Validation-and-Serialization
Validation and Serialization
Fastify uses a schema-based approach, and even if it is not mandatory we recommend using JSON Schema to validate your routes and serialize your outputs. Internally, Fastify compiles the schema into a highly performant function.
Validation will only be attempted if the content type is application-json
, as
described in the documentation for the content type
parser.
All the examples in this section are using the JSON Schema Draft 7 specification.
⚠ Security Notice
Treat the schema definition as application code. Validation and serialization features dynamically evaluate code with
new Function()
, which is not safe to use with user-provided schemas. See Ajv and fast-json-stringify for more details.Regardless the
$async
Ajv feature is supported by Fastify, it should not be used as part of the first validation strategy. This option is used to access Databases and reading them during the validation process may lead to Denial of Service Attacks to your application. If you need to runasync
tasks, use Fastify's hooks instead after validation completes, such aspreHandler
.
Core concepts
The validation and the serialization tasks are processed by two different, and customizable, actors:
- Ajv v8 for the validation of a request
- 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)
.
Adding a shared schema
Thanks to the addSchema
API, you can add multiple schemas to the Fastify
instance and then reuse them in multiple parts of your application. As usual,
this API is encapsulated.
The shared schemas can be reused through the JSON Schema
$ref
keyword. Here is an overview of how references work:
myField: { $ref: '#foo'}
will search for field with$id: '#foo'
inside the current schemamyField: { $ref: '#/definitions/foo'}
will search for fielddefinitions.foo
inside the current schemamyField: { $ref: 'http://url.com/sh.json#'}
will search for a shared schema added with$id: 'http://url.com/sh.json'
myField: { $ref: 'http://url.com/sh.json#/definitions/foo'}
will search for a shared schema added with$id: 'http://url.com/sh.json'
and will use the fielddefinitions.foo
myField: { $ref: 'http://url.com/sh.json#foo'}
will search for a shared schema added with$id: 'http://url.com/sh.json'
and it will look inside of it for object with$id: '#foo'
Simple usage:
fastify.addSchema({
$id: 'http://example.com/',
type: 'object',
properties: {
hello: { type: 'string' }
}
})
fastify.post('/', {
handler () {},
schema: {
body: {
type: 'array',
items: { $ref: 'http://example.com#/properties/hello' }
}
}
})
$ref
as root reference:
fastify.addSchema({
$id: 'commonSchema',
type: 'object',
properties: {
hello: { type: 'string' }
}
})
fastify.post('/', {
handler () {},
schema: {
body: { $ref: 'commonSchema#' },
headers: { $ref: 'commonSchema#' }
}
})
Retrieving the shared schemas
If the validator and the serializer are customized, the .addSchema
method will
not be useful since the actors are no longer controlled by Fastify. To access
the schemas added to the Fastify instance, you can simply use .getSchemas()
:
fastify.addSchema({
$id: 'schemaId',
type: 'object',
properties: {
hello: { type: 'string' }
}
})
const mySchemas = fastify.getSchemas()
const mySchema = fastify.getSchema('schemaId')
As usual, the function getSchemas
is encapsulated and returns the shared
schemas available in the selected scope:
fastify.addSchema({ $id: 'one', my: 'hello' })
// will return only `one` schema
fastify.get('/', (request, reply) => { reply.send(fastify.getSchemas()) })
fastify.register((instance, opts, done) => {
instance.addSchema({ $id: 'two', my: 'ciao' })
// will return `one` and `two` schemas
instance.get('/sub', (request, reply) => { reply.send(instance.getSchemas()) })
instance.register((subinstance, opts, done) => {
subinstance.addSchema({ $id: 'three', my: 'hola' })
// will return `one`, `two` and `three`
subinstance.get('/deep', (request, reply) => { reply.send(subinstance.getSchemas()) })
done()
})
done()
})
Validation
The route validation internally relies upon Ajv v8 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:
body
: validates the body of the request if it is a POST, PUT, or PATCH method.querystring
orquery
: validates the query string.params
: validates the route params.headers
: validates the request headers.
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 latest version of Ajv (v8) you should read how to do it in the
schemaController
section.
Example:
const bodyJsonSchema = {
type: 'object',
required: ['requiredKey'],
properties: {
someKey: { type: 'string' },
someOtherKey: { type: 'number' },
requiredKey: {
type: 'array',
maxItems: 3,
items: { type: 'integer' }
},
nullableKey: { type: ['number', 'null'] }, // or { type: 'number', nullable: true }
multipleTypesKey: { type: ['boolean', 'number'] },
multipleRestrictedTypesKey: {
oneOf: [
{ type: 'string', maxLength: 5 },
{ type: 'number', minimum: 10 }
]
},
enumKey: {
type: 'string',
enum: ['John', 'Foo']
},
notTypeKey: {
not: { type: 'array' }
}
}
}
const queryStringJsonSchema = {
type: 'object',
properties: {
name: { type: 'string' },
excitement: { type: 'integer' }
}
}
const paramsJsonSchema = {
type: 'object',
properties: {
par1: { type: 'string' },
par2: { type: 'number' }
}
}
const headersJsonSchema = {
type: 'object',
properties: {
'x-foo': { type: 'string' }
},
required: ['x-foo']
}
const schema = {
body: bodyJsonSchema,
querystring: queryStringJsonSchema,
params: paramsJsonSchema,
headers: headersJsonSchema
}
fastify.post('/the/url', { schema }, handler)
For body
schema, it is further possible to differentiate the schema per content
type by nesting the schemas inside content
property. The schema validation
will be applied based on the Content-Type
header in the request.
fastify.post('/the/url', {
schema: {
body: {
content: {
'application/json': {
schema: { type: 'object' }
},
'text/plain': {
schema: { type: 'string' }
}
// Other content types will not be validated
}
}
}
}, handler)
Note that Ajv will try to coerce the values
to the types specified in your schema type
keywords, both to pass the
validation and to use the correctly typed data afterwards.
The Ajv default configuration in Fastify supports coercing array parameters in
querystring
. Example:
const opts = {
schema: {
querystring: {
type: 'object',
properties: {
ids: {
type: 'array',
default: []
},
},
}
}
}
fastify.get('/', opts, (request, reply) => {
reply.send({ params: request.query }) // echo the querystring
})
fastify.listen({ port: 3000 }, (err) => {
if (err) throw err
})
curl -X GET "http://localhost:3000/?ids=1
{"params":{"ids":["1"]}}
You can also specify a custom schema validator for each parameter type (body, querystring, params, headers).
For example, the following code disable type coercion only for the body
parameters, changing the ajv default options:
const schemaCompilers = {
body: new Ajv({
removeAdditional: false,
coerceTypes: false,
allErrors: true
}),
params: new Ajv({
removeAdditional: false,
coerceTypes: true,
allErrors: true
}),
querystring: new Ajv({
removeAdditional: false,
coerceTypes: true,
allErrors: true
}),
headers: new Ajv({
removeAdditional: false,
coerceTypes: true,
allErrors: true
})
}
server.setValidatorCompiler(req => {
if (!req.httpPart) {
throw new Error('Missing httpPart')
}
const compiler = schemaCompilers[req.httpPart]
if (!compiler) {
throw new Error(`Missing compiler for ${req.httpPart}`)
}
return compiler.compile(req.schema)
})
For further information see here
Ajv Plugins
You can provide a list of plugins you want to use with the default ajv
instance. Note that the plugin must be compatible with the Ajv version shipped
within Fastify.
Refer to
ajv options
to check plugins format
const fastify = require('fastify')({
ajv: {
plugins: [
require('ajv-merge-patch')
]
}
})
fastify.post('/', {
handler (req, reply) { reply.send({ ok: 1 }) },
schema: {
body: {
$patch: {
source: {
type: 'object',
properties: {
q: {
type: 'string'
}
}
},
with: [
{
op: 'add',
path: '/properties/q',
value: { type: 'number' }
}
]
}
}
}
})
fastify.post('/foo', {
handler (req, reply) { reply.send({ ok: 1 }) },
schema: {
body: {
$merge: {
source: {
type: 'object',
properties: {
q: {
type: 'string'
}
}
},
with: {
required: ['q']
}
}
}
}
})
Validator Compiler
The validatorCompiler
is a function that returns a function that validates the
body, URL parameters, headers, and query string. The default
validatorCompiler
returns a function that implements the
ajv validation interface. Fastify uses it internally to
speed the validation up.
Fastify's baseline ajv configuration is:
{
coerceTypes: 'array', // change data type of data to match type keyword
useDefaults: true, // replace missing properties and items with the values from corresponding default keyword
removeAdditional: true, // remove additional properties if additionalProperties is set to false, see: https://ajv.js.org/guide/modifying-data.html#removing-additional-properties
uriResolver: require('fast-uri'),
addUsedSchema: false,
// Explicitly set allErrors to `false`.
// When set to `true`, a DoS attack is possible.
allErrors: false
}
This baseline configuration can be modified by providing
ajv.customOptions
to your Fastify factory.
If you want to change or set additional config options, you will need to create your own instance and override the existing one like:
const fastify = require('fastify')()
const Ajv = require('ajv')
const ajv = new Ajv({
removeAdditional: 'all',
useDefaults: true,
coerceTypes: 'array',
// any other options
// ...
})
fastify.setValidatorCompiler(({ schema, method, url, httpPart }) => {
return ajv.compile(schema)
})
Note: If you use a custom instance of any validator (even Ajv), you have to
add schemas to the validator instead of Fastify, since Fastify's default
validator is no longer used, and Fastify's addSchema
method has no idea what
validator you are using.
Using other validation libraries
The setValidatorCompiler
function makes it easy to substitute ajv
with
almost any JavaScript validation library (joi,
yup, ...) or a custom one:
const Joi = require('joi')
fastify.post('/the/url', {
schema: {
body: Joi.object().keys({
hello: Joi.string().required()
}).required()
},
validatorCompiler: ({ schema, method, url, httpPart }) => {
return data => schema.validate(data)
}
}, handler)
const yup = require('yup')
// Validation options to match ajv's baseline options used in Fastify
const yupOptions = {
strict: false,
abortEarly: false, // return all errors
stripUnknown: true, // remove additional properties
recursive: true
}
fastify.post('/the/url', {
schema: {
body: yup.object({
age: yup.number().integer().required(),
sub: yup.object().shape({
name: yup.string().required()
}).required()
})
},
validatorCompiler: ({ schema, method, url, httpPart }) => {
return function (data) {
// with option strict = false, yup `validateSync` function returns the
// coerced value if validation was successful, or throws if validation failed
try {
const result = schema.validateSync(data, yupOptions)
return { value: result }
} catch (e) {
return { error: e }
}
}
}
}, handler)
.statusCode property
All validation errors will be added a .statusCode
property set to 400
. This guarantees
that the default error handler will set the status code of the response to 400
.
fastify.setErrorHandler(function (error, request, reply) {
request.log.error(error, `This error has status code ${error.statusCode}`)
reply.status(error.statusCode).send(error)
})
Validation messages with other validation libraries
Fastify's validation error messages are tightly coupled to the default
validation engine: errors returned from ajv
are eventually run through the
schemaErrorFormatter
function which is responsible for building human-friendly
error messages. However, the schemaErrorFormatter
function is written with
ajv
in mind. As a result, you may run into odd or incomplete error messages
when using other validation libraries.
To circumvent this issue, you have 2 main options :
- make sure your validation function (returned by your custom
schemaCompiler
) returns errors in the same structure and format asajv
(although this could prove to be difficult and tricky due to differences between validation engines) - or use a custom
errorHandler
to intercept and format your 'custom' validation errors
To help you in writing a custom errorHandler
, Fastify adds 2 properties to all
validation errors:
validation
: the content of theerror
property of the object returned by the validation function (returned by your customschemaCompiler
)validationContext
: the 'context' (body, params, query, headers) where the validation error occurred
A very contrived example of such a custom errorHandler
handling validation
errors is shown below:
const errorHandler = (error, request, reply) => {
const statusCode = error.statusCode
let response
const { validation, validationContext } = error
// check if we have a validation error
if (validation) {
response = {
// validationContext will be 'body' or 'params' or 'headers' or 'query'
message: `A validation error occurred when validating the ${validationContext}...`,
// this is the result of your validation library...
errors: validation
}
} else {
response = {
message: 'An error occurred...'
}
}
// any additional work here, eg. log error
// ...
reply.status(statusCode).send(response)
}
Serialization
Usually, you will send your data to the clients as JSON, and Fastify has a powerful tool to help you, fast-json-stringify, which is used if you have provided an output schema in the route options. We encourage you to use an output schema, as it can drastically increase throughput and help prevent accidental disclosure of sensitive information.
Example:
const schema = {
response: {
200: {
type: 'object',
properties: {
value: { type: 'string' },
otherValue: { type: 'boolean' }
}
}
}
}
fastify.post('/the/url', { schema }, handler)
As you can see, the response schema is based on the status code. If you want to
use the same schema for multiple status codes, you can use '2xx'
or default
,
for example:
const schema = {
response: {
default: {
type: 'object',
properties: {
error: {
type: 'boolean',
default: true
}
}
},
'2xx': {
type: 'object',
properties: {
value: { type: 'string' },
otherValue: { type: 'boolean' }
}
},
201: {
// the contract syntax
value: { type: 'string' }
}
}
}
fastify.post('/the/url', { schema }, handler)
You can even have a specific response schema for different content types. For example:
const schema = {
response: {
200: {
description: 'Response schema that support different content types'
content: {
'application/json': {
schema: {
name: { type: 'string' },
image: { type: 'string' },
address: { type: 'string' }
}
},
'application/vnd.v1+json': {
schema: {
type: 'array',
items: { $ref: 'test' }
}
}
}
},
'3xx': {
content: {
'application/vnd.v2+json': {
schema: {
fullName: { type: 'string' },
phone: { type: 'string' }
}
}
}
},
default: {
content: {
// */* is match-all content-type
'*/*': {
schema: {
desc: { type: 'string' }
}
}
}
}
}
}
fastify.post('/url', { schema }, handler)
Serializer Compiler
The serializerCompiler
is a function that returns a function that must return
a string from an input object. When you define a response JSON Schema, you can
change the default serialization method by providing a function to serialize
every route where you do.
fastify.setSerializerCompiler(({ schema, method, url, httpStatus, contentType }) => {
return data => JSON.stringify(data)
})
fastify.get('/user', {
handler (req, reply) {
reply.send({ id: 1, name: 'Foo', image: 'BIG IMAGE' })
},
schema: {
response: {
'2xx': {
type: 'object',
properties: {
id: { type: 'number' },
name: { type: 'string' }
}
}
}
}
})
If you need a custom serializer in a very specific part of your code, you can
set one with reply.serializer(...)
.
Error Handling
When schema validation fails for a request, Fastify will automatically return a status 400 response including the result from the validator in the payload. As an example, if you have the following schema for your route
const schema = {
body: {
type: 'object',
properties: {
name: { type: 'string' }
},
required: ['name']
}
}
and fail to satisfy it, the route will immediately return a response with the following payload
{
"statusCode": 400,
"error": "Bad Request",
"message": "body should have required property 'name'"
}
If you want to handle errors inside the route, you can specify the
attachValidation
option for your route. If there is a validation error, the
validationError
property of the request will contain the Error
object with
the raw validation
result as shown below
const fastify = Fastify()
fastify.post('/', { schema, attachValidation: true }, function (req, reply) {
if (req.validationError) {
// `req.validationError.validation` contains the raw validation error
reply.code(400).send(req.validationError)
}
})
schemaErrorFormatter
If you want to format errors yourself, you can provide a sync function that must
return an error as the schemaErrorFormatter
option to Fastify when
instantiating. The context function will be the Fastify server instance.
errors
is an array of Fastify schema errors FastifySchemaValidationError
.
dataVar
is the currently validated part of the schema. (params | body |
querystring | headers).
const fastify = Fastify({
schemaErrorFormatter: (errors, dataVar) => {
// ... my formatting logic
return new Error(myErrorMessage)
}
})
// or
fastify.setSchemaErrorFormatter(function (errors, dataVar) {
this.log.error({ err: errors }, 'Validation failed')
// ... my formatting logic
return new Error(myErrorMessage)
})
You can also use setErrorHandler to define a custom response for validation errors such as
fastify.setErrorHandler(function (error, request, reply) {
if (error.validation) {
reply.status(422).send(new Error('validation failed'))
}
})
If you want a custom error response in the schema without headaches, and
quickly, take a look at
ajv-errors
. Check out the
example
usage.
Make sure to install version 1.0.1 of
ajv-errors
, because later versions of it are not compatible with AJV v6 (the version shipped by Fastify v3).
Below is an example showing how to add custom error messages for each property of a schema by supplying custom AJV options. Inline comments in the schema below describe how to configure it to show a different error message for each case:
const fastify = Fastify({
ajv: {
customOptions: {
jsonPointers: true,
// Warning: Enabling this option may lead to this security issue https://www.cvedetails.com/cve/CVE-2020-8192/
allErrors: true
},
plugins: [
require('ajv-errors')
]
}
})
const schema = {
body: {
type: 'object',
properties: {
name: {
type: 'string',
errorMessage: {
type: 'Bad name'
}
},
age: {
type: 'number',
errorMessage: {
type: 'Bad age', // specify custom message for
min: 'Too young' // all constraints except required
}
}
},
required: ['name', 'age'],
errorMessage: {
required: {
name: 'Why no name!', // specify error message for when the
age: 'Why no age!' // property is missing from input
}
}
}
}
fastify.post('/', { schema, }, (request, reply) => {
reply.send({
hello: 'world'
})
})
If you want to return localized error messages, take a look at ajv-i18n
const localize = require('ajv-i18n')
const fastify = Fastify()
const schema = {
body: {
type: 'object',
properties: {
name: {
type: 'string',
},
age: {
type: 'number',
}
},
required: ['name', 'age'],
}
}
fastify.setErrorHandler(function (error, request, reply) {
if (error.validation) {
localize.ru(error.validation)
reply.status(400).send(error.validation)
return
}
reply.send(error)
})
JSON Schema support
JSON Schema provides utilities to optimize your schemas that, in conjunction with Fastify's shared schema, let you reuse all your schemas easily.
Use Case | Validator | Serializer |
---|---|---|
$ref to $id | ️️✔️ | ✔️ |
$ref to /definitions | ✔️ | ✔️ |
$ref to shared schema $id | ✔️ | ✔️ |
$ref to shared schema /definitions | ✔️ | ✔️ |
Examples
Usage of $ref
to $id
in same JSON Schema
const refToId = {
type: 'object',
definitions: {
foo: {
$id: '#address',
type: 'object',
properties: {
city: { type: 'string' }
}
}
},
properties: {
home: { $ref: '#address' },
work: { $ref: '#address' }
}
}
Usage of $ref
to /definitions
in same JSON Schema
const refToDefinitions = {
type: 'object',
definitions: {
foo: {
$id: '#address',
type: 'object',
properties: {
city: { type: 'string' }
}
}
},
properties: {
home: { $ref: '#/definitions/foo' },
work: { $ref: '#/definitions/foo' }
}
}
Usage $ref
to a shared schema $id
as external schema
fastify.addSchema({
$id: 'http://foo/common.json',
type: 'object',
definitions: {
foo: {
$id: '#address',
type: 'object',
properties: {
city: { type: 'string' }
}
}
}
})
const refToSharedSchemaId = {
type: 'object',
properties: {
home: { $ref: 'http://foo/common.json#address' },
work: { $ref: 'http://foo/common.json#address' }
}
}
Usage $ref
to a shared schema /definitions
as external schema
fastify.addSchema({
$id: 'http://foo/shared.json',
type: 'object',
definitions: {
foo: {
type: 'object',
properties: {
city: { type: 'string' }
}
}
}
})
const refToSharedSchemaDefinitions = {
type: 'object',
properties: {
home: { $ref: 'http://foo/shared.json#/definitions/foo' },
work: { $ref: 'http://foo/shared.json#/definitions/foo' }
}
}
Resources
- JSON Schema
- Understanding JSON Schema
- fast-json-stringify documentation
- Ajv documentation
- Ajv i18n
- Ajv custom errors
- Custom error handling with core methods with error file dumping example