diff --git a/Cargo.lock b/Cargo.lock
index c013ace2..5853fb3b 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -643,6 +643,7 @@ name = "cozo_py"
 version = "0.5.0"
 dependencies = [
  "cozo",
+ "miette",
  "pyo3",
 ]
 
diff --git a/cozo-core/src/fixed_rule/mod.rs b/cozo-core/src/fixed_rule/mod.rs
index feca8c99..b41b9f6f 100644
--- a/cozo-core/src/fixed_rule/mod.rs
+++ b/cozo-core/src/fixed_rule/mod.rs
@@ -9,12 +9,14 @@
 use std::collections::BTreeMap;
 use std::sync::Arc;
 
+use crossbeam::channel::{bounded, Receiver, Sender};
 #[allow(unused_imports)]
 use either::{Left, Right};
 #[cfg(feature = "graph-algo")]
 use graph::prelude::{CsrLayout, DirectedCsrGraph, GraphBuilder};
 use itertools::Itertools;
 use lazy_static::lazy_static;
+use miette::IntoDiagnostic;
 #[allow(unused_imports)]
 use miette::{bail, ensure, Diagnostic, Report, Result};
 use smartstring::{LazyCompact, SmartString};
@@ -586,6 +588,28 @@ impl SimpleFixedRule {
             rule: Box::new(rule),
         }
     }
+    /// Construct a SimpleFixedRule that uses channels for communication.
+    pub fn rule_with_channel(
+        return_arity: usize,
+    ) -> (
+        Self,
+        Receiver<(Vec<NamedRows>, JsonValue)>,
+        Sender<Result<NamedRows>>,
+    ) {
+        let (db2app_sender, db2app_receiver) = bounded(0);
+        let (app2db_sender, app2db_receiver) = bounded(0);
+        (
+            Self {
+                return_arity,
+                rule: Box::new(move |inputs, options| -> Result<NamedRows> {
+                    db2app_sender.send((inputs, options)).into_diagnostic()?;
+                    app2db_receiver.recv().into_diagnostic()?
+                }),
+            },
+            db2app_receiver,
+            app2db_sender,
+        )
+    }
 }
 
 impl FixedRule for SimpleFixedRule {
@@ -635,7 +659,7 @@ impl FixedRule for SimpleFixedRule {
         let results: NamedRows = (self.rule)(inputs, options)?;
         for row in results.rows {
             #[derive(Debug, Error, Diagnostic)]
-            #[error("arity mismatch: expect {0}, got {1}")]
+            #[error("arity mismatch: expect {1}, got {2}")]
             #[diagnostic(code(parser::simple_fixed_rule_arity_mismatch))]
             struct ArityMismatch(#[label] SourceSpan, usize, usize);
 
diff --git a/cozo-core/src/lib.rs b/cozo-core/src/lib.rs
index e160a21a..a0d56975 100644
--- a/cozo-core/src/lib.rs
+++ b/cozo-core/src/lib.rs
@@ -36,6 +36,7 @@ use std::path::Path;
 #[allow(unused_imports)]
 use std::time::Instant;
 
+use crossbeam::channel::Receiver;
 use lazy_static::lazy_static;
 pub use miette::Error;
 use miette::Report;
@@ -375,22 +376,24 @@ impl DbInstance {
 
         self.import_from_backup(&json_payload.path, &json_payload.relations)
     }
+
     /// Dispatcher method. See [crate::Db::register_callback].
     #[cfg(not(target_arch = "wasm32"))]
-    pub fn register_callback<CB>(&self, relation: &str, callback: CB) -> Result<u32>
-    where
-        CB: Fn(CallbackOp, NamedRows, NamedRows) + Send + Sync + 'static,
-    {
+    pub fn register_callback(
+        &self,
+        relation: &str,
+        capacity: Option<usize>,
+    ) -> (u32, Receiver<(CallbackOp, NamedRows, NamedRows)>) {
         match self {
-            DbInstance::Mem(db) => db.register_callback(relation, callback),
+            DbInstance::Mem(db) => db.register_callback(relation, capacity),
             #[cfg(feature = "storage-sqlite")]
-            DbInstance::Sqlite(db) => db.register_callback(relation, callback),
+            DbInstance::Sqlite(db) => db.register_callback(relation, capacity),
             #[cfg(feature = "storage-rocksdb")]
-            DbInstance::RocksDb(db) => db.register_callback(relation, callback),
+            DbInstance::RocksDb(db) => db.register_callback(relation, capacity),
             #[cfg(feature = "storage-sled")]
-            DbInstance::Sled(db) => db.register_callback(relation, callback),
+            DbInstance::Sled(db) => db.register_callback(relation, capacity),
             #[cfg(feature = "storage-tikv")]
-            DbInstance::TiKv(db) => db.register_callback(relation, callback),
+            DbInstance::TiKv(db) => db.register_callback(relation, capacity),
         }
     }
 
@@ -410,11 +413,7 @@ impl DbInstance {
         }
     }
     /// Dispatcher method. See [crate::Db::register_fixed_rule].
-    pub fn register_fixed_rule(
-        &self,
-        name: String,
-        rule_impl: Box<dyn FixedRule>,
-    ) -> Result<()> {
+    pub fn register_fixed_rule<R>(&self, name: String, rule_impl: R) -> Result<()> where R: FixedRule + 'static {
         match self {
             DbInstance::Mem(db) => db.register_fixed_rule(name, rule_impl),
             #[cfg(feature = "storage-sqlite")]
diff --git a/cozo-core/src/runtime/callback.rs b/cozo-core/src/runtime/callback.rs
index 075574be..1f7fdcfc 100644
--- a/cozo-core/src/runtime/callback.rs
+++ b/cozo-core/src/runtime/callback.rs
@@ -9,6 +9,7 @@
 use std::collections::{BTreeMap, BTreeSet};
 use std::fmt::{Display, Formatter};
 
+use crossbeam::channel::Sender;
 use smartstring::{LazyCompact, SmartString};
 
 use crate::{Db, NamedRows, Storage};
@@ -31,10 +32,20 @@ impl Display for CallbackOp {
     }
 }
 
+impl CallbackOp {
+    /// Get the string representation
+    pub fn as_str(&self) -> &'static str {
+        match self {
+            CallbackOp::Put => "Put",
+            CallbackOp::Rm => "Rm"
+        }
+    }
+}
+
 #[cfg(not(target_arch = "wasm32"))]
 pub struct CallbackDeclaration {
     pub(crate) dependent: SmartString<LazyCompact>,
-    pub(crate) callback: Box<dyn Fn(CallbackOp, NamedRows, NamedRows) + Send + Sync>,
+    pub(crate) sender: Sender<(CallbackOp, NamedRows, NamedRows)>,
 }
 
 pub(crate) type CallbackCollector =
@@ -66,11 +77,40 @@ impl<'s, S: Storage<'s>> Db<S> {
     }
     #[cfg(not(target_arch = "wasm32"))]
     pub(crate) fn send_callbacks(&'s self, collector: CallbackCollector) {
-        for (k, vals) in collector {
+        let mut to_remove = vec![];
+
+        for (table, vals) in collector {
             for (op, new, old) in vals {
-                self.callback_sender
-                    .send((k.clone(), op, new, old))
-                    .expect("sending to callback processor failed");
+                let (cbs, cb_dir) = &*self.event_callbacks.read().unwrap();
+                if let Some(cb_ids) = cb_dir.get(&table) {
+                    let mut it = cb_ids.iter();
+                    if let Some(fst) = it.next() {
+                        for cb_id in it {
+                            if let Some(cb) = cbs.get(cb_id) {
+                                if cb.sender.send((op, new.clone(), old.clone())).is_err() {
+                                    to_remove.push(*cb_id)
+                                }
+                            }
+                        }
+
+                        if let Some(cb) = cbs.get(fst) {
+                            if cb.sender.send((op, new, old)).is_err() {
+                                to_remove.push(*fst)
+                            }
+                        }
+                    }
+                }
+            }
+        }
+
+        if !to_remove.is_empty() {
+            let (cbs, cb_dir) = &mut *self.event_callbacks.write().unwrap();
+            for removing_id in &to_remove {
+                if let Some(removed) = cbs.remove(removing_id) {
+                    if let Some(set) = cb_dir.get_mut(&removed.dependent) {
+                        set.remove(removing_id);
+                    }
+                }
             }
         }
     }
diff --git a/cozo-core/src/runtime/db.rs b/cozo-core/src/runtime/db.rs
index cc08f51f..6f86a45a 100644
--- a/cozo-core/src/runtime/db.rs
+++ b/cozo-core/src/runtime/db.rs
@@ -20,8 +20,7 @@ use std::thread;
 #[allow(unused_imports)]
 use std::time::{Duration, SystemTime, UNIX_EPOCH};
 
-#[cfg(not(target_arch = "wasm32"))]
-use crossbeam::channel::{unbounded, Sender};
+use crossbeam::channel::{bounded, unbounded, Receiver};
 use crossbeam::sync::ShardedLock;
 use either::{Left, Right};
 use itertools::Itertools;
@@ -92,10 +91,7 @@ pub struct Db<S> {
     #[cfg(not(target_arch = "wasm32"))]
     callback_count: AtomicU32,
     #[cfg(not(target_arch = "wasm32"))]
-    pub(crate) callback_sender:
-        Sender<(SmartString<LazyCompact>, CallbackOp, NamedRows, NamedRows)>,
-    #[cfg(not(target_arch = "wasm32"))]
-    pub(crate) event_callbacks: Arc<ShardedLock<EventCallbackRegistry>>,
+    pub(crate) event_callbacks: ShardedLock<EventCallbackRegistry>,
     relation_locks: ShardedLock<BTreeMap<SmartString<LazyCompact>, Arc<ShardedLock<()>>>>,
 }
 
@@ -183,8 +179,6 @@ impl<'s, S: Storage<'s>> Db<S> {
     /// You must call [`initialize`](Self::initialize) immediately after creation.
     /// Due to lifetime restrictions we are not able to call that for you automatically.
     pub fn new(storage: S) -> Result<Self> {
-        #[cfg(not(target_arch = "wasm32"))]
-        let (sender, receiver) = unbounded();
         let ret = Self {
             db: storage,
             temp_db: Default::default(),
@@ -194,29 +188,11 @@ impl<'s, S: Storage<'s>> Db<S> {
             fixed_rules: ShardedLock::new(DEFAULT_FIXED_RULES.clone()),
             #[cfg(not(target_arch = "wasm32"))]
             callback_count: Default::default(),
-            #[cfg(not(target_arch = "wasm32"))]
-            callback_sender: sender,
             // callback_receiver: Arc::new(receiver),
             #[cfg(not(target_arch = "wasm32"))]
             event_callbacks: Default::default(),
             relation_locks: Default::default(),
         };
-        #[cfg(not(target_arch = "wasm32"))]
-        {
-            let callbacks = ret.event_callbacks.clone();
-            thread::spawn(move || {
-                while let Ok((table, op, new, old)) = receiver.recv() {
-                    let (cbs, cb_dir) = &*callbacks.read().unwrap();
-                    if let Some(cb_ids) = cb_dir.get(&table) {
-                        for cb_id in cb_ids {
-                            if let Some(cb) = cbs.get(cb_id) {
-                                (cb.callback)(op, new.clone(), old.clone())
-                            }
-                        }
-                    }
-                }
-            });
-        }
         Ok(ret)
     }
 
@@ -548,10 +524,13 @@ impl<'s, S: Storage<'s>> Db<S> {
         }
     }
     /// Register a custom fixed rule implementation.
-    pub fn register_fixed_rule(&self, name: String, rule_impl: Box<dyn FixedRule>) -> Result<()> {
+    pub fn register_fixed_rule<R>(&self, name: String, rule_impl: R) -> Result<()>
+    where
+        R: FixedRule + 'static,
+    {
         match self.fixed_rules.write().unwrap().entry(name) {
             Entry::Vacant(ent) => {
-                ent.insert(Arc::new(rule_impl));
+                ent.insert(Arc::new(Box::new(rule_impl)));
                 Ok(())
             }
             Entry::Occupied(ent) => {
@@ -571,31 +550,22 @@ impl<'s, S: Storage<'s>> Db<S> {
         Ok(self.fixed_rules.write().unwrap().remove(name).is_some())
     }
 
-    /// Register callbacks to run when changes to the requested relation are successfully committed.
-    /// The returned ID can be used to unregister the callbacks.
-    ///
-    /// When any mutation is made against the registered relation,
-    /// the callback will be called with three arguments:
-    ///
-    /// * The type of the mutation (`Put` or `Rm`)
-    /// * The incoming rows
-    ///   * For `Put`, the rows inserted / upserted
-    ///   * For `Rm`, the keys of the rows to be removed. These keys do not necessarily existed in the database.
-    /// * The evicted rows
-    ///   * For `Put`, old rows that were updated
-    ///   * For `Rm`, the rows actually removed.
-    ///
-    /// The database has a single dedicated thread for executing callbacks no matter how many callbacks
-    /// are registered. You should not perform blocking operations in the callback provided to this function.
-    /// If blocking operations are required, send the data to some other thread and perform the operations there.
+    /// Register callback channel to receive changes when the requested relation are successfully committed.
+    /// The returned ID can be used to unregister the callback channel.
     #[cfg(not(target_arch = "wasm32"))]
-    pub fn register_callback<CB>(&self, relation: &str, callback: CB) -> Result<u32>
-    where
-        CB: Fn(CallbackOp, NamedRows, NamedRows) + Send + Sync + 'static,
-    {
+    pub fn register_callback(
+        &self,
+        relation: &str,
+        capacity: Option<usize>,
+    ) -> (u32, Receiver<(CallbackOp, NamedRows, NamedRows)>) {
+        let (sender, receiver) = if let Some(c) = capacity {
+            bounded(c)
+        } else {
+            unbounded()
+        };
         let cb = CallbackDeclaration {
             dependent: SmartString::from(relation),
-            callback: Box::new(callback),
+            sender: sender,
         };
 
         let mut guard = self.event_callbacks.write().unwrap();
@@ -607,10 +577,10 @@ impl<'s, S: Storage<'s>> Db<S> {
             .insert(new_id);
 
         guard.0.insert(new_id, cb);
-        Ok(new_id)
+        (new_id, receiver)
     }
 
-    /// Unregister callbacks to run when changes to relations are committed.
+    /// Unregister callbacks/channels to run when changes to relations are committed.
     #[cfg(not(target_arch = "wasm32"))]
     pub fn unregister_callback(&self, id: u32) -> bool {
         let mut guard = self.event_callbacks.write().unwrap();
diff --git a/cozo-core/src/runtime/tests.rs b/cozo-core/src/runtime/tests.rs
index c47d0097..5068fbe1 100644
--- a/cozo-core/src/runtime/tests.rs
+++ b/cozo-core/src/runtime/tests.rs
@@ -8,7 +8,6 @@
  */
 
 use std::collections::BTreeMap;
-use std::sync::{Arc, Mutex};
 use std::time::Duration;
 
 use itertools::Itertools;
@@ -444,12 +443,8 @@ fn test_trigger() {
 #[test]
 fn test_callback() {
     let db = new_cozo_mem().unwrap();
-    let collected = Arc::new(Mutex::new(vec![]));
-    let copy = collected.clone();
-    db.register_callback("friends", move |op, new, old| {
-        copy.lock().unwrap().push((op, new, old))
-    })
-    .unwrap();
+    let mut collected = vec![];
+    let (_id, receiver) = db.register_callback("friends", None).unwrap();
     db.run_script(
         ":create friends {fr: Int, to: Int => data: Any}",
         Default::default(),
@@ -471,7 +466,10 @@ fn test_callback() {
     )
     .unwrap();
     std::thread::sleep(Duration::from_secs_f64(0.01));
-    let collected = collected.lock().unwrap().clone();
+    while let Ok(d) = receiver.try_recv() {
+        collected.push(d);
+    }
+    let collected = collected;
     assert_eq!(collected[0].0, CallbackOp::Put);
     assert_eq!(collected[0].1.rows.len(), 2);
     assert_eq!(collected[0].1.rows[0].len(), 3);
@@ -578,7 +576,7 @@ fn test_index() {
 
 #[test]
 fn test_custom_rules() {
-    let mut db = new_cozo_mem().unwrap();
+    let db = new_cozo_mem().unwrap();
     struct Custom;
 
     impl FixedRule for Custom {
diff --git a/cozo-lib-python/Cargo.toml b/cozo-lib-python/Cargo.toml
index 551dec67..dda9216a 100644
--- a/cozo-lib-python/Cargo.toml
+++ b/cozo-lib-python/Cargo.toml
@@ -43,3 +43,4 @@ io-uring = ["cozo/io-uring"]
 [dependencies]
 cozo = { version = "0.5.0", path = "../cozo-core", default-features = false }
 pyo3 = { version = "0.17.1", features = ["extension-module", "abi3", "abi3-py37"] }
+miette = "5.5.0"
\ No newline at end of file
diff --git a/cozo-lib-python/src/lib.rs b/cozo-lib-python/src/lib.rs
index aedfa8a2..077b6ef3 100644
--- a/cozo-lib-python/src/lib.rs
+++ b/cozo-lib-python/src/lib.rs
@@ -6,13 +6,26 @@
  * You can obtain one at https://mozilla.org/MPL/2.0/.
  */
 
+use std::collections::BTreeMap;
+use std::thread;
+
+use miette::{IntoDiagnostic, Result};
 use pyo3::exceptions::PyException;
 use pyo3::prelude::*;
-use pyo3::types::{PyDict, PyList};
-use std::collections::BTreeMap;
+use pyo3::types::{PyDict, PyList, PyString, PyTuple};
 
 use cozo::*;
 
+fn py_to_named_rows(ob: &PyAny) -> PyResult<NamedRows> {
+    let rows = ob.extract::<Vec<Vec<&PyAny>>>()?;
+    let res: Vec<Vec<DataValue>> = rows
+        .into_iter()
+        .map(|row| row.into_iter().map(py_to_value).collect::<PyResult<_>>())
+        .collect::<PyResult<_>>()?;
+
+    Ok(NamedRows::new(vec![], res))
+}
+
 fn py_to_value(ob: &PyAny) -> PyResult<DataValue> {
     Ok(if ob.is_none() {
         DataValue::Null
@@ -40,7 +53,9 @@ fn py_to_value(ob: &PyAny) -> PyResult<DataValue> {
         }
         DataValue::List(coll)
     } else {
-        return Err(PyException::new_err(format!("Cannot convert {ob} into Cozo value")));
+        return Err(PyException::new_err(format!(
+            "Cannot convert {ob} into Cozo value"
+        )));
     })
 }
 
@@ -81,10 +96,8 @@ fn value_to_py(val: DataValue, py: Python<'_>) -> PyObject {
     }
 }
 
-fn named_rows_to_py(named_rows: NamedRows, py: Python<'_>) -> PyObject {
-    let rows = named_rows
-        .rows
-        .into_iter()
+fn rows_to_py_rows(rows: Vec<Vec<DataValue>>, py: Python<'_>) -> PyObject {
+    rows.into_iter()
         .map(|row| {
             row.into_iter()
                 .map(|val| value_to_py(val, py))
@@ -92,7 +105,11 @@ fn named_rows_to_py(named_rows: NamedRows, py: Python<'_>) -> PyObject {
                 .into_py(py)
         })
         .collect::<Vec<_>>()
-        .into_py(py);
+        .into_py(py)
+}
+
+fn named_rows_to_py(named_rows: NamedRows, py: Python<'_>) -> PyObject {
+    let rows = rows_to_py_rows(named_rows.rows, py);
     let headers = named_rows.headers.into_py(py);
     BTreeMap::from([("rows", rows), ("headers", headers)]).into_py(py)
 }
@@ -130,22 +147,86 @@ impl CozoDbPy {
     pub fn register_callback(&self, rel: &str, callback: &PyAny) -> PyResult<u32> {
         if let Some(db) = &self.db {
             let cb: Py<PyAny> = callback.into();
-            match db.register_callback(rel, move |op, new, old| {
-                Python::with_gil(|py| {
-                    let callable = cb.as_ref(py);
-                    let _ = callable.call0();
-                })
-            }) {
-                Ok(id) => Ok(id),
-                Err(err) => {
-                    let reports = format_error_as_json(err, None).to_string();
-                    Err(PyException::new_err(reports))
+            let (id, ch) = db.register_callback(rel, None);
+            thread::spawn(move || {
+                for (op, new, old) in ch {
+                    Python::with_gil(|py| {
+                        let op = PyString::new(py, op.as_str()).into();
+                        let new_py = rows_to_py_rows(new.rows, py);
+                        let old_py = rows_to_py_rows(old.rows, py);
+                        let args = PyTuple::new(py, [op, new_py, old_py]);
+                        let callable = cb.as_ref(py);
+                        if let Err(err) = callable.call1(args) {
+                            eprintln!("{}", err);
+                        }
+                    })
+                }
+            });
+            Ok(id)
+        } else {
+            Err(PyException::new_err(DB_CLOSED_MSG))
+        }
+    }
+    pub fn register_fixed_rule(
+        &self,
+        name: String,
+        arity: usize,
+        callback: &PyAny,
+    ) -> PyResult<()> {
+        if let Some(db) = &self.db {
+            let cb: Py<PyAny> = callback.into();
+            let (rule_impl, receiver, sender) = SimpleFixedRule::rule_with_channel(arity);
+            match db.register_fixed_rule(name, rule_impl) {
+                Ok(_) => {
+                    thread::spawn(move || {
+                        for (inputs, options) in receiver {
+                            let res = Python::with_gil(|py| -> Result<NamedRows> {
+                                let json_convert =
+                                    PyModule::import(py, "json").into_diagnostic()?;
+                                let json_convert: Py<PyAny> =
+                                    json_convert.getattr("loads").into_diagnostic()?.into();
+                                let py_inputs = PyList::new(
+                                    py,
+                                    inputs.into_iter().map(|nr| rows_to_py_rows(nr.rows, py)),
+                                );
+                                let opts_str = PyString::new(py, &options.to_string());
+                                let args = PyTuple::new(py, vec![opts_str]);
+                                let py_opts = json_convert.call1(py, args).into_diagnostic()?;
+                                let args =
+                                    PyTuple::new(py, vec![PyObject::from(py_inputs), py_opts]);
+                                let res = cb.as_ref(py).call1(args).into_diagnostic()?;
+                                py_to_named_rows(res).into_diagnostic()
+                            });
+                            if sender.send(res).is_err() {
+                                break;
+                            }
+                        }
+                    });
+                    Ok(())
                 }
+                Err(err) => Err(PyException::new_err(err.to_string())),
             }
         } else {
             Err(PyException::new_err(DB_CLOSED_MSG))
         }
     }
+    pub fn unregister_callback(&self, id: u32) -> bool {
+        if let Some(db) = &self.db {
+            db.unregister_callback(id)
+        } else {
+            false
+        }
+    }
+    pub fn unregister_fixed_rule(&self, name: &str) -> PyResult<bool> {
+        if let Some(db) = &self.db {
+            match db.unregister_fixed_rule(name) {
+                Ok(b) => Ok(b),
+                Err(err) => Err(PyException::new_err(err.to_string())),
+            }
+        } else {
+            Ok(false)
+        }
+    }
     pub fn run_query(&self, py: Python<'_>, query: &str, params: &str) -> String {
         if let Some(db) = &self.db {
             py.allow_threads(|| db.run_script_str(query, params))