diff --git a/pallas-blockfetch/src/lib.rs b/pallas-blockfetch/src/lib.rs
index 787b59c..0ba8d70 100644
--- a/pallas-blockfetch/src/lib.rs
+++ b/pallas-blockfetch/src/lib.rs
@@ -1,5 +1,10 @@
+use std::sync::mpsc::Receiver;
+
 use log::info;
-use pallas_machines::{Agent, CodecError, DecodePayload, EncodePayload, MachineOutput, PayloadDecoder, PayloadEncoder, Transition, primitives::Point};
+use pallas_machines::{
+    primitives::Point, Agent, CodecError, DecodePayload, EncodePayload, MachineOutput,
+    PayloadDecoder, PayloadEncoder, Transition,
+};
 
 #[derive(Debug, PartialEq, Clone)]
 pub enum State {
@@ -20,10 +25,7 @@ pub enum Message {
 }
 
 impl EncodePayload for Message {
-    fn encode_payload(
-        &self,
-        e: &mut PayloadEncoder,
-    ) -> Result<(), Box<dyn std::error::Error>> {
+    fn encode_payload(&self, e: &mut PayloadEncoder) -> Result<(), Box<dyn std::error::Error>> {
         match self {
             Message::RequestRange { range } => {
                 e.array(3)?.u16(0)?;
@@ -57,9 +59,7 @@ impl EncodePayload for Message {
 }
 
 impl DecodePayload for Message {
-    fn decode_payload(
-        d: &mut PayloadDecoder,
-    ) -> Result<Self, Box<dyn std::error::Error>> {
+    fn decode_payload(d: &mut PayloadDecoder) -> Result<Self, Box<dyn std::error::Error>> {
         d.array()?;
         let label = d.u16()?;
 
@@ -87,17 +87,30 @@ impl DecodePayload for Message {
     }
 }
 
-#[derive(Debug)]
-pub struct BlockFetchClient {
-    pub state: State,
-    pub range: (Point, Point),
+pub trait Observer {
+    fn on_block(&self, body: Vec<u8>) -> Result<(), Box<dyn std::error::Error>> {
+        log::debug!("block fetched {:?}", body);
+        Ok(())
+    }
 }
 
-impl BlockFetchClient {
-    pub fn initial(range: (Point, Point)) -> Self {
+pub struct NoopObserver {}
+
+impl Observer for NoopObserver {}
+
+#[derive(Debug)]
+pub struct BatchClient<O> where O: Observer {
+    pub state: State,
+    pub range: (Point, Point),
+    pub observer: O,
+}
+
+impl<O> BatchClient<O> where O: Observer {
+    pub fn initial(range: (Point, Point), observer: O) -> Self {
         Self {
             state: State::Idle,
             range,
+            observer,
         }
     }
 
@@ -115,7 +128,7 @@ impl BlockFetchClient {
     }
 }
 
-impl Agent for BlockFetchClient {
+impl<O> Agent for BatchClient<O> where O: Observer {
     type Message = Message;
 
     fn is_done(&self) -> bool {
@@ -150,6 +163,7 @@ impl Agent for BlockFetchClient {
             }),
             (State::Streaming, Message::Block { body }) => {
                 info!("received block body of size {}", body.len());
+                self.observer.on_block(body)?;
                 Ok(self)
             }
             (State::Streaming, Message::BatchDone) => Ok(Self {
@@ -160,3 +174,84 @@ impl Agent for BlockFetchClient {
         }
     }
 }
+
+#[derive(Debug)]
+pub struct OnDemandClient<O> where O: Observer {
+    pub state: State,
+    pub requests: Receiver<Point>,
+    pub observer: O,
+}
+
+impl<O> OnDemandClient<O> where O: Observer {
+    pub fn initial(requests: Receiver<Point>, observer: O) -> Self {
+        Self {
+            state: State::Idle,
+            requests,
+            observer,
+        }
+    }
+
+    fn wait_for_request_and_send(self, tx: &impl MachineOutput) -> Transition<Self> {
+        let point = self.requests.recv()?;
+
+        let msg = Message::RequestRange {
+            range: (point.clone(), point),
+        };
+
+        tx.send_msg(&msg)?;
+
+        Ok(Self {
+            state: State::Busy,
+            ..self
+        })
+    }
+}
+
+impl<O> Agent for OnDemandClient<O> where O: Observer {
+    type Message = Message;
+
+    // we're never done because we react to external work requests.
+    // TODO: see if we can inspect mpsc channel status and stop if disconnected
+    fn is_done(&self) -> bool {
+        false
+    }
+
+    fn has_agency(&self) -> bool {
+        match self.state {
+            State::Idle => true,
+            State::Busy => false,
+            State::Streaming => false,
+            State::Done => false,
+        }
+    }
+
+    fn send_next(self, tx: &impl MachineOutput) -> Transition<Self> {
+        match self.state {
+            State::Idle => self.wait_for_request_and_send(tx),
+            _ => panic!("I don't have agency, don't know what to do"),
+        }
+    }
+
+    fn receive_next(self, msg: Self::Message) -> Transition<Self> {
+        match (&self.state, msg) {
+            (State::Busy, Message::StartBatch) => Ok(Self {
+                state: State::Streaming,
+                ..self
+            }),
+            (State::Busy, Message::NoBlocks) => Ok(Self {
+                state: State::Idle,
+                ..self
+            }),
+            (State::Streaming, Message::Block { body }) => {
+                info!("received block body of size {}", body.len());
+                self.observer.on_block(body)?;
+                Ok(self)
+            }
+            (State::Streaming, Message::BatchDone) => Ok(Self {
+                state: State::Idle,
+                ..self
+            }),
+            _ => panic!("I have agency, I don't expect messages"),
+        }
+    }
+}