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Concepts

Plan​

The x/upgrade module defines a Plan type in which a live upgrade is scheduled to occur. A Plan can be scheduled at a specific block height. A Plan is created once a (frozen) release candidate along with an appropriate upgrade Handler (see below) is agreed upon, where the Name of a Plan corresponds to a specific Handler. Typically, a Plan is created through a governance proposal process, where if voted upon and passed, will be scheduled. The Info of a Plan may contain various metadata about the upgrade, typically application specific upgrade info to be included on-chain such as a git commit that validators could automatically upgrade to.

Sidecar Process​

If an operator running the application binary also runs a sidecar process to assist in the automatic download and upgrade of a binary, the Info allows this process to be seamless. Namely, the x/upgrade module fulfills the cosmosd Upgradeable Binary Specification specification and cosmosd can optionally be used to fully automate the upgrade process for node operators. By populating the Info field with the necessary information, binaries can automatically be downloaded. See here for more info.

type Plan struct {
  Name   string
  Height int64
  Info   string
}

Handler​

The x/upgrade module facilitates upgrading from major version X to major version Y. To accomplish this, node operators must first upgrade their current binary to a new binary that has a corresponding Handler for the new version Y. It is assumed that this version has fully been tested and approved by the community at large. This Handler defines what state migrations need to occur before the new binary Y can successfully run the chain. Naturally, this Handler is application specific and not defined on a per-module basis. Registering a Handler is done via Keeper#SetUpgradeHandler in the application.

type UpgradeHandler func(Context, Plan, VersionMap) (VersionMap, error)

During each EndBlock execution, the x/upgrade module checks if there exists a Plan that should execute (is scheduled at that height). If so, the corresponding Handler is executed. If the Plan is expected to execute but no Handler is registered or if the binary was upgraded too early, the node will gracefully panic and exit.

StoreLoader​

The x/upgrade module also facilitates store migrations as part of the upgrade. The StoreLoader sets the migrations that need to occur before the new binary can successfully run the chain. This StoreLoader is also application specific and not defined on a per-module basis. Registering this StoreLoader is done via app#SetStoreLoader in the application.

func UpgradeStoreLoader (upgradeHeight int64, storeUpgrades *store.StoreUpgrades) baseapp.StoreLoader

If there's a planned upgrade and the upgrade height is reached, the old binary writes UpgradeInfo to the disk before panic'ing.

type UpgradeInfo struct {
  Name    string
  Height  int64
}

This information is critical to ensure the StoreUpgrades happens smoothly at correct height and expected upgrade. It eliminiates the chances for the new binary to execute StoreUpgrades multiple times everytime on restart. Also if there are multiple upgrades planned on same height, the Name will ensure these StoreUpgrades takes place only in planned upgrade handler.

Proposal​

Typically, a Plan is proposed and submitted through governance via a SoftwareUpgradeProposal. This proposal prescribes to the standard governance process. If the proposal passes, the Plan, which targets a specific Handler, is persisted and scheduled. The upgrade can be delayed or hastened by updating the Plan.Height in a new proposal.

type SoftwareUpgradeProposal struct {
  Title       string
  Description string
  Plan        Plan
}

Cancelling Upgrade Proposals​

Upgrade proposals can be cancelled. There exists a CancelSoftwareUpgrade proposal type, which can be voted on and passed and will remove the scheduled upgrade Plan. Of course this requires that the upgrade was known to be a bad idea well before the upgrade itself, to allow time for a vote.

If such a possibility is desired, the upgrade height is to be 2 * (VotingPeriod + DepositPeriod) + (SafetyDelta) from the beginning of the upgrade proposal. The SafetyDelta is the time available from the success of an upgrade proposal and the realization it was a bad idea (due to external social consensus).

A CancelSoftwareUpgrade proposal can also be made while the original SoftwareUpgradeProposal is still being voted upon, as long as the VotingPeriod ends after the SoftwareUpgradeProposal.