Monday, January 21, 2008

RAC Stretch Clusters - a survey

There's been an interesting digression on the Oracle-L list today on the subject of RAC stretch clusters.

A stretch cluster is one in which nodes are separated - possibly by several miles - mainly as a precaution against a complete data-centre outage. I know of a couple of examples where separations in the region of 20-30 miles (30-50 km) are either in use or planned, and other posters on Oracle-L have mentioned "several" or "a handful" of implementations worldwide.

Obviously, the main performance issue for a stretch cluster is the latency and bandwidth of the interconnect. The bandwidth isn't affected by distance, but the latency certainly is.

I'd be very interested to hear from anyone who has implemented a stretch cluster (in test or production) with as much detail as you are free to pass on, particularly:
  • what is the distance between sites
  • how many nodes at each site
  • is the workload evenly distributed (active/active homogeneous), partitioned (active/active, heterogeneous) or uneven (active/passive)
  • some indication of database size and transaction rates (in whatever units are meaningful to you
  • Any performance issues?

Either email me (nigel at preferisco dot com) or comment below. I would like to publish the results but let me know if any part(s) of your information is too sensitive to be broadcast, even anonymously.

Thanks in advance...

Friday, January 18, 2008


I thought I'd seen all the permutations of Wikis out there: written in php, stored as files; written in PL/SQL and stored in Oracle; and with a range of different markup conventions. And recently I've successfully used wiki style markup to better decorate output from Oracle Designer (of which more later in a separate post).

Today, in one of those serendipitious chances that comes from following a couple of irrelevant but intriguing blog links, I tripped over TiddlyWiki. Rather than giving you a server-side wiki, the whole thing is packaged on the client side. A TiddlyWiki is a single HTML file that contains its own code; which self-edits as you add content. You save the file locally (and can then publish to a website if you like).

As a tool written by web designers it just looks great. You don't just click and get another page; the expanded content (called a Tiddler) zooms out at you. You can choose to expand or collapse whichever tiddlers you like - at the same time. That may be a big advantage over more boring wiki implementations.

For large communities, it's probably all wrong - personal changes in effect fork the entire wiki; but it may be a very effective personal tool - kind of mind mapping on the cheap, but much prettier than editors like treepad (and instantly portable, no software install required - keep it on your USB stick).

Basic usage instructions can be found here - written as a TiddlyWiki so you can try it out. I'm going to see if I can turn a rather unexciting Designer entity report into a thing of beauty...