ADR 046: Lite Client Implementation

Changelog

13-02-2020: Initial draft

Context

A Client struct represents a light client, connected to a single blockchain. As soon as it's started (via Start), it tries to update to the latest header (using bisection algorithm by default).

Cleaning routine is also started to remove headers outside of trusting period. NOTE: since it's periodic, we still need to check header is not expired in TrustedHeader, TrustedValidatorSet methods (and others which are using the latest trusted header).

The user has an option to manually verify headers using VerifyHeader and VerifyHeaderAtHeight methods. To avoid races, UpdatePeriod(0) needs to be passed when initializing the light client (it turns off the auto update).

type Client interface {
	// start and stop updating & cleaning goroutines
	Start() error
	Stop()
	Cleanup() error

	// get trusted headers & validators
	TrustedHeader(height int64, now time.Time) (*types.SignedHeader, error)
	TrustedValidatorSet(height int64, now time.Time) (*types.ValidatorSet, error)
	LastTrustedHeight() (int64, error)
	FirstTrustedHeight() (int64, error)

	// query configuration options
	ChainID() string
	Primary() provider.Provider
	Witnesses() []provider.Provider

	// verify new headers
	VerifyHeaderAtHeight(height int64, now time.Time) (*types.SignedHeader, error)
	VerifyHeader(newHeader *types.SignedHeader, newVals *types.ValidatorSet, now time.Time) error
}

A new light client can either be created from scratch (via NewClient) or using the trusted store (via NewClientFromTrustedStore). When there's some data in the trusted store and NewClient is called, the light client will a) check if stored header is more recent b) optionally ask the user whenever it should rollback (no confirmation required by default).

func NewClient(
	chainID string,
	trustOptions TrustOptions,
	primary provider.Provider,
	witnesses []provider.Provider,
	trustedStore store.Store,
	options ...Option) (*Client, error) {

witnesses as argument (as opposite to Option) is an intentional choice, made to increase security by default. At least one witness is required, although, right now, the light client does not check that primary != witness. When cross-checking a new header with witnesses, minimum number of witnesses required to respond: 1.

Due to bisection algorithm nature, some headers might be skipped. If the light client does not have a header for height X and TrustedHeader(X) or TrustedValidatorSet(X) methods are called, it will download the header from primary provider and perform a backwards verification.

type Provider interface {
	ChainID() string

	SignedHeader(height int64) (*types.SignedHeader, error)
	ValidatorSet(height int64) (*types.ValidatorSet, error)
}

Provider is a full node usually, but can be another light client. The above interface is thin and can accommodate many implementations.

If provider (primary or witness) becomes unavailable for a prolonged period of time, it will be removed to ensure smooth operation.

Both Client and providers expose chain ID to track if there are on the same chain. Note, when chain upgrades or intentionally forks, chain ID changes.

The light client stores headers & validators in the trusted store:

type Store interface {
	SaveSignedHeaderAndNextValidatorSet(sh *types.SignedHeader, valSet *types.ValidatorSet) error
	DeleteSignedHeaderAndNextValidatorSet(height int64) error

	SignedHeader(height int64) (*types.SignedHeader, error)
	ValidatorSet(height int64) (*types.ValidatorSet, error)

	LastSignedHeaderHeight() (int64, error)
	FirstSignedHeaderHeight() (int64, error)

	SignedHeaderAfter(height int64) (*types.SignedHeader, error)
}

At the moment, the only implementation is the db store (wrapper around the KV database, used in Tendermint). In the future, remote adapters are possible (e.g. Postgresql).

func Verify(
	chainID string,
	h1 *types.SignedHeader,
	h1NextVals *types.ValidatorSet,
	h2 *types.SignedHeader,
	h2Vals *types.ValidatorSet,
	trustingPeriod time.Duration,
	now time.Time,
	trustLevel tmmath.Fraction) error {

Verify pure function is exposed for a header verification. It handles both cases of adjacent and non-adjacent headers. In the former case, it compares the hashes directly (2/3+ signed transition). Otherwise, it verifies 1/3+ (trustLevel) of trusted validators are still present in new validators.

Status

Accepted.

Consequences

Positive

single Client struct, which is easy to use
flexible interfaces for header providers and trusted storage

Negative

Verify needs to be aligned with the current spec

Neutral

Verify function might be misused (called with non-adjacent headers in incorrectly implemented sequential verification)

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