Epochs

Abstract

While using the SDK, we often want to run certain code periodically. The epochs module allows other modules to be configured such that they are 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

1. Concepts
2. State
3. Events
4. Keeper
5. Hooks
6. Queries
7. Future Improvements

Concepts

Epochs are on-chain timers that have timer ticks at specific time intervals, triggering the execution of certain logic that is constrained by a specific epoch. The purpose of the epochs module is to provide a generalized epoch interface to other modules so that they can easily implement epochs without keeping their own code for epochs.

Every epoch has a unique identifier. Every epoch will have a start time, and an end time, where end_time = start_time + duration.
When an epoch triggers the execution of code, that code is executed at the first block whose blocktime is greater than end_time. It follows that the start_time of the following epoch will be the end_time of the previous epoch.

Stride uses three epoch identifiers as found in x/epochs/genesis.go

1. DAY_EPOCH: this identifies an epoch that lasts 24 hours.
2. STRIDE_EPOCH: this identifies an epoch that lasts 5 minutes on local mode testnet (although this may be changed) and longer on public testnet and mainnet, and is used in the x/stakeibc/ module as a time interval in accordance with which the Stride app chain performs certain functions, such as autocompound staking rewards.

State

The epochs module keeps EpochInfo objects and modifies the information as epoch 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

EndBlocker

Keeper

Keeper Functions

epochs/keeper/ module provides utility functions to manage epochs.

// Keeper is the interface for lockup 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 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)

The BeforeEpochStart hook does different things depending on the identifier.

If in a day identifier it:

1. begins unbondings
2. sweeps unbonded tokens to the redemption account
3. cleans up old records
4. creates empty epoch unbonding records for the next day

If in a stride_epoch identifier it: 5. creates and deposits records on each host zone 6. sets withdrawal addresses 7. updates redemption rates (if the epoch coincides with the correct interval) 8. processes TRANSFER_QUEUE deposit records to the delegation Interchain Account (if the epoch coincides with the correct interval) 9. processes DELEGATION_QUEUE deposit records to the delegation Interchain Account (if the epoch coincides with the correct interval) 10. Query the rewards account using interchain queries, with the transfer callback to a delegation account as a staked record (if at proper interval)

How modules receive hooks

On the hook receiver functions of other modules, they need to filter epochIdentifier and execute for only a specific epochIdentifier.
Filtering epochIdentifier could be in Params of other modules so that they can be modified by governance.
Governance can change an epoch from week to day as needed.

Queries

epochs module provides the below 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

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.