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PTP/designs/remote/sync

< PTP‎ | designs‎ | remote
Revision as of 22:21, 22 November 2010 by Roland.utk.edu (Talk | contribs)

Introduction

This document should describe the current state of the ideas and designs for a synchronization based file access for remote projects. This document in it it's original version has been created by Roland Schulz based on discussions on bug 316709.

Rationale

Both RemoteTools and RSE use SFTP to access files when editing them. This has several disadvantages:

  • Working on files when not connected to the internet
  • Responsiveness of UI
  • Not all PTP functions are supported using RemoteTools/RDT

The synchronization approach has different disadvantages (see below). Thus this is not meant to replace the current RemoteTools/RDT approach but offer an alternative. This will allow the user to choose the approach with those advantages more important to his working environment. Also this approach will reuse parts from RDT (e.g. scanner/indexer) from RDT.

Responsiveness

The Eclipse core and CDT doing most of the file operations in the main thread (assuming that all file operations are low latency). This causes a responsiveness problems with a remote file system.

Because the file operations are in the main thread they block the GUI until the IO operation finishes and thus preventing the user to continue the work while the IO operation is running. It also often prevents IO operations which could run in parallel to do so. See Bugs 160353, 177994, 195997, 218387, 219169 and wiki.eclipse.org/TM_and_RSE_FAQ from the RSE team regarding the same problem for RSE. Their seems to be no work-around for this problem. While it seams in theory to be possible to improve it somewhat by using Display.readAndDispatch, it is not advised and has been removed from RSE (160353). Having a responsive UI is considered by many extremely important thus this is an important point.

It is very unlikely, at least for the medium-term (meaning the next Eclipse release in 2011), that both Eclipse Core and CDT move all file operations into threads and hide latency by doing IO operations in parallel. Therefore a different approach is needed to have s performant remote IO method.

Disadvantages of Synchronizing approach

  • The entire project must be copied to the local

machine. This only happens once, but could take a very long time for large projects/slow connections.

  • Local indexing is problematic as the local environment will be different

from the remote environment, so macros and includes will be incorrect. Running scanner discovery remotely seems to be the obvious way to solve the macro problem, but scanner discovery is hopelessly broken and not even the CDT people seem to know how it works. In addition, the indexer would need to be modified to copy system and library includes from the remote machine as part of the indexing.

  • Some activities, such as building, will always need to be done remotely, so

the performance problems will always be evident to some degree.

Similar/Prior efforts

Within Eclipse

RSE

Phortran

The current implementation is a strawman prototype of a rsync-based remotely-synchronized project. It adds a new project wizard which creates a C/Fortran project but replaces the standard CDT build command (make) with a call to a custom shell script which uses rsync to copy the project to a remote server and run make remotely. This was definitely a prototype -- I'm sure the final version won't look anything like it (e.g., our build script makes two or three separate connections to the remote machine) -- but this is *simple* and it works, more or less, which gave us something real to try out.

Remote Include Path Support adds remote (Fortran) INCLUDE paths to Photran. Photran's include paths are configured in the project properties. Traditionally, they'd be paths on the local maching (e.g., /usr/include:/usr/local/include). This replaces them with URIs, so they can be on either the local machine or a remote one (e.g., rse://remotehost/usr/include:file:///usr/include). It also changes the properties page to use a remote file selection dialog box.

Outside from Eclipse

Synchronization using GIT

Remote file systems

Options at what time the synchronization is done

  • Synchronizing before any remote operation (build, remote index, ...)
  • Synchronizing after each save

The 2nd option shouldn't wait on the sync but do it asynchronous. Otherwise the responsiveness problem (see above) wouldn't be addressed. Each remote operation would need to call a function to guarantee that all outstanding synchronization calls have finished. The same function would initiate the synchronization for option 1. Which option is better depends on the synchronization back-end and the user preferences and should just be configurable.

Pro after each save:

  • Required for Auto-Build and Indexing on Server
  • Reduces time to build (because is already synced)

Contra

  • Causes larger repository (~2k per commit) and more traffic

Possible Back-ends

Advantages of Rsync

TBD

Disadvantages of Rsync

  • no JAVA implementation is available
  • the synchronization is only one-way

The later is important if, either automatically or by the user, remote files get changed. The one-way synchronization of rsync would usually not synchronize changes to the client and would not detect conflicts caused by changes on both sides very well.

Advantages of GIT

  • It has a java implementation (shipping with Helios)
  • is known to be extremely fast (including the java implementation)
  • supports two way synchronization.

Of course GIT is not meant as a synchronization tool (but a DCVS) but it works as a synchronization tool extremely well. Using git for synchronization would work both for those users using it also for version control and for those users using some other tool for version control. As an example a remote synchronization of a folder containing ~4000files (1 changed - which unknown to GIT), ~100MB, where GIT detects file changes on both sides, over a remote connection (cable), takes less than one second. The performance is mainly limited by the file system for the tree traversal.

Implementation Issues with GIT

To push to a non-bare repository is discouraged

Their are different options

  • Fetch (not good option because it requires SSHD on the client)
  • Push into working branch with post-update hook. Disadvantages: Requires

stat of each file on server (slow over NFS) and doesn't allow merge on client side

  • Push to separate bare repository. Disadvantage: Requires 2 repositories
  • Push to remote branch. Seems best option

Indexing

Local indexing should be supported. Should we also support remote indexing? Local indexing requires remote include files.

Support for other Remote Tools besides Build

TBD

Milestones

I propose the following steps:

  • Define a new Synchronization service type (which add synchronization/replication to the running EFS). It would have as public method guranteeSynchronized. The default server (for a purely local project or for remotetools/RSE) would do nothing.
  • Add to all remote operations (compile, remote index, ..) a call to gureanteeSyncronized
  • Implement a GIT based synchronization service (doing the GIT push in the gureanteeSyncronized call)
  • Add the GUI to configure the synronization service

Later an EFS which would do the asynchronous GIT push after a file modifcation(e.g. save) and the gureanteeSyncronized would just wait for the push to finish.

Timeline: TBD

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