Dive into SObjectizer 5.5. Fourth part. Exception

Dive into SObjectizer-5.5
SObjectizer Team, Jan 2016
Fourth Part: Exceptions
(at v.5.5.15)

This is the next part of the series of presentations with deep
introduction into features of SObjectizer-5.5.
This part is dedicated to usage of exceptions. In particular:
● why exceptions are used in SObjectizer Run-Time;
● what exceptions are used by SObjectizer-5;
● how SObjectizer-5.5 handles exceptions from agents;
● how a custom exception logger could be set;
● when and why SObjectizer can call std::abort().

Why SObjectizer Run-Time Uses Exceptions

Exceptions are the primary way of error reporting in
SObjectizer-5.
There are no methods/functions which use return codes for
indication of success or failure. Any method/function either
finishes successfully or throws an exception.
If a user does a valid call and here are all necessary
resources (like RAM, OS threads and so on) then there is no
source for a failure...

...but if something goes wrong (like insufficient amount of
RAM or inability to start yet another OS thread) then
returning an error code as indication of failure is not a good
idea.

Heavy usage of exceptions is a consequence of experience
from several years with SObjectizer-4 in production.
Error codes were used in SObjectizer-4. And the main
lesson learned was:
If an error handling could be forgotten then
it will be forgotten.

The most unpleasant error in SObjectizer-4 was
unsuccessful subscription.
If a subscription failed but error is not handled then the work
of an agent will be continued.
But the agent will work incorrectly.
And it could take a long time before the incorrectness will be
found and fixed.

The main purpose of exceptions in SObjectizer-5 is to
prevent the disregard of errors.
Ignoring error code is easier than ignoring of an exception.
Because of that all methods/functions like creating coops or
subscribing of events throw exceptions in the case of error.

More than four years of working with SObjectizer-5 show
that SObjectizer-related exceptions are not an issue.
Just because they are thrown when a programmer does
something wrong.
It is hard to get SObjectizer-5 to throw an exception in
normal situation...

For example, if agent creates a legal subscription for existing
mbox there is no reason for exception…
...except the case of lack of memory.
But in this particular case it is better to throw an exception
instead of сontinuation of execution without subscription
made or abortion of a whole application.
That is why SObjectizer-5.5 uses exceptions instead of error
codes.

If you do not like exceptions and prefer dealing with error
code in plain-old-C-style...

If you do not like exceptions and prefer dealing with error
code in plain-old-C-style...
Then you probably is trying to use
an inappropriate framework.
It seems that SObjectizer-5 is a wrong choice for you.
Just try to find something else.

What Exceptions Are Used By SObjectizer?

SObjectizer-5 doesn’t use a complex hierarchy of exception
classes.
There is only one class for representing SObjectizer-related
exceptions: so_5::exception_t.
It is derived from std::runtime_error.

Method exception_t::what(), inherited from
std::exception, returns textual description of an error.
But there is exception_t::error_code() method which returns
internal error code represented as int.
All SObjectizer error codes are defined inside so_5
namespace as constants with prefix rc_ in their names:
rc_disp_start_failed, rc_parent_coop_not_found and so on.

So if it is necessary to catch, analyze and handle
SObjectizer-related exception it could be done like that:
try
{
... // Some action with SObjectizer.
}
catch( const so_5::exception_t & x )
{
if( so_5::rc_named_disp_not_found == x.error_code() )
... // Create named dispatcher and try again.
else
throw;
}

Exceptions From Agents

There is one simple rule:
Normal agents should provide no-throw guarantee!

It means that an exception should not go out of an event-
handler.
It is because SObjectizer doesn’t know what to do with that
exception.
And doesn’t know the actual state of agent: is the agent in
the correct state and could process next message or the
agent’s internals are broken and no more messages must be
delivered to the agent.

So the best way is to catch and handle all exception inside
an agent’s event-handler.
But what if an exception is going out anyway?

When SObjectizer detects an exception of type
std::exception (or derived from it) the following actions are
performed:
● the exception caught is logged by a special exception
logger (more about it below);
● virtual method so_exception_reaction() is called for the
agent who threw the exception;
● perform action(s) in dependence of
so_exception_reaction() result.

Virtual method so_exception_reaction() can return one of the
following values...

so_5::abort_on_exception
This value means that the whole application must be terminated
immediately by calling std::abort().

so_5::shutdown_sobjectizer_on_exception
This value means that:
● the agent who threw the exception will be switched to a special
state (in that state it cannot handle other messages);
● SObjectizer Environment in which the exception has been
caught will be shutdowned the usual way.
This value useful if agent provides basic exception guarantee (no
resources leaks) and application can be shutdowned normally.

so_5::deregister_coop_on_exception
This value means that:
● the agent who threw the exception will be switched to a special
state (in that state it cannot handle other messages);
● the coop which holds the problematic agent will be
deregistered.
This value useful if agent provides basic exception guarantee (no
resources leaks) and application can continue its work after
deregistration of that coop. For example, the coop will be
automatically registered again by some supervisor.

so_5::ignore_exception
This value means that no more actions should be performed by
SObjectizer.
This value is useful if agent provides strong exception guarantee.
SObjectizer assumes that no damage has been made and work
could be safely continued.

so_5::inherit_exception_reaction
This value means that actual exception reaction needs to be
received from agent’s coop.
SObjectizer call exception_reaction() method for agent’s coop.
One of the values described above could be received. The
appropriate action will be performed.

But the value so_5::inherit_exception_reaction could be
returned by coop_t::exception_reaction() method.
In that case SObjectizer will call exception_reaction() method for
the parent coop (if exists). And then for the parent of the parent
and so on.
If the top-most parent coop returns inherit_exception_reaction then
exception_reaction() method will be called for SObjectizer
Exception instance. That method usually returns
abort_on_exception.

By default agent_t::so_exception_reaction() and coop_t::
exception_reaction() return inherit_exception_reaction value.
By default environment_t::exception_reaction() return
abort_on_exception.
So the application will be aborted in the case of an uncaught
exception by default.

A user can set appropriate exception reaction for the whole
coop by coop_t::set_exception_reaction() method:
env.introduce_coop( []( so_5::coop_t & coop ) {
coop.set_exception_reaction( so_5::deregister_coop_on_exception );
...
} );
In that case an exception from any coop’s agent will lead to
deregistration of the coop (but only if agent’s
so_exception_reaction() returns inherit_exception_reaction
value).

An exception reaction for the whole SObjectizer Environment
can be set via Environment’s parameters:
so_5::launch( []( so_5::environment_t & env ) {
... // Starting code.
},
[]( so_5::environment_params_t & params ) {
... // Environment parameters tuning code.
// Setting the exception reaction for the whole Environment.
params.exception_reaction( so_5::shutdown_sobjectizer_on_exception );
} );

Coop Dereg Reason

There is a possibility to know what the reason was behind
the coop deregistration.
When a coop is deregistered a special value must be passed
to environment_t::deregister_coop(). This value is a “coop
dereg reason”.
There are several predefined values which are defined as
constants in so_5::dereg_reason namespace...

● normal. Coop’s deregistration is a part of application
logic;
● shutdown. Deregistration is a part of Environment’s
shutdown process;
● parent_deregistration. Coop is deregistered because its
parent coop is deregistered.
● unhandled_exception. Coop is deregistered because of
unhandled exception.
There are several additional constants but they intended for
use by a programmer, not SObjectizer.

Coop dereg reason is passed to coop dereg notificator.
So a notificator can use this value for performing some
actions.
For example for restarting a cooperation in the case of
exception...

Restart coop on exception example (1/3):
#include <iostream>
#include <so_5/all.hpp>
void start_coop( so_5::environment_t & env )
{
env.introduce_coop( [&]( so_5::coop_t & coop ) {
struct raise_exception : public so_5::signal_t {};
// The single agent of the coop.
// Will throw an exception after one second from registration.
auto agent = coop.define_agent();
agent.on_start( [agent] {
so_5::send_delayed< raise_exception >( agent, std::chrono::seconds(1) );
} )
.event< raise_exception >( agent, [] {
throw std::runtime_error( "Just a test exception" );
} );

// Tell SObjectizer to deregister the coop on exception.
coop.set_exception_reaction( so_5::deregister_coop_on_exception );
// Add notificator which will initiate reregistration of the coop.
coop.add_dereg_notificator(
[]( so_5::environment_t & env,
const std::string & coop_name,
const so_5::coop_dereg_reason_t & why )
{
std::cout << "Deregistered: " << coop_name << ", reason: "
<< why.reason() << std::endl;
if( so_5::dereg_reason::unhandled_exception == why.reason() )
start_coop( env );
} );
} );
}

int main()
{
// Register coop the first time.
start_coop( env );
// Take some time to the example to work.
std::this_thread::sleep_for( std::chrono::seconds( 5 ) );
env.stop();
} );
}

Custom Exception Logger

Every unhandled exception is logged.
Default exception logger uses std::cerr as an output stream.
A user can set its own custom exception logger.

An exception logger must be derived from class
so_5::event_exception_logger_t.
At least one method, log_exception(), must be overridden
and implemented.
It could looks like...

A very simple implementation of event_exception_logger_t:
class spd_exception_logger : public so_5::event_exception_logger_t
{
public :
spd_exception_logger()
: logger_( spdlog::rotating_logger_mt( "file_logger",
"logs/exceptions", 10*1024*1024 ) )
{}
virtual void log_exception( const std::exception & ex, const std::string & coop_name ) override
{
logger_->alert( "Unhandled exception from coop '{}': {}", coop_name, ex.what() );
}
private :
std::shared_ptr< spdlog::logger > logger_;
};

There are two ways to set up a custom exception logger...

The first one is usage of Environment parameters:
},
[]( so_5::environment_params_t & params ) {
... // Environment parameters tuning code.
// Setting the exception logger.
params.event_exception_logger(
so_5::event_exception_logger_unique_ptr_t{new spd_exception_logger{}} );
} );

The second one is usage of Environment’s
install_exception_logger() method:
env.install_exception_logger(
so_5::event_exception_logger_unique_ptr_t{
new spd_exception_logger{} } );
} );
The main difference between those approaches is the ability
to change exception logger on working Environment via
install_exception_logger() method.

When and why SObjectizer calls std::abort()?

There are some situations where SObjectizer cannot throw
an exception and has to call abort() instead.

One of them is nested exceptions during handling an
uncaught exception from some agent’s event handler.
For example:
● SObjectizer calls log_exception() method for custom exception
logger;
● log_exception() throws an exception;
● SObjectizer calls abort().

Another one is related to delayed and periodic messages. If
the timer thread can’t send a message instance then abort()
will be called.
It is because inability to send a delayed/periodic message is a
violation of an important contract: if delayed/periodic message was
successfully scheduled then it must be sent.
There is no other way to report a violation of that contract except
calling the abort() function.

There are some other situations when SObjectizer calls
abort(). They are related to problems during recovery from
some previous errors.
There is a possibility of errors during registration of some coop. An
attempt to create another working thread in a dispatcher can fail.
SObjectizer will revert all changes which have been made before
that failure.
If some exception is thrown during this recovery SObjectizer has
no choice except to call abort().

So there are places in the SObjectizer code where abort() is
called.
But abort() is called by SObjectizer only in the case of a very
serious problem. When there is no other way to continue
correct work of an application.

Additional Information:
Project’s home: http://sourceforge.net/projects/sobjectizer
Documentation: http://sourceforge.net/p/sobjectizer/wiki/
Forum: http://sourceforge.net/p/sobjectizer/discussion/
Google-group: https://groups.google.com/forum/#!forum/sobjectizer
GitHub mirror: https://github.com/masterspline/SObjectizer

Dive into SObjectizer 5.5. Fourth part. Exception

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