Question about AbstractEnv API

[jonathan-laurent]

In the documentation for AbstractEnv, you write the following remark:

So why don't we adopt the step! method like OpenAI Gym here? The reason is that the async manner will simplify a lot of things here.

Would you care to elaborate what you mean here?

[findmyway]

I was asked of this question from at least three persons. So I'd better write down my thoughts here (and in the upcoming new documentation).

Originally, we have a similar API named interact! written by @jbrea . (Though I'm not sure the reason to use this name, it's just the same with step! in OpenAI Gym if I understand it correctly).

step! together with some other functions in OpenAI Gym are undoubtedly becoming the de facto standard APIs in the RL world. And even in Julia, Gym.jl, Reinforce.jl, RLInterface.jl and MinimalRLCore.jl also adopt the step! there. However, that doesn't mean it is the right way to do so.

In my opinion, one critical issue with step! is that it conflates two different operations into one: 1. Feed an action to the environment; 2. Get an observation from the environment. (There's a similar problem with pop! in data structures. But that's another topic though)

In single-agent sequential environments, step! works well with the following process:

# Given policy and env
observation = reset!(env)
while true
    action = policy(observation )
    observation , reward, done, info = step!(env, action)
    done && break
end

But when it comes to simultaneous environments, we have to change the return type of step! function slightly:

# Given policy and env
observation = reset!(env)
while true
    action = policy(observation )
    task = step!(env, action)
    observation , reward, done, info = fetch(task)
    done && break
end

Until now, it's not a big change. After all, the signature of function in Julia doesn't include the return type. So we can return a Future instead. (But do note that it already breaks the definition in OpenAI Gym)

Now consider the multi-agent environments, things become much more complicated. See more discussions at openai/gym#934:

1. reset! should be triggered by a (global) policy
2. reset! and step! must return observations and other info of all the players.
3. We can only get the chance to observe the environment after taking an action in it. (There's a workaround by introducing a no-op action)

The root reason for those complexities is that step! is synchronous by design. I'm not the first person to realize this problem. In Ray/RLlib, environments are treated async by design, see more details here:

In many situations, it does not make sense for an environment to be “stepped” by RLlib. For example, if a policy is to be used in a web serving system, then it is more natural for an agent to query a service that serves policy decisions, and for that service to learn from experience over time. This case also naturally arises with external simulators (e.g. Unity3D, other game engines, or the Gazebo robotics simulator) that run independently outside the control of RLlib, but may still want to leverage RLlib for training.

So we modified the APIs in OpenAI Gym a little:

• reset! is kept but it should only return nothing (or at least package users shouldn't rely on the result of reset!)
• Each environment is a functional object which can take in an action (a vector of actions or action and player_id pairs ...) and return nothing. The only reason to use a functional struct here is to reduce the burden of remembering an extra API name ;)
• An agent can observe the environment independently anytime.
  • We adopt the duck-typing here for the result of observe. Generally get_state(obs), get_reward(obs), and get_terminal(obs) are required. The fact that the result of observe can be of any type is useful for some search based algorithms like MCTS.

I must admit that treating all environments as async does bring in some inconveniences. For those environments which are sync essentially, we have to store the state, reward, info in the environment after applying an action. For example, ReinforcementLearningEnvironments.jl#mountain_car. But I think it's worth to do so.

(Also cc @sebastian-engel, @zsunberg, @mkschleg, @MaximeBouton, @jbrea in case they are interested in discussions here.)

[jbrea]

Though I'm not sure the reason to use this name

This dates still back to the time, when I didn't have a look at how other RL packages work and I thought the agent is basicallically interacting with the environment, since it sends and action and receives and observation, therefore interact!. I really like the new convention, that step! just steps the environment forward and observe is used to observe the current state of the environment.

[jonathan-laurent]

Thanks for your thorough explanation!
Am I correct that with your API, calls to observe, get_terminal and get_reward are blocking until all agents have submitted an action? More generally, do you have a canonical example of a simultaneous environment?

[findmyway]

Am I correct that with your API, calls to observe, get_terminal and get_reward are blocking until all agents have submitted an action?

It depends. It can be blocking either when calling env(action), observe(env), or at the first call of get_*(obs). For the first case, you can refer ReinforcementLearningBase.jl#MultiThreadEnv. For latter two, I don't have an example yet.

More generally, do you have a canonical example of a simultaneous environment?

Unfortunately, no😳.

[jonathan-laurent]

Thanks!
Also, I was wondering: is there currently a function to reset an environment to a previous state (or equivalently create a new environment with a custom initial state)?

I am asking because although Gym environments typically do not offer this functionality, it is essential for tree-based planning algorithms such as AlphaZero.

[findmyway]

is there currently a function to reset an environment to a previous state (or equivalently create a new environment with a custom initial state)?

No. To support this operation we need to separate the environment into two parts: 1) Description part, like action_space, num_of_player, etc. 2) Internal state related part. And I'm not sure how to handle it gracefully yet.

Based on my limited experience with MCTS, I found that implementing a Base.clone(env::AbstractEnv) would be enough to mimic the behavior you said above. I can add one example later. (You may watch https://github.com/JuliaReinforcementLearning/ReinforcementLearningZoo.jl/issues/32)

[zsunberg]

In POMDPs.jl, we made a different decision where the separation of the state and model is central. Instead of using the step! paradigm, we have a generative model, so, for instance, if m is an MDP object and s is any state (that you have stored in your tree for instance), you can call

sp, r = @gen(:sp, :r)(m, s, a, rng)

to generate a new state and reward (in POMDPs 0.8.4 + - we are still finalizing some issues and updating documentation to move to POMDPs 1.0). We've also tried to make it really easy to define simple models in a few lines with QuickPOMDPs.

That being said, it is true that using a step! or RLBase style interface will make it easier to wrap environments that others have written (though it could be done with POMDPs.jl), and the only thing you need to add to a step! style simulator for it to work with MCTS is the ability to copy the environment, not initialize at any arbitrary state.

[zsunberg]

In any case, I don't think it will be too hard to adjust to different interfaces in the future. Probably best to just get it to work with one MDP, and then think hard about the interface in the second round. As mentioned in the RLZoo README, Make it work, make it right, make it fast is a good mantra.

[mkschleg]

I made the choice to go w/ the API in MinimalRLCore for a few reasons. The biggest is just where I'm studying and who I learned RL from initially (i.e. Adam White and at UofA/IU). In our course we heavily use the RLGlue interface which Adam made during his graduate degree w/ Brian Tanner. The API is very much inspired from this and modernized to remove some of the cruft of the original (they had constraints that I don't have to deal w/ in Juia). The focus of MinimalRLCore was also to create an API which lets people do what they need to for research, even if I didn't imagine it initially. I find that I run into walls a lot when adopting an RL API, although Julia helps a lot here w/ multiple dispatch. One example is dealing w/ a non-global RNG which is shared btw the agent and environment, or defining a reset which sets the state to a provided value (very necessary for MonteCarloRollouts when working on prediction).

While it is true the API I provided isn't really designed w/ async in mind, and this was partially on purpose and partially on how I'm actually using this in my research. But users can overload step! for any of their envs that may be async, so I don't really see it as an issue that needs addressed. If this were to be supported later I would probably have a separate abstract type. I don't feel like the assumption should be that all envs async, or that you have multiple agents running around in an env instance (like A3C for example). This usually adds complexity, that I don't really want to deal w/ as a researcher.

[zsunberg]

Oh man, this is great to get us all in the same room talking :) (@MaximeBouton @rejuvyesh, @lassepe you might be interested in this). I think we should make an actually really minimal interface that can be used for MCTS and RL and put it in a package (after a quick look, MinimalRLCore and RLInterface are almost there, but not quite). Should we move the discussion to discourse?

[zsunberg]

I would submit that the minimal interface for MCTS would have:

step!(env, a) # returns an observation, reward, and done
actions(env) # returns only the valid actions at the current state of the environment
reset!(env)
clone(env) # creates a complete copy at the current state - it is assumed that the two environments are now completely independent of each other.

The other option is to explicitly separate the state from the environment.

[rejuvyesh]

A popular example of interfaces that have this concept of explicit observation interface as @findmyway mentioned is dm-lab I believe.

[zsunberg]

Yeah, I must say that the explicit observation interface in RLBase is a very nice feature for some of the more complicated use cases.

This afternoon, I was thinking about a way to have a common core of basic and some optional functionality that we can all link into. My idea is a CommonRL package that all of our packages that are optimized for different use cases depend on and allows for interoperability at least on the reset! and step! level. Here is my sketch: https://gist.github.com/zsunberg/a6cae2f92b5f8fae8f624dc173bc5c6b .

[mkschleg]

I would 100% be up to helping with this.

One thing that I have issue w/ still in Julia is dealing with the implicit enforcement of interfaces (thus why MinimalRLCore separates what is called and what is implemented by users). But I think if we were to have a common package w/ good docs, this shouldn't be an issue (and I guess I should be more trusting of users :P).

I think having someway of expressing what observation types are being returned would be useful, but I never have landed on a design I like. The dict of types is reasonable, but feels really pythony. I was also playing around w/ the idea of dispatching on value types with symbols, this was a bit onerous though. Maybe we should use traits here.

[findmyway]

@zsunberg 's sketch is a really nice starting point. I'd also be glad to support such common core package.

The dict of types is reasonable, but feels really python. I was also playing around w/ the idea of dispatching on value types with symbols, this was a bit onerous though. Maybe we should use traits here.

@mkschleg I'm feeling the same 😄.