bos@559: <!-- vim: set filetype=docbkxml shiftwidth=2 autoindent expandtab tw=77 : -->
bos@559:
bos@559: <chapter id="chap:concepts">
bos@572: <?dbhtml filename="behind-the-scenes.html"?>
bos@559: <title>Behind the scenes</title>
bos@559:
bos@620: <para id="x_2e8">Unlike many revision control systems, the concepts
bos@620: upon which Mercurial is built are simple enough that it's easy to
bos@620: understand how the software really works. Knowing these details
bos@620: certainly isn't necessary, so it is certainly safe to skip this
bos@620: chapter. However, I think you will get more out of the software
bos@620: with a <quote>mental model</quote> of what's going on.</para>
bos@620:
bos@620: <para id="x_2e9">Being able to understand what's going on behind the
bos@620: scenes gives me confidence that Mercurial has been carefully
bos@620: designed to be both <emphasis>safe</emphasis> and
bos@559: <emphasis>efficient</emphasis>. And just as importantly, if it's
bos@559: easy for me to retain a good idea of what the software is doing
bos@559: when I perform a revision control task, I'm less likely to be
bos@672: surprised by its behavior.</para>
bos@559:
bos@584: <para id="x_2ea">In this chapter, we'll initially cover the core concepts
bos@559: behind Mercurial's design, then continue to discuss some of the
bos@559: interesting details of its implementation.</para>
bos@559:
bos@559: <sect1>
bos@559: <title>Mercurial's historical record</title>
bos@559:
bos@559: <sect2>
bos@559: <title>Tracking the history of a single file</title>
bos@559:
bos@584: <para id="x_2eb">When Mercurial tracks modifications to a file, it stores
bos@559: the history of that file in a metadata object called a
bos@559: <emphasis>filelog</emphasis>. Each entry in the filelog
bos@559: contains enough information to reconstruct one revision of the
bos@559: file that is being tracked. Filelogs are stored as files in
bos@559: the <filename role="special"
bos@559: class="directory">.hg/store/data</filename> directory. A
bos@559: filelog contains two kinds of information: revision data, and
bos@559: an index to help Mercurial to find a revision
bos@559: efficiently.</para>
bos@559:
bos@584: <para id="x_2ec">A file that is large, or has a lot of history, has its
bos@559: filelog stored in separate data
bos@559: (<quote><literal>.d</literal></quote> suffix) and index
bos@559: (<quote><literal>.i</literal></quote> suffix) files. For
bos@559: small files without much history, the revision data and index
bos@559: are combined in a single <quote><literal>.i</literal></quote>
bos@559: file. The correspondence between a file in the working
bos@559: directory and the filelog that tracks its history in the
bos@592: repository is illustrated in <xref
bos@559: linkend="fig:concepts:filelog"/>.</para>
bos@559:
bos@591: <figure id="fig:concepts:filelog">
bos@591: <title>Relationships between files in working directory and
bos@591: filelogs in repository</title>
bos@591: <mediaobject>
bos@594: <imageobject><imagedata fileref="figs/filelog.png"/></imageobject>
bos@591: <textobject><phrase>XXX add text</phrase></textobject>
bos@591: </mediaobject>
bos@591: </figure>
bos@559:
bos@559: </sect2>
bos@559: <sect2>
bos@559: <title>Managing tracked files</title>
bos@559:
bos@584: <para id="x_2ee">Mercurial uses a structure called a
bos@559: <emphasis>manifest</emphasis> to collect together information
bos@559: about the files that it tracks. Each entry in the manifest
bos@559: contains information about the files present in a single
bos@559: changeset. An entry records which files are present in the
bos@559: changeset, the revision of each file, and a few other pieces
bos@559: of file metadata.</para>
bos@559:
bos@559: </sect2>
bos@559: <sect2>
bos@559: <title>Recording changeset information</title>
bos@559:
bos@584: <para id="x_2ef">The <emphasis>changelog</emphasis> contains information
bos@559: about each changeset. Each revision records who committed a
bos@559: change, the changeset comment, other pieces of
bos@559: changeset-related information, and the revision of the
bos@559: manifest to use.</para>
bos@559:
bos@559: </sect2>
bos@559: <sect2>
bos@559: <title>Relationships between revisions</title>
bos@559:
bos@584: <para id="x_2f0">Within a changelog, a manifest, or a filelog, each
bos@559: revision stores a pointer to its immediate parent (or to its
bos@559: two parents, if it's a merge revision). As I mentioned above,
bos@559: there are also relationships between revisions
bos@559: <emphasis>across</emphasis> these structures, and they are
bos@559: hierarchical in nature.</para>
bos@559:
bos@584: <para id="x_2f1">For every changeset in a repository, there is exactly one
bos@559: revision stored in the changelog. Each revision of the
bos@559: changelog contains a pointer to a single revision of the
bos@559: manifest. A revision of the manifest stores a pointer to a
bos@559: single revision of each filelog tracked when that changeset
bos@592: was created. These relationships are illustrated in
bos@559: <xref linkend="fig:concepts:metadata"/>.</para>
bos@559:
bos@591: <figure id="fig:concepts:metadata">
bos@591: <title>Metadata relationships</title>
bos@591: <mediaobject>
bos@594: <imageobject><imagedata fileref="figs/metadata.png"/></imageobject>
bos@591: <textobject><phrase>XXX add text</phrase></textobject>
bos@559: </mediaobject>
bos@591: </figure>
bos@559:
bos@584: <para id="x_2f3">As the illustration shows, there is
bos@559: <emphasis>not</emphasis> a <quote>one to one</quote>
bos@559: relationship between revisions in the changelog, manifest, or
bos@701: filelog. If a file that
bos@559: Mercurial tracks hasn't changed between two changesets, the
bos@559: entry for that file in the two revisions of the manifest will
bos@701: point to the same revision of its filelog<footnote>
bos@702: <para id="x_725">It is possible (though unusual) for the manifest to
bos@701: remain the same between two changesets, in which case the
bos@701: changelog entries for those changesets will point to the
bos@701: same revision of the manifest.</para>
bos@701: </footnote>.</para>
bos@559:
bos@559: </sect2>
bos@559: </sect1>
bos@559: <sect1>
bos@559: <title>Safe, efficient storage</title>
bos@559:
bos@584: <para id="x_2f4">The underpinnings of changelogs, manifests, and filelogs are
bos@559: provided by a single structure called the
bos@559: <emphasis>revlog</emphasis>.</para>
bos@559:
bos@559: <sect2>
bos@559: <title>Efficient storage</title>
bos@559:
bos@584: <para id="x_2f5">The revlog provides efficient storage of revisions using a
bos@559: <emphasis>delta</emphasis> mechanism. Instead of storing a
bos@559: complete copy of a file for each revision, it stores the
bos@559: changes needed to transform an older revision into the new
bos@559: revision. For many kinds of file data, these deltas are
bos@559: typically a fraction of a percent of the size of a full copy
bos@559: of a file.</para>
bos@559:
bos@584: <para id="x_2f6">Some obsolete revision control systems can only work with
bos@559: deltas of text files. They must either store binary files as
bos@559: complete snapshots or encoded into a text representation, both
bos@559: of which are wasteful approaches. Mercurial can efficiently
bos@559: handle deltas of files with arbitrary binary contents; it
bos@559: doesn't need to treat text as special.</para>
bos@559:
bos@559: </sect2>
bos@559: <sect2 id="sec:concepts:txn">
bos@559: <title>Safe operation</title>
bos@559:
bos@584: <para id="x_2f7">Mercurial only ever <emphasis>appends</emphasis> data to
bos@559: the end of a revlog file. It never modifies a section of a
bos@559: file after it has written it. This is both more robust and
bos@559: efficient than schemes that need to modify or rewrite
bos@559: data.</para>
bos@559:
bos@584: <para id="x_2f8">In addition, Mercurial treats every write as part of a
bos@559: <emphasis>transaction</emphasis> that can span a number of
bos@559: files. A transaction is <emphasis>atomic</emphasis>: either
bos@559: the entire transaction succeeds and its effects are all
bos@559: visible to readers in one go, or the whole thing is undone.
bos@559: This guarantee of atomicity means that if you're running two
bos@559: copies of Mercurial, where one is reading data and one is
bos@559: writing it, the reader will never see a partially written
bos@559: result that might confuse it.</para>
bos@559:
bos@584: <para id="x_2f9">The fact that Mercurial only appends to files makes it
bos@559: easier to provide this transactional guarantee. The easier it
bos@559: is to do stuff like this, the more confident you should be
bos@559: that it's done correctly.</para>
bos@559:
bos@559: </sect2>
bos@559: <sect2>
bos@559: <title>Fast retrieval</title>
bos@559:
bos@701: <para id="x_2fa">Mercurial cleverly avoids a pitfall common to
bos@701: all earlier revision control systems: the problem of
bos@701: <emphasis>inefficient retrieval</emphasis>. Most revision
bos@701: control systems store the contents of a revision as an
bos@701: incremental series of modifications against a
bos@701: <quote>snapshot</quote>. (Some base the snapshot on the
bos@701: oldest revision, others on the newest.) To reconstruct a
bos@701: specific revision, you must first read the snapshot, and then
bos@701: every one of the revisions between the snapshot and your
bos@701: target revision. The more history that a file accumulates,
bos@701: the more revisions you must read, hence the longer it takes to
bos@701: reconstruct a particular revision.</para>
bos@559:
bos@591: <figure id="fig:concepts:snapshot">
bos@591: <title>Snapshot of a revlog, with incremental deltas</title>
bos@591: <mediaobject>
bos@594: <imageobject><imagedata fileref="figs/snapshot.png"/></imageobject>
bos@591: <textobject><phrase>XXX add text</phrase></textobject>
bos@591: </mediaobject>
bos@591: </figure>
bos@559:
bos@584: <para id="x_2fc">The innovation that Mercurial applies to this problem is
bos@559: simple but effective. Once the cumulative amount of delta
bos@559: information stored since the last snapshot exceeds a fixed
bos@559: threshold, it stores a new snapshot (compressed, of course),
bos@559: instead of another delta. This makes it possible to
bos@559: reconstruct <emphasis>any</emphasis> revision of a file
bos@559: quickly. This approach works so well that it has since been
bos@559: copied by several other revision control systems.</para>
bos@559:
bos@592: <para id="x_2fd"><xref linkend="fig:concepts:snapshot"/> illustrates
bos@559: the idea. In an entry in a revlog's index file, Mercurial
bos@559: stores the range of entries from the data file that it must
bos@559: read to reconstruct a particular revision.</para>
bos@559:
bos@559: <sect3>
bos@559: <title>Aside: the influence of video compression</title>
bos@559:
bos@701: <para id="x_2fe">If you're familiar with video compression or
bos@701: have ever watched a TV feed through a digital cable or
bos@701: satellite service, you may know that most video compression
bos@701: schemes store each frame of video as a delta against its
bos@701: predecessor frame.</para>
bos@701:
bos@701: <para id="x_2ff">Mercurial borrows this idea to make it
bos@701: possible to reconstruct a revision from a snapshot and a
bos@701: small number of deltas.</para>
bos@559:
bos@559: </sect3>
bos@559: </sect2>
bos@559: <sect2>
bos@559: <title>Identification and strong integrity</title>
bos@559:
bos@584: <para id="x_300">Along with delta or snapshot information, a revlog entry
bos@559: contains a cryptographic hash of the data that it represents.
bos@559: This makes it difficult to forge the contents of a revision,
bos@559: and easy to detect accidental corruption.</para>
bos@559:
bos@584: <para id="x_301">Hashes provide more than a mere check against corruption;
bos@559: they are used as the identifiers for revisions. The changeset
bos@559: identification hashes that you see as an end user are from
bos@559: revisions of the changelog. Although filelogs and the
bos@559: manifest also use hashes, Mercurial only uses these behind the
bos@559: scenes.</para>
bos@559:
bos@584: <para id="x_302">Mercurial verifies that hashes are correct when it
bos@559: retrieves file revisions and when it pulls changes from
bos@559: another repository. If it encounters an integrity problem, it
bos@559: will complain and stop whatever it's doing.</para>
bos@559:
bos@584: <para id="x_303">In addition to the effect it has on retrieval efficiency,
bos@559: Mercurial's use of periodic snapshots makes it more robust
bos@559: against partial data corruption. If a revlog becomes partly
bos@559: corrupted due to a hardware error or system bug, it's often
bos@559: possible to reconstruct some or most revisions from the
bos@559: uncorrupted sections of the revlog, both before and after the
bos@559: corrupted section. This would not be possible with a
bos@559: delta-only storage model.</para>
bos@559: </sect2>
bos@559: </sect1>
bos@701:
bos@559: <sect1>
bos@559: <title>Revision history, branching, and merging</title>
bos@559:
bos@584: <para id="x_304">Every entry in a Mercurial revlog knows the identity of its
bos@559: immediate ancestor revision, usually referred to as its
bos@559: <emphasis>parent</emphasis>. In fact, a revision contains room
bos@559: for not one parent, but two. Mercurial uses a special hash,
bos@559: called the <quote>null ID</quote>, to represent the idea
bos@559: <quote>there is no parent here</quote>. This hash is simply a
bos@559: string of zeroes.</para>
bos@559:
bos@592: <para id="x_305">In <xref linkend="fig:concepts:revlog"/>, you can see
bos@559: an example of the conceptual structure of a revlog. Filelogs,
bos@559: manifests, and changelogs all have this same structure; they
bos@559: differ only in the kind of data stored in each delta or
bos@559: snapshot.</para>
bos@559:
bos@584: <para id="x_306">The first revision in a revlog (at the bottom of the image)
bos@559: has the null ID in both of its parent slots. For a
bos@559: <quote>normal</quote> revision, its first parent slot contains
bos@559: the ID of its parent revision, and its second contains the null
bos@559: ID, indicating that the revision has only one real parent. Any
bos@559: two revisions that have the same parent ID are branches. A
bos@559: revision that represents a merge between branches has two normal
bos@559: revision IDs in its parent slots.</para>
bos@559:
bos@591: <figure id="fig:concepts:revlog">
bos@591: <title>The conceptual structure of a revlog</title>
bos@591: <mediaobject>
bos@594: <imageobject><imagedata fileref="figs/revlog.png"/></imageobject>
bos@591: <textobject><phrase>XXX add text</phrase></textobject>
bos@591: </mediaobject>
bos@591: </figure>
bos@559:
bos@559: </sect1>
bos@559: <sect1>
bos@559: <title>The working directory</title>
bos@559:
bos@584: <para id="x_307">In the working directory, Mercurial stores a snapshot of the
bos@559: files from the repository as of a particular changeset.</para>
bos@559:
bos@584: <para id="x_308">The working directory <quote>knows</quote> which changeset
bos@559: it contains. When you update the working directory to contain a
bos@559: particular changeset, Mercurial looks up the appropriate
bos@559: revision of the manifest to find out which files it was tracking
bos@559: at the time that changeset was committed, and which revision of
bos@559: each file was then current. It then recreates a copy of each of
bos@559: those files, with the same contents it had when the changeset
bos@559: was committed.</para>
bos@559:
bos@701: <para id="x_309">The <emphasis>dirstate</emphasis> is a special
bos@701: structure that contains Mercurial's knowledge of the working
bos@701: directory. It is maintained as a file named
bos@701: <filename>.hg/dirstate</filename> inside a repository. The
bos@701: dirstate details which changeset the working directory is
bos@701: updated to, and all of the files that Mercurial is tracking in
bos@701: the working directory. It also lets Mercurial quickly notice
bos@701: changed files, by recording their checkout times and
bos@701: sizes.</para>
bos@559:
bos@584: <para id="x_30a">Just as a revision of a revlog has room for two parents, so
bos@559: that it can represent either a normal revision (with one parent)
bos@559: or a merge of two earlier revisions, the dirstate has slots for
bos@559: two parents. When you use the <command role="hg-cmd">hg
bos@559: update</command> command, the changeset that you update to is
bos@559: stored in the <quote>first parent</quote> slot, and the null ID
bos@559: in the second. When you <command role="hg-cmd">hg
bos@559: merge</command> with another changeset, the first parent
bos@559: remains unchanged, and the second parent is filled in with the
bos@559: changeset you're merging with. The <command role="hg-cmd">hg
bos@559: parents</command> command tells you what the parents of the
bos@559: dirstate are.</para>
bos@559:
bos@559: <sect2>
bos@559: <title>What happens when you commit</title>
bos@559:
bos@584: <para id="x_30b">The dirstate stores parent information for more than just
bos@559: book-keeping purposes. Mercurial uses the parents of the
bos@559: dirstate as <emphasis>the parents of a new
bos@559: changeset</emphasis> when you perform a commit.</para>
bos@559:
bos@591: <figure id="fig:concepts:wdir">
bos@591: <title>The working directory can have two parents</title>
bos@591: <mediaobject>
bos@594: <imageobject><imagedata fileref="figs/wdir.png"/></imageobject>
bos@591: <textobject><phrase>XXX add text</phrase></textobject>
bos@591: </mediaobject>
bos@591: </figure>
bos@559:
bos@592: <para id="x_30d"><xref linkend="fig:concepts:wdir"/> shows the
bos@559: normal state of the working directory, where it has a single
bos@559: changeset as parent. That changeset is the
bos@559: <emphasis>tip</emphasis>, the newest changeset in the
bos@559: repository that has no children.</para>
bos@559:
bos@591: <figure id="fig:concepts:wdir-after-commit">
bos@591: <title>The working directory gains new parents after a
bos@591: commit</title>
bos@591: <mediaobject>
bos@594: <imageobject><imagedata fileref="figs/wdir-after-commit.png"/></imageobject>
bos@591: <textobject><phrase>XXX add text</phrase></textobject>
bos@591: </mediaobject>
bos@591: </figure>
bos@559:
bos@584: <para id="x_30f">It's useful to think of the working directory as
bos@559: <quote>the changeset I'm about to commit</quote>. Any files
bos@559: that you tell Mercurial that you've added, removed, renamed,
bos@559: or copied will be reflected in that changeset, as will
bos@559: modifications to any files that Mercurial is already tracking;
bos@559: the new changeset will have the parents of the working
bos@559: directory as its parents.</para>
bos@559:
bos@592: <para id="x_310">After a commit, Mercurial will update the
bos@592: parents of the working directory, so that the first parent is
bos@592: the ID of the new changeset, and the second is the null ID.
bos@592: This is shown in <xref
bos@592: linkend="fig:concepts:wdir-after-commit"/>. Mercurial
bos@559: doesn't touch any of the files in the working directory when
bos@559: you commit; it just modifies the dirstate to note its new
bos@559: parents.</para>
bos@559:
bos@559: </sect2>
bos@559: <sect2>
bos@559: <title>Creating a new head</title>
bos@559:
bos@584: <para id="x_311">It's perfectly normal to update the working directory to a
bos@559: changeset other than the current tip. For example, you might
bos@559: want to know what your project looked like last Tuesday, or
bos@559: you could be looking through changesets to see which one
bos@559: introduced a bug. In cases like this, the natural thing to do
bos@559: is update the working directory to the changeset you're
bos@559: interested in, and then examine the files in the working
bos@559: directory directly to see their contents as they were when you
bos@559: committed that changeset. The effect of this is shown in
bos@592: <xref linkend="fig:concepts:wdir-pre-branch"/>.</para>
bos@559:
bos@591: <figure id="fig:concepts:wdir-pre-branch">
bos@591: <title>The working directory, updated to an older
bos@591: changeset</title>
bos@591: <mediaobject>
bos@594: <imageobject><imagedata fileref="figs/wdir-pre-branch.png"/></imageobject>
bos@591: <textobject><phrase>XXX add text</phrase></textobject>
bos@591: </mediaobject>
bos@591: </figure>
bos@559:
bos@592: <para id="x_313">Having updated the working directory to an
bos@592: older changeset, what happens if you make some changes, and
bos@592: then commit? Mercurial behaves in the same way as I outlined
bos@559: above. The parents of the working directory become the
bos@559: parents of the new changeset. This new changeset has no
bos@559: children, so it becomes the new tip. And the repository now
bos@559: contains two changesets that have no children; we call these
bos@559: <emphasis>heads</emphasis>. You can see the structure that
bos@592: this creates in <xref
bos@559: linkend="fig:concepts:wdir-branch"/>.</para>
bos@559:
bos@591: <figure id="fig:concepts:wdir-branch">
bos@591: <title>After a commit made while synced to an older
bos@591: changeset</title>
bos@591: <mediaobject>
bos@594: <imageobject><imagedata fileref="figs/wdir-branch.png"/></imageobject>
bos@591: <textobject><phrase>XXX add text</phrase></textobject>
bos@591: </mediaobject>
bos@591: </figure>
bos@559:
bos@559: <note>
bos@701: <para id="x_315">If you're new to Mercurial, you should keep
bos@701: in mind a common <quote>error</quote>, which is to use the
bos@701: <command role="hg-cmd">hg pull</command> command without any
bos@559: options. By default, the <command role="hg-cmd">hg
bos@559: pull</command> command <emphasis>does not</emphasis>
bos@559: update the working directory, so you'll bring new changesets
bos@559: into your repository, but the working directory will stay
bos@559: synced at the same changeset as before the pull. If you
bos@559: make some changes and commit afterwards, you'll thus create
bos@559: a new head, because your working directory isn't synced to
bos@701: whatever the current tip is. To combine the operation of a
bos@701: pull, followed by an update, run <command>hg pull
bos@701: -u</command>.</para>
bos@701:
bos@701: <para id="x_316">I put the word <quote>error</quote> in quotes
bos@701: because all that you need to do to rectify the situation
bos@701: where you created a new head by accident is
bos@701: <command role="hg-cmd">hg merge</command>, then <command
bos@701: role="hg-cmd">hg commit</command>. In other words, this
bos@701: almost never has negative consequences; it's just something
bos@701: of a surprise for newcomers. I'll discuss other ways to
bos@701: avoid this behavior, and why Mercurial behaves in this
bos@701: initially surprising way, later on.</para>
bos@559: </note>
bos@559:
bos@559: </sect2>
bos@559: <sect2>
bos@620: <title>Merging changes</title>
bos@559:
bos@592: <para id="x_317">When you run the <command role="hg-cmd">hg
bos@592: merge</command> command, Mercurial leaves the first parent
bos@592: of the working directory unchanged, and sets the second parent
bos@592: to the changeset you're merging with, as shown in <xref
bos@559: linkend="fig:concepts:wdir-merge"/>.</para>
bos@559:
bos@591: <figure id="fig:concepts:wdir-merge">
bos@591: <title>Merging two heads</title>
bos@591: <mediaobject>
bos@591: <imageobject>
bos@594: <imagedata fileref="figs/wdir-merge.png"/>
bos@591: </imageobject>
bos@591: <textobject><phrase>XXX add text</phrase></textobject>
bos@591: </mediaobject>
bos@591: </figure>
bos@559:
bos@584: <para id="x_319">Mercurial also has to modify the working directory, to
bos@559: merge the files managed in the two changesets. Simplified a
bos@559: little, the merging process goes like this, for every file in
bos@559: the manifests of both changesets.</para>
bos@559: <itemizedlist>
bos@584: <listitem><para id="x_31a">If neither changeset has modified a file, do
bos@559: nothing with that file.</para>
bos@559: </listitem>
bos@584: <listitem><para id="x_31b">If one changeset has modified a file, and the
bos@559: other hasn't, create the modified copy of the file in the
bos@559: working directory.</para>
bos@559: </listitem>
bos@584: <listitem><para id="x_31c">If one changeset has removed a file, and the
bos@559: other hasn't (or has also deleted it), delete the file
bos@559: from the working directory.</para>
bos@559: </listitem>
bos@584: <listitem><para id="x_31d">If one changeset has removed a file, but the
bos@559: other has modified the file, ask the user what to do: keep
bos@559: the modified file, or remove it?</para>
bos@559: </listitem>
bos@584: <listitem><para id="x_31e">If both changesets have modified a file,
bos@559: invoke an external merge program to choose the new
bos@559: contents for the merged file. This may require input from
bos@559: the user.</para>
bos@559: </listitem>
bos@584: <listitem><para id="x_31f">If one changeset has modified a file, and the
bos@559: other has renamed or copied the file, make sure that the
bos@559: changes follow the new name of the file.</para>
bos@559: </listitem></itemizedlist>
bos@584: <para id="x_320">There are more details&emdash;merging has plenty of corner
bos@559: cases&emdash;but these are the most common choices that are
bos@559: involved in a merge. As you can see, most cases are
bos@559: completely automatic, and indeed most merges finish
bos@559: automatically, without requiring your input to resolve any
bos@559: conflicts.</para>
bos@559:
bos@584: <para id="x_321">When you're thinking about what happens when you commit
bos@559: after a merge, once again the working directory is <quote>the
bos@559: changeset I'm about to commit</quote>. After the <command
bos@559: role="hg-cmd">hg merge</command> command completes, the
bos@559: working directory has two parents; these will become the
bos@559: parents of the new changeset.</para>
bos@559:
bos@701: <para id="x_322">Mercurial lets you perform multiple merges, but
bos@701: you must commit the results of each individual merge as you
bos@701: go. This is necessary because Mercurial only tracks two
bos@701: parents for both revisions and the working directory. While
bos@701: it would be technically feasible to merge multiple changesets
bos@701: at once, Mercurial avoids this for simplicity. With multi-way
bos@701: merges, the risks of user confusion, nasty conflict
bos@701: resolution, and making a terrible mess of a merge would grow
bos@701: intolerable.</para>
bos@559:
bos@559: </sect2>
bos@620:
bos@620: <sect2>
bos@620: <title>Merging and renames</title>
bos@620:
bos@676: <para id="x_69a">A surprising number of revision control systems pay little
bos@620: or no attention to a file's <emphasis>name</emphasis> over
bos@620: time. For instance, it used to be common that if a file got
bos@620: renamed on one side of a merge, the changes from the other
bos@620: side would be silently dropped.</para>
bos@620:
bos@676: <para id="x_69b">Mercurial records metadata when you tell it to perform a
bos@620: rename or copy. It uses this metadata during a merge to do the
bos@620: right thing in the case of a merge. For instance, if I rename
bos@620: a file, and you edit it without renaming it, when we merge our
bos@620: work the file will be renamed and have your edits
bos@620: applied.</para>
bos@620: </sect2>
bos@559: </sect1>
bos@620:
bos@559: <sect1>
bos@559: <title>Other interesting design features</title>
bos@559:
bos@584: <para id="x_323">In the sections above, I've tried to highlight some of the
bos@559: most important aspects of Mercurial's design, to illustrate that
bos@559: it pays careful attention to reliability and performance.
bos@559: However, the attention to detail doesn't stop there. There are
bos@559: a number of other aspects of Mercurial's construction that I
bos@559: personally find interesting. I'll detail a few of them here,
bos@559: separate from the <quote>big ticket</quote> items above, so that
bos@559: if you're interested, you can gain a better idea of the amount
bos@559: of thinking that goes into a well-designed system.</para>
bos@559:
bos@559: <sect2>
bos@559: <title>Clever compression</title>
bos@559:
bos@584: <para id="x_324">When appropriate, Mercurial will store both snapshots and
bos@559: deltas in compressed form. It does this by always
bos@559: <emphasis>trying to</emphasis> compress a snapshot or delta,
bos@559: but only storing the compressed version if it's smaller than
bos@559: the uncompressed version.</para>
bos@559:
bos@584: <para id="x_325">This means that Mercurial does <quote>the right
bos@559: thing</quote> when storing a file whose native form is
bos@559: compressed, such as a <literal>zip</literal> archive or a JPEG
bos@559: image. When these types of files are compressed a second
bos@559: time, the resulting file is usually bigger than the
bos@559: once-compressed form, and so Mercurial will store the plain
bos@559: <literal>zip</literal> or JPEG.</para>
bos@559:
bos@584: <para id="x_326">Deltas between revisions of a compressed file are usually
bos@559: larger than snapshots of the file, and Mercurial again does
bos@559: <quote>the right thing</quote> in these cases. It finds that
bos@559: such a delta exceeds the threshold at which it should store a
bos@559: complete snapshot of the file, so it stores the snapshot,
bos@559: again saving space compared to a naive delta-only
bos@559: approach.</para>
bos@559:
bos@559: <sect3>
bos@559: <title>Network recompression</title>
bos@559:
bos@584: <para id="x_327">When storing revisions on disk, Mercurial uses the
bos@559: <quote>deflate</quote> compression algorithm (the same one
bos@559: used by the popular <literal>zip</literal> archive format),
bos@559: which balances good speed with a respectable compression
bos@559: ratio. However, when transmitting revision data over a
bos@559: network connection, Mercurial uncompresses the compressed
bos@559: revision data.</para>
bos@559:
bos@584: <para id="x_328">If the connection is over HTTP, Mercurial recompresses
bos@559: the entire stream of data using a compression algorithm that
bos@559: gives a better compression ratio (the Burrows-Wheeler
bos@559: algorithm from the widely used <literal>bzip2</literal>
bos@559: compression package). This combination of algorithm and
bos@559: compression of the entire stream (instead of a revision at a
bos@559: time) substantially reduces the number of bytes to be
bos@620: transferred, yielding better network performance over most
bos@620: kinds of network.</para>
bos@559:
bos@701: <para id="x_329">If the connection is over
bos@701: <command>ssh</command>, Mercurial
bos@701: <emphasis>doesn't</emphasis> recompress the stream, because
bos@701: <command>ssh</command> can already do this itself. You can
bos@701: tell Mercurial to always use <command>ssh</command>'s
bos@701: compression feature by editing the
bos@701: <filename>.hgrc</filename> file in your home directory as
bos@701: follows.</para>
bos@701:
bos@701: <programlisting>[ui]
bos@701: ssh = ssh -C</programlisting>
bos@559:
bos@559: </sect3>
bos@559: </sect2>
bos@559: <sect2>
bos@559: <title>Read/write ordering and atomicity</title>
bos@559:
bos@592: <para id="x_32a">Appending to files isn't the whole story when
bos@592: it comes to guaranteeing that a reader won't see a partial
bos@592: write. If you recall <xref linkend="fig:concepts:metadata"/>,
bos@701: revisions in the changelog point to revisions in the manifest,
bos@701: and revisions in the manifest point to revisions in filelogs.
bos@592: This hierarchy is deliberate.</para>
bos@559:
bos@584: <para id="x_32b">A writer starts a transaction by writing filelog and
bos@559: manifest data, and doesn't write any changelog data until
bos@559: those are finished. A reader starts by reading changelog
bos@559: data, then manifest data, followed by filelog data.</para>
bos@559:
bos@584: <para id="x_32c">Since the writer has always finished writing filelog and
bos@559: manifest data before it writes to the changelog, a reader will
bos@559: never read a pointer to a partially written manifest revision
bos@559: from the changelog, and it will never read a pointer to a
bos@559: partially written filelog revision from the manifest.</para>
bos@559:
bos@559: </sect2>
bos@559: <sect2>
bos@559: <title>Concurrent access</title>
bos@559:
bos@584: <para id="x_32d">The read/write ordering and atomicity guarantees mean that
bos@559: Mercurial never needs to <emphasis>lock</emphasis> a
bos@559: repository when it's reading data, even if the repository is
bos@559: being written to while the read is occurring. This has a big
bos@559: effect on scalability; you can have an arbitrary number of
bos@559: Mercurial processes safely reading data from a repository
bos@701: all at once, no matter whether it's being written to or
bos@559: not.</para>
bos@559:
bos@584: <para id="x_32e">The lockless nature of reading means that if you're
bos@559: sharing a repository on a multi-user system, you don't need to
bos@559: grant other local users permission to
bos@559: <emphasis>write</emphasis> to your repository in order for
bos@559: them to be able to clone it or pull changes from it; they only
bos@559: need <emphasis>read</emphasis> permission. (This is
bos@559: <emphasis>not</emphasis> a common feature among revision
bos@559: control systems, so don't take it for granted! Most require
bos@559: readers to be able to lock a repository to access it safely,
bos@559: and this requires write permission on at least one directory,
bos@559: which of course makes for all kinds of nasty and annoying
bos@559: security and administrative problems.)</para>
bos@559:
bos@584: <para id="x_32f">Mercurial uses locks to ensure that only one process can
bos@559: write to a repository at a time (the locking mechanism is safe
bos@559: even over filesystems that are notoriously hostile to locking,
bos@559: such as NFS). If a repository is locked, a writer will wait
bos@559: for a while to retry if the repository becomes unlocked, but
bos@559: if the repository remains locked for too long, the process
bos@559: attempting to write will time out after a while. This means
bos@559: that your daily automated scripts won't get stuck forever and
bos@559: pile up if a system crashes unnoticed, for example. (Yes, the
bos@559: timeout is configurable, from zero to infinity.)</para>
bos@559:
bos@559: <sect3>
bos@559: <title>Safe dirstate access</title>
bos@559:
bos@584: <para id="x_330">As with revision data, Mercurial doesn't take a lock to
bos@559: read the dirstate file; it does acquire a lock to write it.
bos@559: To avoid the possibility of reading a partially written copy
bos@559: of the dirstate file, Mercurial writes to a file with a
bos@559: unique name in the same directory as the dirstate file, then
bos@559: renames the temporary file atomically to
bos@559: <filename>dirstate</filename>. The file named
bos@559: <filename>dirstate</filename> is thus guaranteed to be
bos@559: complete, not partially written.</para>
bos@559:
bos@559: </sect3>
bos@559: </sect2>
bos@559: <sect2>
bos@559: <title>Avoiding seeks</title>
bos@559:
bos@584: <para id="x_331">Critical to Mercurial's performance is the avoidance of
bos@559: seeks of the disk head, since any seek is far more expensive
bos@559: than even a comparatively large read operation.</para>
bos@559:
bos@584: <para id="x_332">This is why, for example, the dirstate is stored in a
bos@559: single file. If there were a dirstate file per directory that
bos@559: Mercurial tracked, the disk would seek once per directory.
bos@559: Instead, Mercurial reads the entire single dirstate file in
bos@559: one step.</para>
bos@559:
bos@584: <para id="x_333">Mercurial also uses a <quote>copy on write</quote> scheme
bos@559: when cloning a repository on local storage. Instead of
bos@559: copying every revlog file from the old repository into the new
bos@559: repository, it makes a <quote>hard link</quote>, which is a
bos@559: shorthand way to say <quote>these two names point to the same
bos@559: file</quote>. When Mercurial is about to write to one of a
bos@559: revlog's files, it checks to see if the number of names
bos@559: pointing at the file is greater than one. If it is, more than
bos@559: one repository is using the file, so Mercurial makes a new
bos@559: copy of the file that is private to this repository.</para>
bos@559:
bos@584: <para id="x_334">A few revision control developers have pointed out that
bos@559: this idea of making a complete private copy of a file is not
bos@559: very efficient in its use of storage. While this is true,
bos@559: storage is cheap, and this method gives the highest
bos@559: performance while deferring most book-keeping to the operating
bos@559: system. An alternative scheme would most likely reduce
bos@701: performance and increase the complexity of the software, but
bos@701: speed and simplicity are key to the <quote>feel</quote> of
bos@559: day-to-day use.</para>
bos@559:
bos@559: </sect2>
bos@559: <sect2>
bos@559: <title>Other contents of the dirstate</title>
bos@559:
bos@584: <para id="x_335">Because Mercurial doesn't force you to tell it when you're
bos@559: modifying a file, it uses the dirstate to store some extra
bos@559: information so it can determine efficiently whether you have
bos@559: modified a file. For each file in the working directory, it
bos@559: stores the time that it last modified the file itself, and the
bos@559: size of the file at that time.</para>
bos@559:
bos@584: <para id="x_336">When you explicitly <command role="hg-cmd">hg
bos@559: add</command>, <command role="hg-cmd">hg remove</command>,
bos@559: <command role="hg-cmd">hg rename</command> or <command
bos@559: role="hg-cmd">hg copy</command> files, Mercurial updates the
bos@559: dirstate so that it knows what to do with those files when you
bos@559: commit.</para>
bos@559:
bos@701: <para id="x_337">The dirstate helps Mercurial to efficiently
bos@701: check the status of files in a repository.</para>
bos@701:
bos@701: <itemizedlist>
bos@701: <listitem>
bos@702: <para id="x_726">When Mercurial checks the state of a file in the
bos@701: working directory, it first checks a file's modification
bos@701: time against the time in the dirstate that records when
bos@701: Mercurial last wrote the file. If the last modified time
bos@701: is the same as the time when Mercurial wrote the file, the
bos@701: file must not have been modified, so Mercurial does not
bos@701: need to check any further.</para>
bos@701: </listitem>
bos@701: <listitem>
bos@702: <para id="x_727">If the file's size has changed, the file must have
bos@701: been modified. If the modification time has changed, but
bos@701: the size has not, only then does Mercurial need to
bos@701: actually read the contents of the file to see if it has
bos@701: changed.</para>
bos@701: </listitem>
bos@701: </itemizedlist>
bos@701:
bos@702: <para id="x_728">Storing the modification time and size dramatically
bos@701: reduces the number of read operations that Mercurial needs to
bos@701: perform when we run commands like <command>hg status</command>.
bos@701: This results in large performance improvements.</para>
bos@559: </sect2>
bos@559: </sect1>
bos@559: </chapter>
bos@559:
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