From 4c2f46e2e2dc111b226e929813ee0f3f797987b7 Mon Sep 17 00:00:00 2001
From: Wojciech Stachowski <wojciech.stachowski@nexontis.com>
Date: Tue, 12 Aug 2025 11:33:00 +0200
Subject: [PATCH] Allow `upstream` to be `undici.BalancedPool`, pass
 `maxRetriesOn503` to `replyOpts`

---
 README.md        | 2 ++
 index.js         | 4 +++-
 types/index.d.ts | 2 +-
 3 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/README.md b/README.md
index 154a61d..9820e24 100644
--- a/README.md
+++ b/README.md
@@ -124,6 +124,8 @@ be streamed directly to the destination._
 
 An URL (including protocol) that represents the target server to use for proxying.
 
+It can be an array of URLs, if you want to use [`undici.BalancedPool`](https://undici.nodejs.org/#/docs/api/BalancedPool) (`upstream` is compatible with  [`base`](https://github.com/fastify/fastify-reply-from?tab=readme-ov-file#base) option).
+
 ### `prefix`
 
 The prefix to mount this plugin on. All the requests to the current server starting with the given prefix will be proxied to the provided upstream.
diff --git a/index.js b/index.js
index 10375c9..418e6ab 100644
--- a/index.js
+++ b/index.js
@@ -532,8 +532,10 @@ async function fastifyHttpProxy (fastify, opts) {
 
   const internalRewriteLocationHeader = opts.internalRewriteLocationHeader ?? true
   const oldRewriteHeaders = (opts.replyOptions || {}).rewriteHeaders
+  const maxRetriesOn503 = opts.maxRetriesOn503
   const replyOpts = Object.assign({}, opts.replyOptions, {
-    rewriteHeaders
+    rewriteHeaders,
+    maxRetriesOn503
   })
   fromOpts.rewriteHeaders = rewriteHeaders
 
diff --git a/types/index.d.ts b/types/index.d.ts
index 7edaac1..acf86ea 100644
--- a/types/index.d.ts
+++ b/types/index.d.ts
@@ -60,7 +60,7 @@ declare namespace fastifyHttpProxy {
   ) => string
 
   export interface FastifyHttpProxyOptions extends FastifyReplyFromOptions {
-    upstream: string;
+    upstream: string | string[];
     prefix?: string;
     rewritePrefix?: string;
     proxyPayloads?: boolean;