Key takeaways

• Polkadot is upgrading its core infrastructure from Relay Chains to a new architecture called JAM (Join‑Accumulate Machine).
• JAM enables permissionless deployment, removing the need for costly parachain slot auctions.
• It introduces a multi-core, rollup-friendly virtual machine (PVM) built on RISC‑V, designed for scalability and modularity.
• The new architecture supports decentralized scheduling, off-chain processing, and on-chain finality, enhancing network efficiency.
  

Polkadot’s Next Chapter: What Is JAM and Why It Matters

Polkadot, a leading layer-0 blockchain protocol known for its unique Relay Chain–Parachain model, is making a groundbreaking shift in architecture. The upcoming JAM upgrade, short for Join‑Accumulate Machine, is poised to revolutionize how the Polkadot network handles scalability, smart contract deployment, and interoperability.

This upgrade is not just a technical enhancement; it’s a foundational redesign that brings developer freedom, network efficiency, and Web3 readiness to the forefront.

Relay Chain Limitations: Why Polkadot Needed JAM

Polkadot’s original architecture uses a central Relay Chain that secures multiple parachains, each requiring slot auctions to operate. While innovative, this model comes with several limitations:

• High barriers to entry via costly slot auctions
• Rigid structure with pre-defined chain logic
• Scalability bottlenecks due to synchronous execution

To overcome these challenges, Polkadot is evolving toward a decentralized, modular architecture and that’s where JAM comes in.

What Is JAM (Join‑Accumulate Machine)?

JAM stands for Join-Accumulate Machine, a name that originates from CoreJAM, a model that includes Collect, Refine, Join, and Accumulate phases. While Collect and Refine happen off-chain, the on-chain JAM protocol executes only the Join and Accumulate stages.

This design shift enables JAM to act as a generalized rollup chain, focusing solely on integrating off-chain computation into shared state, much like the original Relay Chain, but with far greater flexibility and modularity.

Why JAM Replaces the Relay Chain

Polkadot’s Relay Chain was innovative, but its governance-heavy upgrades, parachain slot auctions, and rigid protocol definitions imposed high entry barriers and slow innovation.

JAM addresses these limitations with:

• A unified upgrade approach, minimizing long-term breaking changes.
• A transactionless design for efficient, parallel execution.
• Support for permissionless services (instead of governance-approved pallets).
• A domain-specific rollup architecture that generalizes computation without opinionation.

Core Features of JAM Architecture

Core to JAM’s architecture is its modular, service-based design that replaces rigid parachain slots with permissionless deployment. It leverages a multi-core execution model powered by the RISC‑V-based Polkadot Virtual Machine (PVM), enabling parallel processing at scale. 

JAM is transactionless, operates through off-chain computation refined into on-chain state, and supports native rollups and asynchronous communication between services, making it a flexible and scalable foundation for next-generation blockchain applications.

Let’s understand these features in a bit more detail:

Services Instead of Parachains

In JAM, blockchain state is broken into services, which encapsulate code, data, and balances—similar to smart contracts but much more powerful. Deploying a new service is permissionless and requires no auctions or governance votes.

Three Execution Entry Points

JAM services operate through:

• Refine: Stateless, off-chain transformation of work items
• Accumulate: On-chain state integration of Refine results
• onTransfer: Handles asynchronous messages and tokens between services
  

This architecture supports a multi-core, parallel execution model, where each core processes up to 15MB of input per 6-second time slot and produces compact 90kB outputs.

JAM Is Transactionless

Unlike traditional smart contract platforms, JAM has no on-chain transactions. Instead, it processes work packages that are refined off-chain and committed via validators’ attestations. JAM uses five types of extrinsic information to maintain integrity:

• Guarantees and Assurances: Validator confirmations on output correctness
• Judgments: Trigger state rollback if consensus on output validity fails
• Preimages: Allows Refine to request stored data via hashes
• Tickets: Anonymous block production entries for the SAFROLE consensus algorithm

RISC-V Powered Polkadot Virtual Machine (PVM)

JAM introduces the Polkadot Virtual Machine (PVM) based on the open-source RISC-V instruction set. Compared to WebAssembly (Wasm), RISC-V:

• Allows continuation support using a memory-based stack
• Offers superior speed and predictability
• Integrates seamlessly with LLVM toolchains
• Enables asynchronous and parallel execution for better scalability

PVM becomes the universal runtime across JAM, replacing Wasm and enabling efficient core usage across services.

Block Production with SAFROLE

SAFROLE is JAM’s simplified and privacy-focused consensus protocol, evolving from SASSAFRAS. Its goals include:

• Minimal opinionation to enable broader implementation
• Anonymous validator participation through a ticket system
• Fast block propagation and fork resistance

JAM’s SAFROLE protocol helps simplify understanding and replicability, aligning with Polkadot’s broader mission to open up protocol innovation.

Smart Contract-Like Power Without Smart Contracts

Services in JAM behave like smart contracts but are more flexible:

• Can be upgraded independently
• Support asynchronous communication between services
• Operate with no transaction layer, reducing overhead
• Interact through Accords, a trustless messaging system similar to XCM forwarding
  

This allows for use cases like token teleportation across parachains without relying on intermediaries.

Pipelined Processing for High Efficiency

JAM redefines block processing with pipelining:

• Block headers contain the prior state root (not the posterior one)
• Allows 95% of work to be performed after block distribution
• Enables continuous computation with minimal idle time between blocks
• Results in up to 3.5 seconds of actual computation time per 6-second block

This contrasts sharply with traditional blockchains like Ethereum, which rely on synchronous full-block computation.

Networking and Validator Efficiency

JAM utilizes the QUIC protocol for peer-to-peer connections among over 1,000 validators. It avoids gossip-based propagation through grid-diffusal, ensuring low-latency, high-efficiency message delivery even in large validator sets.

JAM Toaster: Large-Scale Testing Infrastructure

To validate JAM at scale, the JAM Toaster testnet simulates a full 1,023-node validator network. Unlike Kusama or small-scale testnets, JAM Toaster allows developers to study emergent behavior, performance bottlenecks, and system dynamics in a fully instrumented environment, crucial for high-assurance deployment.

JAM and Substrate: Compatibility and Flexibility

JAM remains compatible with Substrate and existing Polkadot SDK tools, although it uses PVM instead of Wasm. It introduces a metered system for runtime performance, reducing the need for traditional benchmarking.

JAM also supports full XCMP (Cross-Chain Message Passing) and Agile Coretime, enabling developers to build complex systems with flexible resource usage.

JAM vs Relay Chain: What’s Different?

JAM introduces a radically simplified and developer-friendly architecture compared to the traditional Relay Chain. By eliminating slot auctions and enabling permissionless service deployment, JAM empowers builders with greater flexibility and autonomy.

Its shift from a slot-based, Wasm-dependent execution model to a multi-core, RISC‑V-powered Polkadot Virtual Machine (PVM) allows for faster, parallelized computation. Combined with service-specific upgrades and native rollup support, JAM creates a more scalable, modular, and decentralized framework tailored for the future of Web3.

Benefits of JAM for Developers and Web3 Builders

• Lower entry costs: No slot auctions required
• Faster deployment cycles with modular service updates
• Greater scalability thanks to multi-core execution
• Built-in support for rollups, DAOs, AI agents, and complex apps
• Improved decentralization and security through SNARK-powered consensus
  

JAM positions Polkadot as a future-ready infrastructure layer, ideal for emerging Web3 use cases and high-throughput decentralized apps.

When Will JAM Launch?

JAM is currently in development with the support of Parity Technologies and the Web3 Foundation. Testnets like JAM Toaster are being prepared, and the protocol will undergo community governance approval before going live on Polkadot mainnet (anticipated in Q3 2025).

FAQs:

1. What does JAM mean in Polkadot?
   JAM stands for Join‑Accumulate Machine, a new Polkadot architecture that enables permissionless services and multi-core processing.
2. How is JAM different from Polkadot’s Relay Chain?
   JAM removes slot auctions, uses a lightweight VM, and supports native rollups for better scalability and modularity.
3. Is JAM compatible with existing Polkadot parachains?
   Yes, JAM is designed for backward compatibility, allowing current parachains to integrate or migrate smoothly.
4. What is the PolkaVM (PVM)?
   PolkaVM is a new virtual machine in JAM based on RISC‑V, optimized for blockchain use cases and persistent smart contracts.
5. When is the JAM upgrade expected on Polkadot?
   While there’s no fixed date, testnet deployments and community testing are underway in 2025, with governance approval required for mainnet launch.