`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`](../core/baseapp.md#beginblock) and [`EndBlock`](../core/baseapp.md#endblock) ABCI messages are received from the underlying consensus engine. {synopsis}
`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](./module-manager.md#appmodule). 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`.
A specificity of the `EndBlocker` is that it can return validator updates to the underlying consensus engine in the form of an [`[]abci.ValidatorUpdates`](https://tendermint.com/docs/app-dev/abci-spec.html#validatorupdate). 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](./module-manager.md#manager).
