Runtime upgrades allow Polkadot to change the logic of the chain without the need for a hard fork.
You may have encountered the term "hard fork" before in the blockchain space. A hard fork occurs when a blockchain's logic changes such that nodes that do not include the new changes cannot remain in consensus with nodes that do. Such changes are backward incompatible. Hard forks can be political due to the nature of the upgrades and logistically demanding due to the number (potentially thousands) of nodes in the network that need to upgrade their software. Thus, hard forking is slow, inefficient, and error-prone due to the levels of offline coordination required and, therefore, the propensity to bundle many upgrades into one large-scale event.
By using Wasm in Substrate (the framework powering Polkadot, Kusama, and many connecting chains), parachains are given the ability to upgrade their runtime (a chain's "business logic") without hard forking.
Rather than encoding the runtime in the nodes, Polkadot nodes contain a WebAssembly execution host. They maintain consensus on a very low-level and well-established instruction set. Upgrades can be small, isolated, and very specific by deploying Wasm on-chain and having nodes auto-enact the new logic at a particular block height.
The Polkadot runtime is stored on the Polkadot blockchain itself. Polkadot can upgrade its runtime by upgrading the logic stored on-chain and removes the coordination challenge of requiring thousands of node operators to upgrade in advance of a given block number. Polkadot stakeholders propose and approve upgrades through the on-chain governance system, which also enacts them autonomously.
As a result of storing the Runtime as part of the state, the Runtime code itself becomes state sensitive, and calls to Runtime can change the Runtime code itself. Therefore, the Polkadot Host must always ensure it provides the Runtime corresponding to the state in which the entry point has been called.
The existing runtime logic is followed to update the Wasm runtime stored on the blockchain to a new version. The upgrade is then included in the blockchain itself, meaning that all the nodes on the network execute it. Generally, there is no need to upgrade your nodes manually before the runtime upgrade, as they will automatically start to follow the new logic of the chain. Nodes only need to be updated when the runtime requires new host functions, or there is a change in networking or consensus.
Transactions constructed for a given runtime version will not work on later versions. Therefore, a transaction constructed based on a runtime version will not be valid in later runtime versions. If you can’t submit a transaction before the upgrade, it is better to wait and construct it afterward.
Although upgrading your nodes is generally not necessary to follow an upgrade, we recommend following the Polkadot releases and upgrading promptly, especially for high-priority or critical releases.
Runtime vs Client versions
The runtime and client versions are distinct from each other. The runtime versioning typically looks
network-xxxx, whereas the client versioning looks like
vx.x.xx. For instance, the runtime
version shown on the top left section of Polkadot-JS UI below is
kusama-9370, and the client
(node) version shown on the top right section is
The runtime version can be queried on-chain through Polkadot-JS UI by navigating to the Developer
tab > Chain State > Storage > system and query
The node version can be queried by navigating to the Developer tab > RPC calls > system and query
Runtime Upgrades for Various Users
For Infrastructure Providers
Infrastructure services include but are not limited to the following:
- API services
- Node-as-a-Service (NaaS)
- General infrastructure management (e.g. block explorers, custodians)
For validators, keeping in sync with the network is key. At times, upgrades will require validators to upgrade their clients within a specific time frame, for example, if a release includes breaking changes to networking. It is essential to check the release notes, starting with the upgrade priority and acting accordingly.
General infrastructure providers, aside from following the Polkadot releases and upgrading in a timely manner, should monitor changes to runtime events and auxiliary tooling, such as the Substrate API Sidecar.
Transactions constructed for runtime
n will not work for any other runtime
>n. If a runtime
upgrade occurs before broadcasting a previously constructed transaction, you will need to
reconstruct it with the appropriate runtime version and corresponding metadata.
Runtime upgrades don't require any actions by a nominator, though it is always encouraged to keep up-to-date and participate with the latest runtime upgrade motions and releases while keeping an eye on how the nodes on the network are reacting to a new upgrade.
Monitoring Runtime Changes
You can monitor the chain for upcoming upgrades. The client release notes include the hashes of any proposals related to any on-chain upgrades for easy matching. Monitor the chain for:
democracy(Started)events and log
blockNumber. This event indicates that a referendum has started (although it does not mean it is a runtime upgrade). Get the referendum info*; it should have a status of
Ongoing. Find the ending block number (
end) and the enactment
delay(delay). If the referendum passes, it will execute on block number
end + delay.
democracy(Cancelled)events citing the index. If
Passed, you need to look at the
scheduler(Scheduled)event in the same block for the enactment block.
democracy(PreimageNoted)events with the same hash as the
ReferendumInfoOf(index)item. This may be up to the last block before execution, but it will not work if this is missing.
democracy(Executed)events for actual execution. In the case of a runtime upgrade, there will also be a
You can also monitor Polkassembly for discussions on on-chain proposals and referenda.
* E.g. via
pallets/democracy/storage/ReferendumInfoOf?key1=index&at=blockNumber on Sidecar.