code.vikunja.io/web

Vikunja Web Handler

When I started Vikunja, I started like everyone else, by writing a bunch of functions to do the logic and then a bunch of handler functions to parse the request data and call the implemented functions to do the logic and eventually return a dataset. After I implemented some functions, I've decided to save me a lot of hassle and put most of that "parse the request and call a processing function"-logic to a general interface to facilitate development and not having to have a lot of similar code all over the place.

This webhandler was built to be used in a REST-API, it takes and returns JSON, but can also be used in combination with own other handler implementations, enabling a lot of flexibility while develeoping.

Features

• Easy to use
• Built for REST-APIs
• Beautiful error handling built in
• Manages rights
• Pluggable authentication mechanisms

Table of contents

• Installation
• Todos
• DB Sessions
• CRUDable
• Rights
• Handler Config
  • Auth
  • Logging
  • Full Example
• Preprocessing
  • Pagination
  • Search
• Standard web handler
• Errors
• URL param binder

TODOs

• Improve docs/Merge with the ones of Vikunja
• Description of web.HTTPError
• Rights methods should return errors (I know, this will break a lot of existing stuff)
• optional Before- and after-{load|update|create} methods which do some preprocessing/after processing like making human-readable names from automatically up counting consts
• "Magic": Check if a passed struct implements Crudable methods and use a general (user defined) function if not

Installation

Using the web handler in your application is pretty straight forward, simply run go get -u code.vikunja.io/web and start using it.

In order to use the common web handler, the struct must implement the web.CRUDable and web.Rights interface.

To learn how to use the handler, take a look at the handler config defining routes

DB Sessions

Each request runs in its own db session. This ensures each operation is one atomic entity without any side effects for concurrent requests happening at the same time.

The session is started at the beginning of the request, rolled back in case of any errors and comitted if no errors occur. The rights methods get the same session (for the same request) as the actual crud methods.

See SessionFactory for docs about how to configure it.

CRUDable

This interface defines methods to Create/Read/ReadAll/Update/Delete something. It is defined as followed:

type CRUDable interface {
	Create(*xorm.Session, Auth) error
	ReadOne(*xorm.Session, Auth) error
	ReadAll(s *xorm.Session, auth Auth, search string, page int, perPage int) (result interface{}, resultCount int, numberOfTotalItems int64, err error)
	Update(*xorm.Session, Auth) error
	Delete(*xorm.Session, Auth) error
}

Each of these methods gets called on an instance of a struct like so:

func (l *List) ReadOne() (err error) {
	*l, err = GetListByID(l.ID)
	return
}

In that case, it takes the ID saved in the struct instance, gets the full list object and fills the original object with it. (See parambinder to understand where that ID is coming from in that specific case).

All functions should behave like this, if they create or update something, the struct instance they are called on should contain the created/updated struct instance. The only exception is ReadAll() which returns an interface. Usually this method returns a slice of results because you cannot make an array of a set type (If you know a way to do this, don't hesitate to drop me a message).

Rights

This interface defines methods to check for rights on structs. They accept an Auth-element as parameter and return a bool and error.

The error is handled as usual.

The interface is defined as followed:

type Rights interface {
	CanRead(*xorm.Session, Auth) (bool, int, error) // The int is the max right the user has for this entity.
	CanDelete(*xorm.Session, Auth) (bool, error)
	CanUpdate(*xorm.Session, Auth) (bool, error)
	CanCreate(*xorm.Session, Auth) (bool, error)
}

When using the standard web handler, all methods are called before their CRUD counterparts. Use pointers for methods like CanRead() to get the base data of the model first, then check the right and then add addintional data.

The CanRead method should also return the max right a user has on this entity. This number will be returned in thex-max-right header to enable user interfaces to show/hide ui elements based on the right the user has.

Handler Config

The handler has some options which you can (and need to) configure.

Auth

Auth is an interface with some methods to decouple the action of getting the current user from the web handler. The function defined via Auths should return a struct which implements the Auth interface.

To define the thing which gets the appropriate auth object, you need to call a middleware like so (After all auth middlewares were called):

Logging

You can provide your own instance of logger.Logger (using go-logging) to the handler. It will use this instance to log errors which are not better specified or things like users trying to do something they're not allowed to do and so on.

MaxItemsPerPage

Contains the maximum number of items per page. If the client requests more items than this, the number of items requested is set to this value.

See pagination for more.

SessionFactory

To create a new session for each request, you need to call the SetSessionFactory method before any web request. It has the following signature:

func SetSessionFactory(sessionFactory func() *xorm.Session)

The closure will be called for every request.

Full Example

handler.SetAuthProvider(&web.Auths{
    AuthObject: func(echo.Context) (web.Auth, error) {
        return models.GetCurrentUser(c) // Your functions
    },
})
handler.SetLoggingProvider(&log.Log)
handler.SetSessionFactory(x.NewSession)

Preprocessing

Pagination

The ReadAll-method has a number of parameters:

ReadAll(auth Auth, search string, page int, perPage int) (result interface{}, resultCount int, numberOfItems int64, err error)

The third parameter contains the requested page, the fourth parameter contains the number of items per page. You should calculate the limits accordingly.

If the number of items per page are not set by the client, the web handler will pass the maximum number of items per page instead. This makes items per page optional for clients. Take a look at the config section for information on how to set that value.

You need to return a number of things:

• The result itself, usually a slice
• The number of items you return in result. Most of the time, this is just len(result). You need to return this value to make the clients aware if they requested a number of items > max items per page.
• The total number of items available. We use the total number of items here and not the number pages so the implementations don't have to deal with calculating the number of pages from that. The total number of clients is then calculated and returned to the client, ite can then be used by the clients to build client-side pagination or similar.
• An error.

The number of items and the total number of pages available will be returned in the x-pagination-total-pages and x-pagination-result-count response headers. You should put this in your api documentation.

Search

When using the ReadAll-method, the first parameter is a search term which should be used to search items of your struct. You define the critera inside of that function.

Users can then pass the ?s=something parameter to the url to search, thats something you should put in your api documentation.

As the logic for "give me everything" and "give me everything where the name contains 'something'" is mostly the same, we made the decision to design the function like this, in order to keep the places with mostly the same logic as few as possible. Also just adding ?s=query to the url one already knows and uses is a lot more convenient.

Defining routes using the standard web handler

You can define routes for the standard web handler like so:

models.List needs to implement web.CRUDable and web.Rights.

listHandler := &crud.WebHandler{
    EmptyStruct: func() crud.CObject {
        return &models.List{}
    },
}
a.GET("/lists", listHandler.ReadAllWeb)
a.GET("/lists/:list", listHandler.ReadOneWeb)
a.POST("/lists/:list", listHandler.UpdateWeb)
a.DELETE("/lists/:list", listHandler.DeleteWeb)
a.PUT("/namespaces/:namespace/lists", listHandler.CreateWeb)

The handler will take care of everything like parsing the request, checking rights, pretty-print errors and return appropriate responses.

Errors

Error types with their messages and http-codes should be implemented by you somewhere in your application and then returned by the appropriate function when an error occures. If the error type implements HTTPError, the server returns a user-friendly error message when this error occours. This means it returns a good HTTP status code, a message, and an error code. The error code should be unique across all error codes and can be used on the client to show a localized error message or do other stuff based on the exact error the server returns. That way the client won't have to "guess" that the error message remains the same over multiple versions of your application.

An HTTPError is defined as follows:

type HTTPError struct {
	HTTPCode int    `json:"-"` // Can be any valid HTTP status code, I'd reccomend to use the constants of the http package.
	Code     int    `json:"code"` // Must be a uniqe int identifier for this specific error. I'd reccomend defining a constant for this.
	Message  string `json:"message"` // A user-readable message what went wrong.
}

You can learn more about how exactly custom error types are created in the vikunja docs.

How the url param binder works

The binder binds all values inside the url to their respective fields in a struct. Those fields need to have a tag param with the name of the url placeholder which must be the same as in routes.

Whenever one of the standard CRUD methods is invoked, this binder is called, which enables one handler method to handle all kinds of different urls with different parameters.