Polkadot, Ethereum, Cosmos and NEAR all aim to build a sharded, decentralized and permissionless blockchain. Differences are reflected in their sharded blockchain architecture, consensus mechanism, and validator selection.
The consensus mechanism implemented on NEAR is called Nightshade. Nightshade models the system as a single blockchain. The list of all the transactions in each block is split into physical chunks, one chunk per shard. All chunks accumulate to one block. Note that chunks can only be validated by nodes that maintain the state of that shard.Theoretically, each logical block contains all the transactions for all the shards. However, since transmitting a logical block across the network would be prohibitively expensive, it is never initiated. Instead, each network participant maintains the state that corresponds to the shards that they validate transactions for and any additional shard that they want to track.
The consensus is based on the heaviest chain consensus. Meaning, once a block producer publishes a block, they collect the signatures of validator nodes. The weight of a block is then the cumulative stake of all the signers whose signatures are included in the block. The weight of a chain is the sum of the block weights. Additionally, the consensus utilises a finality gadget that introduces additional slashing conditions for higher chain security.
For more information on security, cross-shard communication, and validation roles, please refer to the Nightshade Paper or our summary in the docs.