ssr-caching: document way to configure cache key (#1133)

* ssr-caching: document way to configure cache key

* simplify comment

examples/ssr-caching/server.js

@@ -36,20 +36,30 @@ app.prepare()
   })
 })
 
+/*
+ * NB: make sure to modify this to take into account anything that should trigger
+ * an immediate page change (e.g a locale stored in req.session)
+ */
+function getCacheKey (req) {
+  return `${req.url}`
+}
+
 function renderAndCache (req, res, pagePath, queryParams) {
+  const key = getCacheKey(req)
+
   // If we have a page in the cache, let's serve it
-  if (ssrCache.has(req.url)) {
-    console.log(`CACHE HIT: ${req.url}`)
-    res.send(ssrCache.get(req.url))
+  if (ssrCache.has(key)) {
+    console.log(`CACHE HIT: ${key}`)
+    res.send(ssrCache.get(key))
     return
   }
 
   // If not let's render the page into HTML
   app.renderToHTML(req, res, pagePath, queryParams)
     .then((html) => {
       // Let's cache this page
-      console.log(`CACHE MISS: ${req.url}`)
-      ssrCache.set(req.url, html)
+      console.log(`CACHE MISS: ${key}`)
+      ssrCache.set(key, html)
 
       res.send(html)
     })