Both protocols are blockchains at their core but serve fundamentally different roles in how they areutilized:
- Ethereum is a general-purpose blockchain that hosts the Ethereum Virtual Machine, an environmentfor executing smart contracts. Ethereum is hom*ogenous but can utilize rollups and layer twosolutions to scale its usage.
- Polkadot is a heterogeneous, multi-chain protocol (a "layer 0" or metaprotocol) that hostsmultiple layer one blockchains and allows them to partake in shared security. Polkadot acts as ameta-protocol allowing multiple protocols to coexist and work together.
Sharding
In the context of blockchains, the term "shards" or "sharded protocol" is typically used to refer tosub-protocols or as a general term to refer to a form ofhorizontal scaling.
High-Level Comparison
Both protocols have fundamentally different goals:
Ethereum is a general-purpose blockchain based on the Ethereum Virtual Machine (EVM). Ethereum isnot specialized nor optimized for any particular application. Instead, its primary focus is theEthereum Virtual Machine for executing smart contracts. Ethereum achieves scalability viarollups are secondary protocols that utilize Ethereum as asettlement layer.
Polkadot is a multi-chain protocol that provides shared security and secure interoperability foreach of its parachains. Each parachain (also called an "appchain" in this context) isspecialized towards a specific focus and optimized towards that goal. Parachains must abide by theParachains Protocol.
Polkadot does not directly run a virtual machine for smart contracts, as Polkadot's primarypurpose is to validate the protocols that operate under it.
However, several parachains providesmart contract functionality.Parachains on Polkadot can even run an EVM for executing smart contracts written in Solidity usingFrontier, an Ethereum compatibility layer for Substrate.
As a general summary, one could also say that Polkadot coordinates and validatessub-protocols that follow the Parachains Protocol (which areakin to an optimistic-style rollup). In contrast, Ethereum coordinates inputs and outputs for theEVM. On Polkadot, any sub-protocol can have its own logic so long as it compiles to WebAssembly.
Scalability Approaches
Ethereum favors a rollup-centric approach for scaling transaction throughput.Danksharding is how Ethereum plans to betteraccommodate and facilitate rollup activity by providing better utilities, such as data availabilityvia Proto-Danksharding, for rollups to record state toEthereum.
Danksharding will allow for much more space to be utilized per block on Ethereum, where blobs ofdata will be verifiable for an amount of time before being pruned from the network. This approachwill enable data availability at layer one and further enable layer two protocols on Ethereum toflourish more readily.
In contrast, Polkadot require parachains toregister themselves in accordance with the Parachains Protocol.Once registered, Polkadot validates the statetransitions of each parachain as per their parachain validation function (PVF).Data availability is anintegral part of validating the parachain state. This approach enables parallelized interactionsbetween parachains. They can trust that each sub-protocol's respective state is valid, as Polkadotcollectively validated them.
Rollups vs. Parachain Creation
Ethereum primarily focuses on optimizing itself for rollups; Polkadot's parachains protocol allowsvalidation to occur on the protocol level without needing a layer two solution.
Rollup vs. Parachain Comparison
For a more in-depth comparison of parachains versus rollups, take a look at therollup comparison page
On Polkadot, each parachain hosts its own corelogic, called a runtime (sometimes called a state transition function).Polkadot uses WebAssembly(Wasm) as a "meta-protocol".
Parachains have the option of using cross-consensus messaging (XCM) to communicatewith one another and facilitate inter-chain reactions. It is also possible to utilize XCM onEthereum as it is merely a format for describing state transitions on a particular network.
Architectural Differences: Polkadot and Ethereum
As previously mentioned, Ethereum is a general-purpose virtual machine that can run sandboxedprograms are written in Solidity, whereas Polkadot is a meta-protocol for other parachains toconnect and interact with each other.
Ethereum operates as a single, hom*ogeneous chain. Each Ethereum node is divided into two layers: theconsensus and execution layers. Each layer handles the block validation information, peerdiscovery, and Proof-of-Stake of the Ethereum client.
Polkadot's primary component is the relay chain, which hosts heterogeneous parachains. Therelay chain aggregates information from each parachain, where validators agree upon consensus andfinality. One can see Polkadot as a series of runtimes, which are state transition functionsused to describe parachains and Polkadot itself.
Forks, Upgrades, and Governance
Ethereum's governance is done off-chain, where various stakeholders come to a consensus through somemedium other than the protocol itself. Upgrades on Ethereum will follow the standard hard forkprocedure, coordinating the community and validators to upgrade their nodes to implement protocolchanges.
Polkadot uses on-chaingovernance, called OpenGov, to facilitate upgrades to the Polkadotruntime. The stakeholders of Polkadot vote on these upgrades, and if successful, the upgrade isenacted automatically in the blocks to come. Polkadot validator operators only upgrade their nodeswhen the client itself gets updated.
Because of this mechanism, Polkadot can enactchain upgrades using the Wasm meta-protocol without a hard fork. As the WebAssembly runtime forPolkadot (and all of its subsequent parachains) are stored on-chain, this involves simply replacingthe runtime with a new WebAssembly blob once governance allowed the upgrade to be enacted.
Anything within the state transition function, the transaction queue, or off-chain workers can beupgraded without forking the chain, as these are all part of the WebAssembly runtime.
Block Production & Finalization
Both Ethereum and Polkadot use hybridconsensus models where block production and finality are decoupled.
For finalization, Ethereum utilizes Casper FFG, whichworks with LMD-GHOST as the fork choice rule forfinalization.
Polkadot utilizes GRANDPA for finalization. Ratherthan decide on a block-by-block basis, GRANDPA can finalize chains of blocks. Both finalizationmechanisms are bothGHOST-based and can bothfinalize batches of blocks in one round.
For block production, both protocols use slot-based protocols that randomly assign validators to aslot and provide a fork choice rule for unfinalized blocks. Polkadot usesBABE for block production. BABE includes twomechanisms for selecting block producers, one of which is a fallback in case the first fails, whichallows for chain liveness. BABE produces unfinalized blocks on top of the chain already finalized byGRANDPA.
There are two main differences between Ethereum andPolkadot consensus:
Polkadot's finality protocol, GRANDPA,finalizes batches of blocks based onavailability and validity checksthat happen as the proposed chain grows. The time to finality varies with the number of checksthat need to be performed (and invalidity reports, which cause extra checks). The expected timeto finality is 30 seconds.
Ethereum typically has many validators per round (called anepoch on Ethereum) to provide strong validityguarantees while Polkadot can providestronger guarantees with fewer validators per round.Polkadot achieves this by makingvalidators distribute an erasure coding to allvalidators in the system, such that anyone - not only the round's validators - can reconstruct aparachain's block and test its validity. This data availability is a core part of Polkadot -ensuring state is valid for its state transitions. The random parachain-validator assignmentsand secondary checks are performed by randomly selected validators, making it less likely forthe small set of validators on each parachain to collude.
Staking Mechanics: Ethereum PoS vs. Polkadot NPoS
Polkadot uses Nominated Proof of Stake (NPoS) to select validators from asmaller set, letting smaller holders nominate validators to run the network while claiming thesystem's rewards without running a node. Polkadot needs about five validators for each parachain inthe network. For more information, see the staking page.
Ethereum is a Proof of Stake (PoS) network that requires 32 ETH to stake for each validatorinstance. Validators run a primary Beacon Chain node and multiple validator clients - one for each32 ETH. These validators get assigned to "committees," randomly selected groups to validate blocksin the network.
Interoperability and Message Passing
Polkadot usesCross-Consensus Messaging (XCM) for parachains to send arbitrary messages to eachother. Parachains open connections with each other and can send messages via their establishedchannels. Given that collators communicate directly to the relay chain, theywill be connected and can relay messages from parachain A to parachain B if needed through thesemessage passing channels (see:HRMP, VMP, and other message passing mechanisms for XCM).
Messages do not pass through the relay chain. Only validity proofs and channel operations do (open,close, etc.). This enhances scalability by keeping data on the edges of the system.
Currently, Ethereum rollups can communicate usingshared sequencers,which provides a common ground of interoperability between layer two solutions.
Polkadot plans to have the concept ofAccords are opt-in treaties for differentprotocols to partake in. Accords ensure that logic about interoperability is kept consistent andcannot be changed and undermined by participating protocols. This helps ensure that any XCM messagecan be properly interpreted and executed as needed on the target protocol in a fully trustlessenvironment.
SPREE (Shared Protected Runtime Execution Enclaves) is the mechanism thatprovides shared logic for cross-consensus messages, and will be used to construct Accords.
DApp Support and Development
Ethereum supports smart contract development using Solidity. These contracts are immutable, andcannot be changed once published on-chain.
Polkadot supports smart contracts throughparachains, usually using the ink! smart contract language, but also Soliditythrough Frontier-enabled parachains. On Ethereum, smart contracts can call each other; however, theyare fixed on-chain to the domain of Ethereum. On Polkadot, smart contracts can call each other inthe same parachain and across parachains.
On Polkadot, developers have the option of either using smart contracts, calling extrinsics frompallets that modify the chain's state in some particular way or merely use Polkadot's RPC todirectly retrieve and act on-chain information. DApps on Polkadot are often composed of thesemultiple components working together to modify, retrieve, and watch state changes live as theyhappen.
For a more comprehensive list of how to build on Polkadot, be sure to check theBuild Section.
Conclusion
Ethereum and Polkadot both use a shardedmodel. Danksharding plans to utilize a rollup-centric approach by focusing on data availability. ThePolkadot ecosystem is secured by a main chain,called the "relay chain," which in turn manages cores and allows tasks, such as parachains, to berun on top of those cores and messages to be sent between them.
The primary differences between the two protocols are:
- Ethereum processes EVM-compatible state transitions, whether through rollups or on the mainnetit*elf, while Polkadot allows its parachainsto have an abstract state transition function implementation.
- Governance processes in Ethereum are planned to be off-chain and thus require coordination for ahard fork to enact governance decisions. In contrast, inPolkadot the decisions are on-chain andenacted autonomously via forkless upgrades.
- Validator selection mechanisms differ asPolkadot can provide strong availability andvalidity guarantees with fewer validators per protocol.
