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73 lines
3.1 KiB
Markdown
73 lines
3.1 KiB
Markdown
# Configuration Pallet
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This module is responsible for managing all configuration of the parachain host in-flight. It provides a central point
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for configuration updates to prevent races between configuration changes and parachain-processing logic. Configuration
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can only change during the session change routine, and as this module handles the session change notification first it
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provides an invariant that the configuration does not change throughout the entire session. Both the
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[scheduler](scheduler.md) and [inclusion](inclusion.md) modules rely on this invariant to ensure proper behavior of the
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scheduler.
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The configuration that we will be tracking is the [`HostConfiguration`](../types/runtime.md#host-configuration) struct.
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## Storage
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The configuration module is responsible for two main pieces of storage.
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```rust
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/// The current configuration to be used.
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Configuration: HostConfiguration;
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/// A pending configuration to be applied on session change.
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PendingConfigs: Vec<(SessionIndex, HostConfiguration)>;
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/// A flag that says if the consistency checks should be omitted.
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BypassConsistencyCheck: bool;
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```
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## Session change
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The session change routine works as follows:
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- If there is no pending configurations, then return early.
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- Take all pending configurations that are less than or equal to the current session index.
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- Get the pending configuration with the highest session index and apply it to the current configuration. Discard the
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earlier ones if any.
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## Routines
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```rust
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enum InconsistentError {
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// ...
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}
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impl HostConfiguration {
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fn check_consistency(&self) -> Result<(), InconsistentError> { /* ... */ }
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}
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/// Get the host configuration.
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pub fn configuration() -> HostConfiguration {
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Configuration::get()
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}
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/// Schedules updating the host configuration. The update is given by the `updater` closure. The
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/// closure takes the current version of the configuration and returns the new version.
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/// Returns an `Err` if the closure returns a broken configuration. However, there are a couple of
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/// exceptions:
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///
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/// - if the configuration that was passed in the closure is already broken, then it will pass the
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/// update: you cannot break something that is already broken.
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/// - If the `BypassConsistencyCheck` flag is set, then the checks will be skipped.
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///
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/// The changes made by this function will always be scheduled at session X, where X is the current session index + 2.
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/// If there is already a pending update for X, then the closure will receive the already pending configuration for
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/// session X.
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///
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/// If there is already a pending update for the current session index + 1, then it won't be touched. Otherwise,
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/// that would violate the promise of this function that changes will be applied on the second session change (cur + 2).
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fn schedule_config_update(updater: impl FnOnce(&mut HostConfiguration<BlockNumberFor<T>>)) -> DispatchResult
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```
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## Entry-points
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The Configuration module exposes an entry point for each configuration member. These entry-points accept calls only from
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governance origins. These entry-points will use the `update_configuration` routine to update the specific configuration
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field.
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