bos@108: \chapter{Behind the scenes}
jeffpc@56: \label{chap:concepts}
jeffpc@56: 
bos@108: Unlike many revision control systems, the concepts upon which
bos@108: Mercurial is built are simple enough that it's easy to understand how
bos@108: the software really works.  Knowing this certainly isn't necessary,
bos@108: but I find it useful to have a ``mental model'' of what's going on.
jeffpc@56: 
bos@109: This understanding gives me confidence that Mercurial has been
bos@109: carefully designed to be both \emph{safe} and \emph{efficient}.  And
bos@111: just as importantly, if it's easy for me to retain a good idea of what
bos@111: the software is doing when I perform a revision control task, I'm less
bos@111: likely to be surprised by its behaviour.
bos@109: 
bos@112: In this chapter, we'll initially cover the core concepts behind
bos@112: Mercurial's design, then continue to discuss some of the interesting
bos@112: details of its implementation.
bos@112: 
bos@109: \section{Mercurial's historical record}
bos@109: 
bos@109: \subsection{Tracking the history of a single file}
jeffpc@56: 
bos@108: When Mercurial tracks modifications to a file, it stores the history
bos@108: of that file in a metadata object called a \emph{filelog}.  Each entry
bos@108: in the filelog contains enough information to reconstruct one revision
bos@108: of the file that is being tracked.  Filelogs are stored as files in
bos@108: the \sdirname{.hg/data} directory.  A filelog contains two kinds of
bos@108: information: revision data, and an index to help Mercurial to find a
bos@108: revision efficiently.  
jeffpc@56: 
bos@109: A file that is large, or has a lot of history, has its filelog stored
bos@109: in separate data (``\texttt{.d}'' suffix) and index (``\texttt{.i}''
bos@109: suffix) files.  For small files without much history, the revision
bos@109: data and index are combined in a single ``\texttt{.i}'' file.  The
bos@109: correspondence between a file in the working directory and the filelog
bos@109: that tracks its history in the repository is illustrated in
bos@109: figure~\ref{fig:concepts:filelog}.
jeffpc@56: 
jeffpc@56: \begin{figure}[ht]
bos@108:   \centering
bos@108:   \grafix{filelog}
bos@108:   \caption{Relationships between files in working directory and
bos@108:     filelogs in repository}
bos@108:   \label{fig:concepts:filelog}
jeffpc@56: \end{figure}
jeffpc@56: 
bos@109: \subsection{Managing tracked files}
bos@109: 
bos@109: Mercurial uses a structure called a \emph{manifest} to collect
bos@109: together information about the files that it tracks.  Each entry in
bos@109: the manifest contains information about the files present in a single
bos@109: changeset.  An entry records which files are present in the changeset,
bos@109: the revision of each file, and a few other pieces of file metadata.
bos@109: 
bos@109: \subsection{Recording changeset information}
bos@109: 
bos@109: The \emph{changelog} contains information about each changeset.  Each
bos@109: revision records who committed a change, the changeset comment, other
bos@109: pieces of changeset-related information, and the revision of the
bos@109: manifest to use.
bos@109: 
bos@109: \subsection{Relationships between revisions}
bos@109: 
bos@109: Within a changelog, a manifest, or a filelog, each revision stores a
bos@109: pointer to its immediate parent (or to its two parents, if it's a
bos@109: merge revision).  As I mentioned above, there are also relationships
bos@109: between revisions \emph{across} these structures, and they are
bos@109: hierarchical in nature.
bos@109: 
bos@109: For every changeset in a repository, there is exactly one revision
bos@109: stored in the changelog.  Each revision of the changelog contains a
bos@109: pointer to a single revision of the manifest.  A revision of the
bos@109: manifest stores a pointer to a single revision of each filelog tracked
bos@109: when that changeset was created.  These relationships are illustrated
bos@109: in figure~\ref{fig:concepts:metadata}.
bos@109: 
bos@109: \begin{figure}[ht]
bos@109:   \centering
bos@109:   \grafix{metadata}
bos@109:   \caption{Metadata relationships}
bos@109:   \label{fig:concepts:metadata}
bos@109: \end{figure}
bos@109: 
bos@110: As the illustration shows, there is \emph{not} a ``one to one''
bos@110: relationship between revisions in the changelog, manifest, or filelog.
bos@110: If the manifest hasn't changed between two changesets, the changelog
bos@110: entries for those changesets will point to the same revision of the
bos@110: manifest.  If a file that Mercurial tracks hasn't changed between two
bos@110: changesets, the entry for that file in the two revisions of the
bos@110: manifest will point to the same revision of its filelog.
bos@110: 
bos@110: \section{Safe, efficient storage}
bos@109: 
bos@109: The underpinnings of changelogs, manifests, and filelogs are provided
bos@109: by a single structure called the \emph{revlog}.
bos@109: 
bos@109: \subsection{Efficient storage}
bos@109: 
bos@109: The revlog provides efficient storage of revisions using a
bos@109: \emph{delta} mechanism.  Instead of storing a complete copy of a file
bos@109: for each revision, it stores the changes needed to transform an older
bos@109: revision into the new revision.  For many kinds of file data, these
bos@109: deltas are typically a fraction of a percent of the size of a full
bos@109: copy of a file.
bos@109: 
bos@109: Some obsolete revision control systems can only work with deltas of
bos@109: text files.  They must either store binary files as complete snapshots
bos@109: or encoded into a text representation, both of which are wasteful
bos@109: approaches.  Mercurial can efficiently handle deltas of files with
bos@109: arbitrary binary contents; it doesn't need to treat text as special.
bos@109: 
bos@109: \subsection{Safe operation}
bos@121: \label{sec:concepts:txn}
bos@109: 
bos@109: Mercurial only ever \emph{appends} data to the end of a revlog file.
bos@109: It never modifies a section of a file after it has written it.  This
bos@109: is both more robust and efficient than schemes that need to modify or
bos@109: rewrite data.
bos@109: 
bos@109: In addition, Mercurial treats every write as part of a
bos@109: \emph{transaction} that can span a number of files.  A transaction is
bos@109: \emph{atomic}: either the entire transaction succeeds and its effects
bos@109: are all visible to readers in one go, or the whole thing is undone.
bos@109: This guarantee of atomicity means that if you're running two copies of
bos@109: Mercurial, where one is reading data and one is writing it, the reader
bos@109: will never see a partially written result that might confuse it.
bos@109: 
bos@109: The fact that Mercurial only appends to files makes it easier to
bos@109: provide this transactional guarantee.  The easier it is to do stuff
bos@109: like this, the more confident you should be that it's done correctly.
bos@109: 
bos@109: \subsection{Fast retrieval}
bos@109: 
bos@109: Mercurial cleverly avoids a pitfall common to all earlier
bos@109: revision control systems: the problem of \emph{inefficient retrieval}.
bos@109: Most revision control systems store the contents of a revision as an
bos@109: incremental series of modifications against a ``snapshot''.  To
bos@109: reconstruct a specific revision, you must first read the snapshot, and
bos@109: then every one of the revisions between the snapshot and your target
bos@109: revision.  The more history that a file accumulates, the more
bos@109: revisions you must read, hence the longer it takes to reconstruct a
bos@109: particular revision.
bos@109: 
bos@110: \begin{figure}[ht]
bos@110:   \centering
bos@110:   \grafix{snapshot}
bos@110:   \caption{Snapshot of a revlog, with incremental deltas}
bos@110:   \label{fig:concepts:snapshot}
bos@110: \end{figure}
bos@110: 
bos@109: The innovation that Mercurial applies to this problem is simple but
bos@109: effective.  Once the cumulative amount of delta information stored
bos@109: since the last snapshot exceeds a fixed threshold, it stores a new
bos@109: snapshot (compressed, of course), instead of another delta.  This
bos@109: makes it possible to reconstruct \emph{any} revision of a file
bos@110: quickly.  This approach works so well that it has since been copied by
bos@110: several other revision control systems.
bos@110: 
bos@110: Figure~\ref{fig:concepts:snapshot} illustrates the idea.  In an entry
bos@110: in a revlog's index file, Mercurial stores the range of entries from
bos@110: the data file that it must read to reconstruct a particular revision.
bos@109: 
bos@109: \subsubsection{Aside: the influence of video compression}
bos@109: 
bos@109: If you're familiar with video compression or have ever watched a TV
bos@109: feed through a digital cable or satellite service, you may know that
bos@109: most video compression schemes store each frame of video as a delta
bos@109: against its predecessor frame.  In addition, these schemes use
bos@109: ``lossy'' compression techniques to increase the compression ratio, so
bos@109: visual errors accumulate over the course of a number of inter-frame
bos@109: deltas.
bos@109: 
bos@109: Because it's possible for a video stream to ``drop out'' occasionally
bos@109: due to signal glitches, and to limit the accumulation of artefacts
bos@109: introduced by the lossy compression process, video encoders
bos@109: periodically insert a complete frame (called a ``key frame'') into the
bos@109: video stream; the next delta is generated against that frame.  This
bos@109: means that if the video signal gets interrupted, it will resume once
bos@109: the next key frame is received.  Also, the accumulation of encoding
bos@109: errors restarts anew with each key frame.
bos@109: 
bos@112: \subsection{Identification and strong integrity}
bos@109: 
bos@109: Along with delta or snapshot information, a revlog entry contains a
bos@109: cryptographic hash of the data that it represents.  This makes it
bos@109: difficult to forge the contents of a revision, and easy to detect
bos@112: accidental corruption.  
bos@112: 
bos@112: Hashes provide more than a mere check against corruption; they are
bos@112: used as the identifiers for revisions.  The changeset identification
bos@111: hashes that you see as an end user are from revisions of the
bos@112: changelog.  Although filelogs and the manifest also use hashes,
bos@112: Mercurial only uses these behind the scenes.
bos@112: 
bos@112: Mercurial verifies that hashes are correct when it retrieves file
bos@112: revisions and when it pulls changes from another repository.  If it
bos@112: encounters an integrity problem, it will complain and stop whatever
bos@112: it's doing.
bos@109: 
bos@109: In addition to the effect it has on retrieval efficiency, Mercurial's
bos@109: use of periodic snapshots makes it more robust against partial data
bos@109: corruption.  If a revlog becomes partly corrupted due to a hardware
bos@109: error or system bug, it's often possible to reconstruct some or most
bos@109: revisions from the uncorrupted sections of the revlog, both before and
bos@109: after the corrupted section.  This would not be possible with a
bos@109: delta-only storage model.
bos@109: 
bos@115: \section{Revision history, branching,
bos@115:   and merging}
bos@115: 
bos@115: Every entry in a Mercurial revlog knows the identity of its immediate
bos@115: ancestor revision, usually referred to as its \emph{parent}.  In fact,
bos@115: a revision contains room for not one parent, but two.  Mercurial uses
bos@115: a special hash, called the ``null ID'', to represent the idea ``there
bos@115: is no parent here''.  This hash is simply a string of zeroes.
bos@115: 
bos@115: In figure~\ref{fig:concepts:revlog}, you can see an example of the
bos@115: conceptual structure of a revlog.  Filelogs, manifests, and changelogs
bos@115: all have this same structure; they differ only in the kind of data
bos@115: stored in each delta or snapshot.
bos@115: 
bos@115: The first revision in a revlog (at the bottom of the image) has the
bos@115: null ID in both of its parent slots.  For a ``normal'' revision, its
bos@115: first parent slot contains the ID of its parent revision, and its
bos@115: second contains the null ID, indicating that the revision has only one
bos@115: real parent.  Any two revisions that have the same parent ID are
bos@115: branches.  A revision that represents a merge between branches has two
bos@115: normal revision IDs in its parent slots.
bos@115: 
bos@115: \begin{figure}[ht]
bos@115:   \centering
bos@115:   \grafix{revlog}
bos@115:   \caption{}
bos@115:   \label{fig:concepts:revlog}
bos@115: \end{figure}
bos@115: 
bos@110: \section{The working directory}
bos@110: 
bos@113: In the working directory, Mercurial stores a snapshot of the files
bos@113: from the repository as of a particular changeset.
bos@113: 
bos@113: The working directory ``knows'' which changeset it contains.  When you
bos@113: update the working directory to contain a particular changeset,
bos@113: Mercurial looks up the appropriate revision of the manifest to find
bos@113: out which files it was tracking at the time that changeset was
bos@113: committed, and which revision of each file was then current.  It then
bos@113: recreates a copy of each of those files, with the same contents it had
bos@113: when the changeset was committed.
bos@113: 
bos@113: The \emph{dirstate} contains Mercurial's knowledge of the working
bos@113: directory.  This details which changeset the working directory is
bos@113: updated to, and all of the files that Mercurial is tracking in the
bos@113: working directory.
bos@113: 
bos@113: Just as a revision of a revlog has room for two parents, so that it
bos@113: can represent either a normal revision (with one parent) or a merge of
bos@113: two earlier revisions, the dirstate has slots for two parents.  When
bos@113: you use the \hgcmd{update} command, the changeset that you update to
bos@113: is stored in the ``first parent'' slot, and the null ID in the second.
bos@113: When you \hgcmd{merge} with another changeset, the first parent
bos@113: remains unchanged, and the second parent is filled in with the
bos@113: changeset you're merging with.  The \hgcmd{parents} command tells you
bos@113: what the parents of the dirstate are.
bos@113: 
bos@113: \subsection{What happens when you commit}
bos@113: 
bos@113: The dirstate stores parent information for more than just book-keeping
bos@113: purposes.  Mercurial uses the parents of the dirstate as \emph{the
bos@113:   parents of a new changeset} when you perform a commit.
bos@113: 
bos@113: \begin{figure}[ht]
bos@113:   \centering
bos@113:   \grafix{wdir}
bos@113:   \caption{The working directory can have two parents}
bos@113:   \label{fig:concepts:wdir}
bos@113: \end{figure}
bos@113: 
bos@113: Figure~\ref{fig:concepts:wdir} shows the normal state of the working
bos@113: directory, where it has a single changeset as parent.  That changeset
bos@113: is the \emph{tip}, the newest changeset in the repository that has no
bos@113: children.
bos@113: 
bos@113: \begin{figure}[ht]
bos@113:   \centering
bos@113:   \grafix{wdir-after-commit}
bos@113:   \caption{The working directory gains new parents after a commit}
bos@113:   \label{fig:concepts:wdir-after-commit}
bos@113: \end{figure}
bos@113: 
bos@113: It's useful to think of the working directory as ``the changeset I'm
bos@113: about to commit''.  Any files that you tell Mercurial that you've
bos@113: added, removed, renamed, or copied will be reflected in that
bos@113: changeset, as will modifications to any files that Mercurial is
bos@113: already tracking; the new changeset will have the parents of the
bos@113: working directory as its parents.
bos@113: 
bos@113: After a commit, Mercurial will update the parents of the working
bos@113: directory, so that the first parent is the ID of the new changeset,
bos@115: and the second is the null ID.  This is shown in
bos@115: figure~\ref{fig:concepts:wdir-after-commit}.  Mercurial doesn't touch
bos@115: any of the files in the working directory when you commit; it just
bos@115: modifies the dirstate to note its new parents.
bos@115: 
bos@115: \subsection{Creating a new head}
bos@115: 
bos@115: It's perfectly normal to update the working directory to a changeset
bos@115: other than the current tip.  For example, you might want to know what
bos@115: your project looked like last Tuesday, or you could be looking through
bos@115: changesets to see which one introduced a bug.  In cases like this, the
bos@115: natural thing to do is update the working directory to the changeset
bos@115: you're interested in, and then examine the files in the working
bos@115: directory directly to see their contents as they werea when you
bos@115: committed that changeset.  The effect of this is shown in
bos@115: figure~\ref{fig:concepts:wdir-pre-branch}.
bos@115: 
bos@115: \begin{figure}[ht]
bos@115:   \centering
bos@115:   \grafix{wdir-pre-branch}
bos@115:   \caption{The working directory, updated to an older changeset}
bos@115:   \label{fig:concepts:wdir-pre-branch}
bos@115: \end{figure}
bos@115: 
bos@115: Having updated the working directory to an older changeset, what
bos@115: happens if you make some changes, and then commit?  Mercurial behaves
bos@115: in the same way as I outlined above.  The parents of the working
bos@115: directory become the parents of the new changeset.  This new changeset
bos@115: has no children, so it becomes the new tip.  And the repository now
bos@115: contains two changesets that have no children; we call these
bos@115: \emph{heads}.  You can see the structure that this creates in
bos@115: figure~\ref{fig:concepts:wdir-branch}.
bos@115: 
bos@115: \begin{figure}[ht]
bos@115:   \centering
bos@115:   \grafix{wdir-branch}
bos@115:   \caption{After a commit made while synced to an older changeset}
bos@115:   \label{fig:concepts:wdir-branch}
bos@115: \end{figure}
bos@115: 
bos@115: \begin{note}
bos@115:   If you're new to Mercurial, you should keep in mind a common
bos@115:   ``error'', which is to use the \hgcmd{pull} command without any
bos@115:   options.  By default, the \hgcmd{pull} command \emph{does not}
bos@115:   update the working directory, so you'll bring new changesets into
bos@115:   your repository, but the working directory will stay synced at the
bos@115:   same changeset as before the pull.  If you make some changes and
bos@115:   commit afterwards, you'll thus create a new head, because your
bos@115:   working directory isn't synced to whatever the current tip is.
bos@115: 
bos@115:   I put the word ``error'' in quotes because all that you need to do
bos@115:   to rectify this situation is \hgcmd{merge}, then \hgcmd{commit}.  In
bos@115:   other words, this almost never has negative consequences; it just
bos@115:   surprises people.  I'll discuss other ways to avoid this behaviour,
bos@115:   and why Mercurial behaves in this initially surprising way, later
bos@115:   on.
bos@115: \end{note}
bos@115: 
bos@115: \subsection{Merging heads}
bos@115: 
bos@115: When you run the \hgcmd{merge} command, Mercurial leaves the first
bos@115: parent of the working directory unchanged, and sets the second parent
bos@115: to the changeset you're merging with, as shown in
bos@115: figure~\ref{fig:concepts:wdir-merge}.
bos@115: 
bos@115: \begin{figure}[ht]
bos@115:   \centering
bos@115:   \grafix{wdir-merge}
bos@115:   \caption{Merging two hehads}
bos@115:   \label{fig:concepts:wdir-merge}
bos@115: \end{figure}
bos@115: 
bos@115: Mercurial also has to modify the working directory, to merge the files
bos@115: managed in the two changesets.  Simplified a little, the merging
bos@115: process goes like this, for every file in the manifests of both
bos@115: changesets.
bos@115: \begin{itemize}
bos@115: \item If neither changeset has modified a file, do nothing with that
bos@115:   file.
bos@115: \item If one changeset has modified a file, and the other hasn't,
bos@115:   create the modified copy of the file in the working directory.
bos@115: \item If one changeset has removed a file, and the other hasn't (or
bos@115:   has also deleted it), delete the file from the working directory.
bos@115: \item If one changeset has removed a file, but the other has modified
bos@115:   the file, ask the user what to do: keep the modified file, or remove
bos@115:   it?
bos@115: \item If both changesets have modified a file, invoke an external
bos@115:   merge program to choose the new contents for the merged file.  This
bos@115:   may require input from the user.
bos@115: \item If one changeset has modified a file, and the other has renamed
bos@115:   or copied the file, make sure that the changes follow the new name
bos@115:   of the file.
bos@115: \end{itemize}
bos@115: There are more details---merging has plenty of corner cases---but
bos@115: these are the most common choices that are involved in a merge.  As
bos@115: you can see, most cases are completely automatic, and indeed most
bos@115: merges finish automatically, without requiring your input to resolve
bos@115: any conflicts.
bos@115: 
bos@115: When you're thinking about what happens when you commit after a merge,
bos@115: once again the working directory is ``the changeset I'm about to
bos@115: commit''.  After the \hgcmd{merge} command completes, the working
bos@115: directory has two parents; these will become the parents of the new
bos@115: changeset.
bos@115: 
bos@115: Mercurial lets you perform multiple merges, but you must commit the
bos@115: results of each individual merge as you go.  This is necessary because
bos@115: Mercurial only tracks two parents for both revisions and the working
bos@115: directory.  While it would be technically possible to merge multiple
bos@115: changesets at once, the prospect of user confusion and making a
bos@115: terrible mess of a merge immediately becomes overwhelming.
bos@112: 
bos@110: \section{Other interesting design features}
bos@110: 
bos@110: In the sections above, I've tried to highlight some of the most
bos@110: important aspects of Mercurial's design, to illustrate that it pays
bos@110: careful attention to reliability and performance.  However, the
bos@110: attention to detail doesn't stop there.  There are a number of other
bos@110: aspects of Mercurial's construction that I personally find
bos@110: interesting.  I'll detail a few of them here, separate from the ``big
bos@110: ticket'' items above, so that if you're interested, you can gain a
bos@110: better idea of the amount of thinking that goes into a well-designed
bos@110: system.
bos@110: 
bos@110: \subsection{Clever compression}
bos@110: 
bos@110: When appropriate, Mercurial will store both snapshots and deltas in
bos@110: compressed form.  It does this by always \emph{trying to} compress a
bos@110: snapshot or delta, but only storing the compressed version if it's
bos@110: smaller than the uncompressed version.
bos@110: 
bos@110: This means that Mercurial does ``the right thing'' when storing a file
bos@110: whose native form is compressed, such as a \texttt{zip} archive or a
bos@110: JPEG image.  When these types of files are compressed a second time,
bos@110: the resulting file is usually bigger than the once-compressed form,
bos@110: and so Mercurial will store the plain \texttt{zip} or JPEG.
bos@110: 
bos@110: Deltas between revisions of a compressed file are usually larger than
bos@110: snapshots of the file, and Mercurial again does ``the right thing'' in
bos@110: these cases.  It finds that such a delta exceeds the threshold at
bos@110: which it should store a complete snapshot of the file, so it stores
bos@110: the snapshot, again saving space compared to a naive delta-only
bos@110: approach.
bos@110: 
bos@110: \subsubsection{Network recompression}
bos@110: 
bos@110: When storing revisions on disk, Mercurial uses the ``deflate''
bos@110: compression algorithm (the same one used by the popular \texttt{zip}
bos@110: archive format), which balances good speed with a respectable
bos@110: compression ratio.  However, when transmitting revision data over a
bos@110: network connection, Mercurial uncompresses the compressed revision
bos@110: data.
bos@110: 
bos@110: If the connection is over HTTP, Mercurial recompresses the entire
bos@110: stream of data using a compression algorithm that gives a etter
bos@110: compression ratio (the Burrows-Wheeler algorithm from the widely used
bos@110: \texttt{bzip2} compression package).  This combination of algorithm
bos@110: and compression of the entire stream (instead of a revision at a time)
bos@110: substantially reduces the number of bytes to be transferred, yielding
bos@110: better network performance over almost all kinds of network.
bos@110: 
bos@110: (If the connection is over \command{ssh}, Mercurial \emph{doesn't}
bos@110: recompress the stream, because \command{ssh} can already do this
bos@110: itself.)
bos@110: 
bos@109: \subsection{Read/write ordering and atomicity}
bos@109: 
bos@109: Appending to files isn't the whole story when it comes to guaranteeing
bos@109: that a reader won't see a partial write.  If you recall
bos@109: figure~\ref{fig:concepts:metadata}, revisions in the changelog point to
bos@109: revisions in the manifest, and revisions in the manifest point to
bos@109: revisions in filelogs.  This hierarchy is deliberate.
bos@109: 
bos@109: A writer starts a transaction by writing filelog and manifest data,
bos@109: and doesn't write any changelog data until those are finished.  A
bos@109: reader starts by reading changelog data, then manifest data, followed
bos@109: by filelog data.
bos@109: 
bos@109: Since the writer has always finished writing filelog and manifest data
bos@109: before it writes to the changelog, a reader will never read a pointer
bos@109: to a partially written manifest revision from the changelog, and it will
bos@109: never read a pointer to a partially written filelog revision from the
bos@109: manifest.
bos@109: 
bos@109: \subsection{Concurrent access}
bos@109: 
bos@109: The read/write ordering and atomicity guarantees mean that Mercurial
bos@109: never needs to \emph{lock} a repository when it's reading data, even
bos@109: if the repository is being written to while the read is occurring.
bos@109: This has a big effect on scalability; you can have an arbitrary number
bos@109: of Mercurial processes safely reading data from a repository safely
bos@109: all at once, no matter whether it's being written to or not.
bos@109: 
bos@109: The lockless nature of reading means that if you're sharing a
bos@109: repository on a multi-user system, you don't need to grant other local
bos@109: users permission to \emph{write} to your repository in order for them
bos@109: to be able to clone it or pull changes from it; they only need
bos@109: \emph{read} permission.  (This is \emph{not} a common feature among
bos@109: revision control systems, so don't take it for granted!  Most require
bos@109: readers to be able to lock a repository to access it safely, and this
bos@109: requires write permission on at least one directory, which of course
bos@109: makes for all kinds of nasty and annoying security and administrative
bos@109: problems.)
bos@109: 
bos@110: Mercurial uses locks to ensure that only one process can write to a
bos@110: repository at a time (the locking mechanism is safe even over
bos@110: filesystems that are notoriously hostile to locking, such as NFS).  If
bos@110: a repository is locked, a writer will wait for a while to retry if the
bos@110: repository becomes unlocked, but if the repository remains locked for
bos@110: too long, the process attempting to write will time out after a while.
bos@110: This means that your daily automated scripts won't get stuck forever
bos@110: and pile up if a system crashes unnoticed, for example.  (Yes, the
bos@110: timeout is configurable, from zero to infinity.)
bos@110: 
bos@110: \subsubsection{Safe dirstate access}
bos@110: 
bos@110: As with revision data, Mercurial doesn't take a lock to read the
bos@110: dirstate file; it does acquire a lock to write it.  To avoid the
bos@110: possibility of reading a partially written copy of the dirstate file,
bos@110: Mercurial writes to a file with a unique name in the same directory as
bos@110: the dirstate file, then renames the temporary file atomically to
bos@110: \filename{dirstate}.  The file named \filename{dirstate} is thus
bos@110: guaranteed to be complete, not partially written.
bos@109: 
bos@111: \subsection{Avoiding seeks}
bos@111: 
bos@111: Critical to Mercurial's performance is the avoidance of seeks of the
bos@111: disk head, since any seek is far more expensive than even a
bos@111: comparatively large read operation.
bos@111: 
bos@111: This is why, for example, the dirstate is stored in a single file.  If
bos@111: there were a dirstate file per directory that Mercurial tracked, the
bos@111: disk would seek once per directory.  Instead, Mercurial reads the
bos@111: entire single dirstate file in one step.
bos@111: 
bos@111: Mercurial also uses a ``copy on write'' scheme when cloning a
bos@111: repository on local storage.  Instead of copying every revlog file
bos@111: from the old repository into the new repository, it makes a ``hard
bos@111: link'', which is a shorthand way to say ``these two names point to the
bos@111: same file''.  When Mercurial is about to write to one of a revlog's
bos@111: files, it checks to see if the number of names pointing at the file is
bos@111: greater than one.  If it is, more than one repository is using the
bos@111: file, so Mercurial makes a new copy of the file that is private to
bos@111: this repository.
bos@111: 
bos@111: A few revision control developers have pointed out that this idea of
bos@111: making a complete private copy of a file is not very efficient in its
bos@111: use of storage.  While this is true, storage is cheap, and this method
bos@111: gives the highest performance while deferring most book-keeping to the
bos@111: operating system.  An alternative scheme would most likely reduce
bos@111: performance and increase the complexity of the software, each of which
bos@111: is much more important to the ``feel'' of day-to-day use.
bos@109: 
bos@115: \subsection{Other contents of the dirstate}
bos@115: 
bos@115: Because Mercurial doesn't force you to tell it when you're modifying a
bos@115: file, it uses the dirstate to store some extra information so it can
bos@115: determine efficiently whether you have modified a file.  For each file
bos@115: in the working directory, it stores the time that it last modified the
bos@115: file itself, and the size of the file at that time.  
bos@115: 
bos@115: When you explicitly \hgcmd{add}, \hgcmd{remove}, \hgcmd{rename} or
bos@116: \hgcmd{copy} files, Mercurial updates the dirstate so that it knows
bos@116: what to do with those files when you commit.
bos@115: 
bos@115: When Mercurial is checking the states of files in the working
bos@115: directory, it first checks a file's modification time.  If that has
bos@115: not changed, the file must not have been modified.  If the file's size
bos@115: has changed, the file must have been modified.  If the modification
bos@115: time has changed, but the size has not, only then does Mercurial need
bos@115: to read the actual contents of the file to see if they've changed.
bos@115: Storing these few extra pieces of information dramatically reduces the
bos@115: amount of data that Mercurial needs to read, which yields large
bos@115: performance improvements compared to other revision control systems.
bos@115: 
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