# <center><i class="fa fa-edit"></i> 2.2 Distributed Consensus </center>
###### tags: `Blockchain`
The following notes are taken from [Coursera](https://www.coursera.org/learn/cryptocurrency/home/week/2)

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Traditional motivation: reliability in distributed systems
- Distributed key-value store: enables various applications (i.e. DNS, public key directory, stock trades, etc)
- Good for **Altcoins**: systems built on Bitcoin-like principles to achieve slightly diff goals

### Definition
Suppose there are `n` nodes
1. Protocol terminates and all *correct* nodes decide on same value
2. Value must been proposed by some correct node

Bitcoin is **peer-to-peer system**
- When Alice wants to pay Bob, she *broadcasts the transaction to ALL Bitcoin nodes*
- Bob's computer not pictured
- Does not have to listen on the network for node to be his. Can choose to if he wants

### How consensus COULD work in Bitcoin
At any given time:
- All nodes have a sequence of *blocks of transactions* they have reached consensus on
- Each node has a set of outstanding transactions it's heard about

Cons - Why Consensus is Hard
- Nodes might
1. Crash
2. Be malicious

- Network is imperfect
1. Not all pairs of nodes connected
2. Faults in network
3. Latency -> no notion of global time

Impossibilities
- Byzantine General's Problem: inconsistency
- Fischer-Lynch-Paterson (deterministic nodes): consensus impossible with a single faulty node

Well Known Protocols
- Paxos: never produces inconsistent results, but can (rarely) get stuck

:::warning
1. Say more about the model than the problem
2. Developed for distributed systems
:::

:::success
THUS consensus in Bitcoin works better in practice than in theroy
BUT theory is important to help predict *unforeseen attacks*
:::

### Bitcoin Does Things Differently
1. Introduces incentives
- possible only because it is currency
2. Embraces randomness
- Does away with notion of specific end-point
- Consensus happens over long time scales (~ 1 hr)

### Summary
- Protection against invalid transactions is cryptographic but enforced by consensus
- Protection against double-spend is purely by consensus
- You are never 100% sure a transaction you are intered in is on the consensus branch, but exponential probability is pretty good