Better structure for request based protocol implementation

Question

I am using a protocol, which is basically a request & response protocol over TCP, similar to other line-based protocols (SMTP, HTTP etc.).

The protocol has about 130 different request methods (e.g. login, user add, user update, log get, file info, files info, ...). All these methods do not map so well to the broad methods as used in HTTP (GET,POST,PUT,...). Such broad methods would introduce some inconsequent twists of the actual meaning.

But the protocol methods can be grouped by type (e.g. user management, file management, session management, ...).

Current server-side implementation uses a class Worker with methods ReadRequest() (reads request, consisting of method plus parameter list), HandleRequest() (see below) and WriteResponse() (writes response code & actual response data).

HandleRequest() will call a function for the actual request method - using a hash map of method name to member function pointer to the actual handler.

The actual handler is a plain member function there is one per protocol method: each one validates its input parameters, does whatever it has to do and sets response code (success yes/no) and response data.

Example code:

class Worker {
    typedef bool (Worker::*CommandHandler)();
    typedef std::map<UTF8String,CommandHandler> CommandHandlerMap;

    // handlers will be initialized once
    //   e.g. m_CommandHandlers["login"] = &Worker::Handle_LOGIN;
    static CommandHandlerMap m_CommandHandlers;

    bool HandleRequest() {
        CommandHandlerMap::const_iterator ihandler;
        if( (ihandler=m_CommandHandlers.find(m_CurRequest.instruction)) != m_CommandHandler.end() ) {
            // call actual handler
            return (this->*(ihandler->second))();
        }
        // error case:
        m_CurResponse.success = false;
        m_CurResponse.info = "unknown or invalid instruction";
        return true;
    }

    //...


    bool Handle_LOGIN() {
        const UTF8String username = m_CurRequest.parameters["username"];
        const UTF8String password = m_CurRequest.parameters["password"];

        // ....

        if( success ) {

            // initialize some state...
            m_Session.Init(...);
            m_LogHandle.Init(...);
            m_AuthHandle.Init(...);

            // set response data
            m_CurResponse.success = true;
            m_CurResponse.Write( "last_login", ... );
            m_CurResponse.Write( "whatever", ... );
        } else {
            m_CurResponse.Write( "error", "failed, because ..." );
        }
        return true;
    }


};

So. The problem is: My worker class now has about 130 "command handler methods". And each one needs access to:

1. request parameters
2. response object (to write response data)
3. different other session-local objects (like a database handle, a handle for authorization/permission queries, logging, handles to various sub-systems of the server etc.)

What is a good strategy for a better structuring of those command handler methods?

One idea was to have one class per command handler, and initializing it with references to request, response objects etc. - but the overhead is IMHO not acceptable (actually, it would add an indirection for any single access to everything the handler needs: request, response, session objects, ...). It could be acceptable if it would provide an actual advantage. However, that doesn't sound much reasonable:

class HandlerBase {
protected:
    Request &request;
    Response &response;
    Session &session;
    DBHandle &db;
    FooHandle &foo;
    // ...
public:
    HandlerBase( Request &req, Response &rsp, Session &s, ... )
    : request(req), response(rsp), session(s), ...
    {}
    //...
    virtual bool Handle() = 0;
};

class LoginHandler : public HandlerBase {
public:
    LoginHandler( Request &req, Response &rsp, Session &s, ... )
    : HandlerBase(req,rsp,s,..)
    {}
    //...
    virtual bool Handle() {
        // actual code for handling "login" request ...
    }
};

Okay, the HandlerBase could just take a reference (or pointer) to the worker object itself (instead of refs to request, response etc.). But that would also add another indirection (this->worker->session instead of this->session). That indirection would be ok, if it would buy some advantage after all.

Some info about the overall architecture

The worker object represents a single worker thread for an actual TCP connection to some client. Each thread (so, each worker) needs its own database handle, authorization handle etc. These "handles" are per-thread-objects that allow access to some sub-system of the server.

This whole architecture is based on some kind of dependency injection: e.g. to create a session object, one has to provide a "database handle" to the session constructor. The session object then uses this database handle to access the database. It will never call global code or use singletons. So, each thread can run undisturbed on its own.

But the cost is, that - instead of just calling out to singleton objects - the worker and its command handlers must access any data or other code of the system through such thread-specific handles. Those handles define its execution context.

Summary & Clarification: My actual question

I am searching for an elegant alternative to the current ("worker object with a huge list of handler methods") solution: It should be maintainable, have low-overhead & should not require writing too much glue-code. Additionally, it MUST still allow each single method control over very different aspects of its execution (that means: if a method "super flurry foo" wants to fail whenever full moon is on, then it must be possible for that implementation to do so). It also means, that I do not want any kind of entity abstraction (create/read/update/delete XFoo-type) at this architectural layer of my code (it exists at different layers in my code). This architectural layer is pure protocol, nothing else.

In the end, it will surely be a compromise, but I am interested in any ideas!

The AAA bonus: a solution with interchangeable protocol implementations (instead of just that current class Worker, which is responsible for parsing requests and writing responses). There maybe could be an interchangeable class ProtocolSyntax, that handles those protocol syntax details, but still uses our new shiny structured command handlers.

Answer 1

You've already got most of the right ideas, here's how I would proceed.

Let's start with your second question: interchangeable protocols. If you have generic request and response objects, you can have an interface that reads requests and writes responses:

class Protocol {
  virtual Request *readRequest() = 0;
  virtual void writeResponse(Response *response) = 0;
}

and you could have an implementation called HttpProtocol for example.

As for your command handlers, "one class per command handler" is the right approach:

class Command {
  virtual void execute(Request *request, Response *response, Session *session) = 0;
}

Note that I rolled up all the common session handles (DB, Foo etc.) into a single object instead of passing around a whole bunch of parameters. Also making these method parameters instead of constructor arguments means you only need one instance of each command.

Next, you would have a CommandFactory which contains the map of command names to command objects:

class CommandFactory {
  std::map<UTF8String, Command *> handlers;

  Command *getCommand(const UTF8String &name) {
    return handlers[name];
  }
}

If you've done all this, the Worker becomes extremely thin and simply coordinates everything:

class Worker {
  Protocol *protocol;
  CommandFactory *commandFactory;
  Session *session;

  void handleRequest() {
    Request *request = protocol->readRequest();
    Response response;

    Command *command = commandFactory->getCommand(request->getCommandName());
    command->execute(request, &response, session);

    protocol->writeResponse(&response);
  }
}

Answer 2

If it were me I would probably use a hybrid solution of the two in your question.
Have a worker base class that can handle multiple related commands, and can allow your main "dispatch" class to probe for supported commands. For the glue, you would simply need to tell the dispatch class about each worker class.

class HandlerBase
{
public:
    HandlerBase(HandlerDispatch & dispatch) : m_dispatch(dispatch) {
        PopulateCommands();
    }
    virtual ~HandlerBase();

    bool CommandSupported(UTF8String & cmdName);

    virtual bool HandleCommand(UTF8String & cmdName, Request & req, Response & res);
    virtual void PopulateCommands();

protected:
    CommandHandlerMap m_CommandHandlers; 
    HandlerDispatch & m_dispatch;
};

class AuthenticationHandler : public HandlerBase
{
public:
    AuthenticationHandler(HandlerDispatch & dispatch) : HandlerBase(dispatch) {}

    bool HandleCommand(UTF8String & cmdName, Request & req, Response & res) {
        CommandHandlerMap::const_iterator ihandler;                     
        if( (ihandler=m_CommandHandlers.find(req.instruction)) != m_CommandHandler.end() ) {                     
            // call actual handler                     
            return (this->*(ihandler->second))(req,res);                     
        }                     
        // error case:                     
        res.success = false;                     
        res.info = "unknown or invalid instruction";                     
        return true; 
    }

    void PopulateCommands() {
        m_CommandHandlers["login"]=Handle_LOGIN;
        m_CommandHandlers["logout"]=Handle_LOGOUT;
    }

    void Handle_LOGIN(Request & req, Response & res) {
        Session & session = m_dispatch.GetSessionForRequest(req);
        // ...
    }
};

class HandlerDispatch
{
public:
    HandlerDispatch();
    virtual ~HandlerDispatch() {  
        // delete all handlers 
    }

    void AddHandler(HandlerBase * pHandler);
    bool HandleRequest() {
        vector<HandlerBase *>::iterator i;
        for ( i=m_handlers.begin() ; i < m_handlers.end(); i++ ) {
            if ((*i)->CommandSupported(m_CurRequest.instruction)) {
                return (*i)->HandleCommand(m_CurRequest.instruction,m_CurRequest,m_CurResponse);
            }
        }
        // error case:                                                            
        m_CurResponse.success = false;
        m_CurResponse.info = "unknown or invalid instruction";

        return true; 
    }
protected:
    std::vector<HandlerBase*> m_handlers;
}

And then to glue it all together you would do something like this:

// Init
m_handlerDispatch.AddHandler(new AuthenticationHandler(m_handlerDispatch));

Answer 3

As for the transport (TCP) specific part, did you have a look at the ZMQ library that supports various distributed computing patterns via messaging sockets/queues? IMHO you should find an appropriate pattern that serves your needs in their Guide document.

For choice of the protocol messages implementation i would personally favorite google protocol buffers which works very well with C++, we are using it for a couple of projects now.

At least you'll boil down to dispatcher and handler implementations for specific requests and their parameters + the necessary return parameters. Google protobuf message extensions allow to to this in a generic way.

EDIT:

To get a bit more concrete, using protobuf messages the main difference of the dispatcher model vs yours will be that you don't need to do the complete message parsing before dispatch, but you can register handlers that tell themselves if they can handle a particular message or not by the message's extensions. The (main) dispatcher class doesn't need to know about the concrete extensions to handle, but just ask the registered handler classes. You can easily extend this mechanism to have certain sub-dispatchers to cover deeper message category hierarchies.

Because the protobuf compiler can already see your messaging data model completely, you don't need any kind of reflection or dynamic class polymorphism tests to figure out the concrete message content. Your C++ code can statically ask for possible extensions of a message and won't compile if such doesn't exist.

I don't know how to explain this in a better way, or to show a concrete example how to improve your existing code with this approach. I'm afraid you already spent some efforts on the de-/serialization code of your message formats, that could have been avoided using google protobuf messages (or what kind of classes are Request and Response?).

The ZMQ library might help to implement your Session context to dispatch requests through the infrastructure.

Certainly you shouldn't end up in a single interface that handles all kinds of possible requests, but a number of interfaces that specialize on message categories (extension points).

Answer 4

I think this is an ideal case for a REST-like implementation. One other way could also be grouping the handler methods based on category/any-other-criteria to several worker classes.

Answer 5

If the protocol methods can only be grouped by type but methods of the same group do not have anything common in their implementation, possibly the only thing you can do to improve maintainability is distributing methods between different files, one file for a group.

But it is very likely that methods of the same group have some of the following common features:

1. There may be some data fields in the Worker class that are used by only one group of methods or by several (but not every) group. For example, if m_AuthHandle may be used only by user management and session management methods.
2. There may be some groups of input parameters, used by every method of some group.
3. There may be some common data, written to the response by every method of some group.
4. There may be some common methods, called by several methods of some group.

If some of these facts is true, there is a good reason to group these features into different classes. Not one class per command handler, but one class per event group. Or, if there are features, common to several groups, a hierarchy of classes.

It may be convenient to group instances of all these group classes in one place:

classe UserManagement: public IManagement {...};
classe FileManagement: public IManagement {...};
classe SessionManagement: public IManagement {...};
struct Handlers {
  smartptr<IManagement> userManagement;
  smartptr<IManagement> fileManagement;
  smartptr<IManagement> sessionManagement;
  ...
  Handlers():
    userManagement(new UserManagement),
    fileManagement(new FileManagement),
    sessionManagement(new SessionManagement),
    ...
  {}
};

Instead of new SomeClass, some template like make_unique may be used. Or, if "interchangeable protocol implementations" are needed, one of the possibilities is to use factories instead of some (or all) new SomeClass operators.

m_CommandHandlers.find() should be split into two map searches: one - to find appropriate handler in this structure, other (in the appropriate implementation of IManagement) - to find a member function pointer to the actual handler.

In addition to finding a member function pointer, HandleRequest method of any IManagement implementation may extract common parameters for its event group and pass them to event handlers (one by one if there are just several of them, or grouped in a structure if there are many).

Also IManagement implementation may contain WriteCommonResponce method to simplify writing responce fields, common to all event handlers.

Answer 6

The Command Pattern is your solution to both aspects of this problem.

Use it to implement your protocol handler with a generalised IProtocol Interface (and/or abstract base class) and different implementations of protocol handler with a different Classes specialised for each protocol.

Then implement your Commands the same way with an ICommand Interface and each Command Methods implemented in seperate class. You are nearly there with this. Split your existing Methods into new Specialised Classes.

Wrap Your Requests and Responses as Mememento objects