cosmos-sdk/docs/core/handlers.md

# Handlers

A handler takes a context and a message and returns a result.  All
information necessary for processing a message should be available in the
context.

While the context holds the entire application state (ie. the 
MultiStore), handlers are restricted in what they can do based on the
capabilities they were given when the application was set up.

For instance, suppose we have a `newFooHandler`: 

```go
func newFooHandler(key sdk.StoreKey) sdk.Handler {
    return func(ctx sdk.Context, msg sdk.Msg) sdk.Result {
        store := ctx.KVStore(key)
        // ...
    }
}
```

This handler can only access one store based on whichever key its given.
So when we register the handler for the `foo` message type, we make sure 
to give it the `fooKey`:

```
app.Router().AddRoute("foo", newFooHandler(fooKey))
```

Now it can only access the `foo` store, but not the `bar` or `cat` stores!