[Info-vax] Databases versus RMS

[Arne Vajhøj]

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.

Arne