store: Better comments/small simplification for least_used_shard

Author: lutter

store/postgres/src/primary.rs

@@ -1232,7 +1232,7 @@ impl<'a> Connection<'a> {
             .iter()
             .map(|(node, count)| (node.as_str(), *count))
             .chain(missing)
-            .min_by(|(_, a), (_, b)| a.cmp(b))
+            .min_by_key(|(_, count)| *count)
             .map(|(node, _)| NodeId::new(node).map_err(|()| node))
             .transpose()
             // This can't really happen since we filtered by valid NodeId's
@@ -1260,6 +1260,8 @@ impl<'a> Connection<'a> {
             .order_by(sql::<i64>("count(*)"))
             .load::<(String, i64)>(self.conn.as_ref())?;
 
+        // Any shards that have no deployments in them will not be in
+        // 'used'; add them in with a count of 0
         let missing = shards
             .into_iter()
             .filter(|shard| !used.iter().any(|(s, _)| s == shard.as_str()))
@@ -1268,7 +1270,7 @@ impl<'a> Connection<'a> {
         used.iter()
             .map(|(shard, count)| (shard.as_str(), *count))
             .chain(missing)
-            .min_by(|(_, a), (_, b)| a.cmp(b))
+            .min_by_key(|(_, count)| *count)
             .map(|(shard, _)| Shard::new(shard.to_string()))
             .transpose()
             // This can't really happen since we filtered by valid shards

store/postgres/src/subgraph_store.rs

@@ -111,11 +111,12 @@ impl ToSql<Text, Pg> for Shard {
     }
 }
 
-/// Decide where a new deployment should be placed based on the subgraph name
-/// and the network it is indexing. If the deployment can be placed, returns
-/// the name of the database shard for the deployment and the names of the
-/// indexers that should index it. The deployment should then be assigned to
-/// one of the returned indexers.
+/// Decide where a new deployment should be placed based on the subgraph
+/// name and the network it is indexing. If the deployment can be placed,
+/// returns a list of eligible database shards for the deployment and the
+/// names of the indexers that should index it. The deployment should then
+/// be assigned to one of the returned indexers and placed into one of the
+/// shards.
 pub trait DeploymentPlacer {
     fn place(&self, name: &str, network: &str)
         -> Result<Option<(Vec<Shard>, Vec<NodeId>)>, String>;