Normally I would consider any system that changes its behaviour over time as stateful.
Now let's consider a REST service. You GET a resource, later you or someone else modifies the resource with PUT and then you GET the resource again. Obviously, according to the above definition, this would be a stateful system.
But in the context of web services it would still be considered as stateles. So the definition of state must be different here.
What I think is actually meant by stateless webservices, is that the result of a given request should be the same for each client at a given time.
Is that correct? And why does it differ from my usual definition of state? Is stateless the same as connectionless here?
The reference you are looking for is Architectural Styles and
the Design of Network-based Software Architectures, by Roy Fielding. Specifically, chapter three, where he enumerates a number of architectural styles, in particular client-stateless-server.
The client-stateless-server style derives from client-server with the additional constraint that no session state is allowed on the server component. Each request from client to server must contain all of the information necessary to understand the request, and cannot take advantage of any stored context on the server. Session state is kept entirely on the client.
Edit:
So is it correct to say that state in this context does not refer to the state of the server but to the state of a (non-existing) session? Or in other terms, that stateless is a synonym for sessionless here?
Yes.
Related
According to this in property 3 and property 4,
Stateless
Roy fielding got inspiration from HTTP, so it reflects in this
constraint. Make all client-server interactions stateless. The server
will not store anything about the latest HTTP request the client made.
It will treat every request as new. No session, no history.
No client context shall be stored on the server between requests. The
client is responsible for managing the state of the application.
But then again,
In REST, caching shall be applied to resources when applicable, and
then these resources MUST declare themselves cacheable. Caching can be
implemented on the server or client-side
How is the server being stateless if it can cache information?
tldr: Stateless refers the behavior of a server to not record any information on the client's behalf between calls.
Caches are used as a server optimization strategy for resources that are requested often (and do not change frequently).
If a server is "stateless" this mean that no information will be held on the server side on the clients behalf between requests. Thus each request that a client makes must contain all of the required information for the server to perform the desired action. Irrespective of how many calls the client has made on this server previously.
Stateless means there is no memory of the past. Every request is performed as if it were being done for the very first time.
Stateful means that there is memory of the past. Previous request are remembered and may impact behavior of the current Request
Caching is merely holding a copy of a resource that the server is responsible of serving. Caching is commonly used for highly requested resources. Caching strategies can be used by both stateless and stateful services.
In REST when designing an api, you can have Stateless iteration with your clients and you can use caching to store highly requested items in memory to save IO calls to disk.
The authoritative reference for REST is Fielding's dissertation.
Fielding's definition of stateless is found in the discussion of network-based architectural styles
The client-stateless-server style derives from client-server with the additional constraint that no session state is allowed on the server component. Each request from client to server must contain all of the information necessary to understand the request, and cannot take advantage of any stored context on the server. Session state is kept entirely on the client.
It may help to contrast this idea with FTP, where the server is expected to track session state.
RETR example.txt
In FTP, to interpret that RETR command correctly, you need to know what the current working directory is for the session, and the clues that tell you that are stored in earlier messages.
Because HTTP requests are self contained, you don't need "sticky" session management.
As Fielding himself notes, Cookies violate the stateless rest constraint:
The same functionality should have been accomplished via anonymous authentication and true client-side state.
Someone asked me this question a couple of days ago and I didn't have an answer:
As HTTP is a stateless protocol. When we open www.google.com, can it
be called a REST call?
What I think:
When we make a search on google.com all info is passed through cookie and URL parameters. It looks like a stateless request.
But the search results are not independent of user's past request. Search results are specific to user interest and behavior. Now, it doesn't look like a stateless request.
I know this is an old question and I have read many SO answers like Why HTTP is a stateless protocol? but I am still not able to understand what happens when user activity is tracked like on google or Amazon(recommendations based on past purchases) or any other user activity based recommendation websites.
Is it RESTful or is it RESTless?
What if I want to create a web app in which I use REST architecture and still provide user-specific responses?
HTTP is stateless, however the Google Application Layer is not. The specific Cookies and their meaning is part of the Application Layer.
Consider the same with TCP/IP. IP is a stateless protocol, but TCP isn't. The existence of state in TCP embedded in IP packets does not mean that IP protocol itself has a state.
So does that make it a REST call? No.
Although HTTP is stateless & I would suspect that www.google.com when requested with cookies disabled, the results would be the same for each request, making it almost stateless (Google still probably tracks IP to limit request frequency).
But the Application Layer is not stateless. One of the principles of REST is that the system does not retain state data about about the client between requests for the purpose of modifying the responses. In the case of Google, that clearly is not happening.
It seems that the meaning of "stateless" is being (hypothetically) taken beyond its practical expression.
Consider a web system with no DB at all. You call a (RESTful) API, you always get the exactly the same results. This is perfectly stateless... But this is perfectly not a real system, either.
A real system, in practically every implementation, holds data. Moreover, that data is the "resources" that RESTful API allows us to access. Of course, data changes, due to API calls as well. So, if you get a resource's value, change its value, and then get its value again, you will get a different value than the first read; however, this clearly does not say that the reads themselves were not stateless. They are stateless in the sense that they represent the very same action (or, more exact, resource) for each call. Change has to be manually done, using another RESTful API, to change the resource value, that will then be reflected in the next call.
However, what will be the case if we have a resource that changes without a manual, standard API verb?
For example, suppose that we have a resource that counts the number of times some other resource was accessed. Or some other resource that is being populated from some other third party data. Clearly, this is still a stateless protocol.
Moreover, in some sense, almost any system -- say, any system that includes an authentication mechanism -- responds differently for the same API calls, depending, for example, on the user's privileges. And yet, clearly, RESTful systems are not forbidden to authenticate their users...
In short, stateless systems are stateless for the sake of that protocol. If Google tracks the calls so that if I call the same resource in the same session I will get different answers, then it breaks the stateless requirement. But so long as the returned response is different due to application level data, and are not session related, this requirement is not broken.
AFAIK, what Google does is not necessarily related to sessions. If the same user will run the same search under completely identical conditions (e.g., IP, geographical location, OS, browser, etc.), they will get the very same response. If a new identical search will produce different results due to what Google have "learnt" in the last call, it is still stateless, because -- again -- that second call would have produced the very same result if it was done in another session but under identical conditions.
You should probably start from Fielding's comments on cookies in his thesis, and then review Fielding's further thoughts, published on rest-discuss.
My interpretation of Fielding's thoughts, applied to this question: no, it's not REST. The search results change depending on the state of the cookie header in the request, which is to say that the representation of the resource changes depending on the cookie, which is to say that part of the resource's identifier is captured in the cookie header.
Most of the problems with cookies are due to breaking visibility,
which impacts caching and the hypertext application engine -- Fielding, 2003
As it happens, caching doesn't seem to be a big priority for Google; the representation returned to be included a cache control private header, which restricts the participation by intermediate components.
Trying to understand REST HATEOAS:
Suppose I have a service that has state; they are: initial, ready, running. I have a client that connects to the service, obtains a page with links that allow it to mutate the service state.
It uses one of the links to change the service's state and obtains another page with new links.
As long as there is 1 client, the state the client holds is identical with the service. But if there is a second client and it changes the service's state, the first client's representation is stale.
How is this resolved in HATEOAS? From what I've read it seems that REST is not applicable and I should maybe look at something else. If so, what?
Thanks!
This is not resolved by HATEOS (entirely). As REST is stateless this is kind of a paradox use case to keep state in client and server aligned.
Assuming I understand your requirements, yes, you're state in client 1 is stale and not the same as the one on the server. But what if the client would make a periodic call to the server to see whether some other client changed it? If so, with HATEOAS you could provide a link to serve the current state and omit the link to change the state.
#Kay - Thanks for answering.
I'm going to try to answer my own question. I realized after reading your answer that the "application" in HATEOAS is really the virtual application the client experiences when it retrieves and processes the resources it gets from the server. Its states are the pages (resources) it transitions between. The server (service?) may have its own state but it's not the same as the client's.
As long as this distinction is kept in mind, it is not unRESTful to have stale links in client 1. The server simply responds with new links reflecting its own state. And the client makes new transitions based on the updated links.
Still trying to understand. If I have it wrong, I'd appreciate some help.
Thanks!
The stateless requirement of REST refers to the ability of the server to understand and process the client request independent from any previous interactions it has made with said client. In other words, the client should be able to send a request "out of the blue" to the server (I.e., without a session saved on said server) and have the request processed. Hence there isn't a concept of login and logout in a purely RESTful architecture.
That's a different constraint than HATEOAS. Basically, "hypermedia as the engine of application state" means that all state is conveyed through the media type being used and not the connection itself. The client can (and often does) keep its own state, and can request snapshots of the state of resources from the server through resource representations (a.k.a media types).
If you want to be notified when a resource changes state, REST is (probably) the wrong choice. You'd likely want to use a different application protocol than, say, HTTP.
As Fielding says: it's not REST without HATEOAS. Don't call your service REST if it's ignore HATEAOS and stateful service can not be REST. You understood HATEOA. The server provides hyperlinks for the client which should be use to change the state located at the CLIENT SIDE.
To solve your problem: omit tend server from any state information. It will easier your life. Then implement REST as using the Richardson Maturity Model while consider information's from here.
My team has inherited a WCF service that serves as a gateway into multiple back-end systems. The first step in every call to this service is a decision point based on a context key that identifies the caller. This decision point is essentially a factory to provide a handler based on which back end system the request should be directed to.
We're looking at simplifying this service into a RESTful service and are considering the benefits and consequences of passing the context key as part of the request header rather than adding the context key as a parameter to every call into the service. On the one hand, when looking at the individual implementations of the service for each of the backend systems, the context of the caller seems like an orthogonal concern. However, using a custom header leaves me with a slightly uncomfortable feeling since an essential detail for calls to the service are masked from the visible interface. I should note that this is a purely internal solution, which mitigates some of my concern about the visibility of the interface, but even internally there's no telling whether the next engineer to attempt to connect to or modify the service is going to be aware of this hidden detail.
I have two wcf services with the same interface hosted in IIS using http binding. Both have only three methods:
OpenFile(userid) which creates or opens userid.txt.
Write(userid, X) which writes X into the file
Close(userid) which closes the file
InstanceContextMode =InstanceContextMode.PerSession is used for Service B.
Service A:
Is it stateless (service technically does not need to remember the user id, it's tracked by the client) or stateful (the service operation is like a state machine. Client must call the methods in a particular order)?
If HTTPS binding is used, is it stateful?
Service B:
Is it stateless, stateful (because IIS session is used)?
I guess a more general question is whether the statefulness of the web service depends on how it's designed and implemented or how it is hosted? Is there like a "checklist" that I can go through to determine if my web service is categorize as stateless or stateful?
Thanks
By default, anything over HTTP is stateless. When you use PerSession, it still depends on whether your web service implementations use Sessions. But whatever the case, your web server remains stateless, which is precisely while you retain state in a special object (Cache, File, Database or Session).
Even a Session is stateless: unless the server sends the cookie in the HTTP request, nothing is remembered between requests.
This doesn't change for HTTPS. While it is a completely different protocol, the statefulness doesn't change.
About your checklist: it will be short, as it is always stateless with HTTP. Whether or not an implementation maintains state doesn't change this. It's up to the implementation how to work around this limitation and to maintain state, you cannot "see" that on the outside.