Introducing Svelto ECS 2.5

Svelto ECS so far…

As I wrote in my previous articles, Svelto.ECS wasn’t born just from the needs of a large team, but also as result of years of reasoning behind software engineering applied to game development (*) and that’s why the main Svelto.ECS goals are currently different than the ones set by Unity ECS. They may converge one day, but at the moment they are different enough to justify the on going development of Svelto.ECS. The biggest difference is that Svelto.ECS hasn’t been built just for performance. Performance gain is just one of the benefits in using Svelto.ECS, as ECS in general is a great way to write cache-friendly code, albeit the main reasons why Svelto.ECS has been written orbit around the shifting of paradigm from Object Oriented Programming, the consequent improvement of the code design and maintainability, the approachability by junior programmers that won’t need to worry too much about the architecture and can focus on the solution of the problems thanks to the rigid directions that the framework gives.

I haven’t used Unity ECS so far, although I have seen some examples, so I can’t give much feedback about it at this moment, but what I have learned from Svelto.ECS is the importance of defining the concept of Entity at code level to keep coders from abusing the pattern and fall in the trap to write spaghetti code again. Let’s be clear, there is a very fine line between writing sensible ECS code and writing plain C code with global accessible data. The anchor that should prevent the coder from lose his way is to see a Systems as a single-responsibility behavior of a specific set of entities and not as a container of generic logic using global accessible data.

All that said, let’s see what is new in Svelto.ECS 2.5.


introducing new struct friendly features in Svelto.ECS 2.5

Until now Svelto.ECS pushed the user to use objects over structs for normal use when high performance wasn’t required. Svelto.ECS 2.5 inverts this trend, making the use of structs actually the standard and the use of classes the exception. Let’s see how this is achieved:

As you know, an Entity is built through an EntityDescriptor and an optional set of Implementors. An EntityDescriptor can generate the so called “EntityViews” and “EntityStructs“. In my examples I didn’t use many EntityStructs, but with the newest version of Svelto I started to rewrite and change a lot of EntityViews into EntityStructs as you can see in the new Survival Example Code.

Implementor is a very interesting concept in Svelto.ECS. They are meant to act as a bridge between the underlying platform (like Unity) and the application engines. This is actually how testable engines, that don’t use any dependencies from the underlying platform, can be written with Svelto. However we used to use implementors to define Entity Components that were intrinsically independent by the platform too. With Svelto.ECS 2.5 Entity Components should preferably be defined as EntityStructs when they don’t need to act as a bridge with the underlying platform. EntityStructs are obviously mandatory to write cache-friendly code, but even when this kind of performance gain is not required, EntityStructs are still beneficial over EntityViews and Implementors because they allow to write allocation free code.

A good implementation detail is that the relation between Entity Components and Implementors is not 1:1. One single implementor object can be used to implement many entity component interfaces. The interface is the abstraction of the underlying platform functionalities that are actually implemented by the implementor. However EntityViews are classes and every time an Entity is built, a set of objects are allocated on the heap. Therefore, why not declaring EntityViews as structs as well? Previously it wasn’t possible because EntityViews references were extensively used in Engines code, stored inside user data structures. With Svelto.ECS 2.5 the use of custom data structures that hold EntityViews references is instead highly discouraged. For this reason, Svelto.ECS 2.5 introduces the new concept of EntityViewStruct, which is a mix between the two worlds. When EntityViewStructs are used, only the implementors (in Unity mainly  as Monobehaviours) are allocated on the heap.

Let’s recap everything:

  1. the use of EntityViews is still supported, but fundamentally obsolete.
  2. when Implementors are needed, EntityViewStructs should be used instead.
  3. EntitViewStructs should be used only when engines need to be abstracted from the underlying platform functionalities, although there are other exceptions. The Engines shouldn’t need know the implementation details behind the change of entity data (i.e.: the Survival demo code uses this design to wrap the Unity RigidBody functionalities, so that the Entity Components can return its parameters without exposing the RigidBody interface).
  4. EntityStructs should be used for all the other cases. When Monobehaviors don’t need to be used, entity components should always be seen as pure structs. EntityStructs come with the bonus to write way less boilerplate code (neither component interfaces nor implementors are needed).
  5. EntityViewStructs and EntityViews can hold only component interfaces.
  6. Component interfaces must declare only setter and getters of primitive or value types. The only exceptions are made by DispatchOnSet<T> and DispatchOnChange<T>
  7. EntityStructs can hold only primitives and value types (no collections allowed either).
  8. Custom data structures defined in engines with the purpose to hold EntityView references should never be used unless strictly necessary.

The main problem about using EntityStructs is that they don’t allow the same fine abstraction that EntityViewStructs do. This is because EntityViewStructs, being defined by the Engines, allow to re-interpret the same component data under different engines point of view, promoting abstraction and modularity. EntityStructs instead are defined by the Entity and the Engine can only use them without semantic reinterpretation. That’s why EntityStructs should be as modular as possible, although the definition of an EntityStruct could have more practical repercussions depending if a SoA or AoS implementation is preferred. Being able to give to the same data different meaning is the only reason why EntityViewStructs could be still preferred to EntityStructs when implementors are not abstraction of the underling platform. For example, an engine can have a read-only access to the same data that another engine can write into. EntityStructs instead are always mutable as making them immutable would defeat the purpose of improving performance.

Extending the Svelto graph, the EntityStructs are placed on the left side:

If there wasn’t any dependency with the underlying platform, it would be possible to write a game with EntityStructs only, however I am still not sure how this would impact the overall design.

Currently this is my thinking:

If there is no dependency to the underlying platform (i.e.: callbacks from Monobehaviours or some Unity related stuff), use EntityStructs as long as they are very abstracted (HealthEntityStruct is a good example). I don’t think there would be much space for very specialized EntityStructs. Implementors could make sense only as Monobehaviours and would store the essential features needed from the platform. EntityViewStructs should always be used in place of EntityViews when implementors are needed. The main question that I have to answer is: would the absolute use of EntityStructs over EntityViewStructs make the design worse? This is something I will analyse in future with our projects. Currently the nice thing about the EntityView(Structs) is that they describe Entities specialization in terms of behaviors running on the entity more than data. Normally, using ECS design, entities are specialized adding more data, however with Svelto EntityDescriptor and EntityViews is possible to read what behaviors will run on the Entity, example:

Since EntityView(Struct)s come with their relative Engines, an entity descriptor shows which behaviors (engines) will run on the entity. Using only EntityStruct I would loose this semantic, moving back to see Entities as a set of data only.


In-depth view of the new functionalities

I understand this is a lot to take in. It shouldn’t be if you know how to use Svelto already, although you will need to change your current way to use it. If you are new to it, the best thing is to follow the survival example, as the code is simpler than the theory.  Let’s start from our MainContext.

-EntityStructs-

Beside cleaning up some code, the most important difference is how I create the Player Entity. The Player Entity now has only EntityViewStructs defined with Implementors as Monobehaviors and EntityStructs for everything that don’t need to act as a bridge (precisely: HealthEntityStruct and PlayerInputDataStruct).

with:

and as example of EntityStructs:

you may notice how creating an Entity still looks quite readable! Especially if you don’t like the Svelto.ECS (minimal, I have to say), boilerplate, you are going to love the EntityStruct, as they are compact and they don’t need to define interfaces or implementors that must come with them.

I put a lot of effort to make structs user friendly in Svelto.ECS 2.5. Unluckily I couldn’t achieve my final goals, as the interfaces I had in mind need the new c# 7.0 by ref features, so at the moment some functionalities around the EntityStructs are not final, but still OK (or at least the best the can be at the moment I guess).

Let’s start from the initialization of the structs. This interface won’t actually change as I am actually happy with it. More often than not, you will want to initialize the value of your structs with initial values. This was quite badly managed by Svelto 2.0, as I wanted to find an allocation free, pass by reference and easy to understand solution.

I found the solution with the EntityStructInitializer struct returned by any IEntityFactory BuildEntity method. After you call BuildEntity, you will use it like this:

in this way the Player Entity will generate a HealthEntityStruct with the starting values you desire.

Alternatively you may create an Engine that inherits from SingleEntityEngine<> or MultiEntitiesEngine<,>. They both implement the Add/Remove methods with a parameter passed by reference. It supports EntityStructs and the modifications made to it will be saved directly in the database.

To summarize:

  • EntityViewStructs are like the good ol’ EntityViews. They hold Component interfaces which must still be filled with reflection (albeit cached and fast). They must implement the IEntityView interface. However they don’t allocate any object on the heap when created, differently than what EntityViews do
  • EntityStructs, which were already present in Svelto, must implement the IEntityStruct interface. They don’t allocate, they are cache friendly and they don’t use reflection. They are the top for performance. Thanks to the new Svelto ECS features, I would cautiously suggest to use them whenever you can. However keep in mind that they could make your code design worse and this is something I want to test in future on large projects.

-Allocation 0 code-

Now that you can use EntityStructs and EntityViewStructs, you are able to achieve zero allocation code while building entities. This would make Entity Pooling unnecessary.

-Handling Entity Structs and EntityViewStructs inside engines-

This is the part that is still a bit awkward due to the lack of the new c# 7.0 features. Essentially EntityStructs should always be managed inside an engine through an array. This is the way to be able to write cache friendly code. To be honest, having an array of structs is not enough, the cache can be easily broken if you don’t know how much memory can fit in the L1 cache and how much memory can be read in a cycle.  One can say that the so much advertised feature is still for advanced users, who know what they are doing and probably don’t need the Unity ECS implementation to achieve it. In more practical terms, EntityStructs can be used where entity cache is not strictly necessary. For this reason, I wanted to support the querying of EntityStructs by entity id too. The only problem is that without the by ref return of c# 7 the only way I had to solve the problem was to use something like:

it’s much less nicer than using the byref return, but still reasonable. This is error prone for junior coders though, as by mistake the lambda could capture external variables, resulting in allocation. This method will be deprecated once c# 7 will be the standard on Unity (probably still a long way to go)

Alternatively the array of entity structs can be used directly, but even in this case, The Junior Coders must know what they are doing:

In both cases the goal is to be able to modify the value back inside the centralized database of Entities. For example, in the second case, if instead of using enemies[i] I would have stored it into a local variable, the enemies[i] value inside the database wouldn’t change, as I would work on a copy of it. The c# 7 return by ref would let me do something like:

and let me using enemy directly not as a copy of the structure, but as a pointer to it.

-the EGID and Entity Groups-

Svelto.ECS 2.5 boasts a new optimized centralized data structure to hold the data of all the entities. With one data structure only it’s able to hold EntityViews, EntityViewStructs and EntityStructs without causing any boxing allocation. The same data structure is able to query a set of entity data of a given type as an array or return a single one indexed by EGID.

EGID stands for EntityGlobalID and it’s a combination of the EntityID and the GroupID where the entity is defined.

in Svelto 2.5, entities are ALWAYS built inside a group. If the group is not defined, a standard exclusive group will be used. Groups help the database query system to be more flexible. It’s possible to give to a group any meaning, for example a groups could be used to create an entity pooling system when a group is used for the “disabled” entities. Another example is to define a group as “all the entities carried by another entity in game”. There are no limitations except:

  • EntityID must be unique inside a group. The same entity ID can be used across several groups.
  • An entity can be present inside one group at time, HOWEVER nothing keeps you from creating two different entities, with the same ID, in two different groups to be shared separately. This is actually an example where the use of implementors “could” be smart. While I said that implementors should always be used as bridge between the platform and the engines, another possible use of implementors is to share the same data between different entities. In fact, several entities could be created using the same implementors. Without this trick, it would be hard to manage the same entity data in two different groups. It’s also possible to swap a specific entity between groups.

At last, it’s important to remember that the EGID of an entity is not always the same over the time. Changing the group would in fact change the value of the EGID.

-Use Entity Groups for pooling-

In the new survival code, I implemented an Entity Pooling system for the enemy spawning, let’s have a look:

When an enemy dies, I don’t destroy the GameObject and remove the entity from the database anymore. Instead I swap its group on death using the method:

I take the dead enemy ID and use the SwapEntityGroup to move it from the Standard Group to a special group ID I use to keep track of the disabled enemies per enemy type.

Then when I have to reuse the Entity I will take its EntityViewStructs from the disabled pool. The EntityStructs must be reset as well and that’s what I do inside the ExecuteOnEntity methods. Note that all the implementors now are only Monobehaviours and they are not garbage collected, so that fetched EntityViewStructs will still point to the implementors of the GameObject linked to the Entity.

-Other important differences-

  • Previously it was possible to retrieve the complete list of EntityViews regardless the group they were belonging to, this is not possible now
  • Previously the RemoveEntity function didn’t need to specify the group where the Entity laid, it’s now needed.

A review of the communication functionalities in Svelto.ECS

Svelto main communication forms are confirmed to be the Sequencer and the DispatchOnSet and DispatchOnChange methods. Currently DispatchOnSet and DispatchOnChange can only be used through component interfaces, but once c# 7 will be available in unity, maybe I will convert them in structs and let them be usable inside EntityStructs as well. DispatchOnSet and DispatchOnChange must be still seen as Data Binding systems and not as Events.

I also improved a bit the Sequencer code, so that a declaration would be even more explanatory now. Remember that the main benefit of the Sequencer is to give to the user the possibility to understand the flow of communication from its declaration without being forced to check the code and reconstruct the communication flow from it.

It would be awesome if someone would be so cool to write an Unity Editor Tool to fetch all the defined Sequencers in the Game assembly and visualize it with a node graph system. I would never have the time neither the interest to do it, but that’s why Svelto is open source, so that other people can help if they wish 😉

Note: Svelto.ECS (together with the companion Svelto.Tasks) is currently compatible with .NetStandard 2.0, Net 3.5, Net 4.5, unity IL2CPP/UWP.

(*) The evolution of my reasoning can be found in these articles that I strongly suggest to read:

3 thoughts on “Introducing Svelto ECS 2.5

  1. I was wondering what plugin you use for the beautiful vertical alignment. Thank you for this great project!

Leave a Reply