Beefy Boxes and Bandwidth Generously Provided by pair Networks
Problems? Is your data what you think it is?

Comment on

( #3333=superdoc: print w/replies, xml ) Need Help??

No, most databases don't support nested transactions, but many provide savepoints which allow roughly equivalent functionality. PostgreSQL and MySQL do; even recent releases of SQLite. Indeed, so does Microsoft SQL Server.

So if you want to write decent cross-database code, use savepoints instead of nesting transactions. Of course, as is often the case in SQL land, syntax for establishing a savepoint, rolling back to a savepoint, or releasing (kinda like committing) a savepoint vary from database to database. But not too badly.

Your code, as I understand it, when it wants a nested transaction, opens a new database connection, runs the nested transaction there, and holds the transaction open until it's sure the outer transaction is ready to commit. It's a reasonable solution in the case where you're on a database that does transactions but not savepoints.

However, I think it probably has some flaws. You'd typically want code running in an inner transaction to be able to "see" database changes already made in the outer transaction. But if your database is doing ACID properly, then an independent transaction on a second database connection will not be able to see the changes made in the initial transaction until that initial transaction has been committed.

Nested transactions do indeed have some advantages over savepoints. Savepoints need to be given names. That means that Perl code that is executing an SQL transaction, and calling other Perl modules to execute inner transactions, needs some co-ordination between modules to ensure that savepoint names are unique. Nested transactions are anonymous so don't have this problem.

In reply to Re: Nested (sql) transactions by tobyink
in thread Nested (sql) transactions by clueless newbie

Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post; it's "PerlMonks-approved HTML":

  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.
  • Log In?

    What's my password?
    Create A New User
    Corion discovers a new shiny toy to try out over the (longish) weekend. Since I've done some more with websockets, maybe I'll try writing a webserver that implements hot-reloading of HTML(+CSS, +Javascript) in the browser. Edit the local file and ...
    [Corion]: ... the browser(s) get a ping to a) refresh the page or b) reload "just" the changed parts, keeping the scroll position etc.
    [Corion]: But I also have to look at how I can make WWW::Mechanize:: RemoteBrowser a reality, and how to make it safe from malicious content ;)
    [Corion]: Part of wanting hot-reloading is that I think I've stumbled on a very simple set of CSS that I maybe want to use for a blog, but I want to try that out on mobile too, and I also want to add/modify it slightly so it has a header too...
    [Corion]: ... and having hot reloading would make it easy to view the changes in multiple browser windows of different sizes, and on Android, simultaneously

    How do I use this? | Other CB clients
    Other Users?
    Others rifling through the Monastery: (7)
    As of 2018-04-26 10:49 GMT
    Find Nodes?
      Voting Booth?