Wednesday, July 7, 2010

Are we ready to ditch string errors?

I can't really figure out why I'm not in the habit of using exception objects. I seem to only reach for them when things are getting very complicated, instead of by default.

I can rationalize that they are better, but it just doesn't feel right to do this all the time.

I've been thinking about what possible reasons (perhaps based on misconceptions) are preventing me from using them more, but I'm also curious about others' opinions.

These are the trouble areas I've managed to think of:

  • Perl's built in exceptions are strings, and everybody is already used to them. [1]
  • There is no convention for inspecting error objects. Even ->isa() is messy when the error could be a string or an object.[2]
  • Defining error classes is a significant barrier, you need to stop, create a new file, etc. Conversely, universal error objects don't provide significant advantages over strings because they can't easily capture additional data apart from the message.[3]
  • Context capture/reporting is finicky
    • There's no convention like croak for exception objects.
    • Where exception objects become useful (for discriminating between different errors), there are usually multiple contexts involved: the error construction, the initial die, and every time the error is rethrown is potentially relevant. Perl's builtin mechanism for string mangling is shitty, but at least it's well understood.
    • Exception objects sort of imply the formatting is partly the responsibility of the error catching code (i.e. full stack or not), whereas Carp and die $str leave it to the thrower to decide.
    • Using Carp::shortmess(), Devel::StrackTrace->new and other caller futzery to capture full information context is perceived as slow.[4]
  • Error instantiation is slower than string concatenation, especially if a string has to be concatenated for reporting anyway.[5]

[1] I think the real problem is that most core errors worth discriminating are usually not thrown at all, but actually written to $! which can be compared as an error code (see also %! which makes this even easier, and autodie which adds an error hierarchy).

The errors that Perl itself throws, on the other hand, are usually not worth catching (typically they are programmer errors, except for a few well known ones like Can't locate in @INC).

Application level errors are a whole different matter though, they might be recoverable, some might need to be silenced while others pass through, etc.

[2] Exception::Class has some precedent here, its caught method is designed to deal with unknown error values gracefully.

[3] Again, Exception::Class has an elegant solution, adhoc class declarations in the use statement go a long way.

[4] XS based stack capture could easily make this a non issue (just walk the cxstack and save pointers to the COPs of appropriate frames). Trace formatting is another matter.

[5] I wrote a small benchmark to try and put the various runtime costs in perspective.


Here are a few ideas to address my concerns.

A die replacement

First, I see merit for an XS based error throwing module that captures a stack trace and the value of $@ using a die replacement. The error info would be recorded in SV magic and would be available via an API.

This could easily be used on any exception object (but not strings, since SV magic is not transitive), without weird globals or something like that.

It could be mixed into any exception system by exporting die, overriding a throw method or even by setting CORE::GLOBAL::die.

A simple API to get caller information from the captured COP could provide all the important information that caller would, allowing existing error formatters to be reused easily.

This would solve any performance concerns by decoupling stack trace capturing from trace formatting, which is much more complicated.

The idea is that die would not merely throw the error, but also tag it with context info, that you could then extract.

Here's a bare bones example of how this might look:

use MyAwesomeDie qw(die last_trace all_traces previous_error); # tentative
use Try::Tiny;

try {
 die [ @some_values ]; # this is not CORE::die
} catch {
 # gets data out of SV magic in $_
 my $trace = last_trace($_);

 # value of $@ just before dying
 my $prev_error = previous_error($_);

 # prints line 5 not line 15
 # $trace probably quacks like Devel::StackTrace
 die "Offending values: @$_" . $trace->as_string;

And of course error classes could use it on $self inside higher level methods.

Throwable::Error sugar

Exception::Class got many things right but a Moose based solution is just much more appropriate for this, since roles are very helpful for creating error taxonomies.

The only significant addition I would add make is having some sort of sugar layer to lazily build a message attribute using a simple string formatting DSL.

I previously thought MooseX::Declare would be necessary for something truly powerful, but I think that can be put on hold for a version 2.0.

A library for exception formatting

This hasn't got anything to do with the error message, that's the responsibility of each error class.

This would have to support all of the different styles of error printing we can have with error strings (i.e. die, croak with and without $Carp::Level futzing, confess...), but also allow recursively doing this for the whole error stack (previous values of $@).

Exposed as a role, the base API should complement Throwable::Error quite well.

Obviously the usefulness should extend beyond plain text, because the dealing with all that data is a task better suited for an IDE or a web app debug screen.

Therefore, things like code snippet extraction or other goodness might be nice to have in a plugin layer of some sort, but it should be easy to do this for errors of any kind, including strings (which means parsing as much info from Carp traces as possible).

Better facilities for inspecting objects

Check::ISA tried to make it easy to figure out what object you are dealing with.

The problem is that it's ugly, it exports an inv routine instead of a more intuitive isa. It's now possible to go with isa as long as namespace::clean is used to remove so it's not accidentally called as a method.

Its second problem is that it's slow, but it's very easy to make it comparable with the totally wrong UNIVERSAL::isa($obj, "foo") in performance by implementing XS acceleration.


It seems to me if I had those things I would have no more excuses for not using exception objects by default.

Did I miss anything?

Tuesday, July 6, 2010

KiokuDB's Leak Tracking

Perl uses reference counting to manage memory. This means that when you create circular structures this causes leaks.

Cycles are often avoidable in practice, but backreferences can be a huge simplification when modeling relationships between objects.

For this reason Scalar::Util exports the weaken function, which can demote a reference so that its referencing doesn't add to the reference count of the referent.

Since cycles are very common in persisted data (because there are many potential entry points in the data), KiokuDB works hard to support them, but it can't weaken cycles for you and prevent them from leaking.

Apart from the waste of memory, there is another major problem.

When objects are leaked, they remain tracked by KiokuDB so you might see stale data in a multi worker style environment (i.e. preforked web servers).

The new leak_tracker attribute takes a code reference which is invoked with the list of leaked objects when the last live object scope dies.

This can be used to report leaks, to break cycles, or whatever.

The other addition, the clear_leaks attribute allows you to work around the second problem by forcibly unregistering leaked objects.

This completely negates the effect of live object caching and doesn't solve the memory leak, but guarantees you'll see fresh data (without needing to call refresh).

my $dir = KiokuDB->connect(

    # this coerces into a new object
    live_objects => {
        clear_leaks  => 1,
        leak_tracker => sub {
            my @leaked = @_;

            warn "leaked " . scalar(@leaked) . " objects";

            # try to mop up.
            use Data::Structure::Util qw(circular_off);
            circular_off($_) for @leaked;

These options were both refactored out of Catalyst::Model::KiokuDB.

Friday, July 2, 2010

Why another caching module?

In the last post I namedropped Cache::Ref. I should explain why I wrote yet another Cache:: module.

On the CPAN most caching modules are concerned with caching data in a way that can be used across process boundaries (for example on subsequent invocations of the same program, or to share data between workers).

Persistent caching behaves more like on disk databases (like a DBM, or a directory of files), Cache::Ref is like an in memory hash with size limiting:

my %cache;

sub get { $cache{$_[0]} }

sub set {
    my ( $key, $value ) = @_;

    if ( keys %cache > $some_limit ) {
        ... # delete a key from %cache

    $cache{$key} = $value; # not a copy, just a shared reference

The different submodules in Cache::Ref are pretty faithful implementations of algorithms originally intended for virtual memory applications, and is therefore appropriate for when the cache is memory resident.

The goal of these algorithms is to try and choose the most appropriate key to delete quickly and without storing too much information about the key, or requiring costly updates on metadata during a cache hit.

This also means less control, for example there is no temporal expiry (i.e. cache something for $x seconds).

If most of CPAN is concerned with L5 caching, then Cache::Ref tries to address L4.

High level interfaces like CHI make persistent caching easy and consistent, but seem to add memory only caching as a sort of an afterthought, with most of the abstractions being appropriate for long term, large scale storage.

Lastly, you can use Cache::Cascade to create a multi level cache hierarchy. This is similar to CHI's l1_cache attribute, but you can have multiple levels and you can mix and match any cache implementation that uses the same basic API.

Thursday, July 1, 2010

KiokuDB's Immutable Object Cache

KiokuDB 0.46 added integration with Cache::Ref.

To enable it just cargo cult this little snippet:

my $dir = KiokuDB->connect(
    live_objects => {
        cache => Cache::Ref::CART->new( size => 1024 ),

To mark a Moose based object as cacheable, include the KiokuDB::Role::Immutable::Transitive role.

Depending on the cache's mood, some of those cacheable objects may survive even after the last live object scope has been destroyed.

Immutable data has the benefit of being cacheable without needing to worry about updates or stale data, so the data you get from lookup will always be consistent, it just might come back faster in some cases.

Just make sure they don't point at any data that can't be cached (that's treated as a leak), and you should notice significant performance improvements.