[Info-vax] Databases versus RMS

[Arne Vajhøj]

On 4/20/2012 7:50 PM, Richard Maher wrote:
> "Arne Vajhøj"<arne at vajhoej.dk>  wrote in message
> news:4f90cee6$0$281$14726298 at news.sunsite.dk...
>> On 4/19/2012 1:57 AM, Dirk Munk wrote:
>>> Richard B. Gilbert wrote:
>>>> On 4/17/2012 2:52 PM, JF Mezei wrote:
>>>>> A web forum has been down for over 24 hours now because of a power
>>>>> failure. Seems their database "broke".
>>>>>
>>>>> Since there are few teenagers here, I figured I might get some "senior"
>>>>> opinions.
>>>>>
>>>>> Are database engines so fragile that a power failure will truly wreak
>>>>> havok on a database requiring time consuming work and debugging ?
>>>>
>>>> Some are. Some are not. The expensive ones, e.g. Oracle, should survive
>>>> without problems.
>>>>
>>>> My failing memory recalls something that was called "two phase commit"
>>>> or something like that. You write what you're going to do and then you
>>>> do it. On a successful commit, you mark the transaction complete. If
>>>> your commit fails, you have all the pieces necessary to recover.
>>>
>>> Two phase commit was something else as far as I remember. Think of a
>>> central location where you have a complete database, and a branch office
>>> with a part of the database that is important for that office. All the
>>> database actions will be done on the local database of every office, and
>>> only when a transaction needs to be written to the databases, both
>>> database instances will communicate to insure that the information in
>>> both databases stays synchronised.
>>
>> That sounds more like a replication scenario.
>>
>> You need 2PC if you need to update two databases
>> let us say an Oracle database and a MySQL database
>> and need to make the combined update atomic.
>
> and consistent, and isolated, and durable.

Also those. But I consider 2PC A centric.

> Could also be the same databae in>1 connection. (Used to be popular before
> other methods of cursor-stability spanning txns and still has uses)

Good point - transactions are tied to connections.

But I would expect that scenario to be very rare (today).

Arne