Editing Legacy system (section)

==Problems posed by legacy computing==
Legacy systems are considered to be potentially problematic by some software engineers for several reasons.<ref>(for example, see Bisbal et al., 1999).</ref>
* If legacy [[software]] runs on only antiquated [[computer hardware|hardware]], the cost of maintaining the system may eventually outweigh the cost of replacing both the software and hardware unless some form of [[emulator|emulation]] or [[backward compatibility]] allows the software to run on new hardware.<ref>{{FOLDOC|Legacy+system}}</ref><ref>{{cite news|last1=Lamb|first1=John|title=Legacy systems continue to have a place in the enterprise|url=http://www.computerweekly.com/feature/Legacy-systems-continue-to-have-a-place-in-the-enterprise|access-date=27 October 2014|work=Computer Weekly|date=June 2008}}</ref>
* These systems can be hard to maintain, improve, and expand because there is a general lack of understanding of the system; the staff who were experts on it have retired or forgotten what they knew about it, and staff who entered the field after it became "legacy" never learned about it in the first place. This can be worsened by lack or loss of documentation. [[Comair (United States)|Comair]] airline company fired its CEO in 2004 due to the failure of an antiquated legacy crew scheduling system that ran into a limitation not known to anyone in the company.<ref>{{cite web |author=Stephanie Overby |date=2005-05-01 |title=Comair's Christmas Disaster: Bound To Fail - CIO.com - Business Technology Leadership |url=https://www.cio.com/article/272043/risk-management-comair-s-christmas-disaster-bound-to-fail.html |access-date=2012-04-29 |publisher=CIO.com}}</ref>
* Legacy systems may have vulnerabilities in older operating systems or applications due to lack of security patches being available or applied. There can also be production configurations that cause security problems. These issues can put the legacy system at risk of being compromised by attackers or knowledgeable insiders.<ref>{{cite web|author=Razermouse |url=http://www.mousesecurity.com/?p=220 |title=The Danger of Legacy Systems |publisher=Mousesecurity.com |date=2011-05-03 |access-date=2012-04-29 |url-status=dead |archive-url=https://web.archive.org/web/20120323165836/http://www.mousesecurity.com/?p=220 |archive-date=March 23, 2012 }}</ref>
* Integration with newer systems may also be difficult because new software may use completely different technologies.  Integration across technology is quite common in computing, but integration between newer technologies and substantially older ones is not common.  There may simply not be sufficient demand for integration technology to be developed.  Some of this "glue" code is occasionally developed by vendors and enthusiasts of particular legacy technologies.
* Budgetary constraints often lead corporations to not address the need of replacement or migration of a legacy system. However, companies often don't consider the increasing supportability costs (people, software and hardware, all mentioned above) and do not take into consideration the enormous loss of capability or business continuity if the legacy system were to fail. Once these considerations are well understood, then based on the proven ROI of a new, more secure, updated technology stack platform is not as costly as the alternative—and the budget is found.
* Due to the fact that most legacy programmers are entering retirement age and the number of young engineers replacing them is very small, there is an alarming shortage of available workforce. This in turn results in difficulty in maintaining legacy systems, as well as an increase in costs of procuring experienced programmers.<ref>{{Cite web|url=http://www.modernizationhub.com|title=Benefits of Mainframe Modernization|website=Modernization Hub|language=en-US|access-date=2017-08-23}}</ref>
* Some legacy systems have a hard limit on their total capacity which may not be enough for today's needs, for example the 4 GB memory limit on many older [[x86]] CPUs, or the [[IPv4 address exhaustion|4 billion address limit]] in [[IPv4]].