# Fault-tolerant persistent storage (DB?) **URL:** \ **Category:** Ruby\ **Created:** [April 24, 2006, 10:07am UTC](https://www.ruby-forum.com/t/fault-tolerant-persistent-storage-db/58632 "2006-04-24T10:07:54Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![visq](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/2acd7d/32.png) [@visq](https://www.ruby-forum.com/u/visq)\ **Post date:** [April 24, 2006, 10:07am UTC](https://www.ruby-forum.com/t/fault-tolerant-persistent-storage-db/58632/1 "2006-04-24T10:07:54Z")
I’m currently looking for a data storage backend for a little pet project of mine. The catch is - two systems are running the software and need to be able to access data (r/w), and it needs to continue function, if either of the two goes down (i.e. just putting a DB on one of the two isn’t going to help). It doesn’t need to be ultra-high performance or anything (peak load would likely be around a dozen accesses a minute). So far I’ve looked at postgres (and pg\_cluster), which doesn’t seem to fail-over/recover all that well. Mysql clusters seem to need three hosts (I’m stuck with two). So, does anyone know of a free DB that could handle this? Alternatively - does rinda offer persistence / fault-tolerance? …any other contenders? (doesn’t need to be sql; the data structures are not too complex) Benedikt ---
**Author:** ![visq](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/2acd7d/32.png) [@visq](https://www.ruby-forum.com/u/visq)\ **Post date:** [April 24, 2006, 4:35pm UTC](https://www.ruby-forum.com/t/fault-tolerant-persistent-storage-db/58632/2 "2006-04-24T16:35:31Z")
On Mon, 24 Apr 2006, Benedikt H. wrote: > So far I’ve looked at postgres (and pg\_cluster), which doesn’t seem to > …any other contenders? (doesn’t need to be sql; the data structures are > not too complex) are your two processes on the same machine? -a ---
**Author:** ![visq](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/2acd7d/32.png) [@visq](https://www.ruby-forum.com/u/visq)\ **Post date:** [April 24, 2006, 4:50pm UTC](https://www.ruby-forum.com/t/fault-tolerant-persistent-storage-db/58632/3 "2006-04-24T16:50:50Z")
Nope - they’re running on two different hosts - those hosts that would then also run the “storage service” (DB; rinda; …whatever) as well. On Mon, 24 Apr 2006, removed\_email\_address@domain.invalid wrote: > > It doesn’t need to be ultra-high performance or anything (peak load would > > - h.h. the 14th dali lama ALLIANCE, n. In international politics, the union of two thieves who have their hands so deeply inserted in each other’s pockets that they cannot separately plunder a third. (Ambrose Bierce, The Devil’s Dictionary) ---
**Author:** ![visq](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/2acd7d/32.png) [@visq](https://www.ruby-forum.com/u/visq)\ **Post date:** [April 24, 2006, 5:21pm UTC](https://www.ruby-forum.com/t/fault-tolerant-persistent-storage-db/58632/4 "2006-04-24T17:21:42Z")
On Mon, 24 Apr 2006, Benedikt H. wrote: > Nope - they’re running on two different hosts - those hosts that would then > also run the “storage service” (DB; rinda; …whatever) as well. so what are your requirements? lose or strict sychronization? how do you want failback to occur (host a goes does for 2 days, then comes back up?)? are all writes on both machines mirrored to the other? -a ---
**Author:** ![visq](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/2acd7d/32.png) [@visq](https://www.ruby-forum.com/u/visq)\ **Post date:** [April 27, 2006, 4:45am UTC](https://www.ruby-forum.com/t/fault-tolerant-persistent-storage-db/58632/5 "2006-04-27T04:45:17Z")
On Mon, 24 Apr 2006, Benedikt H. wrote: > I’m currently looking for a data storage backend for a little pet project of > mine. The catch is - two systems are running the software and need to be able > to access data (r/w), and it needs to continue function, if either of the two > goes down (i.e. just putting a DB on one of the two isn’t going to help). The most common error in designing fault tolerant systems is not the technology choice, but failing to carefully identify the threats, risks and costs. ie. What threat do you want to tolerate? Mad man with Chainsaw? Powerfailure? Human error? That one is the hardest, how do you protect against an idiot with root? Or worse, a smart malicious one with root. Hardware failure? What are the probabilities? ie. How likely is it to occur? What is the likelyhood of double threats? Accidental low, if malicious attack, high. What are the costs of downtime? How hot a hot standby do you need? If controlling something that will go disastrously wrong in milliseconds, very hot standby. If just a web site you may lose 2 browsing customers out of ten thousand browsing customers only 100 of which may actually buy something. ie. Hot standby just adds cost and complexity and increased risk of failure for way more money than it would save. eg. A journalling file system reboots fast. Ask google about “crash only software”. John C. Phone : (64)(3) 358 6639 Tait Electronics Fax : (64)(3) 359 4632 PO Box 1645 Christchurch Email : removed\_email\_address@domain.invalid New Zealand Carter’s Clarification of Murphy’s Law. “Things only ever go right so that they may go more spectacularly wrong later.” From this principle, all of life and physics may be deduced.