Subject: Re: Question regarding how databases support atomicity

From: Siddharth Jain <[email-masked]>
Date: Fri, 3 May 2024 20:02:49 -0700

On Fri, May 3, 2024 at 8:00 PM Siddharth Jain <[email-masked]> wrote:

> I am trying to sharpen my understanding of databases. Let's say there is
> an operation foo as part of the public API that internally translates to
> more than 1 operation - I am sure there are examples like this in postgres.
> So to do foo we have to do following in order in all or none fashion:
>
> 1. Step 1
> 2. Step 2
> 3. Step 3
>
> The way I understand this is that if there is a failure in-between, we
> start undoing and reverting the previous operations one by one. But what if
> there is a failure and we are not able to revert an operation. How is that
> situation handled? e.g., something failed when we tried to do Step 3. now
> we revert Step 2 and succeed. but when we try to revert step 1 we fail.
> what happens now? To me, it seems its impossible to guarantee true
> atomicity in general.
>
> S.
>

---

From: "David G. Johnston" <[email-masked]>
Date: Fri, 3 May 2024 20:10:31 -0700

On Friday, May 3, 2024, Siddharth Jain <[email-masked]> wrote:

>> start undoing and reverting the previous operations one by one.
>>
>
Not in PostgreSQL.  All work performed is considered provisional until a
commit succeeds.  At which point all provisional work, which had been
tagged with the same transaction identifier, becomes reality to the rest of
the system, by virtue of marking the transaction live.  If the commit never
happens, either because of error, rollback, or session end, the transaction
ends up being left unalive and eventually is cleaned up.

You need to ensure a “begin” happens before Step 1 and a “commit” after
Step 3.

David J.