Question

It is possible that my question title is misleading, but here goes —

I am trying out a prototype app which involves three MySQL/Perl Dancer powered web apps.

The user goes to app A which serves up a Google maps base layer. On document ready, app A makes three jQuery ajax calls — two to app B like so

    http://app_B/points.json
    http://app_B/polys.json

and one to app C

    http://app_C/polys.json

Apps B and C query the MySQL database via DBI, and serve up json packets of points and polys that are rendered in the user’s browser.

All three apps are proxied through Apache to Perl Starman running via plackup started like so

    $ plackup -E production -s Starman -w 10 -p 5000 path/to/app_A/app.pl
    $ plackup -E production -s Starman -w 10 -p 5001 path/to/app_B/app.pl
    $ plackup -E production -s Starman -w 10 -p 5002 path/to/app_C/app.pl

From time to time, I start getting errors back from the apps called via Ajax. The initial symptoms were

    {"error":"Warning caught during route 
     execution: DBD::mysql::st fetchall_arrayref 
     failed: fetch() without execute() at 
     <path/to/app_B/app.pm> line 79.\n"}

The offending lines are

    71> my $sql = qq{
    72>     ..
    73>
    74>
    75> };
    76> 
    77> my $sth = $dbh->prepare($sql);
    78> $sth->execute();
    79> my $res = $sth->fetchall_arrayref({});

This is bizarre… how can execute() not take place above? Perl doesn’t have a habit of jumping over lines, does it? So, I turned on DBI_TRACE

    $DBI_TRACE=2=logs/dbi.log plackup -E production -p 5001 -s Starman -w 

10 -a bin/app.pl

And, following is what stood out to me as the potential culprit in the log file

    > Handle is not in asynchronous mode error 2000 recorded: Handle is 
    > not in asynchronous mode
    >    !! ERROR: 2000 CLEARED by call to fetch method

What is going on? Basically, as is, app A is non-functional because the other apps don’t return data “reliably” — I put that in quotes because they do work correctly occasionally, so I know I don’t have any logic or syntax errors in my code. I have some kind of intrinsic plumbing errors.

I did find the following on DBD::mysql about ASYNCHRONOUS_QUERIES and am wondering if this is the cause and the solution of my problem. Essentially, if I want async queries, I have to add {async => 1} to my $dbh-prepare(). Except, I am not sure if I want async true or false. I tried it, it and it doesn’t seem to help.

I would love to learn what is going on here, and what is the right way to solve this.

Answer 1

How are you managing your database handles? If you are opening a connection before starman forks your code then multiple children may be trying to share one database handle and are confusing MySQL. You can solve this problem by always running a DBI->connect in your methods that talk to the database, but that can be inefficient. Many people switch over to some sort of connection pool, but I have no direct experience with any of them.