New parachain runtime skeleton (#1158)

* file structure and initializer skeleton

* ensure session changes happen before initialization

* add a couple tests for initializer flow

* integrate with session handling

* configuration update logic

* configuration methods

* move test mock to its own module

* integrate configuration into initializer

* add note about initialization order

* integrate configuration module into mock

* add some tests for config module

* paras module storage

* implement paras session change operation

* amend past code pruning to fully cover acceptance period

* update guide again

* do pruning of historical validation code

* add weight to initialization

* integrate into mock & leave notes for next session

* clean up un-ended sentence

* alter test to account for double index in past code meta

* port over code-at logic test

* clarify checking for conflicting code upgrades

* add genesis for paras, include in mock, ensure incoming paras are processed

* note on return value of `validation_code_at`

* implement paras routines from implementor's guide

* bring over some existing tests and begin porting

* port over code upgrade tests

* test parachain registration

* test code_at with intermediate block

* fix warnings

* clean up docs and extract to separate struct

* adjust implementor's guide to include replacementtimes

* kill stray println

* rename expected_at to applied_after

* rewrite ParaPastCodeMeta to avoid reversal

* clarify and test interface of validation_code_at

* make FutureCode optional

* rename do_old_code_pruning

* add comment on Option<()> to answer FAQ

* address some more grumbles

Cargo.lock

@@ -4457,6 +4457,48 @@ dependencies = [
  "trie-db",
 ]
 
+[[package]]
+name = "polkadot-runtime-parachains"
+version = "0.8.0"
+dependencies = [
+ "bitvec",
+ "frame-benchmarking",
+ "frame-support",
+ "frame-system",
+ "hex-literal",
+ "libsecp256k1",
+ "log 0.3.9",
+ "pallet-authorship",
+ "pallet-babe",
+ "pallet-balances",
+ "pallet-offences",
+ "pallet-randomness-collective-flip",
+ "pallet-session",
+ "pallet-staking",
+ "pallet-staking-reward-curve",
+ "pallet-timestamp",
+ "pallet-treasury",
+ "pallet-vesting",
+ "parity-scale-codec",
+ "polkadot-primitives",
+ "rustc-hex",
+ "serde",
+ "serde_derive",
+ "serde_json",
+ "sp-api",
+ "sp-application-crypto",
+ "sp-core",
+ "sp-inherents",
+ "sp-io",
+ "sp-keyring",
+ "sp-runtime",
+ "sp-session",
+ "sp-staking",
+ "sp-std",
+ "sp-trie",
+ "trie-db",
+]
+
 [[package]]
 name = "polkadot-service"
 version = "0.8.2"

Cargo.toml

@@ -30,6 +30,7 @@ members = [
 	"overseer",
 	"primitives",
 	"runtime/common",
+	"runtime/parachains",
 	"runtime/polkadot",
 	"runtime/kusama",
 	"runtime/westend",

roadmap/implementors-guide/guide.md

@@ -414,7 +414,7 @@ It's also responsible for managing parachain validation code upgrades as well as
 Utility structs:
 ```rust
 // the two key times necessary to track for every code replacement.
-struct ReplacementTimes {
+pub struct ReplacementTimes {
 	/// The relay-chain block number that the code upgrade was expected to be activated.
 	/// This is when the code change occurs from the para's perspective - after the
 	/// first parablock included with a relay-parent with number >= this value.
@@ -481,7 +481,7 @@ PastCodePruning: Vec<(ParaId, BlockNumber)>;
 /// in the context of a relay chain block with a number >= `expected_at`.
 FutureCodeUpgrades: map ParaId => Option<BlockNumber>;
 /// The actual future code of a para.
-FutureCode: map ParaId => ValidationCode;
+FutureCode: map ParaId => Option<ValidationCode>;
 
 /// Upcoming paras (chains and threads). These are only updated on session change. Corresponds to an
 /// entry in the upcoming-genesis map.
@@ -507,7 +507,7 @@ OutgoingParas: Vec<ParaId>;
 * `schedule_para_cleanup(ParaId)`: schedule a para to be cleaned up at the next session.
 * `schedule_code_upgrade(ParaId, ValidationCode, expected_at: BlockNumber)`: Schedule a future code upgrade of the given parachain, to be applied after inclusion of a block of the same parachain executed in the context of a relay-chain block with number >= `expected_at`.
 * `note_new_head(ParaId, HeadData, BlockNumber)`: note that a para has progressed to a new head, where the new head was executed in the context of a relay-chain block with given number. This will apply pending code upgrades based on the block number provided.
-* `validation_code_at(ParaId, at: BlockNumber, assume_intermediate: Option<BlockNumber>)`: Fetches the validation code to be used when validating a block in the context of the given relay-chain height. A second block number parameter may be used to tell the lookup to proceed as if an intermediate parablock has been included at the given relay-chain height. This may return past, current, or (with certain choices of `assume_intermediate`) future code. `assume_intermediate`, if provided, must be before `at`. If `at` is not within `config.acceptance_period` of the current block number, this will return `None`.
+* `validation_code_at(ParaId, at: BlockNumber, assume_intermediate: Option<BlockNumber>)`: Fetches the validation code to be used when validating a block in the context of the given relay-chain height. A second block number parameter may be used to tell the lookup to proceed as if an intermediate parablock has been included at the given relay-chain height. This may return past, current, or (with certain choices of `assume_intermediate`) future code. `assume_intermediate`, if provided, must be before `at`. If the validation code has been pruned, this will return `None`.
 
 #### Finalization
 
@@ -755,6 +755,7 @@ All failed checks should lead to an unrecoverable error making the block invalid
     1. Return a list of freed cores consisting of the cores where candidates have become available.
   * `process_candidates(BackedCandidates, scheduled: Vec<CoreAssignment>)`:
     1. check that each candidate corresponds to a scheduled core and that they are ordered in ascending order by `ParaId`.
+    1. Ensure that any code upgrade scheduled by the candidate does not happen within `config.validation_upgrade_frequency` of the currently scheduled upgrade, if any, comparing against the value of `Paras::FutureCodeUpgrades` for the given para ID.
     1. check the backing of the candidate using the signatures and the bitfields.
     1. create an entry in the `PendingAvailability` map for each backed candidate with a blank `availability_votes` bitfield.
     1. Return a `Vec<CoreIndex>` of all scheduled cores of the list of passed assignments that a candidate was successfully backed for, sorted ascending by CoreIndex.

runtime/parachains/Cargo.toml

+[package]
+name = "polkadot-runtime-parachains"
+version = "0.8.0"
+authors = ["Parity Technologies <admin@parity.io>"]
+edition = "2018"
+
+[dependencies]
+bitvec = { version = "0.17.4", default-features = false, features = ["alloc"] }
+codec = { package = "parity-scale-codec", version = "1.3.0", default-features = false, features = ["derive"] }
+log = { version = "0.3.9", optional = true }
+rustc-hex = { version = "2.0.1", default-features = false }
+serde = { version = "1.0.102", default-features = false }
+serde_derive = { version = "1.0.102", optional = true }
+
+sp-api = { git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
+inherents = { package = "sp-inherents", git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
+sp-std = { package = "sp-std", git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
+sp-io = { git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
+sp-runtime = { git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
+sp-session = { git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
+sp-staking = { git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
+sp-core = { git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
+
+authorship = { package = "pallet-authorship", git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
+balances = { package = "pallet-balances", git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
+session = { package = "pallet-session", git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
+frame-support = { git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
+staking = { package = "pallet-staking", git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
+system = { package = "frame-system", git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
+timestamp = { package = "pallet-timestamp", git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
+vesting = { package = "pallet-vesting", git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
+offences = { package = "pallet-offences", git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
+frame-benchmarking = { git = "https://github.com/paritytech/substrate", branch = "master", default-features = false, optional = true }
+
+primitives = { package = "polkadot-primitives", path = "../../primitives", default-features = false }
+libsecp256k1 = { version = "0.3.2", default-features = false, optional = true }
+
+[dev-dependencies]
+hex-literal = "0.2.1"
+keyring = { package = "sp-keyring", git = "https://github.com/paritytech/substrate", branch = "master" }
+sp-trie = { git = "https://github.com/paritytech/substrate", branch = "master" }
+babe = { package = "pallet-babe", git = "https://github.com/paritytech/substrate", branch = "master" }
+sp-application-crypto = { git = "https://github.com/paritytech/substrate", branch = "master" }
+randomness-collective-flip = { package = "pallet-randomness-collective-flip", git = "https://github.com/paritytech/substrate", branch = "master" }
+pallet-staking-reward-curve = { git = "https://github.com/paritytech/substrate", branch = "master" }
+treasury = { package = "pallet-treasury", git = "https://github.com/paritytech/substrate", branch = "master" }
+trie-db = "0.20.0"
+serde_json = "1.0.41"
+libsecp256k1 = "0.3.2"
+
+[features]
+default = ["std"]
+no_std = []
+std = [
+	"bitvec/std",
+	"codec/std",
+	"log",
+	"rustc-hex/std",
+	"serde_derive",
+	"serde/std",
+	"primitives/std",
+	"inherents/std",
+	"sp-core/std",
+	"sp-api/std",
+	"sp-std/std",
+	"sp-io/std",
+	"frame-support/std",
+	"authorship/std",
+	"balances/std",
+	"sp-runtime/std",
+	"sp-session/std",
+	"sp-staking/std",
+	"session/std",
+	"staking/std",
+	"system/std",
+	"timestamp/std",
+	"vesting/std",
+]
+runtime-benchmarks = [
+	"libsecp256k1/hmac",
+	"frame-benchmarking",
+	"frame-support/runtime-benchmarks",
+	"system/runtime-benchmarks",
+]

runtime/parachains/src/configuration.rs

+// Copyright 2020 Parity Technologies (UK) Ltd.
+// This file is part of Polkadot.
+
+// Polkadot is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Polkadot is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Polkadot.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Configuration manager for the Polkadot runtime parachains logic.
+//!
+//! Configuration can change only at session boundaries and is buffered until then.
+
+use sp_std::prelude::*;
+use primitives::{
+	parachain::{ValidatorId},
+};
+use frame_support::{
+	decl_storage, decl_module, decl_error,
+	dispatch::DispatchResult,
+	weights::{DispatchClass, Weight},
+};
+use codec::{Encode, Decode};
+use system::ensure_root;
+
+/// All configuration of the runtime with respect to parachains and parathreads.
+#[derive(Clone, Encode, Decode, PartialEq, Default)]
+#[cfg_attr(test, derive(Debug))]
+pub struct HostConfiguration<BlockNumber: Default> {
+	/// The minimum frequency at which parachains can update their validation code.
+	pub validation_upgrade_frequency: BlockNumber,
+	/// The delay, in blocks, before a validation upgrade is applied.
+	pub validation_upgrade_delay: BlockNumber,
+	/// The acceptance period, in blocks. This is the amount of blocks after availability that validators
+	/// and fishermen have to perform secondary checks or issue reports.
+	pub acceptance_period: BlockNumber,
+	/// The maximum validation code size, in bytes.
+	pub max_code_size: u32,
+	/// The maximum head-data size, in bytes.
+	pub max_head_data_size: u32,
+	/// The amount of execution cores to dedicate to parathread execution.
+	pub parathread_cores: u32,
+	/// The number of retries that a parathread author has to submit their block.
+	pub parathread_retries: u32,
+	/// How often parachain groups should be rotated across parachains.
+	pub parachain_rotation_frequency: BlockNumber,
+	/// The availability period, in blocks, for parachains. This is the amount of blocks
+	/// after inclusion that validators have to make the block available and signal its availability to
+	/// the chain. Must be at least 1.
+	pub chain_availability_period: BlockNumber,
+	/// The availability period, in blocks, for parathreads. Same as the `chain_availability_period`,
+	/// but a differing timeout due to differing requirements. Must be at least 1.
+	pub thread_availability_period: BlockNumber,
+	/// The amount of blocks ahead to schedule parachains and parathreads.
+	pub scheduling_lookahead: u32,
+}
+
+pub trait Trait: system::Trait { }
+
+decl_storage! {
+	trait Store for Module<T: Trait> as Configuration {
+		/// The active configuration for the current session.
+		Config get(fn config) config(): HostConfiguration<T::BlockNumber>;
+		/// Pending configuration (if any) for the next session.
+		PendingConfig: Option<HostConfiguration<T::BlockNumber>>;
+	}
+}
+
+decl_error! {
+	pub enum Error for Module<T: Trait> { }
+}
+
+decl_module! {
+	/// The parachains configuration module.
+	pub struct Module<T: Trait> for enum Call where origin: <T as system::Trait>::Origin {
+		type Error = Error<T>;
+
+		/// Set the validation upgrade frequency.
+		#[weight = (1_000, DispatchClass::Operational)]
+		pub fn set_validation_upgrade_frequency(origin, new: T::BlockNumber) -> DispatchResult {
+			ensure_root(origin)?;
+			Self::update_config_member(|config| {
+				sp_std::mem::replace(&mut config.validation_upgrade_frequency, new) != new
+			});
+			Ok(())
+		}
+
+		/// Set the validation upgrade delay.
+		#[weight = (1_000, DispatchClass::Operational)]
+		pub fn set_validation_upgrade_delay(origin, new: T::BlockNumber) -> DispatchResult {
+			ensure_root(origin)?;
+			Self::update_config_member(|config| {
+				sp_std::mem::replace(&mut config.validation_upgrade_delay, new) != new
+			});
+			Ok(())
+		}
+
+		/// Set the acceptance period for an included candidate.
+		#[weight = (1_000, DispatchClass::Operational)]
+		pub fn set_acceptance_period(origin, new: T::BlockNumber) -> DispatchResult {
+			ensure_root(origin)?;
+			Self::update_config_member(|config| {
+				sp_std::mem::replace(&mut config.acceptance_period, new) != new
+			});
+			Ok(())
+		}
+
+		/// Set the max validation code size for incoming upgrades.
+		#[weight = (1_000, DispatchClass::Operational)]
+		pub fn set_max_code_size(origin, new: u32) -> DispatchResult {
+			ensure_root(origin)?;
+			Self::update_config_member(|config| {
+				sp_std::mem::replace(&mut config.max_code_size, new) != new
+			});
+			Ok(())
+		}
+
+		/// Set the max head data size for paras.
+		#[weight = (1_000, DispatchClass::Operational)]
+		pub fn set_max_head_data_size(origin, new: u32) -> DispatchResult {
+			ensure_root(origin)?;
+			Self::update_config_member(|config| {
+				sp_std::mem::replace(&mut config.max_head_data_size, new) != new
+			});
+			Ok(())
+		}
+
+		/// Set the number of parathread execution cores.
+		#[weight = (1_000, DispatchClass::Operational)]
+		pub fn set_parathread_cores(origin, new: u32) -> DispatchResult {
+			ensure_root(origin)?;
+			Self::update_config_member(|config| {
+				sp_std::mem::replace(&mut config.parathread_cores, new) != new
+			});
+			Ok(())
+		}
+
+		/// Set the number of retries for a particular parathread.
+		#[weight = (1_000, DispatchClass::Operational)]
+		pub fn set_parathread_retries(origin, new: u32) -> DispatchResult {
+			ensure_root(origin)?;
+			Self::update_config_member(|config| {
+				sp_std::mem::replace(&mut config.parathread_retries, new) != new
+			});
+			Ok(())
+		}
+
+
+		/// Set the parachain validator-group rotation frequency
+		#[weight = (1_000, DispatchClass::Operational)]
+		pub fn set_parachain_rotation_frequency(origin, new: T::BlockNumber) -> DispatchResult {
+			ensure_root(origin)?;
+			Self::update_config_member(|config| {
+				sp_std::mem::replace(&mut config.parachain_rotation_frequency, new) != new
+			});
+			Ok(())
+		}
+
+		/// Set the availability period for parachains.
+		#[weight = (1_000, DispatchClass::Operational)]
+		pub fn set_chain_availability_period(origin, new: T::BlockNumber) -> DispatchResult {
+			ensure_root(origin)?;
+			Self::update_config_member(|config| {
+				sp_std::mem::replace(&mut config.chain_availability_period, new) != new
+			});
+			Ok(())
+		}
+
+		/// Set the availability period for parathreads.
+		#[weight = (1_000, DispatchClass::Operational)]
+		pub fn set_thread_availability_period(origin, new: T::BlockNumber) -> DispatchResult {
+			ensure_root(origin)?;
+			Self::update_config_member(|config| {
+				sp_std::mem::replace(&mut config.thread_availability_period, new) != new
+			});
+			Ok(())
+		}
+
+		/// Set the scheduling lookahead, in expected number of blocks at peak throughput.
+		#[weight = (1_000, DispatchClass::Operational)]
+		pub fn set_scheduling_lookahead(origin, new: u32) -> DispatchResult {
+			ensure_root(origin)?;
+			Self::update_config_member(|config| {
+				sp_std::mem::replace(&mut config.scheduling_lookahead, new) != new
+			});
+			Ok(())
+		}
+	}
+}
+
+impl<T: Trait> Module<T> {
+	/// Called by the initializer to initialize the configuration module.
+	pub(crate) fn initializer_initialize(_now: T::BlockNumber) -> Weight {
+		0
+	}
+
+	/// Called by the initializer to finalize the configuration module.
+	pub(crate) fn initializer_finalize() { }
+
+	/// Called by the initializer to note that a new session has started.
+	pub(crate) fn initializer_on_new_session(_validators: &[ValidatorId], _queued: &[ValidatorId]) {
+		if let Some(pending) = <Self as Store>::PendingConfig::take() {
+			<Self as Store>::Config::set(pending);
+		}
+	}
+
+	fn update_config_member(
+		updater: impl FnOnce(&mut HostConfiguration<T::BlockNumber>) -> bool,
+	) {
+		let pending = <Self as Store>::PendingConfig::get();
+		let mut prev = pending.unwrap_or_else(Self::config);
+
+		if updater(&mut prev) {
+			<Self as Store>::PendingConfig::set(Some(prev));
+		}
+	}
+}
+
+#[cfg(test)]
+mod tests {
+	use super::*;
+	use crate::mock::{new_test_ext, Initializer, Configuration, Origin};
+
+	use frame_support::traits::{OnFinalize, OnInitialize};
+
+	#[test]
+	fn config_changes_on_session_boundary() {
+		new_test_ext(Default::default()).execute_with(|| {
+			let old_config = Configuration::config();
+			let mut config = old_config.clone();
+			config.validation_upgrade_delay = 100;
+
+			assert!(old_config != config);
+
+			<Configuration as Store>::PendingConfig::set(Some(config.clone()));
+
+			Initializer::on_initialize(1);
+
+			assert_eq!(Configuration::config(), old_config);
+			assert_eq!(<Configuration as Store>::PendingConfig::get(), Some(config.clone()));
+
+			Initializer::on_finalize(1);
+
+			Configuration::initializer_on_new_session(&[], &[]);
+
+			assert_eq!(Configuration::config(), config);
+			assert!(<Configuration as Store>::PendingConfig::get().is_none());
+		})
+	}
+
+	#[test]
+	fn setting_pending_config_members() {
+		new_test_ext(Default::default()).execute_with(|| {
+			let new_config = HostConfiguration {
+				validation_upgrade_frequency: 100,
+				validation_upgrade_delay: 10,
+				acceptance_period: 5,
+				max_code_size: 100_000,
+				max_head_data_size: 1_000,
+				parathread_cores: 2,
+				parathread_retries: 5,
+				parachain_rotation_frequency: 20,
+				chain_availability_period: 10,
+				thread_availability_period: 8,
+				scheduling_lookahead: 3,
+			};
+
+			assert!(<Configuration as Store>::PendingConfig::get().is_none());
+
+			Configuration::set_validation_upgrade_frequency(
+				Origin::ROOT, new_config.validation_upgrade_frequency,
+			).unwrap();
+			Configuration::set_validation_upgrade_delay(
+				Origin::ROOT, new_config.validation_upgrade_delay,
+			).unwrap();
+			Configuration::set_acceptance_period(
+				Origin::ROOT, new_config.acceptance_period,
+			).unwrap();
+			Configuration::set_max_code_size(
+				Origin::ROOT, new_config.max_code_size,
+			).unwrap();
+			Configuration::set_max_head_data_size(
+				Origin::ROOT, new_config.max_head_data_size,
+			).unwrap();
+			Configuration::set_parathread_cores(
+				Origin::ROOT, new_config.parathread_cores,
+			).unwrap();
+			Configuration::set_parathread_retries(
+				Origin::ROOT, new_config.parathread_retries,
+			).unwrap();
+			Configuration::set_parachain_rotation_frequency(
+				Origin::ROOT, new_config.parachain_rotation_frequency,
+			).unwrap();
+			Configuration::set_chain_availability_period(
+				Origin::ROOT, new_config.chain_availability_period,
+			).unwrap();
+			Configuration::set_thread_availability_period(
+				Origin::ROOT, new_config.thread_availability_period,
+			).unwrap();
+			Configuration::set_scheduling_lookahead(
+				Origin::ROOT, new_config.scheduling_lookahead,
+			).unwrap();
+
+			assert_eq!(<Configuration as Store>::PendingConfig::get(), Some(new_config));
+		})
+	}
+
+	#[test]
+	fn non_root_cannot_set_config() {
+		new_test_ext(Default::default()).execute_with(|| {
+			assert!(Configuration::set_validation_upgrade_delay(Origin::signed(1), 100).is_err());
+		});
+	}
+
+	#[test]
+	fn setting_config_to_same_as_current_is_noop() {
+		new_test_ext(Default::default()).execute_with(|| {
+			Configuration::set_validation_upgrade_delay(Origin::ROOT, Default::default()).unwrap();
+			assert!(<Configuration as Store>::PendingConfig::get().is_none())
+		});
+	}
+}

runtime/parachains/src/inclusion.rs

+// Copyright 2020 Parity Technologies (UK) Ltd.
+// This file is part of Polkadot.
+
+// Polkadot is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Polkadot is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Polkadot.  If not, see <http://www.gnu.org/licenses/>.

runtime/parachains/src/initializer.rs

+// Copyright 2020 Parity Technologies (UK) Ltd.
+// This file is part of Polkadot.
+
+// Polkadot is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Polkadot is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Polkadot.  If not, see <http://www.gnu.org/licenses/>.
+
+//! This module is responsible for maintaining a consistent initialization order for all other
+//! parachains modules. It's also responsible for finalization and session change notifications.
+//!
+//! This module can throw fatal errors if session-change notifications are received after initialization.
+
+use sp_std::prelude::*;
+use frame_support::weights::Weight;
+use primitives::{
+	parachain::{ValidatorId},
+};
+use frame_support::{
+	decl_storage, decl_module, decl_error,
+};
+use crate::{configuration, paras};
+
+pub trait Trait: system::Trait + configuration::Trait + paras::Trait { }
+
+decl_storage! {
+	trait Store for Module<T: Trait> as Initializer {
+		/// Whether the parachains modules have been initialized within this block.
+		///
+		/// Semantically a bool, but this guarantees it should never hit the trie,
+		/// as this is cleared in `on_finalize` and Frame optimizes `None` values to be empty values.
+		///
+		/// As a bool, `set(false)` and `remove()` both lead to the next `get()` being false, but one of
+		/// them writes to the trie and one does not. This confusion makes `Option<()>` more suitable for
+		/// the semantics of this variable.
+		HasInitialized: Option<()>;
+	}
+}
+
+decl_error! {
+	pub enum Error for Module<T: Trait> { }
+}
+
+decl_module! {
+	/// The initializer module.
+	pub struct Module<T: Trait> for enum Call where origin: <T as system::Trait>::Origin {
+		type Error = Error<T>;
+
+		fn on_initialize(now: T::BlockNumber) -> Weight {
+			// The other modules are initialized in this order:
+			// - Configuration
+			// - Paras
+			// - Scheduler
+			// - Inclusion
+			// - Validity
+			let total_weight = configuration::Module::<T>::initializer_initialize(now) +
+				paras::Module::<T>::initializer_initialize(now);
+
+			HasInitialized::set(Some(()));
+
+			total_weight
+		}
+
+		fn on_finalize() {
+			paras::Module::<T>::initializer_finalize();
+			configuration::Module::<T>::initializer_finalize();
+			HasInitialized::take();
+		}