Merge branch 'siri/doc-upgrade/OTP-11717'

* siri/doc-upgrade/OTP-11717:
  Add documentation about upgrade
  Add info about upgrade of core applications

7 files changed, 394 insertions, 35 deletions

diff --git a/lib/sasl/doc/src/appup.xml b/lib/sasl/doc/src/appup.xml
index 85fcbed3ba..95f315d269 100644
--- a/lib/sasl/doc/src/appup.xml
+++ b/lib/sasl/doc/src/appup.xml
@@ -4,7 +4,7 @@
 <fileref>
   <header>
     <copyright>
-      <year>1997</year><year>2013</year>
+      <year>1997</year><year>2014</year>
       <holder>Ericsson AB. All Rights Reserved.</holder>
     </copyright>
     <legalnotice>
@@ -84,6 +84,9 @@
     version identifier must be specified as a binary, e.g.</p>
     <code type="none">&lt;&lt;"2\\.1\\.[0-9]+">></code>
     <p>will match all versions <c>2.1.x</c>, where x is any number.</p>
+    <p>Note that the regular expression must match the complete
+    version string, so the above example will work for for
+    e.g. <c>2.1.1</c>, but not for <c>2.1.1.1</c></p>
   </section>
 
   <section>
@@ -339,7 +342,7 @@ restart_new_emulator
       version of erts, kernel, stdlib and sasl are used when the
       emulator restarts. Only one <c>restart_new_emulator</c>
       instruction is allowed in the relup, and it shall be placed
-      first. <seealso marker="systools#make_relup/3">systools:make_relup3,4</seealso>
+      first. <seealso marker="systools#make_relup/3">systools:make_relup/3,4</seealso>
       will ensure this when the relup is generated. The rest of the
       relup script is executed after the restart as a part of the boot
       script.</p>
@@ -347,11 +350,25 @@ restart_new_emulator
       completed. To programatically find out if the upgrade is
       complete,
       call <seealso marker="release_handler#which_releases/0">
-      release_handler:which_releases</seealso> and check if the
+      release_handler:which_releases/0,1</seealso> and check if the
       expected release has status <c>current</c>.</p>
     <p>The new release must still be made permanent after the upgrade
       is completed. Otherwise, the old emulator is started in case of
       an emulator restart.</p>
+    <warning>
+      <p>As stated above, the <c>restart_new_emulator</c>
+	instruction causes the emulator to be restarted with new
+	versions of <c>erts</c>, <c>kernel</c>, <c>stdlib</c> and
+	<c>sasl</c>. All other applications, however, will at startup
+	be running their old versions in this new emulator. In most
+	cases this is no problem, but every now and then there will be
+	incompatible changes to the core applications which may cause
+	trouble in this setting. Such incompatible changes (when
+	functions are removed) are normally preceded by a deprecation
+	over two major releases. To make sure your application is not
+	crashed by an incompatible change, always remove any call to
+	deprecated functions as soon as possible.</p>
+    </warning>
     <pre>
 restart_emulator
     </pre>
diff --git a/lib/sasl/examples/src/target_system.erl b/lib/sasl/examples/src/target_system.erl
index fb9e9aaaaf..a0ae016791 100644
--- a/lib/sasl/examples/src/target_system.erl
+++ b/lib/sasl/examples/src/target_system.erl
@@ -1,7 +1,7 @@
 %%
 %% %CopyrightBegin%
 %%
-%% Copyright Ericsson AB 2011-2013. All Rights Reserved.
+%% Copyright Ericsson AB 2011-2014. All Rights Reserved.
 %%
 %% The contents of this file are subject to the Erlang Public License,
 %% Version 1.1, (the "License"); you may not use this file except in
@@ -65,7 +65,7 @@ create(RelFileName,SystoolsOpts) ->
               [RelFileName, RelFileName]),
     make_script(RelFileName,SystoolsOpts),
 
-    TarFileName = filename:join(Dir,RelFileName ++ ".tar.gz"),
+    TarFileName = RelFileName ++ ".tar.gz",
     io:fwrite("Creating tar file ~tp ...~n", [TarFileName]),
     make_tar(RelFileName,SystoolsOpts),
 
@@ -100,6 +100,12 @@ create(RelFileName,SystoolsOpts) ->
     copy_file(filename:join([ErtsBinDir, "to_erl"]), 
               filename:join([TmpBinDir, "to_erl"]), [preserve]),
 
+    %% This is needed if 'start' script created from 'start.src' shall
+    %% be used as it points out this directory as log dir for 'run_erl'
+    TmpLogDir = filename:join([TmpDir, "log"]),
+    io:fwrite("Creating temporary directory ~tp ...~n", [TmpLogDir]),
+    ok = file:make_dir(TmpLogDir),
+
     StartErlDataFile = filename:join([TmpDir, "releases", "start_erl.data"]),
     io:fwrite("Creating ~tp ...~n", [StartErlDataFile]),
     StartErlData = io_lib:fwrite("~s ~s~n", [ErtsVsn, RelVsn]),
@@ -115,6 +121,7 @@ create(RelFileName,SystoolsOpts) ->
     erl_tar:add(Tar, filename:join(TmpDir,ErtsDir), ErtsDir, []),
     erl_tar:add(Tar, filename:join(TmpDir,"releases"), "releases", []),
     erl_tar:add(Tar, filename:join(TmpDir,"lib"), "lib", []),
+    erl_tar:add(Tar, filename:join(TmpDir,"log"), "log", []),
     erl_tar:close(Tar),
     %% file:set_cwd(Cwd),
     io:fwrite("Removing directory ~tp ...~n",[TmpDir]),
@@ -136,6 +143,11 @@ install(RelFileName, RootDir) ->
     subst_src_scripts(["erl", "start", "start_erl"], ErtsBinDir, BinDir, 
                       [{"FINAL_ROOTDIR", RootDir}, {"EMU", "beam"}],
                       [preserve]),
+    %%! Workaround for pre OTP 17.0: start.src and start_erl.src did
+    %%! not have correct permissions, so the above 'preserve' option did not help
+    ok = file:change_mode(filename:join(BinDir,"start"),8#0755),
+    ok = file:change_mode(filename:join(BinDir,"start_erl"),8#0755),
+
     io:fwrite("Creating the RELEASES file ...\n"),
     create_RELEASES(RootDir, filename:join([RootDir, "releases",
 					    filename:basename(RelFileName)])).
diff --git a/system/doc/design_principles/release_handling.xml b/system/doc/design_principles/release_handling.xml
index 2a5831b89f..ba8a88d1c2 100644
--- a/system/doc/design_principles/release_handling.xml
+++ b/system/doc/design_principles/release_handling.xml
@@ -4,7 +4,7 @@
 <chapter>
   <header>
     <copyright>
-      <year>2003</year><year>2013</year>
+      <year>2003</year><year>2014</year>
       <holder>Ericsson AB. All Rights Reserved.</holder>
     </copyright>
     <legalnotice>
@@ -329,13 +329,26 @@
         automatically ensured.</p>
       <p>When the release handler encounters the instruction, it first
         generates a temporary boot file, which starts the new versions
-        of the emulator and the core applications. Then it shuts down
+        of the emulator and the core applications, and the old version
+        of all other applications. Then it shuts down
         the current emulator by calling <c>init:reboot()</c>, see
         <c>init(3)</c>. All processes are terminated gracefully and
         the system is rebooted by the heart program, using the
         temporary boot file. After the reboot, the rest of the relup
         instructions are executed. This is done as a part of the
         temporary boot script.</p>
+      <warning>
+	<p>Since this mechanism causes the new versions of the
+	emulator and core applications to run with the old version of
+	other applications during startup, extra care must be taken to
+	avoid incompatibility. Incompatible changes in the core
+	applications may in some situations be necessary. If possible,
+	such changes are preceded by deprecation over two major
+	releases before the actual change. To make sure your
+	application is not crashed by an incompatible change, always
+	remove any call to deprecated functions as soon as
+	possible.</p>
+      </warning>
       <p>An info report is written when the upgrade is completed. To
         programatically find out if the upgrade is complete,
         call <c>release_handler:which_releases(current)</c> and check
diff --git a/system/doc/system_principles/create_target.xmlsrc b/system/doc/system_principles/create_target.xmlsrc
index fbc935d708..b5f8d8ac4d 100644
--- a/system/doc/system_principles/create_target.xmlsrc
+++ b/system/doc/system_principles/create_target.xmlsrc
@@ -4,7 +4,7 @@
 <chapter>
   <header>
     <copyright>
-      <year>2002</year><year>2013</year>
+      <year>2002</year><year>2014</year>
       <holder>Ericsson AB. All Rights Reserved.</holder>
     </copyright>
     <legalnotice>
@@ -21,7 +21,7 @@
 
     </legalnotice>
 
-    <title>Creating a First Target System</title>
+    <title>Creating and Upgrading a Target System</title>
     <prepared>Peter H&ouml;gfeldt</prepared>
     <responsible></responsible>
     <docno></docno>
@@ -71,23 +71,24 @@
   </section>
 
   <section>
+    <marker id="create"/>
     <title>Creating a Target System</title>
     <p>It is assumed that you have a working Erlang/OTP system structured
       according to the OTP Design Principles.</p>
-    <p><em>Step 1.</em> First create a <c>.rel</c> file (see
-      <c>rel(4)</c>) that specifies the <c>erts</c> version
-      and lists all applications that should be included in the new
-      basic target system. An example is the following
+    <p><em>Step 1.</em> First create a <c>.rel</c> file (see <seealso
+      marker="sasl:rel">rel(4)</seealso>) that specifies the <c>erts</c>
+      version and lists all applications that should be included in the
+      new basic target system. An example is the following
       <c>mysystem.rel</c> file:</p>
     <code type="none">
 %% mysystem.rel
 {release,
  {"MYSYSTEM", "FIRST"},
- {erts, "5.1"},
- [{kernel, "2.7"},
-  {stdlib, "1.10"},
-  {sasl, "1.9.3"},
-  {pea, "1.0"}]}.    </code>
+ {erts, "5.10.4"},
+ [{kernel, "2.16.4"},
+  {stdlib, "1.19.4"},
+  {sasl, "2.3.4"},
+  {pea, "1.0"}]}.</code>
     <p>The listed applications are not only original Erlang/OTP
       applications but possibly also new applications that you have
       written yourself (here examplified by the application
@@ -116,13 +117,13 @@ os> <input>erl -pa /home/user/target_system/myapps/pea-1.0/ebin</input></pre>
           <c>systools:make_tar/2</c>. That file has the following
           contents:</p>
         <code type="none">
-erts-5.1/bin/
+erts-5.10.4/bin/
 releases/FIRST/start.boot
 releases/FIRST/mysystem.rel
 releases/mysystem.rel
-lib/kernel-2.7/
-lib/stdlib-1.10/
-lib/sasl-1.9.3/
+lib/kernel-2.16.4/
+lib/stdlib-1.19.4/
+lib/sasl-2.3.4/
 lib/pea-1.0/        </code>
         <p>The file <c>releases/FIRST/start.boot</c> is a copy of our
           <c>mysystem.boot</c></p>
@@ -142,16 +143,19 @@ lib/pea-1.0/        </code>
       <item>Creates the temporary directory <c>tmp</c> and extracts the tar file
       <c>mysystem.tar.gz</c> into that directory. </item>
       <item>Deletes the <c>erl</c> and <c>start</c> files from
-      <c>tmp/erts-5.1/bin</c>. These files will be created again from
+      <c>tmp/erts-5.10.4/bin</c>. These files will be created again from
       source when installing the release.</item>
       <item>Creates the directory <c>tmp/bin</c>.</item>
       <item>Copies the previously created file <c>plain.boot</c> to
       <c>tmp/bin/start.boot</c>.</item>
       <item>Copies the files <c>epmd</c>, <c>run_erl</c>, and
-      <c>to_erl</c> from the directory <c>tmp/erts-5.1/bin</c> to
+      <c>to_erl</c> from the directory <c>tmp/erts-5.10.4/bin</c> to
        the directory <c>tmp/bin</c>.</item>
+      <item>Creates the directory <c>tmp/log</c>, which will be used
+       if the system is started as embedded with the <c>bin/start</c>
+       script.</item>
       <item>Creates the file <c>tmp/releases/start_erl.data</c> with
-       the contents "5.1 FIRST". This file is to be passed as data
+       the contents "5.10.4 FIRST". This file is to be passed as data
        file to the <c>start_erl</c> script.
       </item>
       <item>Recreates the file <c>mysystem.tar.gz</c> from the directories
@@ -171,11 +175,11 @@ lib/pea-1.0/        </code>
       <item>Extracts the tar file <c>mysystem.tar.gz</c> into the target
        directory <c>/usr/local/erl-target</c>.</item>
       <item>In the target directory reads the file <c>releases/start_erl.data</c>
-       in order to find the Erlang runtime system version ("5.1").</item>
+       in order to find the Erlang runtime system version ("5.10.4").</item>
       <item>Substitutes <c>%FINAL_ROOTDIR%</c> and <c>%EMU%</c> for
       <c>/usr/local/erl-target</c> and <c>beam</c>, respectively, in
        the files <c>erl.src</c>, <c>start.src</c>, and
-      <c>start_erl.src</c> of the target <c>erts-5.1/bin</c>
+      <c>start_erl.src</c> of the target <c>erts-5.10.4/bin</c>
        directory, and puts the resulting files <c>erl</c>,
       <c>start</c>, and <c>run_erl</c> in the target <c>bin</c>
        directory.</item>
@@ -185,6 +189,7 @@ lib/pea-1.0/        </code>
   </section>
 
   <section>
+    <marker id="start"/>
     <title>Starting a Target System</title>
     <p>Now we have a target system that can be started in various ways.</p>
     <p>We start it as a <em>basic target system</em> by invoking</p>
@@ -193,7 +198,7 @@ os> <input>/usr/local/erl-target/bin/erl</input></pre>
     <p>where only the <c>kernel</c> and <c>stdlib</c> applications are
       started, i.e. the system is started as an ordinary development
       system. There are only two files needed for all this to work:
-      <c>bin/erl</c> file (obtained from <c>erts-5.1/bin/erl.src</c>)
+      <c>bin/erl</c> file (obtained from <c>erts-5.10.4/bin/erl.src</c>)
       and the <c>bin/start.boot</c> file (a copy of <c>plain.boot</c>).</p>
     <p>We can also start a distributed system (requires <c>bin/epmd</c>).</p>
     <p>To start all applications specified in the original
@@ -208,9 +213,10 @@ os> <input>/usr/local/erl-target/bin/erl -boot /usr/local/erl-target/releases/FI
       <c>bin/run_erl</c>, which in turn calls <c>bin/start_erl</c>
       (roughly, <c>start_erl</c> is an embedded variant of
       <c>erl</c>). </p>
-    <p>The shell script <c>start</c> is only an example. You should
-      edit it to suite your needs. Typically it is executed when the
-      UNIX system boots.</p>
+    <p>The shell script <c>start</c>, which is generated from
+      erts-5.10.4/bin/start.src during installation, is only an
+      example. You should edit it to suite your needs. Typically it is
+      executed when the UNIX system boots.</p>
     <p><c>run_erl</c> is a wrapper that provides logging of output from
       the run-time system to file. It also provides a simple mechanism
       for attaching to the Erlang shell (<c>to_erl</c>).</p>
@@ -218,7 +224,7 @@ os> <input>/usr/local/erl-target/bin/erl -boot /usr/local/erl-target/releases/FI
       (<c>"/usr/local/erl-target"</c>), the releases directory
       (<c>"/usr/local/erl-target/releases"</c>), and the location of
       the <c>start_erl.data</c> file. It reads the run-time system
-      version (<c>"5.1"</c>) and release version (<c>"FIRST"</c>) from
+      version (<c>"5.10.4"</c>) and release version (<c>"FIRST"</c>) from
       the <c>start_erl.data</c> file, starts the run-time system of the
       version found, and provides <c>-boot</c> flag specifying the boot
       file of the release version found
@@ -258,6 +264,198 @@ os> <input>/usr/local/erl-target/bin/erl -boot /usr/local/erl-target/releases/FI
   </section>
 
   <section>
+    <title>Creating the Next Version</title>
+
+    <p>
+      In this example the <c>pea</c> application has been changed, and
+      so are <c>erts</c>, <c>kernel</c>, <c>stdlib</c> and
+      <c>sasl</c>.
+    </p>
+
+    <p>
+      <em>Step 1.</em> Create the <c>.rel</c> file:
+    </p>
+    <code type="none">
+%% mysystem2.rel
+{release,
+ {"MYSYSTEM", "SECOND"},
+ {erts, "6.0"},
+ [{kernel, "3.0"},
+  {stdlib, "2.0"},
+  {sasl, "2.4"},
+  {pea, "2.0"}]}.</code>
+    <p>
+      <em>Step 2.</em> Create the application upgrade file (see
+      <seealso marker="sasl:appup">appup(4)</seealso>) for <c>pea</c>,
+      for example:
+    </p>
+    <code type="none">
+%% pea.appup
+{"2.0",
+ [{"1.0",[{load_module,pea_lib}]}],
+ [{"1.0",[{load_module,pea_lib}]}]}.</code>
+    <p>
+      <em>Step 3.</em> From the directory where the
+      <c>mysystem2.rel</c> file reside, start the Erlang/OTP system:
+    </p>
+    <pre>
+os> <input>erl -pa /home/user/target_system/myapps/pea-2.0/ebin</input></pre>
+    <p>giving the path to the new version of <c>pea</c>. </p>
+
+    <p>
+      <em>Step 4.</em> Create the release upgrade file (see <seealso
+      marker="sasl:relup">relup(4)</seealso>):
+    </p>
+    <pre>
+1> <input>systools:make_relup("mysystem2",["mysystem"],["mysystem"],[{path,["/home/user/target_system/myapps/pea-1.0/ebin","/my/old/erlang/lib/*/ebin"]}]).</input></pre>
+    <p>
+      where <c>"mysystem"</c> is the base release and
+      <c>"mysystem2"</c> is the release to upgrade to.
+    </p>
+    <p>
+      Note that the <c>path</c> option is used for pointing out the
+      old version of all applications. (The new versions are already
+      in the code path - assuming of course that the erlang node on
+      which this is executed is running the correct version of
+      Erlang/OTP.)
+    </p>
+    <p>
+      <em>Step 5.</em> Create the new release:
+    </p>
+    <pre>
+2> <input>target_system:create("mysystem2").</input></pre>
+    <p>
+      Given that the <c>relup</c> file generated in step 4 above is
+      now located in the current directory, it will automatically be
+      included in the release package.
+    </p>
+  </section>
+
+  <section>
+    <title>Upgrading the Target System</title>
+    <p>
+      This part is done on the target node, and for this example we
+      want the node to be running as an embedded system with the
+      <c>-heart</c> option, allowing automatic restart of the
+      node. See <seealso marker="#start">Starting a Target
+      System</seealso> above for more information.
+    </p>
+    <p>
+      We add <c>-heart</c> to <c>bin/start</c>:
+    </p>
+    <code type="none">
+#!/bin/sh
+ROOTDIR=/usr/local/erl-target/
+
+if [ -z "$RELDIR" ]
+then
+   RELDIR=$ROOTDIR/releases
+fi
+
+START_ERL_DATA=${1:-$RELDIR/start_erl.data}
+
+$ROOTDIR/bin/run_erl -daemon /tmp/ $ROOTDIR/log "exec $ROOTDIR/bin/start_erl $ROOTDIR $RELDIR $START_ERL_DATA -heart</code>
+    <p>
+      And we use the simplest possible <c>sys.config</c>, which we
+      store in <c>releases/FIRST</c>:
+    </p>
+    <code type="none">
+%% sys.config
+[].</code>
+    <p>
+      Finally, in order to prepare the upgrade, we need to put the new
+      release package in the <c>releases</c> directory of the first
+      target system:
+    </p>
+    <pre>
+os> <input>cp mysystem2.tar.gz /usr/local/erl-target/releases</input></pre>
+    <p>
+      And assuming that the node has been started like this:
+    </p>
+    <pre>
+os> <input>/usr/local/erl-target/bin/start</input></pre>
+    <p>
+      it can be accessed like this:
+    </p>
+    <pre>
+os> <input>/usr/local/erl-target/bin/to_erl /tmp/erlang.pipe.1</input></pre>
+    <p>
+      Also note that logs can be found in
+      <c>/usr/local/erl-target/log</c>. This directory is specified as
+      an argument to <c>run_erl</c>in the start script listed above.
+    </p>
+    <p>
+      <em>Step 1.</em> Unpack the release:
+    </p>
+    <pre>
+1> <input>{ok,Vsn} = release_handler:unpack_release("mysystem2").</input></pre>
+    <p>
+      <em>Step 2.</em> Install the release:
+    </p>
+    <pre>
+2> <input>release_handler:install_release(Vsn).</input>
+<output>{continue_after_restart,"FIRST",[]}
+heart: Tue Apr  1 12:15:10 2014: Erlang has closed.
+heart: Tue Apr  1 12:15:11 2014: Executed "/usr/local/erl-target/bin/start /usr/local/erl-target/releases/new_start_erl.data" -> 0. Terminating.
+[End]</output></pre>
+    <p>
+      The above return value and output after the call to
+      <c>release_handler:install_release/1</c> means that the
+      <c>release_handler</c> has restarted the node by using
+      <c>heart</c>. This will always be done when the upgrade involves
+      a change of <c>erts</c>, <c>kernel</c>, <c>stdlib</c> or
+      <c>sasl</c>. See <seealso marker="upgrade">Upgrade when
+      Erlang/OTP has Changed</seealso> for more infomation about this.
+    </p>
+    <p>
+      The node will be accessable via a new pipe:
+    </p>
+    <pre>
+os> <input>/usr/local/erl-target/bin/to_erl /tmp/erlang.pipe.2</input></pre>
+    <p>
+      Let's see which releases we have in our system:
+    </p>
+    <pre>
+1> <input>release_handler:which_releases().</input>
+<output>[{"MYSYSTEM","SECOND",
+  ["kernel-3.0","stdlib-2.0","sasl-2.4","pea-2.0"],
+  current},
+ {"MYSYSTEM","FIRST",
+  ["kernel-2.16.4","stdlib-1.19.4","sasl-2.3.4","pea-1.0"],
+  permanent}]</output></pre>
+    <p>
+      Our new release, "SECOND", is now the current release, but we
+      can also see that our "FIRST" release is still permanent. This
+      means that if the node would be restarted at this point, it
+      would come up running the "FIRST" release again.
+    </p>
+    <p>
+      <em>Step 3.</em> Make the new release permanent:
+    </p>
+    <pre>
+2> <input>release_handler:make_permanent("SECOND").</input></pre>
+
+    <p>
+      Now look at the releases again:
+    </p>
+
+    <pre>
+3> <input>release_handler:which_releases().</input>
+<output>[{"MYSYSTEM","SECOND",
+  ["kernel-3.0","stdlib-2.0","sasl-2.4","pea-2.0"],
+  permanent},
+ {"MYSYSTEM","FIRST",
+  ["kernel-2.16.4","stdlib-1.19.4","sasl-2.3.4","pea-1.0"],
+  old}]</output></pre>
+
+  <p>
+    Here we see that the new release version is <c>permanent</c>, so
+    it would be safe to restart the node.
+  </p>
+
+  </section>
+
+  <section>
     <title>Listing of target_system.erl</title>
     <p>This module can also be found in the <c>examples</c> directory
       of the <c>sasl</c> application.</p>
diff --git a/system/doc/system_principles/part.xml b/system/doc/system_principles/part.xml
index 811428baae..d05c89fde8 100644
--- a/system/doc/system_principles/part.xml
+++ b/system/doc/system_principles/part.xml
@@ -4,7 +4,7 @@
 <part xmlns:xi="http://www.w3.org/2001/XInclude">
   <header>
     <copyright>
-      <year>1997</year><year>2013</year>
+      <year>1997</year><year>2014</year>
       <holder>Ericsson AB. All Rights Reserved.</holder>
     </copyright>
     <legalnotice>
@@ -31,6 +31,6 @@
   <xi:include href="system_principles.xml"/>
   <xi:include href="error_logging.xml"/>
   <xi:include href="create_target.xml"/>
+  <xi:include href="upgrade.xml"/>
   <xi:include href="versions.xml"/>
 </part>
-
diff --git a/system/doc/system_principles/upgrade.xml b/system/doc/system_principles/upgrade.xml
new file mode 100644
index 0000000000..68e48da0b8
--- /dev/null
+++ b/system/doc/system_principles/upgrade.xml
@@ -0,0 +1,118 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<!DOCTYPE chapter SYSTEM "chapter.dtd">
+
+<chapter>
+  <header>
+    <copyright>
+      <year>2014</year>
+      <holder>Ericsson AB. All Rights Reserved.</holder>
+    </copyright>
+    <legalnotice>
+      The contents of this file are subject to the Erlang Public License,
+      Version 1.1, (the "License"); you may not use this file except in
+      compliance with the License. You should have received a copy of the
+      Erlang Public License along with this software. If not, it can be
+      retrieved online at http://www.erlang.org/.
+
+      Software distributed under the License is distributed on an "AS IS"
+      basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
+      the License for the specific language governing rights and limitations
+      under the License.
+
+    </legalnotice>
+
+    <title>Upgrade when Erlang/OTP has Changed</title>
+    <prepared></prepared>
+    <responsible></responsible>
+    <docno></docno>
+    <approved></approved>
+    <checked></checked>
+    <date>2014-02-19</date>
+    <rev></rev>
+    <file>upgrade.xml</file>
+  </header>
+
+  <section>
+    <title>Introduction</title>
+    <p>
+      As of Erlang/OTP 17, most applications deliver a valid
+      application upgrade (<c>appup</c>) file. In earlier releases, a
+      majority of the applications in Erlang/OTP did not support
+      upgrade at all. Many of the applications use the
+      <c>restart_application</c> instruction. These are applications
+      for which it is not crucial to support real soft upgrade, for
+      instance tools and library applications. The
+      <c>restart_application</c> instruction
+      ensures that all modules in the application are reloaded and
+      thereby running the new code.
+    </p>
+  </section>
+
+  <section>
+    <title>Upgrade of core applications</title>
+    <p>
+      The core applications ERTS, Kernel, STDLIB
+      and SASL never allow real soft upgrade, but require the
+      Erlang emulator to be restarted. This is indicated to the
+      <c>release_handler</c> by the upgrade instruction
+      <c>restart_new_emulator</c>. This instruction will always be the
+      very first instruction executed, and it will restart the
+      emulator with the new versions of the above mentioned core
+      applications and the old versions of all other
+      applications. When the node is back up all other upgrade instructions are
+      executed, making sure each application is finally running its
+      new version.
+    </p>
+
+    <p>
+      It might seem strange to do a two-step upgrade instead of
+      just restarting the emulator with the new version of all
+      applications. The reason for this design decision is to allow
+      <c>code_change</c> functions to have side effects, for example changing
+      data on disk. It also makes sure that the upgrade mechanism for
+      non-core applications does not differ depending on whether or not
+      core applications are changed at the same time.
+    </p>
+
+    <p>
+      If, however, the more brutal variant is preferred, it is
+      possible to handwrite the release upgrade file using only the
+      single upgrade instruction <c>restart_emulator</c>. This
+      instruction, in contrast to <c>restart_new_emulator</c>, will
+      cause the emulator to restart with the new versions of
+      <em>all</em> applications.
+    </p>
+
+    <p>
+      <em>Note</em> that if other instructions are included before
+      <c>restart_emulator</c> in the handwritten <c>relup</c> file,
+      they will be executed in the old emulator. This is a big risk
+      since there is no guarantee that new beam code can be loaded
+      into the old emulator. Adding instructions after
+      <c>restart_emulator</c> has no effect as the
+      <c>release_handler</c> will not do any attempt at executing
+      them.
+    </p>
+
+    <p>
+      See <seealso marker="sasl:relup">relup(4)</seealso> for
+      information about the release upgrade file, and <seealso
+      marker="sasl:appup">appup(4)</seealso> for further information
+      about upgrade instructions.
+    </p>
+  </section>
+
+  <section>
+    <title>Applications that still do not allow code upgrade</title>
+    <p>
+      A few applications, for instance HiPE do not support
+      upgrade at all. This is indicated by an application upgrade file
+      containing only <c>{Vsn,[],[]}</c>. Any attempt at creating a release
+      upgrade file with such input will fail.
+      The only way to force an upgrade involving applications like this is to
+      handwrite the <c>relup</c> file, preferably as described above
+      with only the <c>restart_emulator</c> instruction.
+    </p>
+
+  </section>
+</chapter>
diff --git a/system/doc/system_principles/xmlfiles.mk b/system/doc/system_principles/xmlfiles.mk
index 9743949798..c60ffbad28 100644
--- a/system/doc/system_principles/xmlfiles.mk
+++ b/system/doc/system_principles/xmlfiles.mk
@@ -1,7 +1,7 @@
 #
 # %CopyrightBegin%
 # 
-# Copyright Ericsson AB 2009. All Rights Reserved.
+# Copyright Ericsson AB 2009-2014. All Rights Reserved.
 # 
 # The contents of this file are subject to the Erlang Public License,
 # Version 1.1, (the "License"); you may not use this file except in
@@ -20,4 +20,5 @@ SYSTEM_PRINCIPLES_CHAPTER_FILES = \
 	system_principles.xml \
 	error_logging.xml \
 	create_target.xml \
+	upgrade.xml \
 	versions.xml