diff --git a/src/service/transaction_ids/mod.rs b/src/service/transaction_ids/mod.rs
index 9707dbb9..1c78396f 100644
--- a/src/service/transaction_ids/mod.rs
+++ b/src/service/transaction_ids/mod.rs
@@ -1,28 +1,34 @@
 use std::{collections::HashMap, sync::Arc};
 
-use conduwuit::{Result, SyncRwLock};
+use async_trait::async_trait;
+use conduwuit::{Error, Result, SyncRwLock};
 use database::{Handle, Map};
 use ruma::{
 	DeviceId, OwnedServerName, OwnedTransactionId, TransactionId, UserId,
-	api::federation::transactions::send_transaction_message,
+	api::{
+		client::error::ErrorKind::LimitExceeded,
+		federation::transactions::send_transaction_message,
+	},
 };
 use tokio::sync::watch::{Receiver, Sender};
 
 pub type TxnKey = (OwnedServerName, OwnedTransactionId);
-pub type TxnChanType = (TxnKey, send_transaction_message::v1::Response);
-pub type ActiveTxnsMapType = HashMap<TxnKey, (Sender<TxnChanType>, Receiver<TxnChanType>)>;
+pub type WrappedTransactionResponse = Option<send_transaction_message::v1::Response>;
+pub type ActiveTransactionsMap = HashMap<TxnKey, Receiver<WrappedTransactionResponse>>;
 
 pub struct Service {
 	db: Data,
-	pub servername_txnid_response_cache:
+	servername_txnid_response_cache:
 		Arc<SyncRwLock<HashMap<TxnKey, send_transaction_message::v1::Response>>>,
-	pub servername_txnid_active: Arc<SyncRwLock<ActiveTxnsMapType>>,
+	servername_txnid_active: Arc<SyncRwLock<ActiveTransactionsMap>>,
+	max_active_txns: usize,
 }
 
 struct Data {
 	userdevicetxnid_response: Arc<Map>,
 }
 
+#[async_trait]
 impl crate::Service for Service {
 	fn build(args: crate::Args<'_>) -> Result<Arc<Self>> {
 		Ok(Arc::new(Self {
@@ -31,9 +37,15 @@ impl crate::Service for Service {
 			},
 			servername_txnid_response_cache: Arc::new(SyncRwLock::new(HashMap::new())),
 			servername_txnid_active: Arc::new(SyncRwLock::new(HashMap::new())),
+			max_active_txns: 50, // TODO: fetch from config
 		}))
 	}
 
+	async fn clear_cache(&self) {
+		let mut state = self.servername_txnid_response_cache.write();
+		state.clear();
+	}
+
 	fn name(&self) -> &str { crate::service::make_name(std::module_path!()) }
 }
 
@@ -63,4 +75,67 @@ impl Service {
 		let key = (user_id, device_id, txn_id);
 		self.db.userdevicetxnid_response.qry(&key).await
 	}
+
+	/// Fetches a receiver channel for the given transaction, if any exists.
+	/// If the given txn is not active, None is returned.
+	#[must_use]
+	pub fn get_active_federation_txn(
+		&self,
+		key: &TxnKey,
+	) -> Option<Receiver<WrappedTransactionResponse>> {
+		let state = self.servername_txnid_active.read();
+		state.get(key).cloned()
+	}
+
+	/// Starts a new inbound transaction handler, returning the appropriate
+	/// sender to broadcast the response via.
+	///
+	/// If the given key is already active, a rate-limited response is returned.
+	pub fn start_federation_txn(
+		&self,
+		key: TxnKey,
+	) -> Result<Sender<WrappedTransactionResponse>> {
+		let mut state = self.servername_txnid_active.write();
+		if state.get(&key).is_some() {
+			Err(Error::BadRequest(
+				LimitExceeded { retry_after: None },
+				"Transaction is already being handled",
+			))
+		} else if state.keys().any(|k| k.0 == key.0) {
+			Err(Error::BadRequest(
+				LimitExceeded { retry_after: None },
+				"Still processing another transaction from this origin",
+			))
+		} else if state.len() >= self.max_active_txns {
+			Err(Error::BadRequest(
+				LimitExceeded { retry_after: None },
+				"Server is overloaded, try again later",
+			))
+		} else {
+			let (tx, rx) = tokio::sync::watch::channel(None);
+			state.insert(key, rx);
+			Ok(tx)
+		}
+	}
+
+	/// Finishes a transaction, removing it from the active txns registry.
+	pub fn finish_federation_txn(&self, key: &TxnKey) {
+		let mut state = self.servername_txnid_active.write();
+		state.remove(key);
+	}
+
+	/// Gets a cached transaction response, if the given key has a value.
+	#[must_use]
+	pub fn get_cached_txn(&self, key: &TxnKey) -> Option<send_transaction_message::v1::Response> {
+		let state = self.servername_txnid_response_cache.read();
+		state.get(key).cloned()
+	}
+
+	/// Sets a cached transaction response. The existing key will be overwritten
+	/// if it exists.
+	pub fn set_cached_txn(&self, key: TxnKey, response: send_transaction_message::v1::Response) {
+		let mut state = self.servername_txnid_response_cache.write();
+		// TODO: time-to-live?
+		state.insert(key, response);
+	}
 }