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Version Subscription

XCM is a versioned messaging format. One version may contain more or different instructions than another, so for parties to communicate via XCM, it is important to know which version the other party is using. XCM enables a version subscription model, where parties can subscribe to each other to get notified of version updates. XCM has two instructions to enable this:

  • SubscribeVersion
  • UnsubscribeVersion

The version subscription model can differ per XCVM implementation. The xcm-executor has a SubscriptionService config item. Any type specified as the SubscriptionService must implement the VersionChangeNotifier trait. The XCM pallet is one such implementor. When the SubscribeVersion instruction is sent to a consensus system that uses the XCM pallet as the SubscriptionService in the XCM executor, the system will send back its currently AdvertisedVersion and will keep the subscribed location up to date when the version changes. The subscribed location can unsubscribe to version changes by sending the UnsubscribeVersion instruction.

SubscribeVersion {
    #[codec(compact)]
    query_id: QueryId,
    max_response_weight: Weight,
}

UnsubscribeVersion

Check out the example.