Use secondary nodes of your MongoDB cluster to calculate balances

src/Book.ts

@@ -75,6 +75,16 @@ export class Book<U extends ITransaction = ITransaction, J extends IJournal = IJ
   }
 
   async balance(query: IBalanceQuery, options = {} as IOptions): Promise<{ balance: number; notes: number }> {
+    if (options) {
+      // If there is a session, we must NOT set any readPreference (as per mongo v5 and v6).
+      // https://www.mongodb.com/docs/v6.0/core/transactions/#read-concern-write-concern-read-preference
+      // Otherwise, we are free to use any readPreference.
+      if (!options.session && !options.readPreference) {
+        // Let's try reading from the secondary node, if available.
+        options.readPreference = "secondaryPreferred";
+      }
+    }
+
     const parsedQuery = parseBalanceQuery(query, this);
     const meta = parsedQuery.meta;
     delete parsedQuery.meta;

src/IOptions.ts

@@ -1,6 +1,8 @@
 import { ClientSession } from "mongoose";
+import { ReadPreferenceLike } from "mongodb";
 
 // aggregate of mongoose expects Record<string, unknown> type
 export type IOptions = {
   session?: ClientSession;
+  readPreference?: ReadPreferenceLike;
 };

src/models/balance.ts

@@ -100,8 +100,5 @@ export async function snapshotBalance(
 export function getBestSnapshot(query: FilterQuery<IBalance>, options: IOptions = {}): Promise<IBalance | null> {
   const { book, account, meta, ...extras } = query;
   const key = hashKey(constructKey(book, account, { ...meta, ...extras }));
-  return balanceModel.collection.findOne(
-    { key },
-    { sort: { _id: -1 }, session: options.session }
-  ) as Promise<IBalance | null>;
+  return balanceModel.collection.findOne({ key }, { sort: { _id: -1 }, ...options }) as Promise<IBalance | null>;
 }