Having recently researched Polkadot, as with other projects, I wanted to document what I had learnt, so that others may potentially find it useful. Hopefully providing a balanced view, it will consist of three articles outlined below.
Part One — Polkadot Overview and Benefits (This article)
I will provide links throughout, providing reference to sections, as well as include a list of sources at the bottom of the article for further reading.
Frustrated with the slow development of Ethereum 2.0, Dr. Gavin Wood, co-founder of Ethereum and inventor of Solidity, left to begin work on Polkadot, a next generation scalable blockchain protocol that connects multiple specialised blockchains into one unified network. It achieves scalability through a sharding infrastructure with multiple blockchains running in parallel, called parachains, that connect to a central chain called the Relay Chain.
Whilst it shares some similarities with Ethereum 2.0, one key differentiator is that it uses heterogeneous sharding, where each parachains can be customised through the Substrate development framework, enabling them to be optimised for a specific use case and running in parallel rather than same across all shards. This is important as when it comes to blockchain architecture, one size does not fit all and all blockchains make trade-offs to support different features and use cases.
All parachains connect to the relay chain, which validates the state transition of connected parachains, providing shared state across the entire ecosystem. If the Relay Chain must revert for any reason, then all of the parachains would also revert. This is to ensure that the validity of the entire system can persist, and no individual part is corruptible. The shared state makes it so that the trust assumptions when using parachains are only those of the Relay Chain validator set, and no other. Since the validator set on the Relay Chain is expected to be secure with a large amount of stake put up to back it, it is desirable for parachains to benefit from this security.
This enables seamless interoperability between all parachains and parathreads using the Cross-chain Message Passing (XCMP) protocol, allowing arbitrary data — not just tokens — to be transferred across blockchains. Interoperability is also possible to other ecosystems through bridges, which are specifically designed parachains or parathreads that are custom made to interact with another ecosystem such as Ethereum, Bitcoin and Cosmos for example, enabling interoperability. Because these other ecosystems don’t use the same shared state of Polkadot, finality is incredibly important, because whilst the relay chain can roll back all the parachains, it can’t roll back the Ethereum or Bitcoin blockchains for example. This is discussed further in part three.
The relay chain is responsible for the network’s shared security, consensus and cross-chain interoperability. It is secured by Validators and Nominators staking the native DOT tokens. Ultimately scalability for the ecosystem is determined by how scalable the relay chain can be. The number of parachains is determined by the number of validators on the relay chain. The hope is to reach 1000 validators, which would enable around 100 parachains. With each parachain being capable of around 1,000 transactions per second.
Nominators stake their DOT tokens with validators they trust, with the validators likely charging a small commission to cover running costs. If a validator is found to have performed misconduct a percentage of the their stake but also the nominators stake will be slashed depending upon the severity. For Level 4 security threats such as collusion and including an invalid block then 100% of the stake will be slashed.What’s really important to understand is that both the validators own stake and the nominated stake will be slashed, so you could lose all your DOT that you have staked against a validator if they perform maliciously. Therefore, it’s very important not to just try and maximise rewards and being oblivious to the risk, not only can you lose all your DOT, but you are making the entire system less secure (addressed in part three). There have already been several minor slashing incidents so far, so something to really consider.
Auction for Parachain Slots
Due to the limited number of parachain slots available, there needs to be a method to decide who gets a parachain slot. This is achieved through a candle-auction where participants bid with DOT to secure a lease on a parchain slot to secure a 6 — 24 month period, with the highest bidders winning. DOT isn’t spent, but rather locked for the duration of the lease and unable to participate in staking and earn rewards. In the event they are unsuccessful in securing a further slot, then the lease expires and the DOT will be returned.
Of the 100 parachain slots that they hope to be able to accommodate, between 10 and 30 will be reserved for system parachains, with the remaining available for either auction slots or used for parathreads. Whilst the DOT is returned, due to the limited number of slots available this could result in significant amounts of DOT needing to be acquired to secure a slot. How the auction mechanics effect the price of DOT also remains to be seen, with potentially a rise from the start of the auction, followed by a fall before the lease ends and the DOT are returned. The plan is to continuously have a small amount of parachain auctions going throughout the year, to minimise any unwanted effects. How comfortable developers will be with locking significant amounts of funds in a highly volatile asset for an extended amount of time, also remains to be seen. They could also be in a position where they can no longer afford to keep their lease and have to downgrade to a parathread (providing the application will still function with the reduced performance or migrate to another platform). See this article for more details on the auction mechanism
For applications that don’t require the guaranteed performance of a parachain or don’t want to pay the large fees to secure a parachain slot, then parathreads can be used instead. Parathreads have a fixed fee for registration that would realistically be much lower than the cost of acquiring a parachain slot and compete with other parathreads in a per-block auction to have their transactions included in the next relay chain block. A portion of the parachain slots on the Relay Chain will be designated as part of the parathread pool.
In the event that a parachain loses its slot then it can transition to a parathread (assuming the application can still function with the reduced and varied performance of sharing the slot between many). This also enables small projects to start out with a parathread and then upgrade to a parachain slot when required.
DOT is the native token of the Polkadot network and serves three key functions. (i) It is staked to provide security for the relay chain, (ii) to be bonded to connect a chain to Polkadot as a parachain and (iii) to be used for governance of the network. There is an initial total supply of 1 billion DOT with yearly inflation estimated to be around 10% providing the optimal 50% staking rate is achieved, resulting in rewards of 20% to those that stake (net 10% when take into account inflation). Those that don’t stake lose 10% through dilution. Should the amount staked exceed the optimal 50% then reward rates reduce as well as inflation to make staking less attractive. Likewise if its below 50% then rewards and inflation rate will be higher to encourage staking. Staking isn’t risk free though as mentioned before.
Polkadot employs an on-chain governance model where in order to make any changes to the network, DOT holders vote on a proposal to upgrade the network with the help of the Council. The council is an entity comprising a 23 seats each represented by an on-chain account. Its goals are to represent passive stakeholders, submit sensible and important proposals, and cancel dangerous or malicious proposals. All DOT holders are free to register their candidacy for the Council, and free to vote for any number of candidates, with a voting power proportional to their stake.
Any stakeholder can submit a public proposal by depositing a fixed minimum amount of DOTs, which stays locked for a certain period. If someone agrees with the proposal, they may deposit the same amount of tokens to endorse it. Public proposals are stored in a priority queue, and at regular intervals the proposal with the most endorsements gets tabled for a referendum. The locked tokens are released once the proposal is tabled. Council proposals are submitted by the Council, and are stored in a separate priority queue where the priorities are set at the Council’s discretion.
Every thirty days, a new proposal will be tabled, and a referendum will come up for a vote. The proposal to be tabled is the top proposal from either the public-proposal queue or the Council-proposal queue, alternating between the two queues.
The Technical Committee is composed according to a single vote for each team that has successfully and independently implemented or formally specified the protocol in Polkadot, or in its canary network Kusama. The Technical Committee is the last line of defence for the system. Its sole purpose is detecting present or imminent issues in the system such as bugs in the code or security vulnerabilities, and proposing and fast-tracking emergency referenda.
Whilst parachains aren’t currently implemented at this stage, there is a rapidly growing ecosystem looking to build on Polkadot with substrate. Polkadot’s “cousin”, the canary network Kusama used for experimentation, was launched last year by the same team and contributes to the early growth of the overall ecosystem. See here for a list of the current projects looking to build on Polkadot and filter by Substrate based.
Now that we have covered the basics, in part two I will explain how the consensus mechanism in Polkadot works and covering more of the technical aspects.
For additional reading please see a list of articles / research papers I found particularly useful whilst researching.
Whitepaper, Overview of Polkadot and its Design Considerations, BABE Research Paper, GRANDPA: a Byzantine Finality Gadget, Video with NEAR explaining Polkadot consensus, Polkadot Wiki, Path of a Parachain block, Block Production and Finalization in Polkadot, Availability and Validity Research Paper, Polkadot’s Data Availability and Validity Scheme, Consensus Part 2 — Grandpa, Consensus Part 3 — BABE, Consensus Part 4 — Security, Slashing Mechanisms, Check Validators / Stake, Censorship, Polkadot Bridges, Polkadot, Substrate and Ethereum, Polkadot — shared security or single point of failure?, Projects building on Polkadot, Polkascan Block Explorer, Polkadot, The Ethereum 2.0 or The Mighty Race