# Actor Spec: Cron

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## Contents

[TOC]

## At a Glance

The Cron actor is a special actor invoked at the end of every Epoch. This is handled via an Implicit Message at the end of each Tipset, in which the System actor invokes Cron.EpochTick.

**Actor Type:**
- Singleton
- Address: 3

**Exported Methods:**

1. Constructor
2. EpochTick

## State

The Cron actor `State` is unique: it is completely static post-initialization.

type State struct {
	Entries []Entry
}

type Entry struct {
	Receiver  addr.Address
	MethodNum abi.MethodNum
}

**`Entries`**: A slice of `Entry`s: method callbacks that will be invoked on the next call to `EpochTick`.
* Each `Entry` defines:
* `Receiver`: The destination actor of the callback. Uses an ID address.
* `MethodNum`: The MethodNum of the method that will be invoked.

The static values for `Entries` are listed below, and can also be found in `cron_state.BuiltInEntries`:

st.Entries = []Entry{
    Receiver:  builtin.StoragePowerActorAddr,
    MethodNum: builtin.MethodsPower.OnEpochTickEnd,
},
{
    Receiver:  builtin.StorageMarketActorAddr,
    MethodNum: builtin.MethodsMarket.CronTick,
},

## Exported Methods

#### 1. Constructor

func (a Actor) Constructor(rt vmr.Runtime, params *ConstructorParams) *adt.EmptyValue

The Cron constructor initializes the Cron actor's static `State` from its parameters:

type ConstructorParams struct {
	Entries []Entry
}

**`Entries`**: Will be equivalent to the specific `Entries` specified above.

#### 2. EpochTick

func (a Actor) EpochTick(rt vmr.Runtime, _ *adt.EmptyValue) *adt.EmptyValue

This method is invoked by the System actor as an Implicit Message at the end of each Tipset. Two method callbacks are invoked:

1. StoragePower.OnEpochTickEnd
2. StorageMarket.CronTick

These invoked methods should not accept any parameters (except the `Runtime`). Also, they should both return `EmptyValue`.

##### Failure conditions

Understanding potential failure conditions of the Cron actor is very important. Because the Cron actor is invoked as the final message in each Tipset, a failure within a Cron method callback could result in the Tipset being unable to be processed. **In a worst case scenario, this could lead to a total network halt.**

Assuming Caller is the System actor, the following conditions will cause `EpochTick` to fail:
* `panic` within an invoked callback
* `Abortf` will simply be ignored, but `panic` will cause execution to halt.

## Questions

* What entries will exist on constructor invocation?
* zenground0: Only the values in `BuiltInEntries`
* Is `st.Entries` cleared/deleted after `EpochTick`?
* zenground0: No, state is static.