# Exception Architecture

MongoDB code uses the following types of assertions that are available for use:
-   `uassert` and `internalAssert`
    -   Checks for per-operation user errors. Operation-fatal.
-   `massert`
    -   Checks per-operation invariants. Operation-fatal.
-   `fassert`
    -   Checks fatal process invariants. Process-fatal. Use to detect unexpected situations (such
        as a system function returning an unexpected error status).
-   `invariant`
    -   Checks process invariant. Process-fatal. Use to detect code logic errors ("pointer should
        never be null", "we should always be locked").

__Note__: Calling C function `assert` is not allowed. Use one of the above instead.

The following types of assertions are deprecated:

-   `verify`
    -   Checks per-operation invariants. A synonym for massert but doesn't require an error code.
        Do not use for new code; use invariant or fassert instead.
-   `dassert`
    -   Calls `verify` but only in debug mode. Do not use!

MongoDB uses a series of `ErrorCodes` (defined in [mongo/base/error_codes.yml][error_codes_yml]) to 
identify and categorize error conditions. `ErrorCodes` are defined in a YAML file and converted to 
C++ files using [MongoDB's IDL parser][idlc_py] at compile time. We also use error codes to create 
`Status` objects, which convey the success or failure of function invocations across the code base. 
`Status` objects are also used internally by `DBException`, MongoDB's primary exception class, and 
its children (e.g., `AssertionException`) as a means of maintaining metadata for exceptions. The 
proper usage of these constructs is described below.

## Considerations

When per-operation invariant checks fail, the current operation fails, but the process and
connection persist. This means that `massert`, `uassert`, `internalAssert` and `verify` only 
terminate the current operation, not the whole process. Be careful not to corrupt process state by 
mistakenly using these assertions midway through mutating process state. Examples of this include 
`uassert`, `internalAssert` and `massert` inside of constructors and destructors.

`fassert` failures will terminate the entire process; this is used for low-level checks where
continuing might lead to corrupt data or loss of data on disk.

Both `massert` and `uassert` take error codes, so that all assertions have codes associated with 
them. Currently, programmers are free to provide the error code by either using a unique location 
number or choose from existing `ErrorCodes`. Unique location numbers are assigned incrementally and 
have no meaning other than a way to associate a log message with a line of code.

`internalAssert` provides similar functionality to `uassert`, but it logs at a higher level and 
does not increment user assertion counters. We should always choose `internalAssert` over `uassert` 
when we expect a failure, a failure might be recoverable, or failure accounting is not interesting.


## Exception

A failed operation-fatal assertion throws an `AssertionException` or a child of that.
The inheritance hierarchy resembles:

-   `std::exception`
    -   `mongo::DBException`
        -   `mongo::AssertionException`
            -   `mongo::UserException`
            -   `mongo::MsgAssertionException`

See util/assert_util.h.

Generally, code in the server should be able to tolerate (e.g., catch) a `DBException`. Server 
functions must be structured with exception safety in mind, such that `DBException` can propagate 
upwards harmlessly. The code should also expect, and properly handle, `UserException`. We use 
[Resource Acquisition Is Initialization][raii] heavily.

## ErrorCodes and Status

MongoDB uses `ErrorCodes` both internally and externally: a subset of error codes (e.g., 
`BadValue`) are used externally to pass errors over the wire and to clients. These error codes are 
the means for MongoDB processes (e.g., *mongod* and *mongo*) to communicate errors, and are visible 
to client applications. Other error codes are used internally to indicate the underlying reason for 
a failed operation. For instance, `PeriodicJobIsStopped` is an internal error code that is passed 
to callback functions running inside a [`PeriodicRunner`][periodic_runner_h] once the runner is 
stopped. The internal error codes are for internal use only and must never be returned to clients 
(i.e., in a network response).
 
Zero or more error categories can be assigned to `ErrorCodes`, which allows a single handler to 
serve a group of `ErrorCodes`. `RetriableError`, for instance, is an `ErrorCategory` that includes 
all retriable `ErrorCodes` (e.g., `HostUnreachable` and `HostNotFound`). This implies that an 
operation that fails with any error code in this category can be safely retried. We can use 
`ErrorCodes::isA<${category}>(${error})` to check if `error` belongs to `category`. Alternatively, 
we can use `ErrorCodes::is${category}(${error})` to check error categories. Both methods provide 
similar functionality.

To represent the status of an executed operation (e.g., a command or a function invocation), we 
use `Status` objects, which represent an error state or the absence thereof. A `Status` uses the 
standardized `ErrorCodes` to determine the underlying cause of an error. It also allows assigning 
a textual description, as well as code-specific extra info, to the error code for further 
clarification. The extra info is a subclass of `ErrorExtraInfo` and specific to `ErrorCodes`. Look 
for `extra` in [here][error_codes_yml] for reference.

MongoDB provides `StatusWith` to enable functions to return an error code or a value without 
requiring them to have multiple outputs. This makes exception-free code cleaner by avoiding 
functions with multiple out parameters. We can either pass an error code or an actual value to a 
`StatusWith` object, indicating failure or success of the operation. For examples of the proper 
usage of `StatusWith`, see [mongo/base/status_with.h][status_with_h] and 
[mongo/base/status_with_test.cpp][status_with_test_cpp]. It is highly recommended to use `uassert` 
or `internalAssert` over `StatusWith`, and catch exceptions instead of checking `Status` objects 
returned from functions. Using `StatusWith` to indicate exceptions, instead of throwing via 
`uassert` and `internalAssert`, makes it very difficult to identify that an error has occurred, and 
could lead to the wrong error being propagated.

## Gotchas

Gotchas to watch out for:

-   Generally, do not throw an `AssertionException` directly. Functions like `uasserted()` do work
    beyond just that. In particular, it makes sure that the `getLastError` structures are set up
    properly.
-   Think about the location of your asserts in constructors, as the destructor would not be
    called. But at a minimum, use `wassert` a lot therein, we want to know if something is wrong.
-   Do __not__ throw in destructors or allow exceptions to leak out (if you call a function that
    may throw).


[raii]: https://en.wikipedia.org/wiki/Resource_acquisition_is_initialization
[error_codes_yml]: ../src/mongo/base/error_codes.yml
[periodic_runner_h]: ../src/mongo/util/periodic_runner.h
[status_with_h]: ../src/mongo/base/status_with.h
[idlc_py]: ../buildscripts/idl/idlc.py
[status_with_test_cpp]: ../src/mongo/base/status_with_test.cpp