epochs

Abstract

This document specifies the internal x/epochs module of the Evmos Hub.

Often, when working with the Cosmos SDK, we would like to run certain pieces of code every so often.

The purpose of the epochs module is to allow other modules to maintain that they would like to be signaled once in a time period. So, another module can specify it wants to execute certain code once a week, starting at UTC-time = x. epochs creates a generalized epoch interface to other modules so they can be more easily signaled upon such events.

Contents

1. Concept
2. State
3. Events
4. Keeper
5. Hooks
6. Queries
7. Future improvements

Concepts

The epochs module defines on-chain timers that execute at fixed time intervals. Other Evmos modules can then register logic to be executed at the timer ticks. We refer to the period in between two timer ticks as an "epoch".

Every timer has a unique identifier, and every epoch will have a start time and an end time, where end time = start time + timer interval.

State

State Objects

The x/epochs module keeps the following objects in state:

State Object	Description	Key	Value	Store
EpochInfo	Epoch info bytecode	[]byte{identifier}	[]byte{epochInfo}	KV

EpochInfo

An EpochInfo defines several variables:

1. identifier keeps an epoch identification string
2. start_time keeps the start time for epoch counting: if block height passes start_time, then epoch_counting_started is set
3. duration keeps the target epoch duration
4. current_epoch keeps the current active epoch number
5. current_epoch_start_time keeps the start time of the current epoch
6. epoch_counting_started is a flag set with start_time, at which point epoch_number will be counted
7. current_epoch_start_height keeps the start block height of the current epoch

message EpochInfo {
    string identifier = 1;
    google.protobuf.Timestamp start_time = 2 [
        (gogoproto.stdtime) = true,
        (gogoproto.nullable) = false,
        (gogoproto.moretags) = "yaml:\"start_time\""
    ];
    google.protobuf.Duration duration = 3 [
        (gogoproto.nullable) = false,
        (gogoproto.stdduration) = true,
        (gogoproto.jsontag) = "duration,omitempty",
        (gogoproto.moretags) = "yaml:\"duration\""
    ];
    int64 current_epoch = 4;
    google.protobuf.Timestamp current_epoch_start_time = 5 [
        (gogoproto.stdtime) = true,
        (gogoproto.nullable) = false,
        (gogoproto.moretags) = "yaml:\"current_epoch_start_time\""
    ];
    bool epoch_counting_started = 6;
    reserved 7;
    int64 current_epoch_start_height = 8;
}

The epochs module keeps these EpochInfo objects in state, which are initialized at genesis and are modified on begin blockers or end blockers.

Genesis State

The x/epochs module's GenesisState defines the state necessary for initializing the chain from a previously exported height. It contains a slice containing all the EpochInfo objects kept in state:

// Genesis State defines the epoch module's genesis state
type GenesisState struct {
    // list of EpochInfo structs corresponding to all epochs
    Epochs []EpochInfo `protobuf:"bytes,1,rep,name=epochs,proto3" json:"epochs"`
}

Events

The x/epochs module emits the following events:

BeginBlocker

Type	Attribute Key	Attribute Value
epoch_start	"epoch_number"	{epoch_number}
epoch_start	"start_time"	{start_time}

EndBlocker

Type	Attribute Key	Attribute Value
epoch_end	"epoch_number"	{epoch_number}

Keepers

The x/epochs module only exposes one keeper, the epochs keeper, which can be used to manage epochs.

Epochs Keeper

Presently only one fully-permissioned epochs keeper is exposed, which has the ability to both read and write the EpochInfo for all epochs, and to iterate over all stored epochs.

// Keeper of epoch nodule maintains collections of epochs and hooks.
type Keeper struct {
    cdc      codec.Codec
    storeKey storetypes.StoreKey
    hooks    types.EpochHooks
}

// Keeper is the interface for epoch module keeper
type Keeper interface {
  // GetEpochInfo returns epoch info by identifier
  GetEpochInfo(ctx sdk.Context, identifier string) types.EpochInfo

  // SetEpochInfo set epoch info
  SetEpochInfo(ctx sdk.Context, epoch types.EpochInfo)

  // DeleteEpochInfo delete epoch info
  DeleteEpochInfo(ctx sdk.Context, identifier string)

  // IterateEpochInfo iterate through epochs
  IterateEpochInfo(ctx sdk.Context, fn func(index int64, epochInfo types.EpochInfo) (stop bool))

  // Get all epoch infos
  AllEpochInfos(ctx sdk.Context) []types.EpochInfo
}

Hooks

The x/epochs module implements hooks so that other modules can use epochs to allow facets of the Cosmos SDK to run on specific schedules.

Hooks Implementation

// combine multiple epoch hooks, all hook functions are run in array sequence
type MultiEpochHooks []types.EpochHooks

// AfterEpochEnd is called when epoch is going to be ended, epochNumber is the
// number of epoch that is ending
func (mh MultiEpochHooks) AfterEpochEnd(ctx sdk.Context, epochIdentifier string, epochNumber int64) {...}

// BeforeEpochStart is called when epoch is going to be started, epochNumber is
// the number of epoch that is starting
func (mh MultiEpochHooks) BeforeEpochStart(ctx sdk.Context, epochIdentifier string, epochNumber int64) {...}

// AfterEpochEnd executes the indicated hook after epochs ends
func (k Keeper) AfterEpochEnd(ctx sdk.Context, identifier string, epochNumber int64) {...}

// BeforeEpochStart executes the indicated hook before the epochs
func (k Keeper) BeforeEpochStart(ctx sdk.Context, identifier string, epochNumber int64) {...}

Recieving Hooks

When other modules (outside of x/epochs) recieve hooks, they need to filter the value epochIdentifier, and only do executions for a specific epochIdentifier.

The filtered values from epochIdentifier could be stored in the Params of other modules, so they can be modified by governance.

Governance can change epoch periods from week to day as needed.

Queries

The x/epochs module provides the following queries to check the module's state.

service Query {
  // EpochInfos provide running epochInfos
  rpc EpochInfos(QueryEpochsInfoRequest) returns (QueryEpochsInfoResponse) {}
  // CurrentEpoch provide current epoch of specified identifier
  rpc CurrentEpoch(QueryCurrentEpochRequest) returns (QueryCurrentEpochResponse) {}
}

Future Improvements

Correct Usage

In the current design, each epoch should be at least two blocks, as the start block should be different from the endblock. Because of this, the time allocated to each epoch will be max(block_time x 2, epoch_duration). For example: if the epoch_duration is set to 1s, and block_time is 5s, actual epoch time should be 10s.

It is recommended to configure epoch_duration to be more than two times the block_time, to use this module correctly. If there is a mismatch between the epoch_duration and the actual epoch time, as in the example above, then module logic could become invalid.

Block-Time Drifts

This implementation of the x/epochs module has block-time drifts based on the value of block_time. For example: if we have an epoch of 100 units that ends at t=100, and we have a block at t=97 and a block at t=104 and t=110, this epoch ends at t=104, and the new epoch will start at t=110.

There are time drifts here, varying about 1-2 blocks time, which will slow down epochs.