Add configuration for ISR Memory Cache Size (#21535) (#27337)

This allows the user to adjust the LRU cache size according to the
application need:

- optimize the cache size based on average page HTML & JSON size
- disable the memory cache by setting the size to 0

The hard-coded default of 50MB has been moved from the code to the
default configuration object.

Fixes #21535
See also #27325



## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`

## Feature

- [X] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [X] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [X] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`

## Documentation / Examples

- [ ] Make sure the linting passes

packages/next/server/config-shared.ts

@@ -101,6 +101,7 @@ export type NextConfig = { [key: string]: any } & {
     esmExternals?: boolean | 'loose'
     staticPageGenerationTimeout?: number
     pageDataCollectionTimeout?: number
+    isrMemoryCacheSize?: number
   }
 }
 
@@ -166,6 +167,8 @@ export const defaultConfig: NextConfig = {
     esmExternals: false,
     staticPageGenerationTimeout: 60,
     pageDataCollectionTimeout: 60,
+    // default to 50MB limit
+    isrMemoryCacheSize: 50 * 1024 * 1024,
   },
   future: {
     strictPostcssConfiguration: false,

packages/next/server/incremental-cache.ts

@@ -39,7 +39,7 @@ export class IncrementalCache {
   }
 
   prerenderManifest: PrerenderManifest
-  cache: LRUCache<string, IncrementalCacheEntry>
+  cache?: LRUCache<string, IncrementalCacheEntry>
   locales?: string[]
 
   constructor({
@@ -85,15 +85,16 @@ export class IncrementalCache {
       max = parseInt(process.env.__NEXT_TEST_MAX_ISR_CACHE, 10)
     }
 
-    this.cache = new LRUCache({
-      // default to 50MB limit
-      max: max || 50 * 1024 * 1024,
-      length({ value }) {
-        if (!value || value.kind === 'REDIRECT') return 25
-        // rough estimate of size of cache value
-        return value.html.length + JSON.stringify(value.pageData).length
-      },
-    })
+    if (max) {
+      this.cache = new LRUCache({
+        max,
+        length({ value }) {
+          if (!value || value.kind === 'REDIRECT') return 25
+          // rough estimate of size of cache value
+          return value.html.length + JSON.stringify(value.pageData).length
+        },
+      })
+    }
   }
 
   private getSeedPath(pathname: string, ext: string): string {
@@ -133,7 +134,7 @@ export class IncrementalCache {
     if (this.incrementalOptions.dev) return null
     pathname = normalizePagePath(pathname)
 
-    let data = this.cache.get(pathname)
+    let data = this.cache && this.cache.get(pathname)
 
     // let's check the disk for seed data
     if (!data) {
@@ -157,7 +158,9 @@ export class IncrementalCache {
             pageData,
           },
         }
-        this.cache.set(pathname, data)
+        if (this.cache) {
+          this.cache.set(pathname, data)
+        }
       } catch (_) {
         // unable to get data from disk
       }
@@ -204,10 +207,15 @@ export class IncrementalCache {
     }
 
     pathname = normalizePagePath(pathname)
-    this.cache.set(pathname, {
-      revalidateAfter: this.calculateRevalidate(pathname, new Date().getTime()),
-      value: data,
-    })
+    if (this.cache) {
+      this.cache.set(pathname, {
+        revalidateAfter: this.calculateRevalidate(
+          pathname,
+          new Date().getTime()
+        ),
+        value: data,
+      })
+    }
 
     // TODO: This option needs to cease to exist unless it stops mutating the
     // `next build` output's manifest.

packages/next/server/next-server.ts

@@ -276,6 +276,7 @@ export default class Server {
         'pages'
       ),
       locales: this.nextConfig.i18n?.locales,
+      max: this.nextConfig.experimental.isrMemoryCacheSize,
       flushToDisk: !minimalMode && this.nextConfig.experimental.sprFlushToDisk,
     })
     this.responseCache = new ResponseCache(this.incrementalCache)