Update the documentation of build-remote.pl
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Eelco Dolstra
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<chapter xmlns="http://docbook.org/ns/docbook"
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xmlns:xlink="http://www.w3.org/1999/xlink"
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xml:id='chap-build-farm'>
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xml:id='chap-distributed-builds'>
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<title>Setting up a Build Farm</title>
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<title>Setting Up Distributed Builds</title>
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<para>This chapter provides some sketchy information on how to set up
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a Nix-based build farm. Nix is particularly suited as a basis for a
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build farm, since:
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<itemizedlist>
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<listitem><para>Nix supports distributed builds: a local Nix
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installation can forward Nix builds to other machines over the
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network. This allows multiple builds to be performed in parallel
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(thus improving performance), but more in importantly, it allows Nix
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to perform multi-platform builds in a semi-transparent way. For
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instance, if you perform a build for a
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<literal>powerpc-darwin</literal> on an
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<literal>i686-linux</literal> machine, Nix can automatically forward
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the build to a <literal>powerpc-darwin</literal> machine, if
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available.</para></listitem>
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<listitem><para>The Nix expression language is ideal for describing
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build jobs, plus all their dependencies. For instance, if your
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package has some dependency, you don't have to manually install it
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on all the machines in the build farm; they will be built
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automatically.</para></listitem>
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<listitem><para>Proper release management requires that builds (if
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deployed) are traceable: it should be possible to figure out from
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exactly what sources they were built, in what configuration, etc.;
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and it should be possible to reproduce the build, if necessary. Nix
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makes this possible since Nix's hashing scheme uniquely identifies
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builds, and Nix expressions are self-contained.</para></listitem>
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<listitem><para>Nix will only rebuild things that have actually
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changed. For instance, if the sources of a package haven't changed
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between runs of the build farm, the package won't be rebuilt (unless
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it was garbage-collected). Also, dependencies typically don't
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change very often, so they only need to be built
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once.</para></listitem>
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<listitem><para>The results of a Nix build farm can be made
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available through a channel, so successful builds can be deployed to
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users immediately.</para></listitem>
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</itemizedlist>
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</para>
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<section><title>Overview</title>
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<para>TODO</para>
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<para>The sources of the Nix build farm are at <link
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xlink:href='https://svn.nixos.org/repos/nix/release/trunk'/>.</para>
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</section>
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<section xml:id='sec-distributed-builds'><title>Setting up distributed builds</title>
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<para>Nix supports distributed builds: a local Nix installation can
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forward Nix builds to other machines over the network. This allows
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multiple builds to be performed in parallel (thus improving
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performance) and allows Nix to perform multi-platform builds in a
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semi-transparent way. For instance, if you perform a build for a
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<literal>powerpc-darwin</literal> on an <literal>i686-linux</literal>
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machine, Nix can automatically forward the build to a
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<literal>powerpc-darwin</literal> machine, if available.</para>
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<para>You can enable distributed builds by setting the environment
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variable <envar>NIX_BUILD_HOOK</envar> to point to a program that Nix
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@ -79,22 +29,22 @@ variable</link>.</para>
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<filename>remote-systems.conf</filename></title>
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<programlisting>
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nix@mcflurry.labs.cs.uu.nl powerpc-darwin /home/nix/.ssh/id_quarterpounder_auto 2
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nix@scratchy.labs.cs.uu.nl i686-linux /home/nix/.ssh/id_scratchy_auto 1
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nix@scratchy.labs.cs.uu.nl i686-linux /home/nix/.ssh/id_scratchy_auto 8 1 kvm
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nix@itchy.labs.cs.uu.nl i686-linux /home/nix/.ssh/id_scratchy_auto 8 2
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</programlisting>
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</example>
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<para>An example build hook can be found in the Nix build farm
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sources: <link
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xlink:href='https://svn.nixos.org/repos/nix/release/trunk/common/distributed/build-remote.pl'
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/>. It should be suitable for most purposes, with maybe some minor
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adjustments. It uses <command>ssh</command> and
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<command>rsync</command> to copy the build inputs and outputs and
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perform the remote build. You should define a list of available build
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machines and set the environment variable
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<envar>REMOTE_SYSTEMS</envar> to point to it. An example
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configuration is shown in <xref linkend='ex-remote-systems' />. Each
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line in the file specifies a machine, with the following bits of
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information:
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<para>Nix ships with a build hook that should be suitable for most
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purposes. It uses <command>ssh</command> and
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<command>nix-copy-closure</command> to copy the build inputs and
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outputs and perform the remote build. To use it, you should set
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<envar>NIX_BUILD_HOOK</envar> to
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<filename><replaceable>prefix</replaceable>/libexec/nix/build-remote.pl</filename>.
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You should also define a list of available build machines and point
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the environment variable <envar>NIX_REMOTE_SYSTEMS</envar> to it. An
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example configuration is shown in <xref linkend='ex-remote-systems'
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/>. Each line in the file specifies a machine, with the following
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bits of information:
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<orderedlist>
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@ -104,34 +54,49 @@ information:
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be an alias defined in your
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<filename>~/.ssh/config</filename>.</para></listitem>
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<listitem><para>The Nix platform type identifier, such as
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<literal>powerpc-darwin</literal>.</para></listitem>
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<listitem><para>A comma-separated list of Nix platform type
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identifiers, such as <literal>powerpc-darwin</literal>. It is
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possible for a machine to support multiple platform types, e.g.,
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<literal>i686-linux,x86_64-linux</literal>.</para></listitem>
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<listitem><para>The SSH private key to be used to log in to the
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remote machine. Since builds should be non-interactive, this key
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should not have a passphrase!</para></listitem>
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<listitem><para>The maximum <quote>load</quote> of the remote
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machine. This is just the maximum number of jobs that
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<listitem><para>The maximum number of builds that
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<filename>build-remote.pl</filename> will execute in parallel on the
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machine. Typically this should be equal to the number of
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CPUs.</para></listitem>
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machine. Typically this should be equal to the number of CPU cores.
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For instance, the machine <literal>itchy</literal> in the example
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will execute up to 8 builds in parallel.</para></listitem>
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<listitem><para>The “speed factor”, indicating the relative speed of
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the machine. If there are multiple machines of the right type, Nix
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will prefer the fastest, taking load into account.</para></listitem>
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<listitem><para>A comma-separated list of
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<emphasis>features</emphasis>. If a derivation has the
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<varname>requiredSystemFeatures</varname> attribute, then
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<filename>build-remote.pl</filename> will only perform the
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derivation on a machine that has the specified features. For instance, the attribute
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<programlisting>
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requiredSystemFeatures = [ "kvm" ];
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</programlisting>
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will cause the build to be performed on a machine that has the
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<literal>kvm</literal> feature (i.e., <literal>scratchy</literal> in
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the example above).</para></listitem>
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</orderedlist>
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You should also set up the environment variable
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<envar>CURRENT_LOAD</envar> to point at a file that
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<filename>build-remote.pl</filename> uses to remember how many jobs it
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is currently executing remotely. It doesn't look at the actual load
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on the remote machine, so if you have multiple instances of Nix
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running, they should use the same <envar>CURRENT_LOAD</envar>
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file<footnote><para>Although there are probably some race conditions
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in the script right now.</para></footnote>. Maybe in the future
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<filename>build-remote.pl</filename> will look at the actual remote
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load. The load file should exist, so you should just create it as an
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empty file initially.</para>
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<envar>NIX_CURRENT_LOAD</envar> to point at a directory (e.g.,
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<filename>/var/run/nix/current-load</filename>) that
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<filename>build-remote.pl</filename> uses to remember how many builds
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it is currently executing remotely. It doesn't look at the actual
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load on the remote machine, so if you have multiple instances of Nix
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running, they should use the same <envar>NIX_CURRENT_LOAD</envar>
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file. Maybe in the future <filename>build-remote.pl</filename> will
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look at the actual remote load.</para>
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</section>
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</chapter>
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@ -119,9 +119,10 @@ $ mount -o bind /mnt/otherdisk/nix /nix</screen>
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<para>Specifies the location of the <emphasis>build hook</emphasis>,
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which is a program (typically some script) that Nix will call
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whenever it wants to build a derivation. This is used to implement
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distributed builds (see <xref linkend="sec-distributed-builds"
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/>). The protocol by which the calling Nix process and the build
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hook communicate is as follows.</para>
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distributed builds<phrase condition="manual"> (see <xref
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linkend="chap-distributed-builds" />)</phrase>. The protocol by
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which the calling Nix process and the build hook communicate is as
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follows.</para>
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<para>The build hook is called with the following command-line
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arguments:
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@ -1293,7 +1293,7 @@ set, the attributes of which specify the inputs of the build.</para>
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can only be performed on a machine and operating system matching the
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platform identifier. (Nix can automatically forward builds for
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other platforms by forwarding them to other machines; see <xref
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linkend='sec-distributed-builds' />.)</para></listitem>
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linkend='chap-distributed-builds' />.)</para></listitem>
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<listitem><para>There must be an attribute named
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<varname>name</varname> whose value must be a string. This is used
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