Validation

Okapi provides a powerful validation system that allows you to easily validate incoming request data against defined rules and constraints. This helps ensure that your API receives well-formed and expected data, improving the robustness and reliability of your application.

Validation and Default Values

Okapi provides declarative validation and automatic default value assignment using struct tags.

Basic Validation Tags

Field Type	Tag	Description
string	minLength:"10"	Ensures the string has at least 10 characters.
string	maxLength:"50"	Ensures the string does not exceed 50 characters.
number	min:"5"	Ensures the number is greater than or equal to 5.
number	max:"100"	Ensures the number is less than or equal to 100.
number	exclusiveMin:"0"	Ensures the number is strictly greater than 0.
number	exclusiveMax:"100"	Ensures the number is strictly less than 100.
number	multipleOf:"5"	Ensures the number is a multiple of the given value.
slice	maxItems:"5"	Ensures the slice contains at most 5 items.
slice	minItems:"2"	Ensures the slice contains at least 2 items.
slice	uniqueItems:"true"	Ensures all items in the slice are unique.
map	minProperties:"1"	Ensures the map has at least 1 entry.
map	maxProperties:"10"	Ensures the map has at most 10 entries.
any	required:"true"	Marks the field as required.
any	default:"..."	Assigns a default value when the field is missing/empty.
string / []string	enum:"pending,paid,canceled"	Restricts the field to one of the listed values.
string / []string	const:"active"	Requires the field to equal a fixed value.
string / []string	format:"email"	Enables format validation (e.g. email, uuid, etc.).
string / []string	pattern:"^[a-zA-Z]+$"	Validates the field against a regular expression.

Slices: enum, const, format, and pattern apply to each element of a []string field. Failures are reported per index, e.g. element [2]: ....

Empty values: enum, const, format, and pattern skip empty strings — combine with required:"true" to also enforce presence.

Data Type & Format Validation

Format validation is enabled with the format tag. All formats apply to string fields (and each element of []string fields).

Date & time

Format	Tag	Description
date	format:"date"	Date in YYYY-MM-DD form.
date-time	format:"date-time"	Date and time (RFC3339).
time	format:"time"	Time of day (RFC3339 full-time, e.g. 15:04:05Z).
duration	format:"duration"	Go duration (e.g. 1h30m, 300ms).

Network, web & identifiers

Format	Tag	Description
email	format:"email"	Valid email address.
hostname	format:"hostname"	Valid hostname.
ipv4	format:"ipv4"	Valid IPv4 address.
ipv6	format:"ipv6"	Valid IPv6 address.
mac	format:"mac"	Valid MAC address.
cidr	format:"cidr"	CIDR notation (e.g. 192.168.1.0/24).
uri	format:"uri"	Valid URI (any scheme).
uri-reference	format:"uri-reference"	URI reference (relative references allowed).
url	format:"url"	Absolute URL using the http or https scheme.
uuid	format:"uuid"	Valid UUID.
ulid	format:"ulid"	Valid ULID.
e164 / phone	format:"e164"	Phone number in E.164 format (e.g. +14155552671).
credit-card	format:"credit-card"	Credit card number (passes the Luhn checksum).
semver	format:"semver"	Semantic version (e.g. 1.2.3-alpha.1).
json-pointer	format:"json-pointer"	JSON Pointer (RFC 6901).
byte / base64	format:"byte"	Base64-encoded value.

String content

Format	Tag	Description
alpha	format:"alpha"	Letters only (a–z, A–Z).
alphanumeric	format:"alphanumeric"	Letters and digits only.
numeric	format:"numeric"	A numeric string (e.g. 123, -12.5).
ascii	format:"ascii"	ASCII characters only.
lowercase	format:"lowercase"	No uppercase characters.
uppercase	format:"uppercase"	No lowercase characters.
slug	format:"slug"	URL slug (e.g. my-post-123).
hexcolor	format:"hexcolor"	Hex color (#RGB or #RRGGBB).

Custom pattern

Format	Tag / Attribute	Description
regex	format:"regex" pattern:"^\+?[1-9]\d{1,14}$"	Validates the field using a custom regular expression.

Example

type CreateUserRequest struct {
    Email    string            `json:"email" required:"true" format:"email" example:"user@example.com"`
    Password string            `json:"password" minLength:"8" description:"User password"`
    Age      int               `json:"age" exclusiveMin:"0" max:"120" default:"18"`
    Website  string            `json:"website" format:"url"`
    Kind     string            `json:"kind" const:"user"`
    Roles    []string          `json:"roles" minItems:"1" uniqueItems:"true" enum:"admin,editor,viewer"`
    Metadata map[string]string `json:"metadata" minProperties:"1" maxProperties:"10"`
}

Validation and Binding Methods

Okapi provides multiple ways to validate and bind incoming request data, each suited for different use cases.

Method 1: Using c.Bind()

The simplest approach to bind and validate the request data within your handler:

o.Post("/users", func(c *okapi.Context) error {
    var req CreateUserRequest
    if err := c.Bind(&req); err != nil {
        return c.ErrorBadRequest(err)
    }
    // Proceed with creating the user using validated data
    return c.JSON(http.StatusOK, req)
})

Method 2: Using okapi.Handle() (Input Validation)

Use okapi.Handle() when you want automatic input binding and validation with a typed handler signature:

type Book struct {
    ID     int    `json:"id" path:"id"`
    Name   string `json:"name" form:"name" maxLength:"50" required:"true"`
    Price  int    `json:"price" form:"price" min:"0" max:"500" default:"0"`
    Status string `json:"status" enum:"paid,unpaid,canceled" required:"true"`
}

o.Post("/books", okapi.Handle(func(c *okapi.Context, book *Book) error {
    book.ID = generateID()
    return c.Created(book)
}),
    okapi.DocRequestBody(&Book{}),
    okapi.DocResponse(&Book{}),
)

Method 3: Using okapi.H() (Shorthand for Handle)

okapi.H() is a shorter version of okapi.Handle() when you only need input validation:

type BookDetailInput struct {
    ID int `json:"id" path:"id"`
}

o.Get("/books/{id:int}", okapi.H(func(c *okapi.Context, input *BookDetailInput) error {
    book := findBookByID(input.ID)
    if book == nil {
        return c.AbortNotFound("Book not found")
    }
    return c.OK(book)
}),
    okapi.DocResponse(&Book{}),
)

Method 4: Using okapi.HandleIO() (Input and Output)

Use okapi.HandleIO() when you want to define both input and output structs separately. This is useful for complex operations where the response structure differs from the input:

type BookEditInput struct {
    ID   int  `json:"id" path:"id" required:"true"`
    Body Book `json:"body"`
}

type BookOutput struct {
    Status int
    Body   Book
}

o.Put("/books/{id:int}", okapi.HandleIO(func(c *okapi.Context, input *BookEditInput) (*BookOutput, error) {
    book := updateBook(input.ID, input.Body)
    if book == nil {
        return nil, c.AbortNotFound("Book not found")
    }
    return &BookOutput{Body: *book}, nil
})).WithIO(&BookEditInput{}, &BookOutput{})

Note: WithIO() generates OpenAPI documentation for both input and output schemas. The output struct should follow the body style convention.

Method 5: Using okapi.HandleO() (Output Only)

Use okapi.HandleO() when you only need a custom output struct without specific input validation:

type BooksResponse struct {
    Body []Book `json:"books"`
}

o.Get("/books", okapi.HandleO(func(c *okapi.Context) (*BooksResponse, error) {
    return &BooksResponse{Body: getAllBooks()}, nil
})).WithOutput(&BooksResponse{})

Note: The output struct must follow the body style convention. The response content type is based on the Accept header requested by the client, defaulting to application/json.

Input Sources

Okapi can bind data from multiple sources based on struct tags:

Tag	Source	Example
json	Request body	json:"name"
form	Form data	form:"name"
query	Query parameters	query:"page"
path	Path parameters	path:"id"
header	Request headers	header:"Authorization"

You can combine multiple source tags on the same field:

type BookInput struct {
    ID     int    `json:"id" path:"id"`
    Name   string `json:"name" form:"name" query:"name"`
    Price  int    `json:"price" form:"price" query:"price"`
}

OpenAPI Documentation Helpers

Okapi provides helper methods to generate OpenAPI documentation:

Method	Description
okapi.DocRequestBody(&T{})	Documents the request body schema
okapi.DocResponse(&T{})	Documents the response schema
.WithInput(&T{})	Documents input schema (for okapi.H())
.WithOutput(&T{})	Documents output schema (for okapi.HandleO())
.WithIO(&In{}, &Out{})	Documents both input and output (for okapi.HandleIO())

Complete Example

package main

import (
    "fmt"
    "github.com/jkaninda/okapi"
)

type Book struct {
    ID     int    `json:"id" path:"id"`
    Name   string `json:"name" form:"name" maxLength:"50" example:"The Go Programming Language" required:"true"`
    Price  int    `json:"price" form:"price" query:"price" min:"0" default:"0" max:"500"`
    Qty    int    `json:"qty" form:"qty" query:"qty" default:"0"`
    Status string `json:"status" form:"status" enum:"paid,unpaid,canceled" required:"true" example:"paid"`
}

type BookEditInput struct {
    ID   int  `json:"id" path:"id" required:"true"`
    Body Book `json:"body"`
}

type BookDetailInput struct {
    ID int `json:"id" path:"id"`
}

type BookOutput struct {
    Status int
    Body   Book
}

type BooksResponse struct {
    Body []Book `json:"books"`
}

var books = []Book{
    {ID: 1, Name: "The Go Programming Language", Price: 30, Qty: 100},
}

func main() {
    o := okapi.Default()
    api := o.Group("api")

    // CREATE - Using okapi.Handle with automatic validation
    api.Post("/books", okapi.Handle(func(c *okapi.Context, book *Book) error {
        book.ID = len(books) + 1
        books = append(books, *book)
        return c.Created(book)
    }),
        okapi.DocRequestBody(&Book{}),
        okapi.DocResponse(&Book{}),
    )

    // READ ONE - Using okapi.H (shorthand)
    api.Get("/books/{id:int}", okapi.H(func(c *okapi.Context, input *BookDetailInput) error {
        for _, b := range books {
            if b.ID == input.ID {
                return c.OK(b)
            }
        }
        return c.AbortNotFound(fmt.Sprintf("Book not found: %d", input.ID))
    }),
        okapi.DocResponse(&Book{}),
    )

    // READ ALL - Using okapi.HandleO for custom output
    api.Get("/books", okapi.HandleO(func(c *okapi.Context) (*BooksResponse, error) {
        return &BooksResponse{Body: books}, nil
    })).WithOutput(&BooksResponse{})

    // UPDATE - Using okapi.HandleIO for input/output
    api.Put("/books/{id:int}", okapi.HandleIO(func(c *okapi.Context, input *BookEditInput) (*BookOutput, error) {
        for i, b := range books {
            if b.ID == input.ID {
                books[i] = input.Body
                books[i].ID = input.ID
                return &BookOutput{Body: books[i]}, nil
            }
        }
        return nil, c.AbortNotFound(fmt.Sprintf("Book not found: %d", input.ID))
    })).WithIO(&BookEditInput{}, &BookOutput{})

    // DELETE - Using okapi.H with path parameter
    api.Delete("/books/{id:int}", okapi.H(func(c *okapi.Context, input *BookDetailInput) error {
        for i, b := range books {
            if b.ID == input.ID {
                books = append(books[:i], books[i+1:]...)
                return c.NoContent()
            }
        }
        return c.AbortNotFound(fmt.Sprintf("Book not found: %d", input.ID))
    })).WithInput(&BookDetailInput{})

    if err := o.Start(); err != nil {
        panic(err)
    }
}