epochs

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

Contents

• epochs
• Concepts
• State
  • Epoch information type
• Events
  • BeginBlocker
  • EndBlocker
• Keepers
  • Keeper functions
• Hooks
  • Hooks
  • How modules receive hooks
• Queries
• Future Improvements
  • Lack point using this module
  • Block-time drifts problem

Concepts

The purpose of epochs module is to provide generalized epoch interface to other modules so that they can easily implement epochs without keeping own code for epochs.

State

Epochs module keeps EpochInfo objects and modify the information as epochs info changes. Epochs are initialized as part of genesis initialization, and modified on begin blockers or end blockers.

Epoch information type

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;
}

EpochInfo keeps identifier, start_time,duration, current_epoch, current_epoch_start_time, epoch_counting_started, current_epoch_start_height.

1. identifier keeps epoch identification string.
2. start_time keeps epoch counting start time, if block time passes start_time, epoch_counting_started is set.
3. duration keeps target epoch duration.
4. current_epoch keeps current active epoch number.
5. current_epoch_start_time keeps the start time of current epoch.
6. epoch_number is counted only when epoch_counting_started flag is set.
7. current_epoch_start_height keeps the start block height of current epoch.

Events

The 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

Keeper functions

Epochs keeper module provides utility functions to manage epochs.

// 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

Hooks

  // the first block whose timestamp is after the duration is counted as the end of the epoch
  AfterEpochEnd(ctx sdk.Context, epochIdentifier string, epochNumber int64)
  // new epoch is next block of epoch end block
  BeforeEpochStart(ctx sdk.Context, epochIdentifier string, epochNumber int64)

How modules receive hooks

On hook receiver function of other modules, they need to filter epochIdentifier and only do executions for only specific epochIdentifier. Filtering epochIdentifier could be in Params of other modules so that they can be modified by governance. Governance can change epoch from week to day as their need.

Queries

Epochs module is providing below queries to check the module's state.

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

Future Improvements

Lack point using this module

In current design each epoch should be at least 2 blocks as start block should be different from endblock. Because of this, each epoch time will be max(blocks_time x 2, epoch_duration). If epoch_duration is set to 1s, and block_time is 5s, actual epoch time should be 10s. We definitely recommend configure epoch_duration as more than 2x block_time, to use this module correctly. If you enforce to set it to 1s, it's same as 10s - could make module logic invalid.

TODO for postlaunch: We should see if we can architect things such that the receiver doesn't have to do this filtering, and the epochs module would pre-filter for them.

Block-time drifts problem

This implementation has block time drift based on block time. For instance, we have an epoch of 100 units that ends at t=100, if we have a block at t=97 and a block at t=104 and t=110, this epoch ends at t=104. And new epoch start at t=110. There are time drifts here, for around 1-2 blocks time. It will slow down epochs.

It's going to slow down epoch by 10-20s per week when epoch duration is 1 week. This should be resolved after launch.