Refactor XCM Simulator Example

[shawntabrizi]

This PR does a "developer experience" refactor of the XCM Simulator Example.

I was looking for existing code / documentation where developers could better learn about working with and configuring XCM.

The XCM Simulator was a natural starting point due to the fact that it can emulate end to end XCM scenarios, without needing to spawn multiple real chains.

However, the XCM Simulator Example was just 3 giant files with a ton of configurations, runtime, pallets, and tests mashed together.

This PR breaks down the XCM Simulator Example in a way that I believe is more approachable by a new developer who is looking to navigate the various components of the end to end example, and modify it themselves.

The basic structure is:

• xcm simulator example
  • lib (tries to only use the xcm simulator macros)
  • tests
  • relay-chain
    • mod (basic runtime that developers should be familiar with)
    • xcm-config
      • mod (contains the XcmConfig type
      • various files for each custom configuration
  • parachain
    • mock_msg_queue (custom pallet for simulator example)
    • mod (basic runtime that developers should be familiar with)
    • xcm-config
      • mod (contains the XcmConfig type
      • various files for each custom configuration

I would like to add more documentation to this too, but I think this is a first step to be accepted which will affect how documentation is added to the example

[shawntabrizi]

ping @franciscoaguirre @kianenigma @shaunxw

[shawntabrizi]

In general, I am not super confident about a file named locations and limits, but was the best I could think of.

[franciscoaguirre]

The example serves its purpose better with this, to be a basic example for XCM but mostly of how to use the simulator.

[shawntabrizi]

In 8518b05, I removed:

pub type SovereignAccountOf = (
	SiblingParachainConvertsVia<Sibling, AccountId>,
	AccountId32Aliases<RelayNetwork, AccountId>,
	ParentIsPreset<AccountId>,
);

As it seemed to be a duplicate definition of LocationConverter, but missing Account32Hash, which it probably should have.

Instances where SovereignAccountOf was used, i put LocationConverter instead.

In general, my feedback is also that SovereignAccountOf is a bad name for the config on the XCM Pallet, and should be something more like SovereignAccountIdConverter

[shawntabrizi]

@franciscoaguirre ping to help me get this in. thank you

[kianenigma]

I would give you a dinner and a hug in Singapore if as a part of the XCM tutorials, we can have an simple version of TestDefaultConfig for this as well.

[kianenigma]

Although, #1959

[shawntabrizi]

i will take a look for sure

[kianenigma]

Looks good, I think adding more inline-documentation + integration with #2633 would be great.

In general, I think if you and @franciscoaguirre can make sure the XCM @ PBA5 is reflected in #2633, it would be a great outcome.

[acatangiu]

very nice!

[bkontur]

very nit, what about extracting XcmRouter to the standalone routing.rs? E.g. barrier is also one-liner.

I think, also it makes sense putting a barrier and routing stuff together. The barrier validates what was routed/sent by some SendXcm impl, e.g. good example is WithUniqueTopic vs TrailingSetTopicAsId

[shawntabrizi]

Thanks for the suggestion! I am totally open to the idea, but let me provide some larger context.

I would like to use this refactor for part of a tutorial which teaches readers how to configure XCM for various situations.

Barrier I expect will be a spot which could be configured in multiple ways, which is why it is extracted into one file even if it is a one liner. That file will be used to explore the various possibilities for barrier.

I don't know much about XCM router. Are there other sensible configurations for it? Or usually it is just plugged into the same pallet?

[franciscoaguirre]

The router is usually one of the available transport options, not much configuration there.

[bkontur]

Thanks for the suggestion! I am totally open to the idea, but let me provide some larger context.

Barrier I expect will be a spot which could be configured in multiple ways, which is why it is extracted into one file even if it is a one liner. That file will be used to explore the various possibilities for barrier.

Ok, thank you for a context, so I agree mixing a barrier and router is not a good idea for this kind of tutorial.

I would like to use this refactor for part of a tutorial which teaches readers how to configure XCM for various situations.

I don't know much about XCM router. Are there other sensible configurations for it? Or usually it is just plugged into the same pallet?

E.g. we have a quite complex XcmRouter for AssetHubKusama: https://github.com/polkadot-fellows/runtimes/blob/main/system-parachains/asset-hubs/asset-hub-kusama/src/xcm_config.rs#L619-L643, simplified:

pub XcmRouter = (
    ParentAsUmp,
    XcmpQueue,
    ToPolkadotXcmRouter,
    ToEthereumXcmRouter
);

I can imagine a standalone chapter for XCM routing explaining Location, SendXcm, ExportMessage or creating custom implementation/feature like WithUniqueTopic ... so yes, for that chapter, I would extract routers to standalone files, e.g.:

example/src/parachain/xcm_config/routing.rs:

pub type XcmRouter = EnsureDecodableXcm<
       // XCMP - `(1, [$crate::Parachain(id)])` part of ParachainXcmRouter
       crate::ToSiblingParachainXcmRouter<MsgQueue>,   
       // UMP - `(1, [])` part of ParachainXcmRouter  
       crate::ToRelayChainXcmRouter<MsgQueue>, 
>;

example/src/relay_chain/xcm_config/routing.rs:

pub type XcmRouter = EnsureDecodableXcm<
       // DMP - `(0, [$crate::Parachain(id)])` part of RelayChainXcmRouter
       crate::ToChildParachainXcmRouter<MsgQueue>,
>;

Anyway, what tutorial do you mean exactly? I remember I saw your twitter post DotCodeSchool some time ago?

[shawntabrizi]

@bkontur tutorial will live here, but still in progress: https://github.com/shawntabrizi/polkadot-sdk-workshop-xcm/tree/master

will be ready by the next academy.

I will spend some time to look more into XCM router config, and if it makes sense, i will push more updates to this example in Polkadot, and continue to use it as the base template for the tutorial.