Similarities

• Both Solr and Sphinx satisfy all of your requirements. They're fast and designed to index and search large bodies of data efficiently.
• Both have a long list of high-traffic sites using them (Solr, Sphinx)
• Both offer commercial support. (Solr, Sphinx)
• Both offer client API bindings for several platforms/languages (Sphinx, Solr)
• Both can be distributed to increase speed and capacity (Sphinx, Solr)

Here are some differences

• Solr, being an Apache project, is obviously is Apache2-licensed. Sphinx is GPLv2. This means that if you ever need to embed or extend (not just "use") Sphinx in a commercial application, you'll have to buy a commercial license.
• Solr is easily embeddable in Java applications.
• Solr is built on top of Lucene, which is a proven technology over 7 years old with a huge user base (this is only a small part). Whenever Lucene gets a new feature or speedup, Solr gets it too. Many of the devs committing to Solr are also Lucene committers.
• Sphinx integrates more tightly with RDBMSs, especially MySQL.
• Solr can be integrated with Hadoop to build distributed applications
• Solr can be integrated with Nutch to quickly build a fully-fledged web search engine with crawler.
• Solr can index proprietary formats like Microsoft Word, PDF, etc. Sphinx can't.
• Solr comes with a spell-checker out of the box.
• Solr comes with facet support out of the box. Faceting in Sphinx takes more work.
• Sphinx doesn't allow partial index updates for field data.
• In Sphinx, all document ids must be unique unsigned non-zero integer numbers. Solr doesn't even require a unique key for many operations, and unique keys can be either integers or strings.
• Solr supports field collapsing to avoid duplicating similar results. Sphinx doesn't seem to provide any feature like this.

Related questions

• http://stackoverflow.com/questions/1284083/choosing-a-stand-alone-full-text-search-server-sphinx-or-solr
• http://stackoverflow.com/questions/1132284/full-text-searching-with-rails
• http://stackoverflow.com/questions/737275/pros-cons-of-full-text-search-engine-lucene-sphinx-postgresql-full-text-searc

Conclusion

In my experience, Solr is very-very fast on the query side. It is also very powerful. The indexing side is very CPU and memory intensive and is an unfortunate side effect of having such a feature-rich, fast application. Nevertheless, I highly recommend Solr.

For disclaimer purposes, I have not had much experience with Sphinx and, again, all credit for this comparison goes to mausch.

By the way, here's a really good resource for Solr 1.4 that just came out: Solr 1.4 Enterprise Search. I have this book and it's quite helpful in explaining such topics as multicore setup, search methods, replication, etc.

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I came up with 2, depending on how detailed the information needs to be, which I'm about to share in this post.

1. If you want exact file and directory names, you can run:

svn status -u

If any files need updating, you will see a * before the file name.

svn status wc
M     wc/bar.c
A  +   wc/qax.c

svn status -u wc
M            965    wc/bar.c
       *     965    wc/foo.c
A  +         965    wc/qax.c
Status against revision:   981

Or more verbose

svn status --show-updates --verbose wc
M            965       938 kfogel       wc/bar.c
       *     965       922 sussman      wc/foo.c
A  +         965       687 joe          wc/qax.c
             965       687 joe          wc/zig.c
Status against revision:   981

Parsing the output in Perl, for instance, should be trivial. A connection to the repository is established for this check, so be sure to catch in your code the times when such connection is not available.

2. If you only care about whether a specific directory needs to be updated or not, here's a quicker method:

svn info vs svn info -rHEAD

cd somedir;
svn info -r HEAD | grep -i "Last Changed Rev"
Last Changed Rev: 8544
svn info | grep -i "Last Changed Rev"
Last Changed Rev: 8531

If these numbers are not the same, an update is needed.

What can I be missing? Are there any other creative ways, or is svn update going to support dry-run? Feel free to leave a comment if you know something I don't.

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