This really does come down to the fundamental tradeoff previously
discussed in various places regarding transactions and autocommit;
That is, that the connection is not necessarily monopolized by
dbix-class. When it comes 'online' with the connection from whatever
pool it came from, there may or may not have been any statements
executed since the last commit/rollback! You may also have executed
modification commands within dbix-class outside a txn_do command - do
you want those to dissappear or become part of the transaction?
Thus to my understanding, when you turn off autocommit, dbix-class goes
into a 'safety' mode, stating something like:
"I cannot know how to reconstruct the entire transaction because
previous statements may have been executed before the connection or
before the txn_do. I do not know if its okay to roll them back or if
its okay to commit them, and I certainly can't redo them (since I don't
know of them) if I lose connection - so I won't do anything at all"
I was rather miffed to discover this myself, and half-created a patch to
fix that behavior when I learned that it was intentional!
So I created an alternative I call 'trust_dbic_transactions'. When this
is enabled, I am enforcing the simple application/system rule of:
If you execute any statements on the database without explicitly
commiting it, preferably through a txn_do transaction, it will be rolled
back.
With that rule in place, it no longer becomes necessary to hedge one's
bets against what was done outside the txn_do. Use txn_do to make
modifications to the db or they don't happen.
The maintainers of DBIx::Class don't seem to find my code quality use
case (in a nutshell enforceing the rule break-early-break-often, hence
making bugs easier to find) impressive or useful and though they have
expressed mild interest in what I've done, I have heard no follow-up
interest in adding the feature to the standard module.
David
PS: In some databases doing multiple begins starts a multi-layer
transaction. Though Mysql does not do this, I think sybase may and I'm
pretty sure MSSQL, Oracle and DB2 do. This multi-layer transactional
behavior may be masked by dbic's implementation which only executes one
begin and holds off on further until the final commit.
Robert Heinzmann wrote:
> Hello,
>
> I'm using DBIx::Class and I want to implement manual transactions.
>
> Connection is set to "AutoCommit => 0". I issue manual
>
> ----
> txn_begin()
>
> Insert
> Update
> Select
>
> txn_commit() / txn_rollback()
> ----
>
> However no Transaction takes place. DBIC Trace shows no BEGIN or COMMIT
> / ROLLBACK.
>
> I added the following:
>
> print Dumper($db->storage->{transaction_depth});
> $db->storage()->txn_begin() or $logger->error_die("ERROR Starting
> Transaction");
> print Dumper($db->storage->{transaction_depth});
>
> print Dumper($db->storage->{transaction_depth});
> $db->storage()->txn_rollback() or $logger->error_die("ERROR Starting
> Transaction");
> print Dumper($db->storage->{transaction_depth});
>
>
> This gives
>
> $VAR1 = 1;
> $VAR1 = 2;
>
> $VAR1 = 2;
> $VAR1 = 1;
>
> ----
> /usr/share/perl5/DBIx/Class/Storage/DBI.pm
> (Version Lenny: ii libdbix-class-perl 0.08010-2
> Extensible and flexible object <-> relational mapper)
> ----
> sub txn_begin {
> my $self = shift;
> $self->ensure_connected();
> if($self->{transaction_depth} == 0) {
> $self->debugobj->txn_begin()
> if $self->debug;
> # this isn't ->_dbh-> because
> # we should reconnect on begin_work
> # for AutoCommit users
> $self->dbh->begin_work;
> }
> $self->{transaction_depth}++;
> }
>
> sub txn_commit {
> my $self = shift;
> if ($self->{transaction_depth} == 1) {
> my $dbh = $self->_dbh;
> $self->debugobj->txn_commit()
> if ($self->debug);
> $dbh->commit;
> $self->{transaction_depth} = 0
> if $self->_dbh_autocommit;
> }
> elsif($self->{transaction_depth} > 1) {
> $self->{transaction_depth}--
> }
> }
>
> sub ensure_connected {
> my ($self) = @_;
>
> unless ($self->connected) {
> $self->_populate_dbh;
> }
> }
>
> sub _populate_dbh {
> my ($self) = @_;
> my @info = @{$self->_dbi_connect_info || []};
> $self->_dbh($self->_connect(@info));
>
> # Always set the transaction depth on connect, since
> # there is no transaction in progress by definition
> $self->{transaction_depth} = $self->_dbh_autocommit ? 0 : 1;
>
> if(ref $self eq 'DBIx::Class::Storage::DBI') {
> my $driver = $self->_dbh->{Driver}->{Name};
> if
> ($self->load_optional_class("DBIx::Class::Storage::DBI::${driver}")) {
> bless $self, "DBIx::Class::Storage::DBI::${driver}";
> $self->_rebless() if $self->can('_rebless');
> }
> }
>
> my $connection_do = $self->on_connect_do;
> $self->_do_connection_actions($connection_do) if ref($connection_do);
>
> $self->_conn_pid($$);
> $self->_conn_tid(threads->tid) if $INC{'threads.pm'};
> }
>
>
> For me this means with AutoCommit => 0 I cant do any transactions at all
> ...
>
> Looks like a bug ?
>
> P.s. Storage is Sybase. Any hints ?
>
> Regards,
> Robert
>
>
> _______________________________________________
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>
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