[[webflux-caching]]
= HTTP Caching

[.small]#xref:web/webmvc/mvc-caching.adoc[See equivalent in the Servlet stack]#

HTTP caching can significantly improve the performance of a web application. HTTP caching
revolves around the `Cache-Control` response header and subsequent conditional request
headers, such as `Last-Modified` and `ETag`. `Cache-Control` advises private (for example, browser)
and public (for example, proxy) caches how to cache and re-use responses. An `ETag` header is used
to make a conditional request that may result in a 304 (NOT_MODIFIED) without a body,
if the content has not changed. `ETag` can be seen as a more sophisticated successor to
the `Last-Modified` header.

This section describes the HTTP caching related options available in Spring WebFlux.


[[webflux-caching-cachecontrol]]
== `CacheControl`
[.small]#xref:web/webmvc/mvc-caching.adoc#mvc-caching-cachecontrol[See equivalent in the Servlet stack]#

{spring-framework-api}/http/CacheControl.html[`CacheControl`] provides support for
configuring settings related to the `Cache-Control` header and is accepted as an argument
in a number of places:

* xref:web/webflux/caching.adoc#webflux-caching-etag-lastmodified[Controllers]
* xref:web/webflux/caching.adoc#webflux-caching-static-resources[Static Resources]

While {rfc-site}/rfc7234#section-5.2.2[RFC 7234] describes all possible
directives for the `Cache-Control` response header, the `CacheControl` type takes a
use case-oriented approach that focuses on the common scenarios, as the following example shows:

[tabs]
======
Java::
+
[source,java,indent=0,subs="verbatim,quotes"]
----
	// Cache for an hour - "Cache-Control: max-age=3600"
	CacheControl ccCacheOneHour = CacheControl.maxAge(1, TimeUnit.HOURS);

	// Prevent caching - "Cache-Control: no-store"
	CacheControl ccNoStore = CacheControl.noStore();

	// Cache for ten days in public and private caches,
	// public caches should not transform the response
	// "Cache-Control: max-age=864000, public, no-transform"
	CacheControl ccCustom = CacheControl.maxAge(10, TimeUnit.DAYS).noTransform().cachePublic();
----

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim,quotes"]
----
	// Cache for an hour - "Cache-Control: max-age=3600"
	val ccCacheOneHour = CacheControl.maxAge(1, TimeUnit.HOURS)

	// Prevent caching - "Cache-Control: no-store"
	val ccNoStore = CacheControl.noStore()

	// Cache for ten days in public and private caches,
	// public caches should not transform the response
	// "Cache-Control: max-age=864000, public, no-transform"
	val ccCustom = CacheControl.maxAge(10, TimeUnit.DAYS).noTransform().cachePublic()

----
======


[[webflux-caching-etag-lastmodified]]
== Controllers
[.small]#xref:web/webmvc/mvc-caching.adoc#mvc-caching-etag-lastmodified[See equivalent in the Servlet stack]#

Controllers can add explicit support for HTTP caching. We recommend doing so, since the
`lastModified` or `ETag` value for a resource needs to be calculated before it can be compared
against conditional request headers. A controller can add an `ETag` and `Cache-Control`
settings to a `ResponseEntity`, as the following example shows:

--
[tabs]
======
Java::
+
[source,java,indent=0,subs="verbatim,quotes"]
----
	@GetMapping("/book/{id}")
	public ResponseEntity<Book> showBook(@PathVariable Long id) {

		Book book = findBook(id);
		String version = book.getVersion();

		return ResponseEntity
				.ok()
				.cacheControl(CacheControl.maxAge(30, TimeUnit.DAYS))
				.eTag(version) // lastModified is also available
				.body(book);
	}
----

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim,quotes"]
----
	@GetMapping("/book/{id}")
	fun showBook(@PathVariable id: Long): ResponseEntity<Book> {

		val book = findBook(id)
		val version = book.getVersion()

		return ResponseEntity
				.ok()
				.cacheControl(CacheControl.maxAge(30, TimeUnit.DAYS))
				.eTag(version) // lastModified is also available
				.body(book)
	}
----
======
--

The preceding example sends a 304 (NOT_MODIFIED) response with an empty body if the comparison
to the conditional request headers indicates the content has not changed. Otherwise, the
`ETag` and `Cache-Control` headers are added to the response.

You can also make the check against conditional request headers in the controller,
as the following example shows:

--
[tabs]
======
Java::
+
[source,java,indent=0,subs="verbatim,quotes"]
----
	@RequestMapping
	public String myHandleMethod(ServerWebExchange exchange, Model model) {

		long eTag = ... // <1>

		if (exchange.checkNotModified(eTag)) {
			return null; // <2>
		}

		model.addAttribute(...); // <3>
		return "myViewName";
	}
----
<1> Application-specific calculation.
<2> Response has been set to 304 (NOT_MODIFIED). No further processing.
<3> Continue with request processing.

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim,quotes"]
----
	@RequestMapping
	fun myHandleMethod(exchange: ServerWebExchange, model: Model): String? {

		val eTag: Long = ... // <1>

		if (exchange.checkNotModified(eTag)) {
			return null// <2>
		}

		model.addAttribute(...) // <3>
		return "myViewName"
	}
----
<1> Application-specific calculation.
<2> Response has been set to 304 (NOT_MODIFIED). No further processing.
<3> Continue with request processing.
======
--

There are three variants for checking conditional requests against `eTag` values, `lastModified`
values, or both. For conditional `GET` and `HEAD` requests, you can set the response to
304 (NOT_MODIFIED). For conditional `POST`, `PUT`, and `DELETE`, you can instead set the response
to 412 (PRECONDITION_FAILED) to prevent concurrent modification.


[[webflux-caching-static-resources]]
== Static Resources
[.small]#xref:web/webmvc/mvc-caching.adoc#mvc-caching-static-resources[See equivalent in the Servlet stack]#

You should serve static resources with a `Cache-Control` and conditional response headers
for optimal performance. See the section on configuring
xref:web/webflux/config.adoc#webflux-config-static-resources[Static Resources].
