diff --git a/backend/common/src/assign_id.rs b/backend/common/src/assign_id.rs
index ea35dca..1bcd42f 100644
--- a/backend/common/src/assign_id.rs
+++ b/backend/common/src/assign_id.rs
@@ -1,10 +1,13 @@
 use bimap::BiMap;
 use std::hash::Hash;
 
-/// A struct that allows you to assign an ID to an arbitrary set of
+/// A struct that allows you to assign an Id to an arbitrary set of
 /// details (so long as they are Eq+Hash+Clone), and then access
-/// the assigned ID given those details or access the details given
-/// the ID.
+/// the assigned Id given those details or access the details given
+/// the Id.
+///
+/// The Id can be any type that's convertible to/from a `usize`. Using
+/// a custom type is recommended for increased type safety.
 #[derive(Debug)]
 pub struct AssignId<Id, Details> {
     current_id: usize,
diff --git a/backend/common/src/dense_map.rs b/backend/common/src/dense_map.rs
index f1fe33b..edc267f 100644
--- a/backend/common/src/dense_map.rs
+++ b/backend/common/src/dense_map.rs
@@ -1,3 +1,12 @@
+/// This stores items in contiguous memory, making a note of free
+/// slots when items are removed again so that they can be reused.
+///
+/// This is particularly efficient when items are often added and
+/// seldom removed.
+///
+/// Items are keyed by an Id, which can be any type you wish, but
+/// must be convertible to/from a `usize`. This promotes using a
+/// custom Id type to talk about items in the map.
 pub struct DenseMap<Id, T> {
     /// List of retired indexes that can be re-used
     retired: Vec<usize>,
diff --git a/backend/common/src/id_type.rs b/backend/common/src/id_type.rs
index 669fed6..d3165a5 100644
--- a/backend/common/src/id_type.rs
+++ b/backend/common/src/id_type.rs
@@ -2,7 +2,7 @@
 /// and serialized/deserialized transparently into the inner type.
 #[macro_export]
 macro_rules! id_type {
-    ($( #[$attrs:meta] )* $vis:vis $ty:ident ( $inner:ident ) $(;)? ) => {
+    ($( #[$attrs:meta] )* $vis:vis struct $ty:ident ( $inner:ident ) $(;)? ) => {
         #[derive(Debug,Clone,Copy,PartialEq,Eq,Hash)]
         $( #[$attrs] )*
         $vis struct $ty($inner);
@@ -30,26 +30,45 @@ macro_rules! id_type {
 
 #[cfg(test)]
 mod test {
+    //! Mostly we're just checking that everything compiles OK
+    //! when the macro is used as expected..
+
+    // A basic definition is possible:
+    id_type! {
+        struct Foo(usize)
+    }
+
+    // We can add a ';' on the end:
+    id_type! {
+        struct Bar(usize);
+    }
+
+    // Visibility qualifiers are allowed:
+    id_type! {
+        pub struct Wibble(u64)
+    }
+
+    // Doc strings are possible
+    id_type! {
+        /// We can have doc strings, too
+        pub(crate) struct Wobble(u16)
+    }
+
+    // In fact, any attributes can be added (common
+    // derives are added already):
+    id_type! {
+        /// We can have doc strings, too
+        #[derive(serde::Serialize)]
+        #[serde(transparent)]
+        pub(crate) struct Lark(u16)
+    }
 
     #[test]
     fn create_and_use_new_id_type() {
-        id_type! {
-            Foo(usize)
-        };
         let _ = Foo::new(123);
         let id = Foo::from(123);
-        let _: usize = id.into();
+        let id_num: usize = id.into();
 
-        // Check that these don't lead to compile errors:
-        id_type! {
-            Bar(usize);
-        };
-        id_type! {
-            pub Wibble(u64)
-        };
-        id_type! {
-            /// We can have doc strings, too
-            pub(crate) Wobble(u16)
-        };
+        assert_eq!(id_num, 123);
     }
 }
diff --git a/backend/common/src/internal_messages.rs b/backend/common/src/internal_messages.rs
index f111174..b6a2330 100644
--- a/backend/common/src/internal_messages.rs
+++ b/backend/common/src/internal_messages.rs
@@ -9,7 +9,7 @@ id_type! {
     /// The shard-local ID of a given node, where a single connection
     /// might send data on behalf of more than one chain.
     #[derive(serde::Serialize, serde::Deserialize)]
-    pub ShardNodeId(usize);
+    pub struct ShardNodeId(usize);
 }
 
 /// Message sent from the shard to the backend core
diff --git a/backend/telemetry/src/aggregator/aggregator.rs b/backend/telemetry/src/aggregator/aggregator.rs
index a8ae177..e04a419 100644
--- a/backend/telemetry/src/aggregator/aggregator.rs
+++ b/backend/telemetry/src/aggregator/aggregator.rs
@@ -12,7 +12,7 @@ id_type! {
     /// A unique Id is assigned per websocket connection (or more accurately,
     /// per feed socket and per shard socket). This can be combined with the
     /// [`LocalId`] of messages to give us a global ID.
-    pub ConnId(u64)
+    pub struct ConnId(u64)
 }
 
 #[derive(Clone)]
diff --git a/backend/telemetry/src/state/chain.rs b/backend/telemetry/src/state/chain.rs
index 5a00ee7..08ee985 100644
--- a/backend/telemetry/src/state/chain.rs
+++ b/backend/telemetry/src/state/chain.rs
@@ -12,7 +12,7 @@ use super::node::Node;
 
 id_type! {
     /// A Node ID that is unique to the chain it's in.
-    pub ChainNodeId(usize)
+    pub struct ChainNodeId(usize)
 }
 
 pub type Label = Box<str>;
diff --git a/backend/telemetry/src/state/state.rs b/backend/telemetry/src/state/state.rs
index 5ae06c7..f1ac6bc 100644
--- a/backend/telemetry/src/state/state.rs
+++ b/backend/telemetry/src/state/state.rs
@@ -11,7 +11,7 @@ use super::chain::{self, Chain, ChainNodeId};
 
 id_type! {
     /// A globally unique Chain ID.
-    pub ChainId(usize)
+    pub struct ChainId(usize)
 }
 
 /// A "global" Node ID is a composite of the ID of the chain it's