Fault-tolerant persistent storage (DB?)

Ruby
1

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

2

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

3

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, [email protected] 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)

4

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

5

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 : [email protected]
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.