sessions

Overview ¶

Package gorilla/sessions provides cookie and filesystem sessions and infrastructure for custom session backends.

The key features are:

• Simple API: use it as an easy way to set signed (and optionally encrypted) cookies.
• Built-in backends to store sessions in cookies or the filesystem.
• Flash messages: session values that last until read.
• Convenient way to switch session persistency (aka "remember me") and set other attributes.
• Mechanism to rotate authentication and encryption keys.
• Multiple sessions per request, even using different backends.
• Interfaces and infrastructure for custom session backends: sessions from different stores can be retrieved and batch-saved using a common API.

Let's start with an example that shows the sessions API in a nutshell:

import (
	"net/http"
	"github.com/gorilla/sessions"
)

var store = sessions.NewCookieStore([]byte("something-very-secret"))

func MyHandler(w http.ResponseWriter, r *http.Request) {
	// Get a session. We're ignoring the error resulted from decoding an
	// existing session: Get() always returns a session, even if empty.
	session, err := store.Get(r, "session-name")
	if err != nil {
		http.Error(w, err.Error(), 500)
		return
	}

	// Set some session values.
	session.Values["foo"] = "bar"
	session.Values[42] = 43
	// Save it before we write to the response/return from the handler.
	session.Save(r, w)
}

First we initialize a session store calling NewCookieStore() and passing a secret key used to authenticate the session. Inside the handler, we call store.Get() to retrieve an existing session or a new one. Then we set some session values in session.Values, which is a map[interface{}]interface{}. And finally we call session.Save() to save the session in the response.

Note that in production code, we should check for errors when calling session.Save(r, w), and either display an error message or otherwise handle it.

Save must be called before writing to the response, otherwise the session cookie will not be sent to the client.

Important Note: If you aren't using gorilla/mux, you need to wrap your handlers with context.ClearHandler as or else you will leak memory! An easy way to do this is to wrap the top-level mux when calling http.ListenAndServe:

http.ListenAndServe(":8080", context.ClearHandler(http.DefaultServeMux))

The ClearHandler function is provided by the gorilla/context package.

That's all you need to know for the basic usage. Let's take a look at other options, starting with flash messages.

Flash messages are session values that last until read. The term appeared with Ruby On Rails a few years back. When we request a flash message, it is removed from the session. To add a flash, call session.AddFlash(), and to get all flashes, call session.Flashes(). Here is an example:

func MyHandler(w http.ResponseWriter, r *http.Request) {
	// Get a session.
	session, err := store.Get(r, "session-name")
	if err != nil {
		http.Error(w, err.Error(), 500)
		return
	}

	// Get the previously flashes, if any.
	if flashes := session.Flashes(); len(flashes) > 0 {
		// Use the flash values.
	} else {
		// Set a new flash.
		session.AddFlash("Hello, flash messages world!")
	}
	session.Save(r, w)
}

Flash messages are useful to set information to be read after a redirection, like after form submissions.

There may also be cases where you want to store a complex datatype within a session, such as a struct. Sessions are serialised using the encoding/gob package, so it is easy to register new datatypes for storage in sessions:

import(
	"encoding/gob"
	"github.com/gorilla/sessions"
)

type Person struct {
	FirstName	string
	LastName 	string
	Email		string
	Age			int
}

type M map[string]interface{}

func init() {

	gob.Register(&Person{})
	gob.Register(&M{})
}

As it's not possible to pass a raw type as a parameter to a function, gob.Register() relies on us passing it an empty pointer to the type as a parameter. In the example above we've passed it a pointer to a struct and a pointer to a custom type representing a map[string]interface. This will then allow us to serialise/deserialise values of those types to and from our sessions.

Note that because session values are stored in a map[string]interface{}, there's a need to type-assert data when retrieving it. We'll use the Person struct we registered above:

func MyHandler(w http.ResponseWriter, r *http.Request) {
	session, err := store.Get(r, "session-name")
	if err != nil {
		http.Error(w, err.Error(), 500)
		return
	}

	// Retrieve our struct and type-assert it
	val := session.Values["person"]
	var person = &Person{}
	if person, ok := val.(*Person); !ok {
		// Handle the case that it's not an expected type
	}

	// Now we can use our person object
}

By default, session cookies last for a month. This is probably too long for some cases, but it is easy to change this and other attributes during runtime. Sessions can be configured individually or the store can be configured and then all sessions saved using it will use that configuration. We access session.Options or store.Options to set a new configuration. The fields are basically a subset of http.Cookie fields. Let's change the maximum age of a session to one week:

session.Options = &sessions.Options{
	Path:     "/",
	MaxAge:   86400 * 7,
	HttpOnly: true,
}

Sometimes we may want to change authentication and/or encryption keys without breaking existing sessions. The CookieStore supports key rotation, and to use it you just need to set multiple authentication and encryption keys, in pairs, to be tested in order:

var store = sessions.NewCookieStore(
	[]byte("new-authentication-key"),
	[]byte("new-encryption-key"),
	[]byte("old-authentication-key"),
	[]byte("old-encryption-key"),
)

New sessions will be saved using the first pair. Old sessions can still be read because the first pair will fail, and the second will be tested. This makes it easy to "rotate" secret keys and still be able to validate existing sessions. Note: for all pairs the encryption key is optional; set it to nil or omit it and and encryption won't be used.

Multiple sessions can be used in the same request, even with different session backends. When this happens, calling Save() on each session individually would be cumbersome, so we have a way to save all sessions at once: it's sessions.Save(). Here's an example:

var store = sessions.NewCookieStore([]byte("something-very-secret"))

func MyHandler(w http.ResponseWriter, r *http.Request) {
	// Get a session and set a value.
	session1, _ := store.Get(r, "session-one")
	session1.Values["foo"] = "bar"
	// Get another session and set another value.
	session2, _ := store.Get(r, "session-two")
	session2.Values[42] = 43
	// Save all sessions.
	sessions.Save(r, w)
}

This is possible because when we call Get() from a session store, it adds the session to a common registry. Save() uses it to save all registered sessions.