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According to the WSGI specification, all WSGI applications must be python callable objects, taking a defined parameter list. Application callables can be python functions, class instances with a __call__() method, or indeed any callable object.
There are two different mechanisms by which you can specify application callable objects to modjy. They are
Each of these mechanisms is described below
This mechanism uses a single string which specifies an importable name, which when imported should deliver a python callable object. As described above, this can be any python callable. The importable name is specified as a single string. However, there are some small variations on how that string is specified, depending on the nature of the callable.
A - The importable name mechanism and reloading
Reloading is not supported with the importable name mechanism. When the importable name mechanism is used, the value of the reload_on_mod parameter is ignored.
B - The importable name mechanism and caching
Caching means that the same application callable object will be re-used repeatedly, to save object creation overhead. You have the choice of whether or not to enable caching using the cache_callables parameter, described in the modjy configuration reference.
If caching is disabled when using the importable name mechanism, this means that a new application object will be created for every new request, when appropriate. When the application is a simple callable, as described above, then caching has no meaning, since there is no instance to be re-used. When the application is an instantiable, as described above, then new instances will be created for every request.
If caching is enabled and the application callable is a class instance, then the same instance will be reused for every single request.
An alternative mechanism for locating an application callable is to specify the full path to an application file, and the name of an application callable within that file. The full path is specified using a combination of a directory name and a filename. In order to locate WSGI application callables with this mechanism, modjy needs three pieces of information.
A - The application directory
Firstly, you need to specify the name of a directory containing the python file containing the source code for the application. Notes for this parameter include
B - The application filename
Once it has located the application directory, modjy then looks for an application source file. Modjy uses the same filename for all application source files. The default value for this filename is application.py. You can change this value by setting the modjy configuration variable app_filename.
C - The callable object name
Lastly, having loaded and executed the application source file in response to a request, modjy needs to find a python callable object within the resulting python namespace. There are two different ways to set the name of the callable used for a request. They are
Now modjy knows the pathname and callable name for the application. It now needs to decide whether it needs to load and compile the application source code, to find a definition of the application object, or whether it already has a usable instance of the application object in its cache.
D - The application object cache
With the directory/file/callable mechanism, you can configure modjy to cache application objects for reuse for multiple requests. Application objects are stored in a cache, keyed under (pathname, callablename). If a request arrives which maps to an application object that is in the cache, then the cached object will be reused. Thus (unless the source file has been modified and reload_on_mod is true) the application source file will not be reloaded and recompiled.
Conversely, if caching is disabled, this means that a fresh application object will be created for every new request. This will have the effect of causing the reloading and recompilation of the application source file for every request, which may be less efficient than you would like. So use this option carefully.
Lastly, note that the cache_callables parameter has implications for the wsgi.run_once environment variable. Therefore, when caching is disabled, modjy sets the value of wsgi.run_once to true (thus essentially making cache disablement almost equivalent to running as a CGI script).
E - Reloading source on modification
With the directory/file/callable mechanism, you can configure modjy to reload application objects when their containing source file has been modified. See under reload_on_mod in the Configuration reference for more information.
Obviously, this parameter only takes effect when caching is enabled. Disabled caching means that the application source code is reloaded and recompiled for every request. If reload_on_mod is enabled, then cached objects will only be discarded when their containing source file has been modified. This is obviously a useful facility during a testing/debugging cycle. Also obviously, checking the access time on application source files comes at a small but finite resource cost for every single request, so you may want to disable it in production scenarios.
F - Multi-threading
You can configure modjy to run your application objects in either multi-threaded or single-threaded mode. See under multithread in the Configuration reference for more information.