Bor Module

Table of Contents

• Preliminary terminology
• Overview
• How does it work
• How to propose a span
• Query commands

Preliminary terminology

• A side-transaction is a normal heimdall transaction but the data with which the message is composed needs to be voted on by the validators since the data is obscure to the consensus protocol itself and it has no way of validating the data's correctness.
• A sprint comprises of 16 bor blocks (configured in bor).
• A span comprises of 400 sprints in bor (check heimdall's bor params endpoint ).

Overview

The validators on bor chain produce blocks in sprints and spans. Hence, it is imperative for the protocol to formalise the validators who will be producers in a range of blocks (span). The bor module in heimdall facilitates this by pseudo-randomly selecting validators who will producing blocks (producers) from the current validator set. The bor chain fetches and persists this information before the next span begins. bor module is a crucial component in heimdall since the PoS chain "liveness" depends on it.

How does it work

A Span is defined by the data structure:

type Span struct {
	ID                uint64       `json:"span_id" yaml:"span_id"`
	StartBlock        uint64       `json:"start_block" yaml:"start_block"`
	EndBlock          uint64       `json:"end_block" yaml:"end_block"`
	ValidatorSet      ValidatorSet `json:"validator_set" yaml:"validator_set"`
	SelectedProducers []Validator  `json:"selected_producers" yaml:"selected_producers"`
	ChainID           string       `json:"bor_chain_id" yaml:"bor_chain_id"`
}

where ,

• ID means the id of the span, calculated by monotonically incrementing the ID of the previous span.
• StartBlock corresponds to the block in bor from which the given span would commence.
• EndBlock corresponds to the block in bor at which the given span would conclude.
• ValidatorSet defines the set of active validators.
• SelectedProducers are the validators selected to produce blocks in bor from the validator set.
• ChainID corresponds to bor chain ID.

A validator on heimdall can construct a span proposal message:

type MsgProposeSpan struct {
    ID         uint64                  `json:"span_id"`
    Proposer   hmTypes.HeimdallAddress `json:"proposer"`
    StartBlock uint64                  `json:"start_block"`
    EndBlock   uint64                  `json:"end_block"`
    ChainID    string                  `json:"bor_chain_id"`
    Seed       common.Hash             `json:"seed"`
}

Upon broadcasting the message, it is initially checked by HandleMsgProposeSpan handler for basic sanity (verify whether the proposed span is in continuity, appropriate span duration, correct chain ID, etc.). Since this is a side-transaction, the validators then vote on the data present in MsgProposeSpan on the basis of its correctness. All these checks are done in SideHandleMsgSpan (verifying Seed, span continuity, etc) and if correct, the validator would vote YES. Finally, if there are 2/3+ YES votes, the PostHandleMsgEventSpan persists the proposed span in the state via the keeper :

err := k.FreezeSet(ctx, msg.ID, msg.StartBlock, msg.EndBlock, msg.ChainID, msg.Seed)
if err != nil {
	k.Logger(ctx).Error("Unable to freeze validator set for span", "Error", err)
	return common.ErrUnableToFreezeValSet(k.Codespace()).Result()
}

FreezeSet internally invokes SelectNextProducers, which pseudo-randomly picks producers from the validaor set, leaning more towards validators with higher voting power based on stake:

// select next producers
newProducers, err := k.SelectNextProducers(ctx, seed)
if err != nil {
	return err
}

and then intialises and stores the span:

newSpan := hmTypes.NewSpan(
	id,
	startBlock,
	endBlock,
	k.sk.GetValidatorSet(ctx),
	newProducers,
	borChainID,
)

return k.AddNewSpan(ctx, newSpan)

How to propose a span

A validator can leverage the CLI to propose a span like so :

heimdallcli tx bor propose-span --proposer <VALIDATOR ADDRESS> --start-block <BOR_START_BLOCK> --span-id <SPAN_ID> --bor-chain-id <BOR_CHAIN_ID>

Or the REST server :

curl -X POST "localhost:1317/bor/propose-span?bor-chain-id=<BOR_CHAIN_ID>&start-block=<BOR_START_BLOCK>&span-id=<SPAN_ID>"

Query commands

One can run the following query commands from the bor module :

• span - Query the span corresponding to the given span id:

via CLI

heimdallcli query bor span --span-id=<SPAN_ID>

via REST

curl localhost:1317/bor/span/<SPAN_ID>

• latest span - Query the latest span :

via CLI

heimdallcli query bor latest-span

via REST

curl localhost:1317/bor/latest-span

• params - Fetch the parameters associated to bor module :

via CLI

heimdallcli query bor params

via REST

curl localhost:1317/bor/params

• spanlist - Fetch span list :

via CLI

heimdallcli query bor spanlist --page=<PAGE_NUM> --limit=<LIMIT>

• next-span-seed - Query the seed for the next span :

via CLI

heimdallcli query bor next-span-seed

via REST

curl localhost:1317/bor/next-span-seed

• propose-span - Print the propose-span command :

via CLI

heimdallcli query bor propose-span --proposer <VALIDATOR ADDRESS> --start-block <BOR_START_BLOCK> --span-id <SPAN_ID> --bor-chain-id <BOR_CHAIN_ID>

via REST

curl "localhost:1317/bor/prepare-next-span?span_id=<SPAN_ID>&start_block=<BOR_START_BLOCK>&chain_id="<BOR_CHAIN_ID>""