diff --git a/finality-aleph/src/sync/mod.rs b/finality-aleph/src/sync/mod.rs
index 54f03c743e..9907ec1ca9 100644
--- a/finality-aleph/src/sync/mod.rs
+++ b/finality-aleph/src/sync/mod.rs
@@ -1 +1,97 @@
+use std::{
+    fmt::{Debug, Display},
+    hash::Hash,
+};
+
+mod substrate;
 mod ticker;
+
+/// The identifier of a block, the least amount of knowledge we can have about a block.
+pub trait BlockIdentifier: Clone + Hash + Debug + Eq {
+    /// The block number, useful when reasoning about hopeless forks.
+    fn number(&self) -> u32;
+}
+
+/// Informs the sync that it should attempt to acquire the specified data.
+pub trait Requester<BI: BlockIdentifier> {
+    /// The sync should attempt to acquire justifications for this block.
+    fn request_justification(&self, id: BI);
+}
+
+/// The header of a block, containing information about the parent relation.
+pub trait Header: Clone {
+    type Identifier: BlockIdentifier;
+
+    /// The identifier of this block.
+    fn id(&self) -> Self::Identifier;
+
+    /// The identifier of this block's parent.
+    fn parent_id(&self) -> Option<Self::Identifier>;
+}
+
+/// The verified justification of a block, including a header.
+pub trait Justification: Clone {
+    type Header: Header;
+    type Unverified;
+
+    /// The header of the block.
+    fn header(&self) -> &Self::Header;
+
+    /// Return an unverified version of this, for sending over the network.
+    fn into_unverified(self) -> Self::Unverified;
+}
+
+/// A verifier of justifications.
+pub trait Verifier<J: Justification> {
+    type Error: Display;
+
+    /// Verifies the raw justification and returns a full justification if successful, otherwise an
+    /// error.
+    fn verify(&self, justification: J::Unverified) -> Result<J, Self::Error>;
+}
+
+/// A facility for finalizing blocks using justifications.
+pub trait Finalizer<J: Justification> {
+    type Error: Display;
+
+    /// Finalize a block using this justification. Since the justification contains the header, we
+    /// don't need to additionally specify the block.
+    fn finalize(&self, justification: J) -> Result<(), Self::Error>;
+}
+
+/// A notification about the chain state changing.
+pub enum ChainStateNotification<BI: BlockIdentifier> {
+    /// A block has been imported.
+    BlockImported(BI),
+    /// A block has been finalized.
+    BlockFinalized(BI),
+}
+
+/// A stream of notifications about the chain state in the database changing.
+#[async_trait::async_trait]
+pub trait ChainStateNotifier<BI: BlockIdentifier> {
+    /// Returns a chain state notification when it is available.
+    async fn next(&self) -> ChainStateNotification<BI>;
+}
+
+/// The state of a block in the database.
+pub enum BlockState<J: Justification> {
+    /// The block is justified and thus finalized.
+    Justified(J),
+    /// The block is present, might be finalized if a descendant is justified.
+    Present(J::Header),
+    /// The block is not known.
+    Unknown,
+}
+
+/// The knowledge about the chain state.
+pub trait ChainState<J: Justification> {
+    /// The state of the block.
+    fn state_of(&self, id: <J::Header as Header>::Identifier) -> BlockState<J>;
+
+    /// The header of the best block.
+    fn best_block(&self) -> J::Header;
+
+    /// The justification of the top finalized block.
+    fn top_finalized(&self) -> J;
+}
diff --git a/finality-aleph/src/sync/substrate.rs b/finality-aleph/src/sync/substrate.rs
new file mode 100644
index 0000000000..5516f80ba2
--- /dev/null
+++ b/finality-aleph/src/sync/substrate.rs
@@ -0,0 +1,47 @@
+use std::hash::{Hash, Hasher};
+
+use aleph_primitives::BlockNumber;
+use sp_runtime::traits::{CheckedSub, Header as SubstrateHeader, One};
+
+use crate::sync::{BlockIdentifier, Header};
+
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub struct BlockId<H: SubstrateHeader<Number = BlockNumber>> {
+    hash: H::Hash,
+    number: H::Number,
+}
+
+impl<SH: SubstrateHeader<Number = BlockNumber>> Hash for BlockId<SH> {
+    fn hash<H>(&self, state: &mut H)
+    where
+        H: Hasher,
+    {
+        self.hash.hash(state);
+        self.number.hash(state);
+    }
+}
+
+impl<H: SubstrateHeader<Number = BlockNumber>> BlockIdentifier for BlockId<H> {
+    fn number(&self) -> u32 {
+        self.number
+    }
+}
+
+impl<H: SubstrateHeader<Number = BlockNumber>> Header for H {
+    type Identifier = BlockId<H>;
+
+    fn id(&self) -> Self::Identifier {
+        BlockId {
+            hash: self.hash(),
+            number: *self.number(),
+        }
+    }
+
+    fn parent_id(&self) -> Option<Self::Identifier> {
+        let number = self.number().checked_sub(&One::one())?;
+        Some(BlockId {
+            hash: *self.parent_hash(),
+            number,
+        })
+    }
+}