Posts in category ‘Framework’.
CppCMS meets Comet
One of the major requirements for framework refactoring was support of Comet. Now, with introduction of asynchronous request handling and persistent application servers it becomes reality.
Client Side
There is a HTML source of simple chat client, that uses Dojo toolkit. It does following:
Submits new messages to the server application by posting form using XHR:
function send_data() { var kw = { url : "/chat/post", form : "theform" }; dojo.xhrPost(kw); dojo.byId("message").value=""; return false; }Receives new messages from the server using long poll via XHR:
var message_count = 0; function read_data() { dojo.xhrGet( { url: "/chat/get/" + message_count, timeout: 120000, handleAs: "text", load: function(response, ioArgs) { dojo.byId("messages").innerHTML = response + '<br/>' + dojo.byId("messages").innerHTML; message_count++; read_data(); return response; }, error: function(response,ioArgs) { read_data(); return response; } }); } dojo.addOnLoad(read_data);
So, the client side is quite simple (however error handling should be quite better).
Server Side
First we create our long running asynchronous application, that receives two kinds
for requests: "/post" – with new data, and "/get/NN" – receive message nuber NN, we assign these calls to two member functions post and get.
class chat : public cppcms::application {
public:
chat(cppcms::service &srv) : cppcms::application(srv)
{
dispatcher().assign("^/post$",&chat::post,this);
dispatcher().assign("^/get/(\\d+)$",&chat::get,this,1);
}
Now, this class includes two data members:
private:
std::vector<std::string> messages_;
std::vector<cppcms::intrusive_ptr<cppcms::http::context> > waiters_;
The history of all chat messages – messages_ and all pending get requests
that can’t be satisfied, because the message still not exists – waiters_
Each, "waiter" is actually pointer to request/response context that can be used for message transport.
Now, when new message arrives, post member function is called:
void post()
{
if(request().request_method()=="POST") {
if(request().post().find("message")!=request().post().end()) {
messages_.push_back(request().post().find("message")->second);
broadcast();
}
}
release_context()->async_complete_response();
}
If the requested message was found, it is added to messages_ list and all waiters are notified using broadcast() member function.
At the end, the current request context is released and completed.
The broadcasting is done as following:
void broadcast()
{
for(unsigned i=0;i<waiters_.size();i++) {
waiters_[i]->response().set_plain_text_header();
waiters_[i]->response().out() << messages_.back();
waiters_[i]->async_complete_response();
waiters_[i]=0;
}
waiters_.clear();
}
For each pending request the last message is written and the request closed. After that, all pending request are cleaned.
When get request arrives, it is handled by get(std::string no) member function, first of all
we check if requested message exists, if so we just return it to user.
unsigned pos=atoi(no.c_str());
if(pos < messages_.size()) {
response().set_plain_text_header();
response().out()<<messages_[pos];
release_context()->async_complete_response();
}
Otherwise, if the requested message is the last one, that does not exists, we
add the request context to pending list waiters
else if(pos == messages_.size()) {
waiters_.push_back(release_context());
}
If requested message it too late – probably client error, we just set status to "404 Not Found" and return the response.
else {
response().status(404);
release_context()->async_complete_response();
}
No, all we need to do is to add application to the main running loop under script name "/char" and start the service.
cppcms::service service(argc,argv); cppcms::intrusive_ptr<chat> app=new chat(service); service.applications_pool().mount(app,"/chat"); service.run();
Summary
So, the simple chat service was written with about 50 lines of C++ code and about same amount of JavaScript code.
I must admit, that it is too simplistic and not efficient, for example: if new client connects it receives all messages one by one and not as bulk (can be easily fixed), I do not handle timeouts and disconnects. But the general idea is quite clear:
- Asynchronous long running application that handles all request is created.
- It manages all outstanding request and uses them for server side push.
This is actually a base for future development of tools like XML-RPC and JSON-RPC that allow client to call asynchronously server side objects, it can be used for implementation of any other Comet protocols.
Progress Report on CppCMS v1
Its quite long time that most of the work is done in new refactoring branch… Meanwhile trunk stays silent. So, I decided to open a window and show some new changes:
Dependencies:
I had removed almost all dependencies with a big exception of Boost libraries.
Because of internal structure changed — mostly introduction of asynchronous event handling I could not use existing implementations of FastCGI because of its synchronous API. Also I decided to remove CgiCC that was very problematic in terms of installation, portability and most important the quality of implementation and ability to communicate with its primary developer.
So, at this point you only need latest boost library… Thats all. When the job would be complete it would be very easy to create deb/rpm packages for most popular distributions.
Server APIs:
In addition to supported FastCGI and SCGI protocols, direct HTTP protocol is supported, so you do not need to use external web server for debug purposes any more. It is also useful for embedding web applications.
Localization is now fully integrated with C++
std::localeand allows using correct facets for each supported language and translationWindows is now would be one of the officially supported platforms.
There is still lot of work to make new version as useful as current CppCMS stable version:
- Integrate all template system back.
- Integrate cache and sessions management back.
- Rewrite forms classes that currently work with CgiCC.
- Rewrite support of CGI API for embedded systems.
But there are many good points that are already visible.
What's Next?
The road map of the project includes two important milestones:
- CppCMS core components refactoring including following:
- Removal of dependency on CgiCC – today there is about 5% of CgiCC library is used, many features are not supported by it or are not supported well. For example: file upload handling in CgiCC is very primitive, limited and error prone, support of cookies buggy and so on.
- Using of Boost.Asio as internal event handler, because:
- It provides transparent synchronous and asynchronous event handling allowing future implementation of server push technologies.
- It provides efficient timer based event handling.
- Removal dependency of libfcgi and writing Boost.Asio friendly implementation of FastCGI/SCGI connectors. Implementation of HTTP connectors as well.
- Support of plug-in applications in CppCMS framework.
- Improving compilation speed by representing more
pimplidioms and removal of unnecessary classes.
- Better support of i18n and and l10n:
- Transparent support of
std::wstringwith forms including automatic encoding testing and conversion. - Support of
std::localefor localization for outputs like numbers, dates, monetary, translation and so on. - Optional support of ICU and icu::UnicodeString and icu::Locale that
would add unsupported features by
std::localeand allow replacementstd::localefeatures with more correct implementations provided by ICU.
- Transparent support of
These changes will significantly break API backward compatibility, but it would be possible to adopt the code almost "mechanically" to the new API.
Unicode in 2009? Why is it so hard?
From my point of view, one of the most missing features in C++ is the lack of good Unicode support. C++ provides some support via std::wstring and std::locale, but it is quite limited for real live purposes.
This definitely makes the life of C++ (Web) Developers harder.
However there are several tools and toolkits that provide such support. I had checked 6 of them: ICU library with bindings to C++, Java and Python, Qt3 and Qt4, glib/pango and native support of Java/JDK, C++ and Python.
I did little bit challenging test for correctness:
- To Upper: Is German ß converted to SS?
- To Lower: Is Greek Σ converted to σ in the middle of the word and to ς at its end?
- Word Boundaries: Are Chinese 中文 actually two words?
Basic features like encoding conversions and simple case conversion like "Артём" (my name in Russian) to "АРТЁМ" worked well in all tools. But more complicated test results were quite bad:
Results
| Tookit | To Upper Case | To Lower Case | Word Boundaries |
|---|---|---|---|
| C++ | Fail | Fail | No Support |
| C++/ICU | Ok | Ok | Ok |
| C++/Qt4 | Ok | Fail | Ok |
| C++/Qt3 | Fail | Fail | No Support |
| C/glib+pango | Ok | Ok | Fail |
| Java/JDK | Ok | Ok | Fail |
| Java/ICU4j | Ok | Ok | Ok |
| Python | Fail | Fail | No Support |
| Python/PyICU | Ok | Ok | Ok |
Description
ICU: Provides great support but… it has very unfriendly and old API in terms of C++ development. The documentation is really bad.
Qt4: Gives good results and friendly API, has great documentation, but as we can see, some tests are failed. Generally, useful in web projects.
Qt3: Provides very basic Unicode support, no reason to use any more, especially when Qt4.5 is released under LGPL.
C++/STL: Even basic support exists, the API is not too friendly to STL containers and requires explicit usage of char * or wchar_t * and manual buffers allocation.
Glib: Gives quite good basic functionality. But finding word boundaries with Pango is really painful and does not work with Chinese. It has very nice C API and quite well documented. It uses internally utf-8 which makes the life easier when working with C strings. It still requires wrapping its functionality with C++ classes or grabbing huge GtkMM.
Python: has very basic native Unicode support. PyICU has terrible documentation.
Java: JDK provides quite good Unicode support, it can be quite easily replaced by ICU4J (actually most of JDK is based on ICU).
Summary
It is a shame that in 2009 there is no high quality, well documented, C++ friendly toolkit to work with Unicode.
- For real purposes I would take QtCode part of Qt4 or wrap ICU library with friendly API.
- Glib is good as well and, what is very important is its high availability on most UNIX systems.
When there will be Boost.ICU or Boost.Unicode just like there is Boost.Math or Boost.Asio?
CppCMS 0.0.4 Released
Version 0.0.4 of CppCMS had released.
It includes optimizations required for using it in embedded systems.
Normal Embedded Build:
- Caching is completely removed. Small memory footprint is very important for embedded system thus, caching stuff in memory is quite useless.
- Zlib compression are removed – it removes dependency on boost::iostreams, zlib and bzip2 libraries.
- Removed mod-prefork.
- Removed dynamic templates loading — this feature requires export of symbols to binary and increases its size in order to make RTTI work. Thus, all templates should be statically compiled into the binary.
Embedded CGI Mode:
- FastCGI and SCGI APIs are removed
- Mod-thread and mod process are removed including all thread pool facilities
- Changes in files based session backend to work properly with CGI mode including garbage collection (sessions that had time-out).
Downloads are avialable from Sf Project Page.