Move pruning strategy to runtime level (#128)

* move pruning strategy to runtim level

* cargo fmt --all

* Update modules/ethereum/src/lib.rs

Co-authored-by: Hernando Castano <HCastano@users.noreply.github.com>

* finalize_headers -> finalize_and_prune_headers

* PruningStrategy::default()

* fn import_of_non_best_block_may_finalize_blocks()

Co-authored-by: Hernando Castano <HCastano@users.noreply.github.com>

bridges/bin/node/runtime/src/kovan.rs

@@ -14,11 +14,16 @@
 // You should have received a copy of the GNU General Public License
 // along with Parity Bridges Common.  If not, see <http://www.gnu.org/licenses/>.
 
+use frame_support::RuntimeDebug;
 use hex_literal::hex;
-use pallet_bridge_eth_poa::{AuraConfiguration, ValidatorsConfiguration, ValidatorsSource};
+use pallet_bridge_eth_poa::{AuraConfiguration, PruningStrategy, ValidatorsConfiguration, ValidatorsSource};
 use sp_bridge_eth_poa::{Address, Header, U256};
 use sp_std::prelude::*;
 
+/// Max number of finalized headers to keep. It is equivalent of ~24 hours of
+/// finalized blocks on current Kovan chain.
+const FINALIZED_HEADERS_TO_KEEP: u64 = 20_000;
+
 /// Aura engine configuration for Kovan chain.
 pub fn kovan_aura_configuration() -> AuraConfiguration {
 	AuraConfiguration {
@@ -102,3 +107,45 @@ pub fn kovan_genesis_header() -> Header {
 		],
 	}
 }
+
+/// Kovan headers pruning strategy.
+///
+/// We do not prune unfinalized headers because exchange module only accepts
+/// claims from finalized headers. And if we're pruning unfinalized headers, then
+/// some claims may never be accepted.
+#[derive(Default, RuntimeDebug)]
+pub struct KovanPruningStrategy;
+
+impl PruningStrategy for KovanPruningStrategy {
+	fn pruning_upper_bound(&mut self, _best_number: u64, best_finalized_number: u64) -> u64 {
+		best_finalized_number
+			.checked_sub(FINALIZED_HEADERS_TO_KEEP)
+			.unwrap_or(0)
+	}
+}
+
+#[cfg(test)]
+mod tests {
+	use super::*;
+
+	#[test]
+	fn pruning_strategy_keeps_enough_headers() {
+		assert_eq!(
+			KovanPruningStrategy::default().pruning_upper_bound(100_000, 10_000),
+			0,
+			"10_000 <= 20_000 => nothing should be pruned yet",
+		);
+
+		assert_eq!(
+			KovanPruningStrategy::default().pruning_upper_bound(100_000, 20_000),
+			0,
+			"20_000 <= 20_000 => nothing should be pruned yet",
+		);
+
+		assert_eq!(
+			KovanPruningStrategy::default().pruning_upper_bound(100_000, 30_000),
+			10_000,
+			"20_000 <= 30_000 => we're ready to prune first 10_000 headers",
+		);
+	}
+}

bridges/bin/node/runtime/src/lib.rs

@@ -225,6 +225,7 @@ impl pallet_bridge_eth_poa::Trait for Runtime {
 	type AuraConfiguration = KovanAuraConfiguration;
 	type FinalityVotesCachingInterval = FinalityVotesCachingInterval;
 	type ValidatorsConfiguration = KovanValidatorsConfiguration;
+	type PruningStrategy = kovan::KovanPruningStrategy;
 	type OnHeadersSubmitted = ();
 }
 

bridges/modules/ethereum/src/import.rs

@@ -18,19 +18,10 @@ use crate::error::Error;
 use crate::finality::finalize_blocks;
 use crate::validators::{Validators, ValidatorsConfiguration};
 use crate::verification::{is_importable_header, verify_aura_header};
-use crate::{AuraConfiguration, ChangeToEnact, Storage};
+use crate::{AuraConfiguration, ChangeToEnact, PruningStrategy, Storage};
 use primitives::{Header, HeaderId, Receipt};
 use sp_std::{collections::btree_map::BTreeMap, prelude::*};
 
-/// Maximal number of headers behind best blocks that we are aiming to store. When there
-/// are too many unfinalized headers, it slows down finalization tracking significantly.
-/// That's why we won't consider imports/reorganizations to blocks of PRUNE_DEPTH age.
-/// If there's more headers than that, we prune the oldest. The only exception is
-/// when unfinalized header schedules validators set change. We can't compute finality
-/// for pruned headers => we won't know when to enact validators set change. That's
-/// why we never prune headers with scheduled changes.
-pub(crate) const PRUNE_DEPTH: u64 = 4096;
-
 /// Imports bunch of headers and updates blocks finality.
 ///
 /// Transactions receipts must be provided if `header_import_requires_receipts()`
@@ -40,11 +31,11 @@ pub(crate) const PRUNE_DEPTH: u64 = 4096;
 /// we have NOT imported.
 /// Returns error if fatal error has occured during import. Some valid headers may be
 /// imported in this case.
-pub fn import_headers<S: Storage>(
+pub fn import_headers<S: Storage, PS: PruningStrategy>(
 	storage: &mut S,
+	pruning_strategy: &mut PS,
 	aura_config: &AuraConfiguration,
 	validators_config: &ValidatorsConfiguration,
-	prune_depth: u64,
 	submitter: Option<S::Submitter>,
 	headers: Vec<(Header, Option<Vec<Receipt>>)>,
 	finalized_headers: &mut BTreeMap<S::Submitter, u64>,
@@ -54,9 +45,9 @@ pub fn import_headers<S: Storage>(
 	for (header, receipts) in headers {
 		let import_result = import_header(
 			storage,
+			pruning_strategy,
 			aura_config,
 			validators_config,
-			prune_depth,
 			submitter.clone(),
 			header,
 			receipts,
@@ -85,11 +76,11 @@ pub fn import_headers<S: Storage>(
 /// has returned true.
 ///
 /// Returns imported block id and list of all finalized headers.
-pub fn import_header<S: Storage>(
+pub fn import_header<S: Storage, PS: PruningStrategy>(
 	storage: &mut S,
+	pruning_strategy: &mut PS,
 	aura_config: &AuraConfiguration,
 	validators_config: &ValidatorsConfiguration,
-	prune_depth: u64,
 	submitter: Option<S::Submitter>,
 	header: Header,
 	receipts: Option<Vec<Receipt>>,
@@ -126,10 +117,9 @@ pub fn import_header<S: Storage>(
 	// (because otherwise we'll have inconsistent storage if transaction will fail)
 
 	// and finally insert the block
-	let (_, best_total_difficulty) = storage.best_block();
+	let (best_id, best_total_difficulty) = storage.best_block();
 	let total_difficulty = import_context.total_difficulty() + header.difficulty;
 	let is_best = total_difficulty > best_total_difficulty;
-	let header_number = header.number;
 	storage.insert_header(import_context.into_import_header(
 		is_best,
 		header_id,
@@ -140,15 +130,19 @@ pub fn import_header<S: Storage>(
 		finalized_blocks.votes,
 	));
 
-	// now mark finalized headers && prune old headers
-	storage.finalize_headers(
-		finalized_blocks.finalized_headers.last().map(|(id, _)| *id),
-		match is_best {
-			true => header_number.checked_sub(prune_depth),
-			false => None,
-		},
+	// compute upper border of updated pruning range
+	let new_best_block_id = if is_best { header_id } else { best_id };
+	let new_best_finalized_block_id = finalized_blocks.finalized_headers.last().map(|(id, _)| *id);
+	let pruning_upper_bound = pruning_strategy.pruning_upper_bound(
+		new_best_block_id.number,
+		new_best_finalized_block_id
+			.map(|id| id.number)
+			.unwrap_or(finalized_id.number),
 	);
 
+	// now mark finalized headers && prune old headers
+	storage.finalize_and_prune_headers(new_best_finalized_block_id, pruning_upper_bound);
+
 	Ok((header_id, finalized_blocks.finalized_headers))
 }
 
@@ -169,7 +163,7 @@ mod tests {
 	use super::*;
 	use crate::mock::{
 		block_i, custom_block_i, custom_test_ext, genesis, signed_header, test_aura_config, test_validators_config,
-		validator, validators, validators_addresses, TestRuntime,
+		validator, validators, validators_addresses, KeepSomeHeadersBehindBest, TestRuntime, GENESIS_STEP,
 	};
 	use crate::validators::ValidatorsSource;
 	use crate::{BlocksToPrune, BridgeStorage, Headers, PruningRange};
@@ -179,19 +173,19 @@ mod tests {
 	fn rejects_finalized_block_competitors() {
 		custom_test_ext(genesis(), validators_addresses(3)).execute_with(|| {
 			let mut storage = BridgeStorage::<TestRuntime>::new();
-			storage.finalize_headers(
+			storage.finalize_and_prune_headers(
 				Some(HeaderId {
 					number: 100,
 					..Default::default()
 				}),
-				None,
+				0,
 			);
 			assert_eq!(
 				import_header(
 					&mut storage,
+					&mut KeepSomeHeadersBehindBest::default(),
 					&test_aura_config(),
 					&test_validators_config(),
-					PRUNE_DEPTH,
 					None,
 					Default::default(),
 					None,
@@ -210,9 +204,9 @@ mod tests {
 			assert_eq!(
 				import_header(
 					&mut storage,
+					&mut KeepSomeHeadersBehindBest::default(),
 					&test_aura_config(),
 					&test_validators_config(),
-					PRUNE_DEPTH,
 					None,
 					block.clone(),
 					None,
@@ -223,9 +217,9 @@ mod tests {
 			assert_eq!(
 				import_header(
 					&mut storage,
+					&mut KeepSomeHeadersBehindBest::default(),
 					&test_aura_config(),
 					&test_validators_config(),
-					PRUNE_DEPTH,
 					None,
 					block,
 					None,
@@ -250,9 +244,9 @@ mod tests {
 			assert_eq!(
 				import_header(
 					&mut storage,
+					&mut KeepSomeHeadersBehindBest::default(),
 					&test_aura_config(),
 					&validators_config,
-					PRUNE_DEPTH,
 					None,
 					header,
 					None
@@ -285,9 +279,9 @@ mod tests {
 				let header = block_i(i, &validators);
 				let (rolling_last_block_id, finalized_blocks) = import_header(
 					&mut storage,
+					&mut KeepSomeHeadersBehindBest::default(),
 					&test_aura_config(),
 					&validators_config,
-					10,
 					Some(100),
 					header,
 					None,
@@ -316,9 +310,9 @@ mod tests {
 			});
 			let (rolling_last_block_id, finalized_blocks) = import_header(
 				&mut storage,
+				&mut KeepSomeHeadersBehindBest::default(),
 				&test_aura_config(),
 				&validators_config,
-				10,
 				Some(101),
 				header11.clone(),
 				Some(vec![crate::validators::tests::validators_change_recept(
@@ -352,9 +346,9 @@ mod tests {
 				expected_blocks.push((header.compute_id(), Some(102)));
 				let (rolling_last_block_id, finalized_blocks) = import_header(
 					&mut storage,
+					&mut KeepSomeHeadersBehindBest::default(),
 					&test_aura_config(),
 					&validators_config,
-					10,
 					Some(102),
 					header,
 					None,
@@ -387,9 +381,9 @@ mod tests {
 			let header = signed_header(&validators, header, step as _);
 			let (_, finalized_blocks) = import_header(
 				&mut storage,
+				&mut KeepSomeHeadersBehindBest::default(),
 				&test_aura_config(),
 				&validators_config,
-				10,
 				Some(103),
 				header,
 				None,
@@ -405,4 +399,76 @@ mod tests {
 			);
 		});
 	}
+
+	#[test]
+	fn import_of_non_best_block_may_finalize_blocks() {
+		const TOTAL_VALIDATORS: u8 = 3;
+		let validators_addresses = validators_addresses(TOTAL_VALIDATORS);
+		custom_test_ext(genesis(), validators_addresses.clone()).execute_with(move || {
+			let validators = validators(TOTAL_VALIDATORS);
+			let validators_config = ValidatorsConfiguration::Single(ValidatorsSource::Contract(
+				[0; 20].into(),
+				validators_addresses.clone(),
+			));
+			let mut storage = BridgeStorage::<TestRuntime>::new();
+			let mut pruning_strategy = KeepSomeHeadersBehindBest::default();
+
+			// insert headers (H1, validator1), (H2, validator1), (H3, validator1)
+			// making H3 the best header, without finalizing anything (we need 2 signatures)
+			let mut expected_best_block = Default::default();
+			for i in 1..4 {
+				let step = GENESIS_STEP + i * TOTAL_VALIDATORS as u64;
+				let header = custom_block_i(i, &validators, |header| {
+					header.author = validators_addresses[0];
+					header.seal[0][0] = step as u8;
+				});
+				let header = signed_header(&validators, header, step);
+				expected_best_block = header.compute_id();
+				import_header(
+					&mut storage,
+					&mut pruning_strategy,
+					&test_aura_config(),
+					&validators_config,
+					None,
+					header,
+					None,
+				)
+				.unwrap();
+			}
+			let (best_block, best_difficulty) = storage.best_block();
+			assert_eq!(best_block, expected_best_block);
+			assert_eq!(storage.finalized_block(), genesis().compute_id());
+
+			// insert headers (H1', validator1), (H2', validator2), finalizing H2, even though H3
+			// has better difficulty than H2' (because there are more steps involved)
+			let mut expected_finalized_block = Default::default();
+			let mut parent_hash = genesis().compute_hash();
+			for i in 1..3 {
+				let header = custom_block_i(i, &validators, |header| {
+					header.gas_limit += 1.into();
+					header.parent_hash = parent_hash;
+				});
+				let header = signed_header(&validators, header, GENESIS_STEP + i);
+				parent_hash = header.compute_hash();
+				if i == 1 {
+					expected_finalized_block = header.compute_id();
+				}
+
+				import_header(
+					&mut storage,
+					&mut pruning_strategy,
+					&test_aura_config(),
+					&validators_config,
+					None,
+					header,
+					None,
+				)
+				.unwrap();
+			}
+			let (new_best_block, new_best_difficulty) = storage.best_block();
+			assert_eq!(new_best_block, expected_best_block);
+			assert_eq!(new_best_difficulty, best_difficulty);
+			assert_eq!(storage.finalized_block(), expected_finalized_block);
+		});
+	}
 }

bridges/modules/ethereum/src/lib.rs

@@ -286,15 +286,31 @@ pub trait Storage {
 	fn scheduled_change(&self, hash: &H256) -> Option<ScheduledChange>;
 	/// Insert imported header.
 	fn insert_header(&mut self, header: HeaderToImport<Self::Submitter>);
-	/// Finalize given block and prune all headers with number < prune_end.
+	/// Finalize given block and schedules pruning of all headers
+	/// with number < prune_end.
+	///
 	/// The headers in the pruning range could be either finalized, or not.
 	/// It is the storage duty to ensure that unfinalized headers that have
 	/// scheduled changes won't be pruned until they or their competitors
 	/// are finalized.
-	fn finalize_headers(&mut self, finalized: Option<HeaderId>, prune_end: Option<u64>);
+	fn finalize_and_prune_headers(&mut self, finalized: Option<HeaderId>, prune_end: u64);
 }
 
-/// Decides whether the session should be ended.
+/// Headers pruning strategy.
+pub trait PruningStrategy: Default {
+	/// Return upper bound (exclusive) of headers pruning range.
+	///
+	/// Every value that is returned from this function, must be greater or equal to the
+	/// previous value. Otherwise it will be ignored (we can't revert pruning).
+	///
+	/// Module may prune both finalized and unfinalized blocks. But it can't give any
+	/// guarantees on when it will happen. Example: if some unfinalized block at height N
+	/// has scheduled validators set change, then the module won't prune any blocks with
+	/// number >= N even if strategy allows that.
+	fn pruning_upper_bound(&mut self, best_number: u64, best_finalized_number: u64) -> u64;
+}
+
+/// Callbacks for header submission rewards/penalties.
 pub trait OnHeadersSubmitted<AccountId> {
 	/// Called when valid headers have been submitted.
 	///
@@ -322,6 +338,9 @@ impl<AccountId> OnHeadersSubmitted<AccountId> for () {
 pub trait Trait: frame_system::Trait {
 	/// Aura configuration.
 	type AuraConfiguration: Get<AuraConfiguration>;
+	/// Validators configuration.
+	type ValidatorsConfiguration: Get<validators::ValidatorsConfiguration>;
+
 	/// Interval (in blocks) for for finality votes caching.
 	/// If None, cache is disabled.
 	///
@@ -329,8 +348,9 @@ pub trait Trait: frame_system::Trait {
 	/// be any significant finalization delays), or something that is bit larger
 	/// than average finalization delay.
 	type FinalityVotesCachingInterval: Get<Option<u64>>;
-	/// Validators configuration.
-	type ValidatorsConfiguration: Get<validators::ValidatorsConfiguration>;
+	/// Headers pruning strategy.
+	type PruningStrategy: PruningStrategy;
+
 	/// Handler for headers submission result.
 	type OnHeadersSubmitted: OnHeadersSubmitted<Self::AccountId>;
 }
@@ -344,9 +364,9 @@ decl_module! {
 
 			import::import_header(
 				&mut BridgeStorage::<T>::new(),
+				&mut T::PruningStrategy::default(),
 				&T::AuraConfiguration::get(),
 				&T::ValidatorsConfiguration::get(),
-				crate::import::PRUNE_DEPTH,
 				None,
 				header,
 				receipts,
@@ -365,9 +385,9 @@ decl_module! {
 			let mut finalized_headers = BTreeMap::new();
 			let import_result = import::import_headers(
 				&mut BridgeStorage::<T>::new(),
+				&mut T::PruningStrategy::default(),
 				&T::AuraConfiguration::get(),
 				&T::ValidatorsConfiguration::get(),
-				crate::import::PRUNE_DEPTH,
 				Some(submitter.clone()),
 				headers_with_receipts,
 				&mut finalized_headers,
@@ -539,16 +559,14 @@ impl<T: Trait> BridgeStorage<T> {
 	}
 
 	/// Prune old blocks.
-	fn prune_blocks(&self, mut max_blocks_to_prune: u64, finalized_number: u64, prune_end: Option<u64>) {
+	fn prune_blocks(&self, mut max_blocks_to_prune: u64, finalized_number: u64, prune_end: u64) {
 		let pruning_range = BlocksToPrune::get();
 		let mut new_pruning_range = pruning_range.clone();
 
 		// update oldest block we want to keep
-		if let Some(prune_end) = prune_end {
 		if prune_end > new_pruning_range.oldest_block_to_keep {
 			new_pruning_range.oldest_block_to_keep = prune_end;
 		}
-		}
 
 		// start pruning blocks
 		let begin = new_pruning_range.oldest_unpruned_block;
@@ -770,7 +788,7 @@ impl<T: Trait> Storage for BridgeStorage<T> {
 		);
 	}
 
-	fn finalize_headers(&mut self, finalized: Option<HeaderId>, prune_end: Option<u64>) {
+	fn finalize_and_prune_headers(&mut self, finalized: Option<HeaderId>, prune_end: u64) {
 		// remember just finalized block
 		let finalized_number = finalized
 			.as_ref()
@@ -928,7 +946,7 @@ pub(crate) mod tests {
 			});
 
 			// try to prune blocks [5; 10)
-			storage.prune_blocks(0xFFFF, 10, Some(5));
+			storage.prune_blocks(0xFFFF, 10, 5);
 			assert_eq!(HeadersByNumber::get(&5).unwrap().len(), 5);
 			assert_eq!(
 				BlocksToPrune::get(),
@@ -949,7 +967,7 @@ pub(crate) mod tests {
 			});
 
 			// try to prune blocks [5; 10)
-			storage.prune_blocks(0xFFFF, 10, Some(3));
+			storage.prune_blocks(0xFFFF, 10, 3);
 			assert_eq!(
 				BlocksToPrune::get(),
 				PruningRange {
@@ -964,7 +982,7 @@ pub(crate) mod tests {
 	fn blocks_are_not_pruned_if_limit_is_zero() {
 		with_headers_to_prune(|storage| {
 			// try to prune blocks [0; 10)
-			storage.prune_blocks(0, 10, Some(10));
+			storage.prune_blocks(0, 10, 10);
 			assert!(HeadersByNumber::get(&0).is_some());
 			assert!(HeadersByNumber::get(&1).is_some());
 			assert!(HeadersByNumber::get(&2).is_some());
@@ -983,7 +1001,7 @@ pub(crate) mod tests {
 	fn blocks_are_pruned_if_limit_is_non_zero() {
 		with_headers_to_prune(|storage| {
 			// try to prune blocks [0; 10)
-			storage.prune_blocks(7, 10, Some(10));
+			storage.prune_blocks(7, 10, 10);
 			// 1 headers with number = 0 is pruned (1 total)
 			assert!(HeadersByNumber::get(&0).is_none());
 			// 5 headers with number = 1 are pruned (6 total)
@@ -999,7 +1017,7 @@ pub(crate) mod tests {
 			);
 
 			// try to prune blocks [2; 10)
-			storage.prune_blocks(11, 10, Some(10));
+			storage.prune_blocks(11, 10, 10);
 			// 4 headers with number = 2 are pruned (4 total)
 			assert!(HeadersByNumber::get(&2).is_none());
 			// 5 headers with number = 3 are pruned (9 total)
@@ -1023,7 +1041,7 @@ pub(crate) mod tests {
 			// last finalized block is 5
 			// and one of blocks#7 has scheduled change
 			// => we won't prune any block#7 at all
-			storage.prune_blocks(0xFFFF, 5, Some(10));
+			storage.prune_blocks(0xFFFF, 5, 10);
 			assert!(HeadersByNumber::get(&0).is_none());
 			assert!(HeadersByNumber::get(&1).is_none());
 			assert!(HeadersByNumber::get(&2).is_none());
@@ -1071,7 +1089,7 @@ pub(crate) mod tests {
 				oldest_unpruned_block: interval - 1,
 				oldest_block_to_keep: interval - 1,
 			});
-			storage.finalize_headers(None, Some(interval + 1));
+			storage.finalize_and_prune_headers(None, interval + 1);
 			assert_eq!(FinalityCache::<TestRuntime>::get(&header_with_entry_hash), None);
 		});
 	}
@@ -1152,7 +1170,7 @@ pub(crate) mod tests {
 			let mut storage = BridgeStorage::<TestRuntime>::new();
 			insert_header(&mut storage, example_header_parent());
 			insert_header(&mut storage, example_header());
-			storage.finalize_headers(Some(example_header().compute_id()), None);
+			storage.finalize_and_prune_headers(Some(example_header().compute_id()), 0);
 			assert_eq!(
 				verify_transaction_finalized(&storage, example_header_parent().compute_hash(), 0, &vec![example_tx()],),
 				true,
@@ -1206,7 +1224,7 @@ pub(crate) mod tests {
 			insert_header(&mut storage, example_header_parent());
 			insert_header(&mut storage, example_header());
 			insert_header(&mut storage, finalized_header_sibling);
-			storage.finalize_headers(Some(example_header().compute_id()), None);
+			storage.finalize_and_prune_headers(Some(example_header().compute_id()), 0);
 			assert_eq!(
 				verify_transaction_finalized(&storage, finalized_header_sibling_hash, 0, &vec![example_tx()],),
 				false,
@@ -1225,7 +1243,7 @@ pub(crate) mod tests {
 			insert_header(&mut storage, example_header_parent());
 			insert_header(&mut storage, finalized_header_uncle);
 			insert_header(&mut storage, example_header());
-			storage.finalize_headers(Some(example_header().compute_id()), None);
+			storage.finalize_and_prune_headers(Some(example_header().compute_id()), 0);
 			assert_eq!(
 				verify_transaction_finalized(&storage, finalized_header_uncle_hash, 0, &vec![example_tx()],),
 				false,

bridges/modules/ethereum/src/mock.rs

@@ -16,7 +16,7 @@
 
 use crate::finality::FinalityVotes;
 use crate::validators::{ValidatorsConfiguration, ValidatorsSource};
-use crate::{AuraConfiguration, GenesisConfig, HeaderToImport, HeadersByNumber, Storage, Trait};
+use crate::{AuraConfiguration, GenesisConfig, HeaderToImport, HeadersByNumber, PruningStrategy, Storage, Trait};
 use frame_support::StorageMap;
 use frame_support::{impl_outer_origin, parameter_types, weights::Weight};
 use parity_crypto::publickey::{sign, KeyPair, Secret};
@@ -78,11 +78,15 @@ parameter_types! {
 
 impl Trait for TestRuntime {
 	type AuraConfiguration = TestAuraConfiguration;
-	type FinalityVotesCachingInterval = TestFinalityVotesCachingInterval;
 	type ValidatorsConfiguration = TestValidatorsConfiguration;
+	type FinalityVotesCachingInterval = TestFinalityVotesCachingInterval;
+	type PruningStrategy = KeepSomeHeadersBehindBest;
 	type OnHeadersSubmitted = ();
 }
 
+/// Step of genesis header.
+pub const GENESIS_STEP: u64 = 42;
+
 /// Aura configuration that is used in tests by default.
 pub fn test_aura_config() -> AuraConfiguration {
 	AuraConfiguration {
@@ -105,7 +109,7 @@ pub fn test_validators_config() -> ValidatorsConfiguration {
 /// Genesis header that is used in tests by default.
 pub fn genesis() -> Header {
 	Header {
-		seal: vec![vec![42].into(), vec![].into()],
+		seal: vec![vec![GENESIS_STEP as _].into(), vec![].into()],
 		..Default::default()
 	}
 }
@@ -123,12 +127,12 @@ pub fn custom_block_i(number: u64, validators: &[KeyPair], customize: impl FnOnc
 		parent_hash: HeadersByNumber::get(number - 1).unwrap()[0].clone(),
 		gas_limit: 0x2000.into(),
 		author: validator(validator_index).address(),
-		seal: vec![vec![number as u8 + 42].into(), vec![].into()],
+		seal: vec![vec![(number + GENESIS_STEP) as u8].into(), vec![].into()],
 		difficulty: number.into(),
 		..Default::default()
 	};
 	customize(&mut header);
-	signed_header(validators, header, number + 42)
+	signed_header(validators, header, number + GENESIS_STEP)
 }
 
 /// Build signed header from given header.
@@ -182,3 +186,18 @@ pub fn insert_header<S: Storage>(storage: &mut S, header: Header) {
 		finality_votes: FinalityVotes::default(),
 	});
 }
+
+/// Pruning strategy that keeps 10 headers behind best block.
+pub struct KeepSomeHeadersBehindBest(pub u64);
+
+impl Default for KeepSomeHeadersBehindBest {
+	fn default() -> KeepSomeHeadersBehindBest {
+		KeepSomeHeadersBehindBest(10)
+	}
+}
+
+impl PruningStrategy for KeepSomeHeadersBehindBest {
+	fn pruning_upper_bound(&mut self, best_number: u64, _: u64) -> u64 {
+		best_number.checked_sub(self.0).unwrap_or(0)
+	}
+}