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Add more test to the userguide.
This commit is contained in:
Andreas Schneider
parent
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commit
0f6a55bb23
2 changed files with 194 additions and 32 deletions
109
doc/csync.txt
109
doc/csync.txt
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@ -13,13 +13,13 @@ Introduction
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It is often the case that we have multiple copies (called replicas) of a
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filesystem or part of a filesystem (for example on a notebook and on a desktop
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computer). Changes to each replica are often made independently and as a
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computer). Changes to each replica are often made independently and as a
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result they do not contain the same information. In that case a file
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synchronizer is used to make them consistent again, without loosing any
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information.
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The goal is to detect conflicting <<X13, updates>> (files which has been
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modified) and propagate non-conflicting updates to each replica. If there
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modified) and propagate non-conflicting updates to each replica. If there
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are no conflicts left we are done and the replicas are identical.
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Basics
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@ -115,10 +115,28 @@ filesystem. It decides which file:
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A wrong decision of the reconciler leads in most cases to a loss of data. So there
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are several conditions a the file synchronizer has to follow.
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Specification
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^^^^^^^^^^^^^
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Algorithms
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^^^^^^^^^^
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TODO
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For conflict resolution several different algorithms could be implemented. The
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most common algorithm are the merge and and the conflict algorithm. The first
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is a batch algortihm and the second is one which needs user interaction.
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Merge algorithm
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+++++++++++++++
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The merge algorithm is an algorithm which doesn't need any user interaction. It
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is simple and used for example by Microsoft for Roaming Proflies. If it detects
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a conflict (the same file changed on both replicas) then it will use the most
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recent file and overwrite the other. This means you can loose some data, but
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normally you want the latest file.
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Conflict algorithm
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++++++++++++++++++
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This is not implemented yet.
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If a file has a conflict the user has to decicde which file should be used.
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Propagation
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~~~~~~~~~~~
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@ -126,28 +144,73 @@ Propagation
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The next instance of the file synchronizer the propagator. It uses the
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calculated records to apply them on the current replica.
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* 2-phase-copy
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* merge trees and write journal
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The propagator uses a 2-phase-commit mechanism to simulate an atomic filesystem
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operation.
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In the first phase we copy the file to a temporary file on the opposite
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replica. This has the advantage that we can check if file which has been copied
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to the opposite replica has been transfered successfully. If the connection
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gets interruppted during the transfer we still have the orignal states of the
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file. This means no data will be lost.
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In the second phase the the file on the opposite replica will be overwritten by
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the temporary file.
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After a successfull propagation we have to merge the trees to reflect the
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current state of the filesystem tree. This updated tree will be written as a
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journal into a database. The database is called the state database. It will be
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used during the update detection of the next synchronization. See above.
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Robustness
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~~~~~~~~~~
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TODO
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This is a really important topic. The file synchronizer should not crash and if
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it crashed, there should be no loss of data. To achieve this goal there are
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several mechanism to prevent this. These mechnanism will be discussed in the
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following sections.
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Crash resistance
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^^^^^^^^^^^^^^^^
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TODO
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The synchronization process can be interrupted by different events, this can
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be:
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* the system could be halted due to errors.
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* the disk could be full or the quota exceeded.
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* the network or power cable could be pulled out.
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* the user could force a stop of the synchronization process.
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* different communication errors could occur.
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That no data will be lost due to the occurance we enforce the following
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invariant:
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IMPORTANT: At every moment of the synchronization each file has either its
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original content or its correct final content.
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So each interupted synchronization process is a partial sync and can be
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continued and completed by simply running csync again. The only problem could
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be an error of the filesystem. So we reach this invariant only approximatly.
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Transfer errors
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^^^^^^^^^^^^^^^
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TODO
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With the Two-Phase-Commit we check the file size after the file has
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transferred. So we can detect transfer erros. Better would be a transfer
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protocol with checksums. This could possibly done in the future.
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Future filesystems like btrfs will help to compare checksums instead of the
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filesize. This will make the synchronization itself safer.
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Database loss
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^^^^^^^^^^^^^
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TODO
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It could be possible, that the state database get corrupted. If this happens
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all files get evaluated. In this case the file synchronizer wont delete any
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file, but it could occur that deleted files will be restored from the other
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replica.
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To prevent a corruption or loss of the database if an error occurs or the user
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forces an abort, the synchronizer is working on a copy of the database and will
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use a 2-Phase-Commit to save it at the end.
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Getting started
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---------------
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@ -160,17 +223,33 @@ procedures. Packagers take a look at <<X90, Appendix B: Packager Notes>>.
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Using the commandline client
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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TODO
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csync /home/csync sftp://TODO:secret@server:port/profile/TODO
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The synopsis of the commandline client is
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csync [OPTION...] SOURCE DESTINATION
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It synchronizes the content of SOURCE with DESTINATION and vice versa. The
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DESTINATION can be a local directory or a remote file server.
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csync /home/csync scheme://user:password@server:port/full/path
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The remote destination is supported by plugins. By default csync ships with smb
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and sftp support. For more information, see the manpage of csync(1).
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The PAM module
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~~~~~~~~~~~~~~
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TODO
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pam_csync is a PAM module to provide roaming home directories for a user
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session. This module is aimed at environments with central file servers a user
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wishes to store his home directory. The Authentication Module verifies the
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identity of a user and triggers a synchronization with the server on the first
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login and the last logout. More information can be found in the manpage of the
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module pam_csync(8).
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[[X90]]
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Appendix A: Packager Notes
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--------------------------
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Read the `README` and `INSTALL` files (in the distribution root
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Read the `README`, `INSTALL` and `FAQ` files (in the distribution root
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directory).
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@ -437,12 +437,12 @@ is a user-level program which means you don't need to be a superuser.</p></div>
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<div class="sectionbody">
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<div class="para"><p>It is often the case that we have multiple copies (called replicas) of a
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filesystem or part of a filesystem (for example on a notebook and on a desktop
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computer). Changes to each replica are often made independently and as a
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computer). Changes to each replica are often made independently and as a
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result they do not contain the same information. In that case a file
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synchronizer is used to make them consistent again, without loosing any
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information.</p></div>
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<div class="para"><p>The goal is to detect conflicting <a href="#X13">updates</a> (files which has been
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modified) and propagate non-conflicting updates to each replica. If there
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modified) and propagate non-conflicting updates to each replica. If there
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are no conflicts left we are done and the replicas are identical.</p></div>
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</div>
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<h2 id="_basics">2. Basics</h2>
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@ -566,31 +566,98 @@ or gets <strong>deleted</strong>
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</ul></div>
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<div class="para"><p>A wrong decision of the reconciler leads in most cases to a loss of data. So there
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are several conditions a the file synchronizer has to follow.</p></div>
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<h4 id="_specification">3.2.1. Specification</h4>
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<div class="para"><p>TODO</p></div>
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<h4 id="_algorithms">3.2.1. Algorithms</h4>
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<div class="para"><p>For conflict resolution several different algorithms could be implemented. The
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most common algorithm are the merge and and the conflict algorithm. The first
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is a batch algortihm and the second is one which needs user interaction.</p></div>
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<h5 id="_merge_algorithm">Merge algorithm</h5>
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<div class="para"><p>The merge algorithm is an algorithm which doesn't need any user interaction. It
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is simple and used for example by Microsoft for Roaming Proflies. If it detects
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a conflict (the same file changed on both replicas) then it will use the most
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recent file and overwrite the other. This means you can loose some data, but
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normally you want the latest file.</p></div>
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<h5 id="_conflict_algorithm">Conflict algorithm</h5>
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<div class="para"><p>This is not implemented yet.</p></div>
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<div class="para"><p>If a file has a conflict the user has to decicde which file should be used.</p></div>
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<h3 id="_propagation">3.3. Propagation</h3><div style="clear:left"></div>
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<div class="para"><p>The next instance of the file synchronizer the propagator. It uses the
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calculated records to apply them on the current replica.</p></div>
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<div class="para"><p>The propagator uses a 2-phase-commit mechanism to simulate an atomic filesystem
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operation.</p></div>
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<div class="para"><p>In the first phase we copy the file to a temporary file on the opposite
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replica. This has the advantage that we can check if file which has been copied
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to the opposite replica has been transfered successfully. If the connection
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gets interruppted during the transfer we still have the orignal states of the
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file. This means no data will be lost.
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In the second phase the the file on the opposite replica will be overwritten by
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the temporary file.</p></div>
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<div class="para"><p>After a successfull propagation we have to merge the trees to reflect the
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current state of the filesystem tree. This updated tree will be written as a
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journal into a database. The database is called the state database. It will be
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used during the update detection of the next synchronization. See above.</p></div>
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<h3 id="_robustness">3.4. Robustness</h3><div style="clear:left"></div>
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<div class="para"><p>This is a really important topic. The file synchronizer should not crash and if
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it crashed, there should be no loss of data. To achieve this goal there are
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several mechanism to prevent this. These mechnanism will be discussed in the
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following sections.</p></div>
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<h4 id="_crash_resistance">3.4.1. Crash resistance</h4>
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<div class="para"><p>The synchronization process can be interrupted by different events, this can
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be:</p></div>
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<div class="ilist"><ul>
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<li>
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<p>
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2-phase-copy
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the system could be halted due to errors.
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</p>
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</li>
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<li>
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<p>
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merge trees and write journal
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the disk could be full or the quota exceeded.
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</p>
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</li>
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<li>
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<p>
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the network or power cable could be pulled out.
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</p>
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</li>
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<li>
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<p>
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the user could force a stop of the synchronization process.
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</p>
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</li>
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<li>
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<p>
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different communication errors could occur.
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</p>
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</li>
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</ul></div>
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<h3 id="_robustness">3.4. Robustness</h3><div style="clear:left"></div>
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<div class="para"><p>TODO</p></div>
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<h4 id="_crash_resistance">3.4.1. Crash resistance</h4>
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<div class="para"><p>TODO</p></div>
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<div class="para"><p>That no data will be lost due to the occurance we enforce the following
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invariant:</p></div>
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<div class="admonitionblock">
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<table><tr>
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<td class="icon">
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<img src="./images/icons/important.png" alt="Important" />
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</td>
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<td class="content">At every moment of the synchronization each file has either its
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original content or its correct final content.</td>
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</tr></table>
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</div>
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<div class="para"><p>So each interupted synchronization process is a partial sync and can be
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continued and completed by simply running csync again. The only problem could
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be an error of the filesystem. So we reach this invariant only approximatly.</p></div>
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<h4 id="_transfer_errors">3.4.2. Transfer errors</h4>
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<div class="para"><p>TODO</p></div>
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<div class="para"><p>With the Two-Phase-Commit we check the file size after the file has
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transferred. So we can detect transfer erros. Better would be a transfer
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protocol with checksums. This could possibly done in the future.</p></div>
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<div class="para"><p>Future filesystems like btrfs will help to compare checksums instead of the
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filesize. This will make the synchronization itself safer.</p></div>
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<h4 id="_database_loss">3.4.3. Database loss</h4>
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<div class="para"><p>TODO</p></div>
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<div class="para"><p>It could be possible, that the state database get corrupted. If this happens
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all files get evaluated. In this case the file synchronizer wont delete any
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file, but it could occur that deleted files will be restored from the other
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replica.
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To prevent a corruption or loss of the database if an error occurs or the user
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forces an abort, the synchronizer is working on a copy of the database and will
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use a 2-Phase-Commit to save it at the end.</p></div>
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</div>
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<h2 id="_getting_started">4. Getting started</h2>
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<div class="sectionbody">
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@ -598,19 +665,35 @@ merge trees and write journal
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<div class="para"><p>See the <tt>README</tt> and <tt>INSTALL</tt> files for install prerequisites and
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procedures. Packagers take a look at <a href="#X90">Appendix B: Packager Notes</a>.</p></div>
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<h3 id="_using_the_commandline_client">4.2. Using the commandline client</h3><div style="clear:left"></div>
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<div class="para"><p>TODO
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csync /home/csync sftp://TODO:secret@server:port/profile/TODO</p></div>
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<div class="para"><p>The synopsis of the commandline client is</p></div>
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<div class="literalblock">
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<div class="content">
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<pre><tt>csync [OPTION...] SOURCE DESTINATION</tt></pre>
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</div></div>
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<div class="para"><p>It synchronizes the content of SOURCE with DESTINATION and vice versa. The
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DESTINATION can be a local directory or a remote file server.</p></div>
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<div class="literalblock">
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<div class="content">
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<pre><tt>csync /home/csync scheme://user:password@server:port/full/path</tt></pre>
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</div></div>
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<div class="para"><p>The remote destination is supported by plugins. By default csync ships with smb
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and sftp support. For more information, see the manpage of <tt>csync(1)</tt>.</p></div>
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<h3 id="_the_pam_module">4.3. The PAM module</h3><div style="clear:left"></div>
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<div class="para"><p>TODO</p></div>
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<div class="para"><p>pam_csync is a PAM module to provide roaming home directories for a user
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session. This module is aimed at environments with central file servers a user
|
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wishes to store his home directory. The Authentication Module verifies the
|
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identity of a user and triggers a synchronization with the server on the first
|
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login and the last logout. More information can be found in the manpage of the
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module pam_csync(8).</p></div>
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</div>
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<h2 id="X90">5. Appendix A: Packager Notes</h2>
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<div class="sectionbody">
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<div class="para"><p>Read the <tt>README</tt> and <tt>INSTALL</tt> files (in the distribution root
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<div class="para"><p>Read the <tt>README</tt>, <tt>INSTALL</tt> and <tt>FAQ</tt> files (in the distribution root
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directory).</p></div>
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</div>
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<div id="footer">
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<div id="footer-text">
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Last updated 2008-11-20 12:16:02 CEST
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Last updated 2008-12-17 15:38:27 CEST
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</div>
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</div>
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</body>
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|
|
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