package DBD::Gofer::Transport::Base;

#   $Id: Base.pm 14120 2010-06-07 19:52:19Z H.Merijn $
#
#   Copyright (c) 2007, Tim Bunce, Ireland
#
#   You may distribute under the terms of either the GNU General Public
#   License or the Artistic License, as specified in the Perl README file.

use strict;
use warnings;

use base qw(DBI::Gofer::Transport::Base);

our $VERSION = "0.014121";

__PACKAGE__->mk_accessors(qw(
    trace
    go_dsn
    go_url
    go_policy
    go_timeout
    go_retry_hook
    go_retry_limit
    go_cache
    cache_hit
    cache_miss
    cache_store
));
__PACKAGE__->mk_accessors_using(make_accessor_autoviv_hashref => qw(
    meta
));


sub new {
    my ($class, $args) = @_;
    $args->{$_} = 0 for (qw(cache_hit cache_miss cache_store));
    $args->{keep_meta_frozen} ||= 1 if $args->{go_cache};
    #warn "args @{[ %$args ]}\n";
    return $class->SUPER::new($args);
}


sub _init_trace { $ENV{DBD_GOFER_TRACE} || 0 }


sub new_response {
    my $self = shift;
    return DBI::Gofer::Response->new(@_);
}


sub transmit_request {
    my ($self, $request) = @_;
    my $trace = $self->trace;
    my $response;

    my ($go_cache, $request_cache_key);
    if ($go_cache = $self->{go_cache}) {
        $request_cache_key
            = $request->{meta}{request_cache_key}
            = $self->get_cache_key_for_request($request);
        if ($request_cache_key) {
            my $frozen_response = eval { $go_cache->get($request_cache_key) };
            if ($frozen_response) {
                $self->_dump("cached response found for ".ref($request), $request)
                    if $trace;
                $response = $self->thaw_response($frozen_response);
                $self->trace_msg("transmit_request is returning a response from cache $go_cache\n")
                    if $trace;
                ++$self->{cache_hit};
                return $response;
            }
            warn $@ if $@;
            ++$self->{cache_miss};
            $self->trace_msg("transmit_request cache miss\n")
                if $trace;
        }
    }

    my $to = $self->go_timeout;
    my $transmit_sub = sub {
        $self->trace_msg("transmit_request\n") if $trace;
        local $SIG{ALRM} = sub { die "TIMEOUT\n" } if $to;

        my $response = eval {
            local $SIG{PIPE} = sub {
                my $extra = ($! eq "Broken pipe") ? "" : " ($!)";
                die "Unable to send request: Broken pipe$extra\n";
            };
            alarm($to) if $to;
            $self->transmit_request_by_transport($request);
        };
        alarm(0) if $to;

        if ($@) {
            return $self->transport_timedout("transmit_request", $to)
                if $@ eq "TIMEOUT\n";
            return $self->new_response({ err => 1, errstr => $@ });
        }

        return $response;
    };

    $response = $self->_transmit_request_with_retries($request, $transmit_sub);

    if ($response) {
        my $frozen_response = delete $response->{meta}{frozen};
        $self->_store_response_in_cache($frozen_response, $request_cache_key)
            if $request_cache_key;
    }

    $self->trace_msg("transmit_request is returning a response itself\n")
        if $trace && $response;

    return $response unless wantarray;
    return ($response, $transmit_sub);
}


sub _transmit_request_with_retries {
    my ($self, $request, $transmit_sub) = @_;
    my $response;
    do {
        $response = $transmit_sub->();
    } while ( $response && $self->response_needs_retransmit($request, $response) );
    return $response;
}


sub receive_response {
    my ($self, $request, $retransmit_sub) = @_;
    my $to = $self->go_timeout;

    my $receive_sub = sub {
        $self->trace_msg("receive_response\n");
        local $SIG{ALRM} = sub { die "TIMEOUT\n" } if $to;

        my $response = eval {
            alarm($to) if $to;
            $self->receive_response_by_transport($request);
        };
        alarm(0) if $to;

        if ($@) {
            return $self->transport_timedout("receive_response", $to)
                if $@ eq "TIMEOUT\n";
            return $self->new_response({ err => 1, errstr => $@ });
        }
        return $response;
    };

    my $response;
    do {
        $response = $receive_sub->();
        if ($self->response_needs_retransmit($request, $response)) {
            $response = $self->_transmit_request_with_retries($request, $retransmit_sub);
            $response ||= $receive_sub->();
        }
    } while ( $self->response_needs_retransmit($request, $response) );

    if ($response) {
        my $frozen_response = delete $response->{meta}{frozen};
        my $request_cache_key = $request->{meta}{request_cache_key};
        $self->_store_response_in_cache($frozen_response, $request_cache_key)
            if $request_cache_key && $self->{go_cache};
    }

    return $response;
}


sub response_retry_preference {
    my ($self, $request, $response) = @_;

    # give the user a chance to express a preference (or undef for default)
    if (my $go_retry_hook = $self->go_retry_hook) {
        my $retry = $go_retry_hook->($request, $response, $self);
        $self->trace_msg(sprintf "go_retry_hook returned %s\n",
            (defined $retry) ? $retry : 'undef');
        return $retry if defined $retry;
    }

    # This is the main decision point.  We don't retry requests that got
    # as far as executing because the error is probably from the database
    # (not transport) so retrying is unlikely to help. But note that any
    # severe transport error occurring after execute is likely to return
    # a new response object that doesn't have the execute flag set. Beware!
    return 0 if $response->executed_flag_set;

    return 1 if ($response->errstr || '') =~ m/induced by DBI_GOFER_RANDOM/;

    return 1 if $request->is_idempotent; # i.e. is SELECT or ReadOnly was set

    return undef; # we couldn't make up our mind
}


sub response_needs_retransmit {
    my ($self, $request, $response) = @_;

    my $err = $response->err
        or return 0; # nothing went wrong

    my $retry = $self->response_retry_preference($request, $response);

    if (!$retry) {  # false or undef
        $self->trace_msg("response_needs_retransmit: response not suitable for retry\n");
        return 0;
    }

    # we'd like to retry but have we retried too much already?

    my $retry_limit = $self->go_retry_limit;
    if (!$retry_limit) {
        $self->trace_msg("response_needs_retransmit: retries disabled (retry_limit not set)\n");
        return 0;
    }

    my $request_meta = $request->meta;
    my $retry_count = $request_meta->{retry_count} || 0;
    if ($retry_count >= $retry_limit) {
        $self->trace_msg("response_needs_retransmit: $retry_count is too many retries\n");
        # XXX should be possible to disable altering the err
        $response->errstr(sprintf "%s (after %d retries by gofer)", $response->errstr, $retry_count);
        return 0;
    }

    # will retry now, do the admin
    ++$retry_count;
    $self->trace_msg("response_needs_retransmit: retry $retry_count\n");

    # hook so response_retry_preference can defer some code execution
    # until we've checked retry_count and retry_limit.
    if (ref $retry eq 'CODE') {
        $retry->($retry_count, $retry_limit)
            and warn "should return false"; # protect future use
    }

    ++$request_meta->{retry_count};         # update count for this request object
    ++$self->meta->{request_retry_count};   # update cumulative transport stats

    return 1;
}


sub transport_timedout {
    my ($self, $method, $timeout) = @_;
    $timeout ||= $self->go_timeout;
    return $self->new_response({ err => 1, errstr => "DBD::Gofer $method timed-out after $timeout seconds" });
}


# return undef if we don't want to cache this request
# subclasses may use more specialized rules
sub get_cache_key_for_request {
    my ($self, $request) = @_;

    # we only want to cache idempotent requests
    # is_idempotent() is true if GOf_REQUEST_IDEMPOTENT or GOf_REQUEST_READONLY set
    return undef if not $request->is_idempotent;

    # XXX would be nice to avoid the extra freeze here
    my $key = $self->freeze_request($request, undef, 1);

    #use Digest::MD5; warn "get_cache_key_for_request: ".Digest::MD5::md5_base64($key)."\n";

    return $key;
}


sub _store_response_in_cache {
    my ($self, $frozen_response, $request_cache_key) = @_;
    my $go_cache = $self->{go_cache}
        or return;

    # new() ensures that enabling go_cache also enables keep_meta_frozen
    warn "No meta frozen in response" if !$frozen_response;
    warn "No request_cache_key" if !$request_cache_key;

    if ($frozen_response && $request_cache_key) {
        $self->trace_msg("receive_response added response to cache $go_cache\n");
        eval { $go_cache->set($request_cache_key, $frozen_response) };
        warn $@ if $@;
        ++$self->{cache_store};
    }
}

1;

__END__

=head1 NAME

DBD::Gofer::Transport::Base - base class for DBD::Gofer client transports

=head1 SYNOPSIS

  my $remote_dsn = "..."
  DBI->connect("dbi:Gofer:transport=...;url=...;timeout=...;retry_limit=...;dsn=$remote_dsn",...)

or, enable by setting the DBI_AUTOPROXY environment variable:

  export DBI_AUTOPROXY='dbi:Gofer:transport=...;url=...'

which will force I<all> DBI connections to be made via that Gofer server.

=head1 DESCRIPTION

This is the base class for all DBD::Gofer client transports.

=head1 ATTRIBUTES

Gofer transport attributes can be specified either in the attributes parameter
of the connect() method call, or in the DSN string. When used in the DSN
string, attribute names don't have the C<go_> prefix.

=head2 go_dsn

The full DBI DSN that the Gofer server should connect to on your behalf.

When used in the DSN it must be the last element in the DSN string.

=head2 go_timeout

A time limit for sending a request and receiving a response. Some drivers may
implement sending and receiving as separate steps, in which case (currently)
the timeout applies to each separately.

If a request needs to be resent then the timeout is restarted for each sending
of a request and receiving of a response.

=head2 go_retry_limit

The maximum number of times an request may be retried. The default is 2.

=head2 go_retry_hook

This subroutine reference is called, if defined, for each response received where $response->err is true.

The subroutine is pass three parameters: the request object, the response object, and the transport object.

If it returns an undefined value then the default retry behaviour is used. See L</RETRY ON ERROR> below.

If it returns a defined but false value then the request is not resent.

If it returns true value then the request is resent, so long as the number of retries does not exceed C<go_retry_limit>.

=head1 RETRY ON ERROR

The default retry on error behaviour is:

 - Retry if the error was due to DBI_GOFER_RANDOM. See L<DBI::Gofer::Execute>.

 - Retry if $request->is_idempotent returns true. See L<DBI::Gofer::Request>.

A retry won't be allowed if the number of previous retries has reached C<go_retry_limit>.

=head1 TRACING

Tracing of gofer requests and responses can be enabled by setting the
C<DBD_GOFER_TRACE> environment variable. A value of 1 gives a reasonably
compact summary of each request and response. A value of 2 or more gives a
detailed, and voluminous, dump.

The trace is written using DBI->trace_msg() and so is written to the default
DBI trace output, which is usually STDERR.

=head1 METHODS

I<This section is currently far from complete.>

=head2 response_retry_preference

  $retry = $transport->response_retry_preference($request, $response);

The response_retry_preference is called by DBD::Gofer when considering if a
request should be retried after an error.

Returns true (would like to retry), false (must not retry), undef (no preference).

If a true value is returned in the form of a CODE ref then, if DBD::Gofer does
decide to retry the request, it calls the code ref passing $retry_count, $retry_limit.
Can be used for logging and/or to implement exponential back-off behaviour.
Currently the called code must return using C<return;> to allow for future extensions.

=head1 AUTHOR

Tim Bunce, L<http://www.tim.bunce.name>

=head1 LICENCE AND COPYRIGHT

Copyright (c) 2007-2008, Tim Bunce, Ireland. All rights reserved.

This module is free software; you can redistribute it and/or
modify it under the same terms as Perl itself. See L<perlartistic>.

=head1 SEE ALSO

L<DBD::Gofer>, L<DBI::Gofer::Request>, L<DBI::Gofer::Response>, L<DBI::Gofer::Execute>.

and some example transports:

L<DBD::Gofer::Transport::stream>

L<DBD::Gofer::Transport::http>

L<DBI::Gofer::Transport::mod_perl>

=cut
