Lincoln A Baxter > Sys-SigAction-0.21 > dbd-oracle-timeout.pod

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NAME ^

dbd-oracle-timeout.pod - test timing out DBD-Oracle operations with Sys::SigAction

ABTRACT ^

This article discuss the problems I encountered using SIGALRM to timeout certain DDB-Oracle operations in a in a perl OLTP service. Perl 5.8.0 and later versions on platforms that support sigaction() implements 'safe' signal handling. Unfortunately, techniques that worked in perl versions earlier than 5.8, do not work in perl 5.8 and later versions. Several solutions to this problem are presented.

DESCRIPTION ^

If you are implementing a real-time service, your software must be both responsive, and well behaved from a resource utilization perspective. It is imperative that no operation take a long time to complete, and that resources are quickly freed, so that the service can respond to new requests. In this situation, it is generally preferable to time out or fail returning an error, than to allow requests to hang for long periods of time, potentially bringing down an entire service because system resources are consumed by all the hanging requests.

My team has implemented a number of real time services using perl and the DBI interface using the DBD-Oracle driver. This article is specific to the problems encountered with Oracle, but I believe that the problems we encountered on moving from perl 5.6 to perl 5.8, are generic, and could affect any database driver that uses a client library that makes restartable system calls like connect(). The techniques presented here can be used to solve this kind of problem with any DBD driver, or for any system resource that could hang, for which SIGALRM has been used to break out of the call.

Using the DBI interface prior to Perl 5.8.0, it was fairly easy to set code references into $SIG{'ALRM'}, and then use alarm() to implement time-outs. The signal handler could then die() or otherwise abort the call in progress. The two operations I have found that require this treatment are:

1 Database Host is Down -- connect() hangs

With SQL*Net, the DBI->connect() call will hang for about 4 minutes. Here is how we handled this situation in perls earlier than 5.8.x:

   eval {
      local $SIG{ALRM} = sub { die "open timed out"; };
      eval {
         alarm(2); #implement 2 second time out
         $dbh = DBI->connect("dbi:Oracle:$dbn" ... );
         alarm(0);
      };
      alarm(0);
      die $@ if $@;
   };
   if ( $@ ) { print "connection to $dbn timed out\n" ; }

Because $SIG{ALRM} has been 'localized', this code restores the original value of $SIG{ALRM} (the original signal handler) when the eval block is exited.

2 Long Running Statements

Long running statements can occur for a variety of reasons out side of the control of the script. Timing out calls to execute() avoids stacking of resources on the server on which the perl script is executing. The following example is similar to the that above:

   eval {
      local $SIG{ALRM} = sub { $sth->cancel(); };
      eval {
         alarm(2); #implement 2 second time out
         $sth->execute( ... );
         alarm(0);
      };
      alarm(0);
      die $@ if $@;
   };
   if ( $@ ) { print "execute timed out\n" }

Again, perl restores the original $SIG{ALRM} handler when the eval block is exited.

Note on eval of eval

The reader might note that the "double evals" in the code samples above. CPAN bug #50628 was filed against Sys::SigAction noting that the sample code was "buggy" because the evals that wrapped the code we wanted to timeout might die for an unanticipated reason, before the alarm could be cleared. In that case, if the alarm expires before the final alarm(0) can be called, either the code will completely die because there is no SIGALRM handler in place to catch the signal, or the wrong handler (not the local handler) will be called.

All the code samples here have been adjusted to execute the code to be timed out in an inner eval to correct for this problem.

The Problem

Many of us have been using perl 5.6.x for several years now, and the above code has worked just fine. We understood that with perl 5.6 (and prior) signal handling was 'unsafe', and we accepted the risk that the signal handler could be called at an in-opportune time, causing non-reentrant system routines to fail. We accepted the possibility of a perl core dump, and program termination. For real-time services this is considered an acceptable risk since failing quickly is preferable to hanging around without returning.

We, like most programmers facing this problem, simply built mechanisms to restart things should such a catastrophic failure (perl core dump) occur. Another technique we use, is to take ourselves out on error, letting a new (clean) instance of our service be created (by the above mechanism).

Upon moving to perl 5.8 or higher however, we discovered that the above code (especially the connect code) no longer works. Instead, it just hangs. This is a result of the changes to the way Unix signal handlers are implemented in perl 5.8 (and later versions).

From the perl 5.8.2 perlvar man page:

   The default delivery policy of signals changed in Perl 5.8.0 
   from immediate (also known as "unsafe") to deferred, also 
   known as "safe signals".  

Unfortunately this 'safe signals' approach causes some system calls to be retried (depending on how they are called) prior to the actual execution of the signal handler depending on how the library making the system call is implemented. The result when this happens is that some calls never return, even though a signal fired. This is the case with the DBD-Oracle connect() call (case 1 above). So the 'standard' mechanism for implementing time outs (above) no longer works with perl 5.8 and later versions.

The Solution

The solution to this problem (documented in the perlvar man page) is to install the signal handler with POSIX::sigaction(). This provides low level access to the POSIX sigaction() system API -- assuming (of course) your system has sigaction(). If your system does not have sigaction(), then you probably do not have this problem, as in that case perl implements the original (unsafe) signal handling approach. With POSIX::sigaction(), we get control over both the signal mask, and the sa_flags that are used to install the handler, and further, with perl 5.8.2 and later, a 'safe' switch is provided which can be used to ask for safe signal handling, in which perl promises to call the signal handler between perl op codes.

Using POSIX::sigaction() does ensure that the signal handler is called when the signal is fired. Calling die() within the signal handler, will cause the system call will be interrupted, and control will return to the perl script. But doing this effectively implements returns us to the 'unsafe' signals behavior -- at least in perl 5.8.0. In perl 5.8.2, it is possible to ask for 'deferred' signal handling while still controlling the sa_flags used to install the signal handler. The does this with perl 5.8.2 is safer than perl 5.6.x.

The usage of POSIX::sigaction() however is not well documented (except for several examples in the posix.t test in the perl core). And in perl versions less than 5.8.0, while POSIX::sigaction() is defined, it appears to be broken. But that's OK, because just setting $SIG{NAME} works.

The Pain

The down side of using POSIX::sigaction() besides the fact that it does not work in perl versions less than 5.8 is that it requires approximately 4 or 5 lines of code where previously you only had to set a localized $SIG{ALRM}.

The POSIX::sigaction() code looks something like this (for the connect() case):

   use POSIX ':signal_h';

   eval {
      my $mask = POSIX::SigSet->new( SIGALRM ); #list of signals to mask in the handler
      my $action = POSIX::SigAction->new( 
          sub { die "connect failed" ; } #the handler code ref
         ,$mask ); #assumes we're not using an specific flags or 'safe' switch
      my $oldaction = POSIX::SigAction->new();
      sigaction( 'ALRM' ,$action ,$oldaction );
      eval {
         alarm(2); #implement 2 second time out
         $dbh = DBI->connect("dbi:Oracle:$dbn" ... );
         alarm(0);
      };
      alarm(0);
      sigaction( 'ALRM' ,$oldaction ); #restore original signal handler
      die $@ if $@;
   };
   if ( $@ ) ....

This is not a pretty replacement for what was a single line of code in perl 5.6.x and before. And, to make matters worse (because POSIX::sigaction() does not work in perl versions less than 5.8, we now have to make it conditional on the perl version.

The Pain Reliever -- Sys::SigAction

Fortunately, having been bitten by this problem, and not wishing to have to replicate all that code every where I had timeout logic, I implemented a module that makes using POSIX::sigaction() as easy as setting a localized $SIG{ALRM} was in perl 5.6.x. The Sys::SigAction module can be retrieved from CPAN by going to:

   http://search.cpan.org/~lbaxter/Sys-SigAction/

The Sys::SigAction module wraps up all of the above POSIX:: code into a single function call which returns an object reference. When the object goes out of scope, its destructor resets the signal handler. So the above code is rewritten as follows:

   use Sys::SigAction qw( set_sig_handler );

   eval {
      my $h = set_sig_handler( 'ALRM' ,sub { die "connect failed" ; } );
      eval {
         alarm(2); #implement 2 second time out
         $dbh = DBI->connect("dbi:Oracle:$dbn" ... );
         alarm(0);
      };
      alarm(0);
      die $@ if $@;
   }; #original signal handler restored here when $h goes out of scope
   if ( $@ ) ....

And the nice thing about using Sys::SigAction, is that it works with older perls back to perl 5.005. So, even though POSIX::sigaction() is not fully functional in perl versions less than 5.8, Sys::SigAction can be used with to facilitate migration to newer perls, while still supporting the older perls. Thus, there is no need to write code conditioned on the perl version, because Sys::SigAction does that for you.

Sample Script

The following test script illustrates the use of Sys::SigAction, with the DBI interface (DBD-Oracle driver) to implement time out of both connects to databases on hosts that are down, and long running sql statements.

Note that with Sys::SigAction version 0.06, this script was changed to explicitly set safe=>0 (instead of safe=>1). The reason is that Sys::SigAction (version 0.04 and less) did not correctly set this parameter on the POSIX::sigaction call. When that was fixed with version 0.06 this script had to be fixed.

   #!/usr/bin/env perl
   use 5.006;
   use strict;
   use warnings; #if your perl is < 5.6 comment this out
   use Test::More ;
   use Cwd;

   use POSIX ':signal_h' ;
   my $iterations = $ENV{TIMEOUT_TEST_ITERATIONS};
   $iterations = 1 if not defined $iterations;

   my $tests = 9 + ($iterations * 2 );
   plan tests => $tests;

   use_ok('Sys::SigAction');
   use_ok('DBI');

   ok( $ENV{ORACLE_USERID} ,"ORACLE_USERID (<validuser>/<passwd>@<database>) is defined\n" );
   die "please export ORACLE_USERID=<validuser>/<passwd>@<database>\n" 
         if not defined $ENV{'ORACLE_USERID'};

   #find a private IP address which does not respond to ping
   my $last_octet = 256;
   my $got_down_host = 0;
   my $down_host ;
   do {
      $last_octet--;
      $down_host = "10.255.255.$last_octet";
   } until $got_down_host = system( "ping -c 1 -t 1 $down_host 2>&1 > /dev/null" )
     or $last_octet == 0;

   ok( $got_down_host ,"Found IP addr ($down_host) for missing system test\n" );

   #parse ORACLE_USERID
   my $dbn='';
   my $usr='';
   my $pwd='';

   ( $usr ,$pwd ,$dbn ) = split( /[\/\@]/ ,$ENV{'ORACLE_USERID'} );
   ok( $usr ,"database user: '$usr' defined" );
   ok( $pwd ,"password for $usr is defined" );
   ok( $dbn ,"database name: '$dbn' defined" );

   #I'm lazy... this stuff is unix specific...  but then,
   #if you are using SigAction that is pretty unix specific too!
   #
   #we need a locally writeable tns_admin directory
   #so we copy it from $TNS_ADMIN and then redefine
   #TNS_ADMIN to the local copy:

   my $save_TNS_ADMIN = $ENV{'TNS_ADMIN'};
   die if not ok( $save_TNS_ADMIN ,'$TNS_ADMIN is defined' );
   my $tmp_tns = cwd() . '/tmp_tns_admin' ;
   system( "rm -rf $tmp_tns" ) if -d $tmp_tns;
   mkdir $tmp_tns;
   system( "cp $save_TNS_ADMIN/*.* $tmp_tns/" );

   open( TNSNAMES ,">>$tmp_tns/tnsnames.ora" ) 
      or die "could not open $tmp_tns/tnsnames.org: $!\n" ;
   my $testdbfail = qq(testdbfail =
     (DESCRIPTION =
       (ADDRESS_LIST =
         (ADDRESS = (PROTOCOL = TCP)(HOST = $down_host)(PORT = 1521))
       )
       (CONNECT_DATA =
         (SERVICE_NAME = testdbfail)
       )
     )
   );
   #ok... we have a local TNS_ADMIN directory
   $ENV{TNS_ADMIN} = $tmp_tns;
   print "redefining TNS_ADMIN=$tmp_tns\n" ;
   print "appending to $tmp_tns/tnsnames.ora:\n$testdbfail\n" ;
   print TNSNAMES $testdbfail;
   close TNSNAMES;


   use Sys::SigAction qw( set_sig_handler );

   my $dbh;
   print "trying missing host test ($iterations iterations will be run)\n" ;
   for ( my $i = 1; $i < $iterations+1; $i++ ) {
      eval {
         my $code = sub {
               die "timed out on connect to database on missing host\n" ;
         };
         #note that if you ask for safe, it will not work...
         my $h = set_sig_handler( 'ALRM' ,$code ,{ flags=>0 ,safe=>0 } ); 
         eval {
            alarm(1);
            print "opening testdbfail (missing host test)\n" ;
            $dbh = DBI->connect("dbi:Oracle:testdbfail" ,"na" ,"na" );
            alarm(0);
            print "connect failed!\n" if not $dbh;
            ok( 0 ,"after missing_host connect... how did we get here?\n" );
         };
         alarm(0);
         die $@ if $@;
      };
      if ( $@ )
      {
         ok( 1 ,"exception: $@" );
      }
      print "completed iteration $i\n" ;
   } #iterate over this test
   print "after missing_host test\n" ;


   print "connecting to $dbn as $usr\n" ;
   $dbh = DBI->connect(
         "dbi:Oracle:$dbn" ,$usr ,$pwd 
        ,{ RaiseError=>1 ,AutoCommit=>0 ,PrintError => 0 } );

   ok( $dbh ,"connected" );

   my $sql = qq{
      BEGIN
         WHILE ( 1 > 0 ) LOOP
            NULL;
         END LOOP;
      END; 
   };

   print "execute timeout test...  ($iterations iterations will be run)\n" ;
   print "using sql:\n$sql\n" ;

   for ( my $i = 1; $i < $iterations+1; $i++ ) {
      print "calling \$dbh->prepare()\n" ;
      my $sth = $dbh->prepare( $sql );
      my $canceled = 0;
      eval {
         my $h = set_sig_handler( 'ALRM' 
                                 ,sub { $canceled = 1; 
                                        $sth->cancel(); 
                                        #dont die (oracle spills its guts)
                                        }
                                 ,{ mask=>[ qw( INT ALRM ) ] ,safe => 0 } 
                                 );
         eval {
            my $timeout = 1;
            print "\ncalling execute with $timeout second timeout\n" ;
            alarm($timeout);
            $sth->execute();
            alarm(0);
            ok( 0 ,"after execute of infinite statement (how did we get here?)\n" );
         };
         alarm(0);
         die $@ if $@;
      };
      if ( $@ )
      {
         print $@ if not $@ =~ m/DBD::Oracle/;
         ok( $canceled ,'execute timed out -- sighandler called' );
      }
      else
      {
         ok( 0 ,"how come \$\@ was not set?" );
      }
      print "completed iteration $i\n" ;
   } #for iterations...

   $dbh->rollback();
   $dbh->disconnect();
   exit;

   #end of test script

AUTHOR ^

   Lincoln A Baxter <lab-at-lincolnbaxter-dot-com>

COPYRIGHT ^

   Copyright (c) 2004-2009 by Lincoln A Baxter
   All rights reserved.

   This file may be distributed under the terms of either the GNU 
   General Public License or the Artistic License, as specified in 
   the Perl README file,

SEE ALSO ^

   perldoc perlvar 
   perldoc POSIX
   perldoc Sys::SigAction
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