3.2.3.1. Session: Introduction to Consensus Mechanisms

Synchronization of the network (15 minutes)

Begin by recapping the key ideas from the previous two modules (decentralization, keeping records in a chain of blocks).
Open discussion on synchronization in blockchain networks. Emphasize that all nodes in the network must agree on the order and content of transactions in the blockchain.
Pose questions to students, such as:
- "Can all nodes in the blockchain network add new blocks to the blockchain at the same time?”
- "What challenges might arise if nodes in the network do not synchronize effectively?”
- “What would be the necessary activities for the blockchain network to add a new block to the blockchain?”
Recap the discussion by highlighting the importance of coordination in the blockchain network. Define the process of blockchain network synchronization as the consensus mechanism.

Defining the Consensus Mechanism (15 minutes)

In this phase provide a concise explanation of the concept of consensus mechanism. ⇒ using slides (2.3-consensus_mechanism.pptx)
Explain that consensus mechanisms determine how new blocks are added to the blockchain and how conflicts are resolved.
Define two main processes that are necessary for the nodes in the blockchain network to reach a consensus on the new block (candidate selection for block proposal and propagation of the block).
Explain that due to the asynchronous nature of the blockchain network, the process of reaching consensus is not deterministic but is probabilistic. It is achieved with some probability over time, due to the propagation of the information in the network.
Explain that there exist multiple approaches for candidate selection for block proposals. Highlight Proof-of-Work (PoW) as the first such approach that is also implemented in the case of the Bitcoin blockchain network.
Clarify that candidate selection in the case of PoW is based on the competition between nodes in the network to be the first one to solve a complex cryptographic question (in other words a “math problem”). The way to solve the question is by the “try and error” approach, where a lot of computations are performed.
Highlight that it is very easy for all nodes in the network to verify if the result is correct and that the result serves as proof that the creator of the block has won the competition and can propose a new block in the network.
Conclude the session by recapping the important ideas.