Long-term support (LTS) is a type of special versions or editions of software designed to be supported for a longer than normal period. It is particularly applicable to open-source software and projects.
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It is a product lifecycle management policy for computer software, that applies the tenets of reliability engineering to the software development process and software release life cycle. Long-term support extends the period of software maintenance; it also alters the type and frequency of software updates (patches) to reduce the risk, expense, and disruption of software deployment, while promoting the dependability of the software. It does not, however, imply technical support.
At the beginning of a long-term support period, the software developers impose a feature freeze: They make patches to correct software bugs and vulnerabilities, but do not introduce new features that may cause regression. The software maintainer either distributes patches individually, or packages them in maintenance releases, point releases, or service packs. At the conclusion of the support period, the product reaches end-of-life.
The term long-term-support is usually reserved for special versions or editions of software that otherwise has a much shorter release life cycle. Typically, a publisher of LTS software maintains it for at least two years.
Need for long-term support
The traditional software lifecycle in many open source projects is release early, release often, or a regular time-based release schedule. In either case, each new release includes both fixes for security vulnerabilities and new functionality.
Large organisations, or users with mission critical projects, need the security fixes but would often prefer to retain the same base version for an extended period without any new or changed functionality. Their concern is, that as software developers add new features they can accidentally introduce new bugs or break old functionality. While in theory they could backport just the security fixes from each new release to their deployed version, in practise this would often be prohibitively difficult or costly.
Even without the added risks, for these types of users, new functionality is also often expensive. Updating a web application with a sensitive configuration, for example, may require the cooperation of many people: Developers for retrofitting; a database administrator for database schema changes; software testers for regression testing; a project manager for scheduling, liaising, and facilitating; a system administrator or release manager for software deployment oversight; and IT operations personnel for backups, installation, and disaster recovery.
LTS versions of a software package typically address these concerns by releasing only security-related updates for the LTS version - such that installing them should always be less risky than not installing them.
Software with separate LTS versions
This table only lists those have a specific LTS version in addition to a normal release cycle. Many projects, such as CentOS, provide a long period of support for every release.
1.^ The support period for Ubuntu's parent distribution, Debian, is one year after the release of the next stable version. For Debian 6.0 "Squeeze" there is an LTS project to provide security updates until February 2016. Decision on providing LTS for later releases is pending.