Better documentation for explicit dependencies (#1428)

* More in-depth ambiguity checker docs
* Updated ecs_guide example with explicit dependencies

crates/bevy_ecs/src/schedule/stage.rs

@@ -21,11 +21,24 @@ pub trait Stage: Downcast + Send + Sync {
 
 impl_downcast!(Stage);
 
-/// When this resource is present in `Resources`, `SystemStage` will log a report containing
-/// pairs of systems with ambiguous execution order - i.e., those systems might induce different
-/// results depending on the order they're executed in, yet don't have an explicit execution order
-/// constraint between them.
-/// This is not necessarily a bad thing - you have to make that judgement yourself.
+/// When this resource is present in the `AppBuilder`'s `Resources`,
+/// each `SystemStage` will log a report containing
+/// pairs of systems with ambiguous execution order.
+///
+/// Systems that access the same Component or Resource within the same stage
+/// risk an ambiguous order that could result in logic bugs, unless they have an
+/// explicit execution ordering constraint between them.
+///
+/// This occurs because, in the absence of explicit constraints, systems are executed in
+/// an unstable, arbitrary order within each stage that may vary between runs and frames.
+///
+/// Some ambiguities reported by the ambiguity checker may be warranted (to allow two systems to run without blocking each other)
+/// or spurious, as the exact combination of archetypes used may prevent them from ever conflicting during actual gameplay.
+/// You can resolve the warnings produced by the ambiguity checker by adding `.before` or `.after` to one of the conflicting systems
+/// referencing the other system to force a specific ordering.
+///
+/// The checker may report a system more times than the amount of constraints it would actually need to have
+/// unambiguous order with regards to a group of already-constrained systems.
 pub struct ReportExecutionOrderAmbiguities;
 
 struct VirtualSystemSet {

crates/bevy_ecs/src/schedule/system_descriptor.rs

@@ -12,7 +12,7 @@ use std::borrow::Cow;
 /// been applied.
 ///
 /// All systems can have a label attached to them; other systems in the same group can then specify
-/// that they have to run before or after the system with that label.
+/// that they have to run before or after the system with that label using the `before` and `after` methods.
 ///
 /// # Example
 /// ```rust

examples/ecs/ecs_guide.rs

@@ -261,19 +261,29 @@ fn main() {
         //
         // SYSTEM EXECUTION ORDER
         //
-        // By default, all systems run in parallel. This is efficient, but sometimes order matters.
+        // Each system belongs to a `Stage`, which controls the execution strategy and broad order of the systems within each tick.
+        // Startup stages (which startup systems are registered in) will always complete before ordinary stages begin,
+        // and every system in a stage must complete before the next stage advances.
+        // Once every stage has concluded, the main loop is complete and begins again.
+        //
+        // By default, all systems run in parallel, except when they require mutable access to a piece of data.
+        // This is efficient, but sometimes order matters.
         // For example, we want our "game over" system to execute after all other systems to ensure we don't
         // accidentally run the game for an extra round.
         //
-        // First, if a system writes a component or resource (ComMut / ResMut), it will force a synchronization.
-        // Any systems that access the data type and were registered BEFORE the system will need to finish first.
-        // Any systems that were registered _after_ the system will need to wait for it to finish. This is a great
-        // default that makes everything "just work" as fast as possible without us needing to think about it ... provided
-        // we don't care about execution order. If we do care, one option would be to use the rules above to force a synchronization
-        // at the right time. But that is complicated and error prone!
+        // Rather than splitting each of your systems into separate stages, you should force an explicit ordering between them
+        // by giving the relevant systems a label with `.label`, then using the `.before` or `.after` methods.
+        // Systems will not be scheduled until all of the systems that they have an "ordering dependency" on have completed.
+        //
+        // Doing that will, in just about all cases, lead to better performance compared to
+        // splitting systems between stages, because it gives the scheduling algorithm more
+        // opportunities to run systems in parallel.
+        // Stages are still necessary, however: end of a stage is a hard sync point
+        // (meaning, no systems are running) where `Commands` issued by systems are processed.
+        // This is required because commands can perform operations that are incompatible with
+        // having systems in flight, such as spawning or deleting entities,
+        // adding or removing resources, etc.
         //
-        // This is where "stages" come in. A "stage" is a group of systems that execute (in parallel). Stages are executed in order,
-        // and the next stage won't start until all systems in the current stage have finished.
         // add_system(system) adds systems to the UPDATE stage by default
         // However we can manually specify the stage if we want to. The following is equivalent to add_system(score_system)
         .add_system_to_stage(stage::UPDATE, score_system.system())
@@ -285,11 +295,22 @@ fn main() {
         .add_stage_after(stage::UPDATE, "after_round", SystemStage::parallel())
         .add_system_to_stage("before_round", new_round_system.system())
         .add_system_to_stage("before_round", new_player_system.system())
-        .add_system_to_stage("after_round", score_check_system.system())
-        .add_system_to_stage("after_round", game_over_system.system())
-        // score_check_system will run before game_over_system because score_check_system modifies GameState and game_over_system
-        // reads GameState. This works, but it's a bit confusing. In practice, it would be clearer to create a new stage that runs
-        // before "after_round"
+        // We can ensure that game_over system runs after score_check_system using explicit ordering constraints
+        // First, we label the system we want to refer to using `.label`
+        // Then, we use either `.before` or `.after` to describe the order we want the relationship
+        .add_system_to_stage(
+            "after_round",
+            score_check_system.system().label("score_check"),
+        )
+        .add_system_to_stage(
+            "after_round",
+            game_over_system.system().after("score_check"),
+        )
+        // We can check our systems for execution order ambiguities by examining the output produced in the console
+        // by adding the following Resource to our App :)
+        // Be aware that not everything reported by this checker is a potential problem, you'll have to make
+        // that judgement yourself.
+        .insert_resource(ReportExecutionOrderAmbiguities)
         // This call to run() starts the app we just built!
         .run();
 }