docs: how to use ajv8 within fastify3 (#3265)

docs/Server.md

@@ -402,7 +402,7 @@ If `false`, the server routes the incoming request as usual.
 <a name="factory-ajv"></a>
 ### `ajv`
 
-Configure the Ajv instance used by Fastify without providing a custom one.
+Configure the Ajv v6 instance used by Fastify without providing a custom one.
 
 + Default:
 
@@ -895,6 +895,9 @@ It can be useful when your schemas are stored in another data structure that is
 See [issue #2446](https://github.com/fastify/fastify/issues/2446) for an example of what
 this property helps to resolve.
 
+Another use case is to tweak all the schemas processing.
+Doing so it is possible to use Ajv v8, instead of the default v6! We will see an example of this later.
+
 ```js
 const fastify = Fastify({
   schemaController: {
@@ -968,6 +971,37 @@ const fastify = Fastify({
 });
 ```
 
+##### Ajv 8 as default schema validator
+
+Ajv 8 is the evolution of Ajv 6, and it has a lot of improvements and new features.
+To use the new Ajv 8 features such as JTD or the Standalone mode, refers to the [`@fastify/ajv-compiler` documentation](https://github.com/fastify/ajv-compiler#usage).
+
+To use Ajv 8 as default schema validator, you can use the following code:
+
+```js
+const AjvCompiler = require('@fastify/ajv-compiler') // It must be the v2.x.x version
+
+// Note that the `format` schema's keyword is no longer supported on Ajv 8 by default.
+// So you need to add it manually.
+const ajvFormats = require('ajv-formats')
+
+const app = fastify({
+  ajv: {
+    customOptions: {
+      validateFormats: true
+    },
+    plugins: [ajvFormats]
+  },
+  schemaController: {
+    compilersFactory: {
+      buildValidator: AjvCompiler()
+    }
+  }
+})
+
+// Done! You can now use Ajv 8 options and keywords into your schemas!
+```
+
 <a name="set-not-found-handler"></a>
 #### setNotFoundHandler
 

docs/Validation-and-Serialization.md

@@ -14,7 +14,7 @@ Fastify uses a schema-based approach, and even if it is not mandatory we recomme
 
 ### Core concepts
 The validation and the serialization tasks are processed by two different, and customizable, actors:
-- [Ajv](https://www.npmjs.com/package/ajv) for the validation of a request
+- [Ajv v6](https://www.npmjs.com/package/ajv/v/6.12.6) for the validation of a request
 - [fast-json-stringify](https://www.npmjs.com/package/fast-json-stringify) for the serialization of a response's body
 
 These two separate entities share only the JSON schemas added to Fastify's instance through `.addSchema(schema)`.
@@ -120,7 +120,7 @@ fastify.register((instance, opts, done) => {
 
 
 ### Validation
-The route validation internally relies upon [Ajv](https://www.npmjs.com/package/ajv), which is a high-performance JSON Schema validator.
+The route validation internally relies upon [Ajv v6](https://www.npmjs.com/package/ajv/v/6.12.6) which is a high-performance JSON Schema validator.
 Validating the input is very easy: just add the fields that you need inside the route schema, and you are done!
 
 The supported validations are:
@@ -131,6 +131,8 @@ The supported validations are:
 
 All the validations can be a complete JSON Schema object (with a `type` property of `'object'` and a `'properties'` object containing parameters) or a simpler variation in which the `type` and `properties` attributes are forgone and the parameters are listed at the top level (see the example below).
 
+> ℹ If you need to use the lastest version of Ajv (v8) you should read how to do it in the [`schemaController`](Server.md#schema-controller) section. It is explained the easier way to avoid to implement a custom validator.
+
 Example:
 ```js
 const bodyJsonSchema = {
@@ -232,7 +234,7 @@ curl -X GET "http://localhost:3000/?ids=1
 {"statusCode":400,"error":"Bad Request","message":"querystring/hello should be array"}
 ```
 
-Using `coerceTypes` as 'array' should fix it:
+Using `coerceTypes` as 'array' will fix it:
 
 ```js
 const ajv = new Ajv({
@@ -258,7 +260,8 @@ For further information see [here](https://ajv.js.org/coercion.html)
 <a name="ajv-plugins"></a>
 #### Ajv Plugins
 
-You can provide a list of plugins you want to use with Ajv:
+You can provide a list of plugins you want to use with the default `ajv` instance.  
+Note that the plugin must be **compatible with Ajv v6**.
 
 > Refer to [`ajv options`](Server.md#ajv) to check plugins format
 

package.json

@@ -115,7 +115,7 @@
   },
   "homepage": "https://www.fastify.io/",
   "devDependencies": {
-    "@fastify/ajv-compiler-8": "github:fastify/ajv-compiler#ajv-8",
+    "@fastify/ajv-compiler-8": "npm:@fastify/ajv-compiler@^2.0.0",
     "@fastify/pre-commit": "^2.0.1",
     "@hapi/joi": "^17.1.1",
     "@sinonjs/fake-timers": "^7.0.0",