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Dive into SObjectizer 5.5. Fourth part. Exception

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Dive into SObjectizer 5.5. Fourth part. Exception

  1. 1. Dive into SObjectizer-5.5 SObjectizer Team, Jan 2016 Fourth Part: Exceptions (at v.5.5.15)
  2. 2. 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(). SObjectizer Team, Jan 2016
  3. 3. Why SObjectizer Run-Time Uses Exceptions SObjectizer Team, Jan 2016
  4. 4. 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... SObjectizer Team, Jan 2016
  5. 5. ...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. SObjectizer Team, Jan 2016
  6. 6. 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. SObjectizer Team, Jan 2016
  7. 7. 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. SObjectizer Team, Jan 2016
  8. 8. 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. SObjectizer Team, Jan 2016
  9. 9. 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... SObjectizer Team, Jan 2016
  10. 10. 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. SObjectizer Team, Jan 2016
  11. 11. If you do not like exceptions and prefer dealing with error code in plain-old-C-style... SObjectizer Team, Jan 2016
  12. 12. 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. SObjectizer Team, Jan 2016
  13. 13. What Exceptions Are Used By SObjectizer? SObjectizer Team, Jan 2016
  14. 14. 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. SObjectizer Team, Jan 2016
  15. 15. 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. SObjectizer Team, Jan 2016
  16. 16. 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; } SObjectizer Team, Jan 2016
  17. 17. Exceptions From Agents SObjectizer Team, Jan 2016
  18. 18. There is one simple rule: Normal agents should provide no-throw guarantee! SObjectizer Team, Jan 2016
  19. 19. 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. SObjectizer Team, Jan 2016
  20. 20. 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? SObjectizer Team, Jan 2016
  21. 21. 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. SObjectizer Team, Jan 2016
  22. 22. Virtual method so_exception_reaction() can return one of the following values... SObjectizer Team, Jan 2016
  23. 23. so_5::abort_on_exception This value means that the whole application must be terminated immediately by calling std::abort(). SObjectizer Team, Jan 2016
  24. 24. 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. SObjectizer Team, Jan 2016
  25. 25. 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. SObjectizer Team, Jan 2016
  26. 26. 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. SObjectizer Team, Jan 2016
  27. 27. 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. SObjectizer Team, Jan 2016
  28. 28. 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. SObjectizer Team, Jan 2016
  29. 29. 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. SObjectizer Team, Jan 2016
  30. 30. 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). SObjectizer Team, Jan 2016
  31. 31. 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 ); } ); SObjectizer Team, Jan 2016
  32. 32. Coop Dereg Reason SObjectizer Team, Jan 2016
  33. 33. 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... SObjectizer Team, Jan 2016
  34. 34. ● 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. SObjectizer Team, Jan 2016
  35. 35. 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... SObjectizer Team, Jan 2016
  36. 36. 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" ); } ); SObjectizer Team, Jan 2016
  37. 37. Restart coop on exception example (2/3): // 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 ); } ); } ); } SObjectizer Team, Jan 2016
  38. 38. Restart coop on exception example (3/3): int main() { so_5::launch( []( so_5::environment_t & env ) { // 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(); } ); } SObjectizer Team, Jan 2016
  39. 39. Custom Exception Logger SObjectizer Team, Jan 2016
  40. 40. Every unhandled exception is logged. Default exception logger uses std::cerr as an output stream. A user can set its own custom exception logger. SObjectizer Team, Jan 2016
  41. 41. 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... SObjectizer Team, Jan 2016
  42. 42. 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_; }; SObjectizer Team, Jan 2016
  43. 43. There are two ways to set up a custom exception logger... SObjectizer Team, Jan 2016
  44. 44. The first one is usage of Environment parameters: so_5::launch( []( so_5::environment_t & env ) { ... // Starting code. }, []( 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{}} ); } ); SObjectizer Team, Jan 2016
  45. 45. The second one is usage of Environment’s install_exception_logger() method: so_5::launch( []( so_5::environment_t & env ) { env.install_exception_logger( so_5::event_exception_logger_unique_ptr_t{ new spd_exception_logger{} } ); ... // Starting code. } ); The main difference between those approaches is the ability to change exception logger on working Environment via install_exception_logger() method. SObjectizer Team, Jan 2016
  46. 46. When and why SObjectizer calls std::abort()? SObjectizer Team, Jan 2016
  47. 47. There are some situations where SObjectizer cannot throw an exception and has to call abort() instead. SObjectizer Team, Jan 2016
  48. 48. 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(). SObjectizer Team, Jan 2016
  49. 49. 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. SObjectizer Team, Jan 2016
  50. 50. 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(). SObjectizer Team, Jan 2016
  51. 51. 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. SObjectizer Team, Jan 2016
  52. 52. 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

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