# BeginBlocker and EndBlocker

BeginBlocker and EndBlocker are optional methods module developers can implement in their module. They will be triggered at the beginning and at the end of each block respectively, when the BeginBlock and EndBlock ABCI messages are received from the underlying consensus engine.

# Pre-requisite Readings

# BeginBlocker and EndBlocker

BeginBlocker and EndBlocker are a way for module developers to add automatic execution of logic to their module. This is a powerful tool that should be used carefully, as complex automatic functions can slow down or even halt the chain.

When needed, BeginBlocker and EndBlocker are implemented as part of the AppModule interface. The BeginBlock and EndBlock methods of the interface implemented in module.go generally defer to BeginBlocker and EndBlocker methods respectively, which are usually implemented in abci.go.

The actual implementation of BeginBlocker and EndBlocker in abci.go are very similar to that of a Msg service:

  • They generally use the keeper and ctx to retrieve information about the latest state.
  • If needed, they use the keeper and ctx to trigger state-transitions.
  • If needed, they can emit events via the ctx's EventManager.

A specificity of the EndBlocker is that it can return validator updates to the underlying consensus engine in the form of an []abci.ValidatorUpdates (opens new window). This is the preferred way to implement custom validator changes.

It is possible for developers to define the order of execution between the BeginBlocker/EndBlocker functions of each of their application's modules via the module's manager SetOrderBeginBlocker/SetOrderEndBlocker methods. For more on the module manager, click here.

See an example implementation of BeginBlocker from the distribution module:

Copy // BeginBlocker sets the proposer for determining distribution during endblock // and distribute rewards for the previous block func BeginBlocker(ctx sdk.Context, req abci.RequestBeginBlock, k keeper.Keeper) { defer telemetry.ModuleMeasureSince(types.ModuleName, time.Now(), telemetry.MetricKeyBeginBlocker) // determine the total power signing the block var previousTotalPower, sumPreviousPrecommitPower int64 for _, voteInfo := range req.LastCommitInfo.GetVotes() { previousTotalPower += voteInfo.Validator.Power if voteInfo.SignedLastBlock { sumPreviousPrecommitPower += voteInfo.Validator.Power } } // TODO this is Tendermint-dependent // ref https://github.com/cosmos/cosmos-sdk/issues/3095 if ctx.BlockHeight() > 1 { previousProposer := k.GetPreviousProposerConsAddr(ctx) k.AllocateTokens(ctx, sumPreviousPrecommitPower, previousTotalPower, previousProposer, req.LastCommitInfo.GetVotes()) } // record the proposer for when we payout on the next block consAddr := sdk.ConsAddress(req.Header.ProposerAddress) k.SetPreviousProposerConsAddr(ctx, consAddr) }

and an example implementation of EndBlocker from the staking module:

Copy // Called every block, update validator set func EndBlocker(ctx sdk.Context, k keeper.Keeper) []abci.ValidatorUpdate { defer telemetry.ModuleMeasureSince(types.ModuleName, time.Now(), telemetry.MetricKeyEndBlocker) return k.BlockValidatorUpdates(ctx) }

# Next

Learn about keepers