|Programming in Stateless Environments|
Why a document on programming in stateless environments and why this under the IIS help pages? Pretty easy: Applications on the web are running in an stateless environment. Thus you need to know how to deal with this in order to write great apps.
This document tries to build a solid understanding of what a stateless environment is, how it affects your application design and which solutions are commonly used. It describes the whole topic from the web point of view. It's main focus, however, is the general theory. It won't cover any special server statements etc (although it might refer to some in the examples). You might find this document interesting even if you do no web development but work in other stateless environments (like some mainframe transaction monitors or MTS).
This is version 0.1 of this document. It has been compiled on Sun, August 11th 1996. Keep watching for updates! It has been ported to the Adiscon site in late December 1997 and is scheduled to undergo a major "renovation" in the first quarter of 1998. Your opinion on this document is happly read. Simple email us.
This document is a free public service of Adiscon GmbH, Germany. The information contained in this document is to the best of our knowledge. However, we do not guarantee its accuracy. Use the information contained herein at your sole risk.
What is a Stateless Environment?
Stateless vs. Stateful
In ususal interactive programming, you deal with a stateful environments. That means, your app is in a session with the user. You have multiple queries and responses during this single session and you keep some sort of processing state (usually in main memory).
A typical example is a mailorder application. Following is a typical pseudo-code for such an application:
This is a typical sample in a procedure oriented environment. Not really neat for an event driven GUI interface, but one thing is common to such applications: the application keeps state information of the user session. This processing state is preserved across multiple interactions (here the product selection phase). The preservation of the processing state is done by the environment itself, typically by letting you application staying loaded during the multiple requests. Your application is always in control of what's going on and can handle it. This is a typical example of a stateful environment.
A stateless environment, on the other hand, does not offer these benefits to your application. The environment won't do anything to preserve the session state. Each request and response is a new one and not related to any other.
As you might guess, there are of course ways to implement stateful sessions in stateless environements. This is what this document is all about.
Why a stateless environement?
Good question, you probably say. Why haven't these real cracks, the OS programmes, created stateful environments only? Why do the bother me with the burden to keep things together that naturally belong together? Shouldn't that be a job of the OS?
Well, stateless environments have their adavantages. They are relatively easy to implement and thus efficient. The are inherently well adopted to non reliable environments, as a recovery is simple (as there is no state information). They are very well suited for retrieval type application.
A key thing here is the efficiency. I bet this is the main thing why this had been implemented in almost all of the mainframe transaction monitors.
Is the web stateless?
Indeed it is. We think the main reason for this is its initial design as a retrieval system. However, there are a lot of initiatives to adopt it to the new needs of stateful applications. The basic protocol (http), however, is and will remain to be stateless.
Stateless vs. Transactional
A stateless environment is often mistaken with a transactional environment: in a transactional environment a single transaction (logical group of actions) is eiteher completed fully or not done at all. Many transactional environments are also stateless, which means that a transaction is formed by a single request and response. However, a stateless environment is not necessarily transactional.
Take an http request as an example: let's say that a single request and response makes up a logical group of actions. So it seems to be both stateless and transactional. However, the following might happen:
So the logical group of actions could not be completed successfully: processing was done OK, but the user could not be notified of the success. In a transactional environment, the database update would now be invalidated (rolled back). In the Web, however, this doesn't happen. It is not even sure that the server will note the session abort! If the user would resubmit the data, there will be duplicate databse entries!
So keep in mind: http is stateless, but not transactional.
In order to get any further, we need to use some common terms. There are a lot of definitions for all of this things out there. Many use different names for the same thing and some use same names for different things. Thus we have decided to define some terms by ourselfs and refer to this definitions later on in this document. Note that these definitions are neither "official" nor "self made". We've based them on common understanding but want to make sure you know what we are talking about. Thus we'd like to remove any doubt about our usage of terms.
This is the logical session your application has with the user. It is started by the log on to your application and ended by the user (or any abnormal termination). There is in infinite number of requests and responses during a single session. However, all of this requests and responses logically belong to the same application and are used to create a single "effect". Thus, there might be multiple sessions during one logon / logoff (e.g. if you have a warehouse applications and start the selection and order process multiple times).
Begin and end of a session is only determined by application logic.
The session identifier describes a single session. It can be used to describe a "master session" containing multiple sub-sessions (as described above) or can identify a sub session. It will most likely describe the master session if there a multiple operations going on during one session. The sub-session will most likely be implemented in the form of multiple contexts.
The context is often also refered to as the state of an operation / application. In this document, context describes all the information that is needed by the application to describe the current state of the application. It includes all pre-entered data as well as any housekeeping information. The context is a logical rather than a physical entity. It's physical representation might be stored in memory, in a database or even "on the wire" (be part of the request/response).
This is an entity (usually a key) that describes a given context. It can be equivalent to a session identifier, but it must not (as a session might be used to create several contexts).
How to handle stateless environments
There is just one way to deal with them: implement some kind of state awareness. The key to stateless environment handling is the design of appropriate sessions and contexts. For each of this entities, a identifier is then created and be passed between requests. Thus you have at least two levels of communication:
This is just as with usual stuff. Consider which need your application should serve. Think about which steps the user is required to take and which he might take. Have a look at which data (and how many!) does make up a context.
There are some special considerations for stateless (http) environments:
Keeping track of Sessions
In most cases, sessions have a single context and both this context and the session is identified by the session context. Thus, we will refer from now on to "session" for the logical group of actions and to "context" for all the sessions' data as well es its identifier.
Saving the Context
In general, you need to decide a way to save the context between multiple requests. This saved context is restored which each new request and thus can be used to build the logical session. There are two basic ways to save the context:
Each of this options will be discussed on greater detail in the following sections.
Saving the Context in the Request itself
This is especially popular if your context information is small. All of its data is simply put into the request and being retransmitted by the client in the response. It is relatively easy to do this kind of context saving if you have full control over your html files.
A major advantage of this approach is its inherent handling of session aborts etc: if the user deciced to discontinue a session (or a fatal error happens) the data doesn't get retransmitted to the server and thus the action is really aborted. As there is nothing stored on the server side, there is no need to clean up anything. So this is a pretty elegant solution.
However, there are two major drawbacks of this method: the context is transmitted in each and every request. This works well with small data size but soon becomes impractical as data sizes increase. Thus it can only be used for small contexts. Another drawback is that the context must be transmitted in every request. If one request doesn't transmit it, the whole context (and thus session) is lost. This effectivly prohibits any static html inside such a session.
Saving the Context at the Server
This method effectivly eliminates the drawbacks of saving the context in the request itself: static html is no problem (well, mostly...) and you can save rather large contexts. However, here are some other drawbacks: you have to handle session aborts with some special logic, as there is a need for cleanup of resources (the orphaned contexts). The other - major - drawback is the lack of any really standardized way to handle server context saving. The thing closest to the standard is the cookie proposal by Netscape as well as an related IETF draft, but this is a moving target and might change.
.... to be extended
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