
bos@16: \chapter{Introduction}
bos@16: \label{chap:intro}
bos@16: 
bos@217: \section{About revision control}
bos@155: 
bos@217: Revision control is the management of multiple versions of a piece of
bos@217: information.  In its simplest form, it's a process that many people
bos@217: perform by hand: every time you modify a file, save it under a new
bos@217: name that contains a number, each one higher than the number of the
bos@217: preceding version.
bos@217: 
bos@217: Manually managing multiple versions of even a single file is an
bos@217: error-prone task, though, so software tools to help automate this
bos@217: process have long been available.  The earliest automated revision
bos@217: control tools were intended to help a single user to manage revisions
bos@217: to a single file.  Over the past several decades, the scope of
bos@217: revision control tools has expanded greatly; they now manage multiple
bos@217: files, and help multiple people to work together.  The best modern
bos@217: revision control tools will have no problem coping with thousands of
bos@217: people working together on a single project, which might consist of
bos@217: hundreds of thousands of files.
bos@217: 
bos@217: \subsection{Why use revision control?}
bos@217: 
bos@217: There are a number of reasons why you or your team might want to use
bos@217: an automated revision control tool for a project.
bos@217: \begin{itemize}
bos@217: \item The software gives you a unified way of working with your
bos@218:   project's files.
bos@218: \item When you're working with other people, it makes it easier for
bos@218:   you to collaborate.  For example, when people more or less
bos@218:   simultaneously make potentially incompatible changes, the software
bos@218:   will help you to identify and resolve those conflicts.
bos@217: \item It will track the history of your project.  For every change,
bos@217:   you'll have a log of \emph{who} made it; \emph{why} they made it;
bos@217:   \emph{when} they made it; and \emph{what} the change was.
bos@217: \item It can help you to recover from mistakes.  If you make a change
bos@217:   that later turns out to be in error, you can revert to an earlier
bos@217:   version of one or more files.  In fact, a \emph{really} good
bos@217:   revision control tool will even help you to efficiently figure out
bos@217:   exactly when a problem was introduced (see
bos@217:   section~\ref{sec:undo:bisect} for details).
bos@218: \item It will help you to work simultaneously on, and manage the drift
bos@218:   between, multiple versions of your project.
bos@217: \end{itemize}
bos@218: Most of these reasons are equally valid---at least in theory---whether
bos@218: you're working on a project by yourself, or with a hundred other
bos@218: people.
bos@218: 
bos@218: A key question about the practicality of revision control at these two
bos@218: different scales (``lone hacker'' and ``huge team'') is how its
bos@218: \emph{benefits} compare to its \emph{costs}.  A revision control tool
bos@218: that's difficult to understand or use is going to impose a high cost.
bos@218: 
bos@218: For example, a five-hundred-person project is likely to collapse under
bos@218: its own weight almost immediately without a revision control tool and
bos@218: process.  In this case, the cost of using revision control might
bos@218: hardly seem worth considering, since \emph{without} it, failure is
bos@218: almost guaranteed.
bos@218: 
bos@218: On the other hand, a one-person ``quick hack'' might seem like a poor
bos@218: place to use a revision control tool, because surely the cost of using
bos@218: one must be close to the overall cost of the project.  Right?
bos@218: 
bos@218: Mercurial uniquely supports \emph{both} of these scales of
bos@218: development.  You can learn the basics in just a few minutes, and due
bos@218: to its low overhead, you can apply revision control to the smallest of
bos@218: projects with ease.  Its simplicity means you won't have a lot of
bos@218: abstruse concepts or command sequences competing for mental space with
bos@218: whatever you're \emph{really} trying to do.  At the same time,
bos@218: Mercurial's high performance and peer-to-peer nature let you scale
bos@218: painlessly to handle large projects.
bos@217: 
bos@217: \subsection{The many names of revision control}
bos@217: 
bos@217: Revision control is a diverse field, so much so that it doesn't
bos@217: actually have a single name or acronym.  Here are a few of the more
bos@217: common names and acronyms you'll encounter:
bos@217: \begin{itemize}
bos@217: \item Configuration management (CM)
bos@217: \item Revision control (RCS)
bos@217: \item Software configuration management (SCM)
bos@218: \item Source code management
bos@218: \item Source control
bos@217: \item Version control (VCS)
bos@217: \end{itemize}
bos@217: Some people claim that these terms actually have different meanings,
bos@217: but in practice they overlap so much that there's no agreed or even
bos@217: useful way to tease them apart.
bos@155: 
bos@218: \section{A short history and hierarchy of revision control}
bos@155: 
bos@218: The best known of the old-time revision control tools is SCCS (Source
bos@218: Code Control System), which Marc Rochkind wrote at Bell Labs, in the
bos@218: early 1970s.  SCCS operated on individual files, and required every
bos@218: person working on a project to have access to a shared workspace on a
bos@218: single system.  Only one person could modify a file at any time;
bos@218: arbitration for access to files was via locks.  It was common for
bos@218: people to lock files, and later forget to unlock them, preventing
bos@218: anyone else from modifying those files without the help of an
bos@218: administrator.  
bos@218: 
bos@218: Walter Tichy developed a free alternative to SCCS in the early 1980s;
bos@218: he called his program RCS (Revison Control System).  Like SCCS, RCS
bos@218: required developers to work in a single shared workspace, and to lock
bos@218: files to prevent multiple people from modifying them simultaneously.
bos@218: 
bos@218: Later in the 1980s, Dick Grune used RCS as a building block for a set
bos@218: of shell scripts he initially called cmt, but then renamed to CVS
bos@218: (Concurrent Versions System).  The big innovation of CVS was that it
bos@218: let developers work simultaneously and somewhat independently in their
bos@218: own personal workspaces.  The personal workspaces prevented developers
bos@218: from stepping on each other's toes all the time, as was common with
bos@218: SCCS and RCS.  Each developer had a copy of every project file, and
bos@218: could modify their copies independently.  They had to merge their
bos@218: edits prior to committing changes to the central repository.
bos@218: 
bos@218: Brian Berliner took Grune's original scripts and rewrote them in~C,
bos@218: releasing in 1989 the code that has since developed into the modern
bos@218: version of CVS.  CVS subsequently acquired the ability to operate over
bos@218: a network connection, giving it a client/server architecture.  CVS's
bos@218: architecture is centralised; only the server has a copy of the history
bos@218: of the project.  Client workspaces just contain copies of recent
bos@218: versions of the project's files, and a little metadata to tell them
bos@218: where the server is.  CVS has been enormously successful; it is
bos@218: probably the world's most widely used revision control system.
bos@218: 
bos@218: In the early 1990s, Sun Microsystems developed an early distributed
bos@218: revision control system, called TeamWare.  A TeamWare workspace
bos@218: contains a complete copy of the project's history.  TeamWare has no
bos@218: notion of a central repository.  (CVS relied upon RCS for its history
bos@218: storage; TeamWare used SCCS.)
bos@218: 
bos@218: As the 1990s progressed, awareness grew of a number of problems with
bos@218: CVS.  It records simultaneous changes to multiple files individually,
bos@218: instead of grouping them together as a single logically atomic
bos@218: operation.  It does not manage its file hierarchy well; it is easy to
bos@218: make a mess of a repository by renaming files and directories.  Worse,
bos@218: its source code is difficult to read and maintain, which made the
bos@218: ``pain level'' of fixing these architectural problems prohibitive.
bos@218: 
bos@218: In 2001, Jim Blandy and Karl Fogel, two developers who had worked on
bos@218: CVS, started a project to replace it with a tool that would have a
bos@218: better architecture and cleaner code.  The result, Subversion, does
bos@218: not stray from CVS's centralised client/server model, but it adds
bos@218: multi-file atomic commits, better namespace management, and a number
bos@218: of other features that make it a generally better tool than CVS.
bos@218: Since its initial release, it has rapidly grown in popularity.
bos@218: 
bos@218: More or less simultaneously, Graydon Hoare began working on an
bos@218: ambitious distributed revision control system that he named Monotone.
bos@218: While Monotone addresses many of CVS's design flaws and has a
bos@218: peer-to-peer architecture, it goes beyond earlier (and subsequent)
bos@218: revision control tools in a number of innovative ways.  It uses
bos@218: cryptographic hashes as identifiers, and has an integral notion of
bos@218: ``trust'' for code from different sources.
bos@218: 
bos@218: Mercurial began life in 2005.  While a few aspects of its design are
bos@218: influenced by Monotone, Mercurial focuses on ease of use, high
bos@218: performance, and scalability to very large projects.
bos@155: 
bos@155: \subsection{On a single system}
bos@155: 
bos@155: \subsection{Network-based, but centralised}
bos@155: 
bos@155: \subsection{Fully distributed}
bos@155: 
bos@155: 
bos@155: \section{Advantages of distributed revision control}
bos@155: 
bos@155: \subsection{For open source projects}
bos@155: 
bos@155: \subsection{For commercial projects}
bos@155: 
bos@155: \subsection{Myths about distributed revision control}
bos@155: 
bos@155: \section{Why choose Mercurial?}
bos@155: 
bos@16: 
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