add docs for backfill orders

Author: kwannoel

src/meta/model/src/streaming_job.rs

@@ -33,6 +33,15 @@ pub struct Model {
     pub specific_resource_group: Option<String>,
 }
 
+/// This data structure contains an adjacency list of
+/// backfill nodes.
+/// Each edge represents a backfill order.
+/// For instance, given:
+/// `BackfillOrders[1] = [2, 3, 4]`
+/// It means that node 1 must be backfilled before nodes 2, 3, and 4.
+/// Concretely, these node ids are the fragment ids.
+/// This is because each fragment will only have 1 stream scan,
+/// and stream scan corresponds to a backfill node.
 #[derive(Clone, Debug, PartialEq, Eq, FromJsonQueryResult, Serialize, Deserialize, Default)]
 pub struct BackfillOrders(pub HashMap<u32, Vec<u32>>);