==============
-In Seastar, a `future` represents a value not yet available but that can become
-available later. `future` can have one of following states:
+In Seastar, a ``future`` represents a value not yet available but that can become
+available later. ``future`` can have one of following states:
* unavailable: value is not available yet,
* value,
* failed: an exception was thrown when computing the value. This exception has
- been captured and stored in the `future` instance via `std::exception_ptr`.
+ been captured and stored in the ``future`` instance via ``std::exception_ptr``.
-In the last case, the exception can be processed using `future::handle_exception()` or
-`future::handle_exception_type()`. Seastar even provides `future::or_terminate()` to
+In the last case, the exception can be processed using ``future::handle_exception()`` or
+``future::handle_exception_type()``. Seastar even provides ``future::or_terminate()`` to
terminate the program if the future fails.
But in Crimson, quite a few errors are not serious enough to fail the program entirely.
performed by compiler to spit a warning if any enum value is not handled in a ``switch-case``
statement.
-Unfortunately, `seastar::future` is not able to satisfy these two requirements.
+Unfortunately, ``seastar::future`` is not able to satisfy these two requirements.
* Seastar imposes re-throwing an exception to dispatch between different types of
exceptions. This is not very performant nor even scalable as locking in the language's
runtime can occur.
* Seastar does not encode the expected exception type in the type of the returned
- `seastar::future`. Only the type of the value is encoded. This imposes huge
+ ``seastar::future``. Only the type of the value is encoded. This imposes huge
mental load on programmers as ensuring that all intended errors are indeed handled
requires manual code audit.
.. highlight:: c++
-So, "errorator" is created. It is a wrapper around the vanilla `seastar::future`.
+So, "errorator" is created. It is a wrapper around the vanilla ``seastar::future``.
It addresses the performance and scalability issues while embedding the information
about all expected types-of-errors to the type-of-future.::
In above example we defined an errorator that allows for two error types:
-* `crimson::ct_error::enoent` and
-* `crimson::ct_error::einval`.
+* ``crimson::ct_error::enoent`` and
+* ``crimson::ct_error::einval``.
-These (and other ones in the `crimson::ct_error` namespace) are basically
-unthrowable wrappers over `std::error_code` to exclude accidental throwing
+These (and other ones in the ``crimson::ct_error`` namespace) are basically
+unthrowable wrappers over ``std::error_code`` to exclude accidental throwing
and ensure signaling errors in a way that enables compile-time checking.
-The most fundamental thing in an errorator is a descendant of `seastar::future`
+The most fundamental thing in an errorator is a descendant of ``seastar::future``
which can be used as e.g. function's return type::
static ertr::future<int> foo(int bar) {
The errorator concept goes further. It not only provides callers with the information
about all potential errors embedded in the function's type; it also ensures at the caller
site that all these errors are handled. As the reader probably know, the main method
-in `seastar::future` is `then()`. On errorated future it is available but only if errorator's
-error set is empty (literally: `errorator<>::future`); otherwise callers have
-to use `safe_then()` instead::
+in ``seastar::future`` is ``then()``. On errorated future it is available but only if errorator's
+error set is empty (literally: ``errorator<>::future``); otherwise callers have
+to use ``safe_then()`` instead::
seastar::future<> baz() {
return foo(42).safe_then(
});
}
-In the above example `ertr::all_same_way` has been used to handle all errors in the same
+In the above example ``ertr::all_same_way`` has been used to handle all errors in the same
manner. This is not obligatory -- a caller can handle each of them separately. Moreover,
it can provide a handler for only a subset of errors. The price for that is the availability
-of `then()`::
+of ``then()``::
using einval_ertr = crimson::errorator<crimson::ct_error::einval>;
}
That is, handling errors removes them from errorated future's error set. This works
-in the opposite direction too -- returning new errors in `safe_then()` appends them
-the error set. Of course, this set must be compliant with error set in the `baz()`'s
+in the opposite direction too -- returning new errors in ``safe_then()`` appends them
+the error set. Of course, this set must be compliant with error set in the ``baz()``'s
signature::
using broader_ertr = crimson::errorator<crimson::ct_error::enoent,
ertr::pass_further{});
}
-As it can be seen, handling and signaling errors in `safe_then()` is basically
+As it can be seen, handling and signaling errors in ``safe_then()`` is basically
an operation on the error set checked at compile-time.
More details can be found in `the slides from ceph::errorator<> throw/catch-free,