If REST applications are supposed to be stateless,

问题:

I\'m in need of some clarification. I\'ve been reading about REST, and building RESTful applications. According to wikipedia, REST itself is defined to be Representational State Transfer. I therefore don\'t understand all this stateless gobbledeygook that everyone keeps spewing.

From wikipedia:

At any particular time, a client can either be in transition between application states or \"at rest\". A client in a rest state is able to interact with its user, but creates no load and consumes no per-client storage on the set of servers or on the network.

Are they just saying don\'t use session/application level data store???

I get that one goal of REST is to make URI access consistent and available, for instance, instead of hiding paging requests inside posts, making the page number of a request a part of the GET URI. Makes sense to me. But it seems like it is just going overboard saying that no per client data (session data) should ever be stored server side.

What if I had a queue of messages, and my user wanted to read the messages, but as he read them, wanted to block certain senders messages coming through for the duration of his session? Wouldn\'t it make sense to store this in a place on the server side, and have the server only send messages (or message ID\'s) that were not blocked by the user?

Do I really have to send the entire list of message senders to block each time I request the new message list? The message list pertinent to me wouldn\'t/shouldn\'t even be a publicly available resource in the first place..

Again, just trying to understand this. Someone please clarify.

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Update:

I have found a stack overflow question that has an answer that doesn\'t quite get me all the way there: How to manage state in REST which says that the client state that is important should all be transferred on every request.... Ugg.. seems like a lot of overhead... Is this right??

回答1:

The fundamental explanation is:

No client session state on the server.

By stateless it means that the server does not store any state about the client session on the server side.

The client session is stored on the client. The server is stateless means that every server can service any client at any time, there is no session affinity or sticky sessions. The relevant session information is stored on the client and passed to the server as needed.

That does not preclude other services that the web server talks to from maintaining state about business objects such as shopping carts, just not about the client\'s current application/session state.

The client\'s application state should never be stored on the server, but passed around from the client to every place that needs it.

That is where the ST in REST comes from, State Transfer. You transfer the state around instead of having the server store it. This is the only way to scale to millions of concurrent users. If for no other reason than because millions of sessions is millions of sessions.

The load of session management is amortized across all the clients, the clients store their session state and the servers can service many orders of magnitude or more clients in a stateless fashion.

Even for a service that you think will only need in the 10\'s of thousands of concurrent users, you still should make your service stateless. Tens of thousands is still tens of thousands and there will be time and space cost associated with it.

Stateless is how the HTTP protocol and the web in general was designed to operate and is an overall simpler implementation and you have a single code path instead of a bunch of server side logic to maintain a bunch of session state.

There are some very basic implementation principles:

These are principles not implementations, how you meet these principles may vary.

In summary, the five key principles are:

1. Give every “thing” an ID
2. Link things together
3. Use standard methods
4. Resources with multiple representations
5. Communicate statelessly

There is nothing about authentication or authorization in the REST dissertation.

Because there is nothing different from authenticating a request that is RESTful from one that is not. Authentication is irrelevant to the RESTful discussion.

Explaining how to create a stateless application for your particular requirements, is too-broad for StackOverflow.

Implementing Authentication and Authorization as it pertains to REST is even more so too-broad and various approaches to implementations are explained in great detail on the internet in general.

Comments asking for help/info on this will/should just be flagged as No Longer Needed.

回答2:

Statelessness means that every HTTP request happens in complete isolation. When the client makes an HTTP request, it includes all the information necessary for the server to fulfill that request. The server never relies on information from previous requests. If that information was important, the client would have to send it again in subsequent request. Statelessness also brings new features. It’s easier to distribute a stateless application across load-balanced servers. A stateless application is also easy to cache.

There are actually two kinds of state. Application State that lives on the client and Resource State that lives on the server.

A web service only needs to care about your application state when you’re actually making a request. The rest of the time, it doesn’t even know you exist. This means that whenever a client makes a request, it must include all the application states the server will need to process it.

Resource state is the same for every client, and its proper place is on the server. When you upload a picture to a server, you create a new resource: the new picture has its own URI and can be the target of future requests. You can fetch, modify, and delete this resource through HTTP.

Hope this helps differentiate what statelessness and various states mean.

回答3:

Are they just saying don\'t use session/application level data store???

No. They aren\'t saying that in a trivial way.

They\'re saying do not define a \"session\". Don\'t login. Don\'t logout. Provide credentials with the request. Each request stands alone.

You still have data stores. You still have authentication and authorization. You just don\'t waste time establishing sessions and maintaining session state.

The point is that each request (a) stands completely alone and (b) can be trivially farmed out to a giant parallel server farm without any actual work. Apache or Squid can pass RESTful requests around blindly and successfully.

What if I had a queue of messages, and my user wanted to read the messages, but as he read them, wanted to block certain senders messages coming through for the duration of his session?

If the user wants a filter, then simply provide the filter on each request.

Wouldn\'t it make sense to ... have the server only send messages (or message ID\'s) that were not blocked by the user?

Yes. Provide the filter in the RESTful URI request.

Do I really have to send the entire list of message senders to block each time I request the new message list?

Yes. How big can this \"list of message senders to block\" be? A short list of PK\'s?

A GET request can be very large. If necessary, you can try a POST request even though it sounds like a kind of query.

回答4:

You are absolutely right, supporting completely stateless interactions with the server does put an additional burden on the client. However, if you consider scaling an application, the computation power of the clients is directly proportional to the number of clients. Therefore scaling to high numbers of clients is much more feasible.

As soon as you put a tiny bit of responsibility on the server to manage some information related to a specific client\'s interactions, that burden can quickly grow to consume the server.

It\'s a trade off.

回答5:

Historical view of user application state management

Sessions in the traditional sense keep the user\'s state in the application inside the server. This may be the current page in a flow or what has been previously entered but not persisted to the main database yet.

The reason for this need was the lack of standards on the client side to effectively maintain the state without making client specific (i.e. browser specific) applications or plug-ins.

HTML5 and XML Header Request has over time standardized the notion of storing complex data including application state in standard way on the client (i.e. browser) side without resorting to going back and forth between the server.

General usage of REST services

REST services are generally called when there is a transaction that needs to be performed or if it needs to retrieve data.

REST services are meant to be called by the client-side application and not the end user directly.

Authenticating

For any request to the server, part of the request should contain the authorization token. How it is implemented is application specific, but in general is either a BASIC or CERTIFICATE form of authentication.

Form based authentication is not used by REST services. However, as noted above REST services are not meant to be called by the user, but by the application. The application needs to manage getting the authentication token. In my case I used cookies with JASPIC with OAuth 2.0 to connect to Google for authentication and simple HTTP Authentication for automated testing. I also used HTTP Header authentication via JASPIC for local testing as well (though the same approach can be performed in SiteMinder)

As per those examples, the authentication is managed on the client side (though SiteMinder or Google would store the authentication session on their end), there\'s nothing that can be done about that state, but it is not part of the REST service application.

Retrieval requests

Retrieval requests in REST are GET operations where a specific resource is requested and is cacheable. There is no need for server sessions because the request has everything it would need to retrieve the data: authentication and the URI.

Transaction scripts

As noted above, the client-side application itself calls the REST services along with the authentication that it manages on the client side as well.

What this means for REST services [if done correctly] is to take a single request to the REST server will contain everything that is needed for a single user operation that does everything that is needed in a single transaction, a Transaction Script is what the pattern is called.

This is done through a POST request usually, but others such as PUT can also be used.

A lot of contrived examples of REST (I myself did this) tried to follow as much of what has been defined in the HTTP protocol, after going through that I decided to be more pragmatic and left it to GET and POST only. The POST method does not even have to implement the POST-REDIRECT-GET pattern.

Regardless though, as I had noted above, the client-side application will be the one calling the service and it will only call the POST request with all the data when it needs to (not every time). This prevents constant requests to the server.

Polling

Though REST can be used for polling as well, I won\'t recommend it unless you have to use it because of browser compatibility. For that I would use WebSockets which I had designed an API contract for as well. Another alternative for older browsers is CometD.

回答6:

REST is very abstract. It helps to have some good, simple, real-world examples.

Take for example all major social media apps -- Tumblr, Instagram, Facebook, and Twitter. They all have a forever-scrolling view where the farther you scroll down, the more content you see, further and further back in time. However, we\'ve all experienced that moment where you lose where you were scrolled to, and the app resets you back to the top. Like if you quit the app, then when you reopen it, you\'re back at the top again.

The reason why, is because the server did not store your session state. Sadly, your scroll position was just stored in RAM on the client.

Fortunately you don\'t have to log back in when you reconnect, but that\'s only because your client-side also stored login certificate has not expired. Delete and reinstall the app, and you\'re going to have to log back in, because the server did not associate your IP address with your session.

You don\'t have a login session on the server, because they abide by REST.

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Now the above examples don\'t involve a web browser at all, but on the back end, the apps are communicating via HTTPS with their host servers. My point is that REST does not have to involve cookies and browsers etc. There are various means of storing client-side session state.

But lets talk about web browsers for a second, because that brings up another major advantage of REST that nobody here is talking about.

If the server tried to store session state, how is it supposed to identify each individual client?

It could not use their IP address, because many people could be using that same address on a shared router. So how, then?

It can\'t use MAC address for many reasons, not the least of which because you can be logged into multiple different Facebook accounts simultaneously on different browsers plus the app. One browser can easily pretend to be another one, and MAC addresses are just as easy to spoof.

If the server has to store some client-side state to identify you, it has to store it in RAM longer than just the time it takes to process your requests, or else it has to cache that data. Servers have limited amounts of RAM and cache, not to mention processor speed. Server-side state adds to all three, exponentially. Plus if the server is going to store any state about your sessions then it has to store it separately for each browser and app you\'re currently logged in with, and also for each different device you use.

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So... I hope that you see now why REST is so important for scalability. I hope you can start to see why server-side session state is to server scalability what welded-on anvils are to car acceleration.

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Where people get confused is by thinking that \"state\" refers to, like, information stored in a database. No, it refers to any information that needs to be in the RAM of the server when you\'re using it.

回答7:

Stateless means the state of the service doesn’t persist between subsequent requests and response. Each request carries its own user credentials and is individually authenticated. But in stateful each request is known from any prior request. All stateful requests are session-oriented i.e. each request need to know and retain changes made in previous requests.

Banking application is an example of stateful application. Where user first login then make transaction and logs out. If after logout user will try to make the transaction, he will not be able to do so.

Yes, http protocol is essentially a stateless protocol but to make it stateful we make us of HTTP cookies. So, is SOAP by default. But it can be make stateful likewise, depends upon framework you are using.

HTTP is stateless but still we can maintain session in our java application by using different session tracking mechanism.

Yes, We can also maintain session in webservice whether it is REST or SOAP. It can be implemented by using any third party library or you can implement by our own.

Taken from http://gopaldas.org/webservices/soap/webservice-is-stateful-or-stateless-rest-soap

回答8:

I see that the basic issue here is mixing up Session with State. And while REST specifies that you should NOT store the State on the server, nothing prevents you from storing a user Session.

Managing the State on the server means that your server knows exactly what the client is doing (what page they\'re viewing in which section of the application). And this is what you shouldn\'t need to do.

I agree with the other people saying that you should keep the session storage to a minimum size; and while that\'s common sense, it\'s actually also dependent on the application. So, in short, you can still keep a session with cached data to handle the requests with less load on the server, and manage the authentication by providing a temporary authentication/access token for the client to use. Whenever the session/token is expired, generate a new one and ask the client to use it.

Someone might argue that the client should better generate the token. I say it works both ways, and it would depend on the application, and who\'s going to work with the API.

Also keeping some sensitive session data on the server should be the right way to do. You cannot trust the client to keep their shopping cart that (for example) contains a field named \"isFreeGift\". Such information should be kept on the server.

The video link provided by Santanu Dey in his answer is helpful. Watch it if you haven\'t.

Just a side note: It seems all the answers already given seem to disregard the fact that some operations could cause a heavy load on the server. That\'s relevant in terms of power consumption, hardware consumption, and cost (for servers rented by CPU cycle). A good developer shouldn\'t be lazy in optimizing their application, even if the operation can be done very quickly on a modern CPU on some rented server for which they don\'t pay its electricity and maintenance bill.

Althoght the question is a few years old, I hope that my answer would still be helpful.

回答9:

Have a look at this presentation.

http://youtu.be/MRxTP-rQ-S8

According to this pattern - create transient restful resources to manage state if and when really needed. Avoid explicit sessions.

回答10:

The major difference between stateless vs Stateful is the data being passed back to the server every time. In case of stateless, the client has to provide all the info so lot of parameters may need to be passed in each request. In Stateful, the cliet passes those parameters once and they are maintained by the server until modified by the client again.

IMO, API should be stateless which gives allows to scale up really quickly.

回答11:

You have to manage client session on the client side. This means that you have to send authentication data with every request, and you probably, but not necessary have an in-memory cache on the server, which pairs auth data to user information like identity, permissions, etc...

This REST statelessness constraint is very important. Without applying this constraint, your server side application won\'t scale well, because maintaining every single client session will be its Achilles\' heel.

回答12:

The whole concept is different... You don\'t need to manage sessions if you are trying to implement RESTFul protocol. In that case it is better to do authentication procedure on every request (whereas there is an extra cost to it in terms of performance - hashing password would be a good example. not a big deal...). If you use sessions - how can you distribute load across multiple servers? I bet RESTFul protocol is meant to eliminate sessions whatsoever - you don\'t really need them... That\'s why it is called \"stateless\". Sessions are only required when you cannot store anything other than Cookie on a client side after a reqest has been made (take old, non Javascript/HTML5-supporting browser as an example). In case of \"full-featured\" RESTFul client it is usually safe to store base64(login:password) on a client side (in memory) until the applictation is still loaded - the application is used to access to the only host and the cookie cannot be compromised by the third party scripts...

I would stronly recommend to disable cookie authentication for RESTFul sevices... check out Basic/Digest Auth - that should be enough for RESTFul based services.

回答13:

REST is stateless and doesn’t maintain any states between the requests. Client cookies / headers are set to maintain the user state like authentication. Say Client username/password are validated by third part authentication mechanism – 2nd level OTP gerneation etc. Once user get authenticated – headers /cookies comes to rest service end point exposed and we can assume user as auth since user is coming with valid headers/cookies. Now certain info of user like IP is either maintained in the cache and after that if request is coming from same Ip (mac address) for listed resources User is allowed. And cache is maintained for some particular time which get invalidated once time lapses. So either cache can be used or DB entries can be used to persist info b/w the requests.

回答14:

There is no spoon. Don\'t think of statelessness like \"sending all your stuff to the server again and again\". No way. There will be state, always - database itself is a kind of state after all, you\'re a registered user, so any set of client-side info won\'t be valid without the server side. Technically, you\'re never truly stateless.

But what you can do, and what makes real sense, is thin your webserver footprint to the minimum. Languages like PHP make it very easy to just stuff everything in the session storage; you can do this, but if you have many webservers, they must use something shared among them (NFS, Redis, Memcached, something) so that your next request knows what you\'ve done one click earlier. It\'s because of load balancing - you might end up on another webserver with your next request. And while there MUST be a shared storage among them (mostly because they will have to tell if you\'re logged in or not), you should not use it as an alternative database. It\'s not for that.

So you\'re saying, keep session storage to the minimum?

Again, it\'s your decision. You can store stuff there for performance reasons (database is almost always slower than Redis), you can store information redundantly, implement your own caching, whatever - just keep in mind that web servers will have a bigger load if you store a lot of rubbish on them. Also, if they break under heavy loads (and they will), you lose valuable information; with the REST way of thinking, all that happens in this case is the client sends the same (!) request again and it gets served this time.

How to do it right then?

No one-fits-all solution here. I\'d say choose a level of statelessness and go with that. Sessions may be loved by some and hated by others but they\'re not going anywhere. With every request, send as much information as makes sense, a bit more perhaps; but don\'t interpret statelessness as not having a session, nor as logging in every time. Somehow the server must know it\'s you; PHP session ids are one good way, manually generated tokens are another.

Think and decide, don\'t let design trends think for you.