[Info-vax] Databases versus RMS

[Arne Vajhøj]

On 4/20/2012 2:29 AM, Jan-Erik Soderholm wrote:
> Arne Vajhøj wrote 2012-04-20 04:50:
>> 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.
>>
>
> I'd say so too. Like a price on a product that is changed and
> has to be "replicated" to all offices. 2PC is more when both
> updates *has* to be done in *both* systems or not at all.
> Like withdraw money in one account/system and add it in the
> the other. They have to match up.
>
>> 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.
>
> Could be two Rdb, Oracle or MySQL (or any other) databases. I does
> not have to be *different* engines,

True.

 >                                but it has to be two engines
 > both supporting transactions.

And 2PC/XA.

Arne