Does user experience have a role throughout the entire development lifecycle of a product (in industry), even as far back as determining what stack developers will use? What are the key influences and roles for each part of the project? If it is present in the entire lifecycle, are these done by other people such as business analysts or project managers or do we allow a user experience designer to look after these areas?
In many corporate environments, UX begins after all the requirements have been gathered and ends before any code is written.
So that's a good example of how it should not be done. Alas, it's a very common model in a lot of organizations.
On the front end, the problem with waiting until the product or service idea is formulated and requirements are built up around it is that often you end up solving for the wrong problem. Bringing UX thinking into the very beginning of the process helps you focus on what the real problem is that a solution is needed for.
All to often I've been brought into projects at a point where after I do a bit of work, it suddenly becomes clear to the team as a whole that they made a bad left turn some 6 months ago. The most common issue I see is that a project has been built up around what some people think the customer wants, rather than an analysis of what the customer actually wants--or even more importantly--what the customer needs.
On the back end, whatever has been designed needs to be built. And how something is built plays a major role in how it works for the end-user. Development needs to be involved along the entire process as well, and working side-by-side with business and UX to make sure that not only what is designed can be built, but also to show what the technology can do above and beyond the current thinking.
It begins when the idea of a product is being formed
If the user experience isn't part of the discussion as soon as the org decides to pursue an idea, then you're not taking a user-first approach and you're missing opportunity. You need a UX designer, product manager, and an engineer at the table to maximize your chances of successfully answering the business need.
It ends when the product is retired
Constantly evolving a product is a necessity if you want to stay in the game for the long haul. There is always something to learn from the customer. And new technology is always opening the doors to new interaction patterns.
The discover and run approach
Some products are developed with a good first effort and then tossed into the market to carry itself. If you go that route, you obviously don't need UX (or much of anyone else) to stick with the product. You need those resources to go work on the next big innovation. And at some point you hope one of those innovations sticks so you can settle into refinement and live a healthier life ;)
User Experience is a process. This design chain is commonly called "User Centered Design". A quick visit to your favorite search engines yields many results, with slightly different flows, but here is just one example:
(different examples call out different stages and names, but all to the same end goal)
UX is involved in every stage and it never ends, with Iterations happening either before you release the product (to fix oversights) or at the same time as product release (to start working on the next version).
During strategy UX is involved with defining what product we are attempting to make. This includes the base set of requirements. A focus on the UX at this point both helps to trim out requirements that don't help the product and make sure the requirements that remain ultimately benefit the experience.
The UX department shines during analysis. Performing user analysis, environmental analysis, task analysis, needs analysis. Finding and defining the product around making sure the user can effectively do their job and to reduce the potential for error.
Your question seems to point a little towards specification where requirements are refined and made specific to what the research shows they should be! This may include what stack developers use -- as going one direction may make certain interaction routes very difficult or impossible.
The design phase takes the analysis and specification and executes on it. Developers do their developy thing, and UX designers do there designy thing.
Finally, UX performs evaluation makes sure all our research and design hold up, through usability testing. Can the user effectively do their job?
... and after finally, we move into the new cycle where UX goes back out to perform analysis to help define the next batch of features and implementation goals. Just because the product has released, doesn't mean our job stops!
This represents the ideal world. Companies rarely include UX through the full cycle, or certainly not as much as we would like to be. Integrating UX into this cycle as soon as possible is preferred.
The user experience designer does typically have a role throughout the entire lifecycle of an application or website.
It is very important for a UX designer to be a part of the discovery phase to understand all business goals and requirements (even if their role isn't always that large at certain stages).
The UX designer should also be present during / after development, and here is why: functionality IS a part of a good UX.
If a website is very slow to load, then it simply has a bad user experience, which is why the UXD needs to be a part of the entire project lifecycle. They say that if a website takes more than 3 seconds to load, the user will just bounce off of the page (obviously a bad user experience).
The sooner you add a UX specialist to your project, the better your product will be. Or more correct, the further a project goes without a UX professional, the more time consuming (and costly) your project will be. This is because you need to do the same work twice, and that's bad project management.
So the second person you assign to a project is the UX pro in a perfect world. And she/he needs to stay on the project until release and be sure to have UX competence available during the entire lifecycle of the product life.