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Courses/App School I/4. Lifecycle

4. Lifecycle

In the last lesson we looked at a couple of useful things used as boilerplate in most agents. Now we're going to get into the guts of how agents work, and start looking at what the agent arms do. The first thing we'll look at is the agent's state, and the three arms for managing it: on-init, on-save, and on-load. These arms handle what we call an agent's "lifecycle".

Lifecycle

An agent's lifecycle starts when it's first installed. At this point, the agent's on-init arm is called. This is the only time on-init is ever called - its purpose is just to initialize the agent. The on-init arm might be very simple and just set an initial value for the state, or even do nothing at all and return the agent core exactly as-is. It may also be more complicated, and perform some scries to obtain extra data or check that another agent is also installed. It might send off some cards to other agents or vanes to do things like load data in to the %settings-store agent, bind an Eyre endpoint, or anything else. It all depends on the needs of your particular application. If on-init fails for whatever reason, the agent installation will fail and be aborted.

Once initialized, an agent will just go on doing its thing - processing events, updating its state, producing effects, etc. At some point, you'll likely want to push an update for your agent. Maybe it's a bug fix, maybe you want to add extra features. Whatever the reason, you need to change the source code of your agent, so you commit a modified version of the file to Clay. When the commit completes, Gall updates the app as follows:

  • The agent's on-save arm is called, which packs the agent's state in a vase and exports it.
  • The new version of the agent is built and loaded into Gall.
  • The previously exported vase is passed to the on-load arm of the newly built agent. The on-load arm will process it, convert it to the new version of the state if necessary, and load it back into the state of the agent.

A vase is just a cell of [type-of-the-noun the-noun]. Most data an agent sends or receives will be encapsulated in a vase. A vase is made with the zapgar (!>) rune like !>(some-data), and unpacked with the zapgal (!<) rune like !<(type-to-extract vase). Have a read through the vase section of the type reference for details.

We'll look at the three arms described here in a little more detail, but first we need to touch on the state itself.

Versioned state type

In the previous lesson we introduced the idea of composing additional cores into the subject of the agent core. Here we'll look at using such a core to define the type of the agent's state. In principle, we could make it as simple as this:

|%
+$  my-state-type  @ud
--

However, when you update your agent as described in the Lifecycle section, you may want to change the type of the state itself. This means on-load might find different versions of the state in the vase it receives, and it might not be able to distinguish between them.

For example, if you were creating an agent for a To-Do task management app, your tasks might initially have a ?(%todo %done) union to specify whether they're complete or not. Something like:

(map task=@t status=?(%todo %done))

At some point, you might want to add a third status to represent "in progress", which might involve changing status like:

(map title=@t status=?(%todo %done %work))

The conventional way to keep this managable and reliably differentiate possible state types is to have versioned states. The first version of the state would typically be called state-0, and its head would be tagged with %0. Then, when you change the state's type in an update, you'd add a new structure called state-1 and tag its head with %1. The next would then be state-2, and so on.

In addition to each of those individual state versions, you'd also define a structure called versioned-state, which just contains a union of all the possible states. This way, the vase on-load receives can be unpacked to a versioned-state type, and then a wuthep (?-) expression can switch on the head (%0, %1, %2, etc) and process each one appropriately.

For example, your state definition core might initially look like:

|%
+$  versioned-state
  $%  state-0
  ==
+$  state-0  [%0 tasks=(map title=@t status=?(%todo %done))]
--

When you later update your agent with a new state version, you'd change it to:

|%
+$  versioned-state
  $%  state-0
      state-1
  ==
+$  state-0  [%0 tasks=(map title=@t status=?(%todo %done))]
+$  state-1  [%1 tasks=(map title=@t status=?(%todo %done %work))]
--

Another reason for versioning the state type is that there may be cases where the state type doesn't change, but you still want to apply special transition logic for an old state during upgrade. For example, you may need to reprocess the data for a new feature or to fix a bug.

Adding the state

Along with a core defining the type of the state, we also need to actually add it to the subject of the core. The conventional way to do this is by adding the following immediately before the agent core itself:

=|  state-0
=*  state  -

The first line bunts (produces the default value) of the state type we defined in the previous core, and adds it to the head of the subject without a face. The next line uses tistar to give it the name of state. You might wonder why we don't just give it a face when we bunt it and skip the tistar part. If we did that, we'd have to refer to tasks as tasks.state. With tistar, we can just reference tasks while also being able to reference the whole state when necessary.

Note that adding the state like this only happens when the agent is built - from then on the arms of our agent will just modify it.

State management arms

We've described the basic lifecycle process and the purpose of each state management arm. Now let's look at each arm in detail:

on-init

This arm takes no argument, and produces a (quip card _this). It's called exactly once, when the agent is first installed. Its purpose is to initialize the agent.

(quip a b) is equivalent to [(list a) b], see the types reference for details.

A card is a message to another agent or vane. We'll discuss cards in detail later.

this is our agent core, which we give the this alias in the virtual arm described in the previous lesson. The underscore at the beginning is the irregular syntax for the buccab ($_) rune. Buccab is like an inverted bunt - instead of producing the default value of a type, instead it produces the type of some value. So _this means "the type of this" - the type of our agent core.

Recall that in the last lesson, we said that most arms return a cell of [effects new-agent-core]. That's exactly what (quip card _this) is.

on-save

This arm takes no argument, and produces a vase. Its purpose is to export the state of an agent - the state is packed into the vase it produces. The main time it's called is when an agent is upgraded. When that happens, the agent's state is exported with on-save, the new version of the agent is compiled and loaded, and then the state is imported back into the new version of the agent via the on-load arm.

As well as the agent upgrade process, on-save is also used when an agent is suspended or an app is uninstalled, so that the state can be restored when it's resumed or reinstalled.

The state is packed in a vase with the zapgar (!>) rune, like !>(state).

on-load

This arm takes a vase and produces a (quip card _this). Its purpose is to import a state previously exported with on-save. Typically you'd have used a versioned state as described above, so this arm would test which state version the imported data has, convert data from an old version to the new version if necessary, and load it into the state wing of the subject.

The vase would be unpacked with a zapgal (!<) rune, and then typically you'd test its version with a wuthep (?-) expression.

Example

Here's a new agent to demonstrate the concepts we've discussed here:

Click to expand

/+  default-agent, dbug
|%
+$  versioned-state
  $%  state-0
  ==
+$  state-0  [%0 val=@ud]
+$  card  card:agent:gall
--
%-  agent:dbug
=|  state-0
=*  state  -
^-  agent:gall
|_  =bowl:gall
+*  this  .
    def   ~(. (default-agent this %.n) bowl)
::
++  on-init
  ^-  (quip card _this)
  `this(val 42)
::
++  on-save
  ^-  vase
  !>(state)
::
++  on-load
  |=  old-state=vase
  ^-  (quip card _this)
  =/  old  !<(versioned-state old-state)
  ?-  -.old
    %0  `this(state old)
  ==
::
++  on-poke   on-poke:def
++  on-watch  on-watch:def
++  on-leave  on-leave:def
++  on-peek   on-peek:def
++  on-agent  on-agent:def
++  on-arvo   on-arvo:def
++  on-fail   on-fail:def
--

Let's break it down and have a look at the new parts we've added. First, the state core:

|%
+$  versioned-state
  $%  state-0
  ==
+$  state-0  [%0 val=@ud]
+$  card  card:agent:gall
--

In state-0 we've defined the structure of our state, which is just a @ud. We've tagged the head with a %0 constant representing the version number, so on-load can easily test the state version. In versioned-state we've created a union and just added our state-0 type. We've added an extra card arm as well, just so we can use card as a type, rather than the unweildy card:agent:gall.

After that core, we have the usual agent:dbug call, and then we have this:

=|  state-0
=*  state  -

We've just bunted the state-0 type, which will produce [%0 val=0], pinning it to the head of the subject. Then, we've use tistar (=*) to give it a name of state.

Inside our agent core, we have on-init:

++  on-init
  ^-  (quip card _this)
  `this(val 42)

The a(b c) syntax is the irregular form of the centis (%=) rune. You'll likely be familiar with this from recursive functions, where you'll typically call the buc arm of a trap like $(a b, c d, ...). It's the same concept here - we're saying this (our agent core) with val replaced by 42. Since on-init is only called when the agent is first installed, we're just initializing the state.

Next we have on-save:

++  on-save
  ^-  vase
  !>(state)

This exports our agent's state, and is called during upgrades, suspensions, etc. We're having it pack the state value in a vase.

Finally, we have on-load:

++  on-load
  |=  old-state=vase
  ^-  (quip card _this)
  =/  old  !<(versioned-state old-state)
  ?-  -.old
    %0  `this(state old)
  ==

It takes in the old state in a vase, then unpacks it to the versioned-state type we defined earlier. We test its head for the version, and load it back into the state of our agent if it matches. This test is a bit redundant at this stage since we only have one state version, but you'll soon see the purpose of it.

You can save it as /app/lifecycle.hoon in the %base desk and |commit %base. Then, run |rein %base [& %lifecycle] to start it.

Let's try inspecting our state with dbug:

>   [%0 val=42]
> :lifecycle +dbug
>=

dbug can also dig into the state with the %state argument, printing the value of the specified face:

>   42
> :lifecycle +dbug [%state %val]
>=

Next, we're going to modify our agent and change the structure of the state so we can test out the upgrade process. Here's a modified version, which you can again save in /app/lifecycle.hoon and |commit %base:

Click to expand

/+  default-agent, dbug
|%
+$  versioned-state
  $%  state-0
      state-1
  ==
+$  state-0  [%0 val=@ud]
+$  state-1  [%1 val=[@ud @ud]]
+$  card  card:agent:gall
--
%-  agent:dbug
=|  state-1
=*  state  -
^-  agent:gall
|_  =bowl:gall
+*  this  .
    def   ~(. (default-agent this %.n) bowl)
::
++  on-init
  ^-  (quip card _this)
  `this(val [27 32])
::
++  on-save
  ^-  vase
  !>(state)
::
++  on-load
  |=  old-state=vase
  ^-  (quip card _this)
  =/  old  !<(versioned-state old-state)
  ?-  -.old
    %1  `this(state old)
    %0  `this(state 1+[val.old val.old])
  ==
::
++  on-poke   on-poke:def
++  on-watch  on-watch:def
++  on-leave  on-leave:def
++  on-peek   on-peek:def
++  on-agent  on-agent:def
++  on-arvo   on-arvo:def
++  on-fail   on-fail:def
--

As soon as you |commit it, Gall will immediately export the existing state with on-save, build the new version of the agent, then import the state back in with on-load.

In the state definition core, you'll see we've added a new state version with a different structure:

+$  versioned-state
  $%  state-0
      state-1
  ==
+$  state-0  [%0 val=@ud]
+$  state-1  [%1 val=[@ud @ud]]
+$  card  card:agent:gall
--

We've also changed the part that adds the state, so it uses the new version instead:

=|  state-1
=*  state  -

In on-init, we've updated it to initialize the state with a value that fits the new type we've defined:

++  on-init
  ^-  (quip card _this)
  `this(val [27 32])

on-init won't be called in this case, but if someone were to directly install this new version of the agent, it would be, so we still need to update it.

on-save has been left unchanged, but on-load has been updated like so:

++  on-load
  |=  old-state=vase
  ^-  (quip card _this)
  =/  old  !<(versioned-state old-state)
  ?-  -.old
    %1  `this(state old)
    %0  `this(state 1+[val.old val.old])
  ==

We've updated the ?- expression with a new case that handles our new state type, and for the old state type we've added a function that converts it to the new type - in this case by duplicating val and changing the head-tag from %0 to %1. This is an extremely simple state type transition function - it would likely be more complicated for an agent with real functionality.

Note: the a+b syntax (as in 1+[val.old val.old]) forms a cell of the constant %a and the noun b. The constant may either be an integer or a @tas. For example:

> foo+'bar'
[%foo 'bar']


> 42+'bar'
[%42 'bar']

Let's now use dbug to confirm our state has successfully been updated to the new type:

>   [%1 val=[42 42]]
> :lifecycle +dbug
>=

Summary

  • The app lifecycle rougly consists of initialization, state export, upgrade, state import and state version transition.
  • This is managed by three arms: on-init, on-save and on-load.
  • on-init initializes the agent and is called when it's first installed.
  • on-save exports the agent's state and is called during upgrade or when an app is suspended.
  • on-load imports an agent's state and is called during upgrade or when an app is unsuspended. It also handles converting data from old state versions to new state versions.
  • The type of an agent's state is typically defined in a separate core.
  • The state type is typically versioned, with a new type definition for each version of the state.
  • The state is initially added by bunting the state type and then naming it state with the tistar (=*) rune, so its contents can be referenced directly.
  • A vase is a cell of [type-of-the-noun the-noun].
  • (quip a b) is the same as [(list a) b], and is the [effects new-agent-core] pair returned by many arms of an agent core.

Exercises