# x/upgrade
# Abstract
x/upgrade is an implementation of a Cosmos SDK module that facilitates smoothly
upgrading a live Cosmos chain to a new (breaking) software version. It accomplishes this by
providing a BeginBlocker hook that prevents the blockchain state machine from
proceeding once a pre-defined upgrade block height has been reached.
The module does not prescribe anything regarding how governance decides to do an upgrade, but just the mechanism for coordinating the upgrade safely. Without software support for upgrades, upgrading a live chain is risky because all of the validators need to pause their state machines at exactly the same point in the process. If this is not done correctly, there can be state inconsistencies which are hard to recover from.
# 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. This tool is Cosmovisor (opens new window).
# 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.
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.
If there's a planned upgrade and the upgrade height is reached, the old binary writes Plan to the disk before panicking.
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 proposal
containing a MsgSoftwareUpgrade message.
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.
# Cancelling Upgrade Proposals
Upgrade proposals can be cancelled. There exists a gov-enabled MsgCancelUpgrade
message type, which can be embedded in a proposal, voted on and, if passed, 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 MsgCancelUpgrade proposal can also be made while the original
MsgSoftwareUpgrade proposal is still being voted upon, as long as the VotingPeriod
ends after the MsgSoftwareUpgrade proposal.
# State
The internal state of the x/upgrade module is relatively minimal and simple. The
state contains the currently active upgrade Plan (if one exists) by key
0x0 and if a Plan is marked as "done" by key 0x1. The state
contains the consensus versions of all app modules in the application. The versions
are stored as big endian uint64, and can be accessed with prefix 0x2 appended
by the corresponding module name of type string. The state maintains a
Protocol Version which can be accessed by key 0x3.
- Plan:
0x0 -> Plan - Done:
0x1 | byte(plan name) -> BigEndian(Block Height) - ConsensusVersion:
0x2 | byte(module name) -> BigEndian(Module Consensus Version) - ProtocolVersion:
0x3 -> BigEndian(Protocol Version)
The x/upgrade module contains no genesis state.
# Events
The x/upgrade does not emit any events by itself. Any and all proposal related
events are emitted through the x/gov module.
# Client
# CLI
A user can query and interact with the upgrade module using the CLI.
# Query
The query commands allow users to query upgrade state.
# applied
The applied command allows users to query the block header for height at which a completed upgrade was applied.
If upgrade-name was previously executed on the chain, this returns the header for the block at which it was applied. This helps a client determine which binary was valid over a given range of blocks, as well as more context to understand past migrations.
Example:
Example Output:
# module versions
The module_versions command gets a list of module names and their respective consensus versions.
Following the command with a specific module name will return only that module's information.
Example:
Example Output:
Example:
Example Output:
# plan
The plan command gets the currently scheduled upgrade plan, if one exists.
Example:
Example Output:
# REST
A user can query the upgrade module using REST endpoints.
# Applied Plan
AppliedPlan queries a previously applied upgrade plan by its name.
Example:
Example Output:
# Current Plan
CurrentPlan queries the current upgrade plan.
Example:
Example Output:
# Module versions
ModuleVersions queries the list of module versions from state.
Example:
Example Output:
# gRPC
A user can query the upgrade module using gRPC endpoints.
# Applied Plan
AppliedPlan queries a previously applied upgrade plan by its name.
Example:
Example Output:
# Current Plan
CurrentPlan queries the current upgrade plan.
Example:
Example Output:
# Module versions
ModuleVersions queries the list of module versions from state.
Example:
Example Output:
# Resources
A list of (external) resources to learn more about the x/upgrade module.
- Cosmos Dev Series: Cosmos Blockchain Upgrade (opens new window) - The blog post that explains how software upgrades work in detail.