What is the purpose of Polkadot's parachains?

Core Functionality of Parachains

1. Parachains are sovereign blockchains that operate in parallel within the Polkadot ecosystem, each maintaining its own state and logic while leveraging shared security from the Relay Chain.

2. They enable specialized use cases such as DeFi, identity management, oracles, and gaming without compromising on scalability or interoperability.

3. Each parachain connects to Polkadot via a dedicated slot secured through an auction mechanism, ensuring resource allocation is both competitive and transparent.

4. Runtime upgrades for parachains are executed without hard forks, allowing seamless evolution of logic and features while preserving continuity of operations.

5. Developers deploy custom WebAssembly-based runtimes, granting full control over consensus parameters, transaction formats, and governance models.

Data Availability and Validity Guarantees

1. Validators on the Relay Chain periodically sample parachain block data to verify availability, preventing malicious withholding of critical information.

2. Each parachain block must be accompanied by a validity certificate signed by a subset of Relay Chain validators before finalization.

3. Disputes over block validity trigger a slashing protocol where misbehaving validators lose bonded DOT tokens, reinforcing honest participation.

4. The inclusion queue mechanism ensures fair ordering of parachain blocks based on priority and availability proofs, not arbitrary validator preference.

5. Fraud proofs are not natively supported; instead, Polkadot relies on synchronous validity checking during block production to enforce correctness.

Cross-Chain Message Passing (XCMP)

1. XCMP enables trust-minimized communication between parachains using horizontal relay-routed messages anchored in Relay Chain consensus.

2. Messages are queued per destination parachain and processed in FIFO order upon successful validation and availability confirmation.

3. Delivery guarantees are probabilistic but strongly bounded—messages either arrive within a known timeframe or are dropped with notification.

4. Message payloads are limited to 100KB to ensure predictable processing overhead across heterogeneous parachain environments.

5. Routing logic resides entirely on the Relay Chain, eliminating the need for parachains to maintain direct peer-to-peer connections.

Economic Incentives and Slot Allocation

1. Parachain slots are allocated via candle auctions where bidders lock DOT tokens for lease periods ranging from three to twelve months.

2. Winning bidders retain exclusive usage rights for their lease duration, after which they must re-enter the auction or risk disconnection.

3. Crowdloans allow token holders to contribute DOT to support parachain campaigns, earning rewards denominated in the respective parachain’s native token.

4. Unused lease time cannot be transferred or sold; early termination results in forfeiture of locked funds unless coordinated through parachain governance.

5. DOT staked in crowdloans remains locked until the end of the relevant lease period, regardless of parachain operational status.

Security Model Integration

1. Parachains inherit finality guarantees from the Relay Chain’s NPoS consensus, eliminating the need for independent validator sets or economic security bootstrapping.

2. Shared security means no single parachain can compromise the integrity of others—even if compromised, its impact is strictly isolated.

3. Validator rotation occurs every era (approximately 24 hours), ensuring continuous redistribution of responsibilities and minimizing long-term collusion risks.

4. The Relay Chain does not execute parachain logic; it only verifies cryptographic proofs of correct execution provided by collators.

5. Collators are responsible for assembling valid parachain blocks and submitting them to validators, but hold no consensus power themselves.

Frequently Asked Questions

Q: Can a parachain change its runtime without losing its slot?A: Yes. Runtime upgrades are permissionless and do not affect slot ownership as long as the parachain continues submitting valid blocks under the updated logic.

Q: What happens if a parachain fails to produce blocks for several sessions?A: It may be marked as inactive and temporarily removed from the active set until it resumes consistent block production and passes health checks.

Q: Are parachain blocks finalized independently or only after Relay Chain confirmation?A: Finality is conferred exclusively by the Relay Chain. A parachain block is not considered irreversible until included and finalized in the Relay Chain state.

Q: Do parachains have access to native DOT for transaction fees?A: No. Each parachain defines its own fee token and economic model. DOT is used solely for staking, governance, and auction participation—not as gas.