three phases of communication. Signup and view all the answers

Which statement about the FLP theorem is correct?

It indicates that network partitions prevent achieving both liveness and safety. Signup and view all the answers

What is the primary function of a replicated state machine?

To allow machines to operate on identical copies of the same state. Signup and view all the answers

What challenge does the Paxos algorithm primarily address?

Facilitating fault tolerance in a single decision. Signup and view all the answers

In a replicated state machine, what ensures that all non-failing machines execute the same command?

A replicated log that maintains command order. Signup and view all the answers

What is a consequence of an asynchronous system as mentioned in the content?

Protocols may not terminate due to undetermined connections. Signup and view all the answers

What does the term 'liveness' refer to in distributed systems?

The system makes progress under normal conditions. Signup and view all the answers

Which of the following describes a potential failure in a distributed system?

Messages may be duplicated or lost. Signup and view all the answers

What indicates a possible blocking situation in the 2PC protocol?

A node fails before sending a commit message. Signup and view all the answers

What characteristic makes 3PC protocols more robust than 2PC?

They provide a mechanism for recovery from failure. Signup and view all the answers

Which statement about the guarantees provided by replicated state machines is accurate?

Non-failing machines execute commands in the same order. Signup and view all the answers

What is a common property of modern data systems in relation to the CAP theorem?

They sacrifice consistency for higher availability. Signup and view all the answers

Study Notes

2PC

2PC is a blocking commit protocol, it waits for all participants to vote before committing.
Vulnerable to single-node failure (coordinator)
Blocks the entire system until the coordinator recovers.

3PC / Three-phase commit

Aims to solve 2PC problems by adding another phase to the protocol.
Three phases: prepare, pre-commit, commit.
Less vulnerable to failure, as it doesn't lead to an impossible commit state.
More expensive than 2PC as it always requires 3 phases of communication.

Raft

Consensus algorithm for state machine replication.
Guarantees all non-failing machines execute the same command in the same order.
Offers a continuous service even if some machines fail.
Provides high availability if a majority of machines are up and can communicate.

Blockchains

A specific use case of replicated state machines.
Examples include Bitcoin
Uses proof of work to achieve consensus.
Transactions are digitally signed and stored in a replicated ledger (blockchain).
Guarantees are probabilistic, not provable, as there's no single entity controlling the majority of computing power.

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Description

This quiz explores various commit protocols used in distributed systems, including Two-Phase Commit (2PC), Three-Phase Commit (3PC), consensus algorithms like Raft, and the application of blockchain technology. Understand their functionalities, advantages, and vulnerabilities in maintaining consistency across systems. Test your knowledge on these critical concepts in distributed transactions.