diff --git a/book/src/common_patterns.md b/book/src/common_patterns.md
index 1f8c4f1e..439dabc6 100644
--- a/book/src/common_patterns.md
+++ b/book/src/common_patterns.md
@@ -1 +1,52 @@
 # Common patterns
+
+## On Demnd (Lazy) Inputs
+
+Salsa input quries work best if you can easily provide all of the inputs upfront.
+However sometimes the set of inputs is not known beforehand.
+
+A typical example is reading files from disk.
+While it is possible to eagarly scan a particular directory and create an in-memory file tree in a salsa input query, a more straight forward approach is to read the files lazily.
+That is, when someone requests the text of a file for the first time:
+
+1. Read the file from disk and cache it.
+2. Setup a file-system watcher for this path.
+3. Innvalidate the cached file once the watcher sends a change notification.
+
+This is possible to achive in salsa, using a derived query and `report_synthetic_read` and `invalidate` queries.
+The setup looks roughtly like this:
+
+```rust
+
+#[salsa::query_group(VfsDatabaseStorage)]
+trait VfsDatabase: salsa::Database + FileWathcer {
+    fn read(&self, path: PathBuf) -> String;
+}
+
+trait FileWatcher {
+    fn watch(&self, path: &Path);
+    fn did_change_file(&self, path: &Path);
+}
+
+fn read(db: &impl salsa::Database, path: PathBuf) -> String {
+    db.salsa_runtime()
+        .report_synthetic_read(salsa::Durability::LOW);
+    db.watch(&path);
+    std::fs::read_to_string(&path).unwrap_or_default()
+}
+
+#[salsa::database(VfsDatabaseStorage)]
+struct MyDatabase { ... }
+
+impl FileWatcher for MyDatabase {
+    fn watch(&self, path: &Path) { ... }
+    fn did_change_file(&self, path: &Path) {
+        self.query_mut(ReadQuery).invalidate(path);
+    }
+}
+```
+
+* We declare the query as a derived query (which is the default).
+* In the query implementation, we don't call any other query and just directly read file from disk.
+* Because the query doesn't read any inputs, it will be assigned a `HIGH` durability by default, which we override with `report_synthetic_read`.
+* The result of the query is cached, and we must call `invalidate` to clear this cache.
diff --git a/tests/on_demand_inputs.rs b/tests/on_demand_inputs.rs
new file mode 100644
index 00000000..e7ba67e3
--- /dev/null
+++ b/tests/on_demand_inputs.rs
@@ -0,0 +1,110 @@
+//! Test that "on-demand" input pattern works.
+//!
+//! On-demand inputs are inputs computed lazily on the fly. They are simulated
+//! via a b query with zero inputs, which uses `add_synthetic_read` to
+//! tweak durability and `invalidate` to clear the input.
+
+use std::{cell::Cell, collections::HashMap, rc::Rc};
+
+use salsa::{Database as _, Durability};
+
+#[salsa::query_group(QueryGroupStorage)]
+trait QueryGroup: salsa::Database + AsRef<HashMap<u32, u32>> {
+    fn a(&self, x: u32) -> u32;
+    fn b(&self, x: u32) -> u32;
+    fn c(&self, x: u32) -> u32;
+}
+
+fn a(db: &impl QueryGroup, x: u32) -> u32 {
+    let durability = if x % 2 == 0 {
+        Durability::LOW
+    } else {
+        Durability::HIGH
+    };
+    db.salsa_runtime().report_synthetic_read(durability);
+    let external_state: &HashMap<u32, u32> = db.as_ref();
+    external_state[&x]
+}
+
+fn b(db: &impl QueryGroup, x: u32) -> u32 {
+    db.a(x)
+}
+
+fn c(db: &impl QueryGroup, x: u32) -> u32 {
+    db.b(x)
+}
+
+#[salsa::database(QueryGroupStorage)]
+#[derive(Default)]
+struct Database {
+    runtime: salsa::Runtime<Database>,
+    external_state: HashMap<u32, u32>,
+    on_event: Option<Box<dyn Fn(salsa::Event<Database>)>>,
+}
+
+impl salsa::Database for Database {
+    fn salsa_runtime(&self) -> &salsa::Runtime<Database> {
+        &self.runtime
+    }
+
+    fn salsa_event(&self, event_fn: impl Fn() -> salsa::Event<Self>) {
+        if let Some(cb) = &self.on_event {
+            cb(event_fn())
+        }
+    }
+}
+
+impl AsRef<HashMap<u32, u32>> for Database {
+    fn as_ref(&self) -> &HashMap<u32, u32> {
+        &self.external_state
+    }
+}
+
+#[test]
+fn on_demand_input_works() {
+    let mut db = Database::default();
+
+    db.external_state.insert(1, 10);
+    assert_eq!(db.b(1), 10);
+    assert_eq!(db.a(1), 10);
+
+    // We changed external state, but haven't signaled about this yet,
+    // so we expect to see the old answer
+    db.external_state.insert(1, 92);
+    assert_eq!(db.b(1), 10);
+    assert_eq!(db.a(1), 10);
+
+    db.query_mut(AQuery).invalidate(&1);
+    assert_eq!(db.b(1), 92);
+    assert_eq!(db.a(1), 92);
+}
+
+#[test]
+fn on_demand_input_durability() {
+    let mut db = Database::default();
+    db.external_state.insert(1, 10);
+    db.external_state.insert(2, 20);
+    assert_eq!(db.b(1), 10);
+    assert_eq!(db.b(2), 20);
+
+    let validated = Rc::new(Cell::new(0));
+    db.on_event = Some(Box::new({
+        let validated = Rc::clone(&validated);
+        move |event| match event.kind {
+            salsa::EventKind::DidValidateMemoizedValue { .. } => validated.set(validated.get() + 1),
+            _ => (),
+        }
+    }));
+
+    db.salsa_runtime().synthetic_write(Durability::LOW);
+    validated.set(0);
+    assert_eq!(db.c(1), 10);
+    assert_eq!(db.c(2), 20);
+    assert_eq!(validated.get(), 2);
+
+    db.salsa_runtime().synthetic_write(Durability::HIGH);
+    validated.set(0);
+    assert_eq!(db.c(1), 10);
+    assert_eq!(db.c(2), 20);
+    assert_eq!(validated.get(), 4);
+}