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Difference "Thread Safety" ver. 1 versus ver. 2

Content:

## Rule of Thumb
All classes are not thread safe for access from multiple
threads simultaneously unless it is stated explicitly otherwise.
In general there are several thread safe classes
in the CppCMS that their member functions
can be accessed simultaneously from several threads:
- `cppcms::service`
- `cppcms::applications_pool`
- `cppcms::thread_pool`
- `cppcms::views::pool`
- `booster::aio::io_service`
In all other cases special measures should be
taken in order to prevent accessing same class
from multiple threads.
## Threads in CppCMS
There are several types of threads in CppCMS:
- Event Loop - the main thread, the thread that runs
the `cppcms::service::run()` threads.
- Thread Pool - the special pool of threads that
every synchronous requests are executed there
- Some background threads auxiliary threads
that are in general do not interest you.
that are in general of no interest to you.
## Cross Thread Interactions
Once the CppCMS service started all synchronous
applications are executed in the thread pool.
If CppCMS synchronous application wants to perform
some operation withing event loop, for example,
interact with some asynchronous application by
sending some data or by transferring its HTTP
context to it, it should do it using
`cppcms::service::post()` member function
and an appropriate execution handler.
This handler would be dispatched to the event loop
queue and executed in its thread.
If asynchronous application or any other object
that runs in the event wants to execute some long
running, blocking or heavy operation, it may do it
by submitting an execution handler to the thread pool.
Note, once the job is complete, the handler should notify
its completion by posting some handler to the event
loop, the same way as described above.
Different applications should never access their
internal states directly as it is not safe.

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