silverstripe-framework/docs/en/topics/error-handling.md

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# Error Handling
SilverStripe has its own error trapping and handling support.
## Error Levels
SilverStripe recognises two basic levels of error:
* **WARNING:** Something strange has happened; the system has attempted to continue as best it can, but the developers
need to look at this. This category also include areas where a newer version of SilverStripe requires changes to the
site's customised code.
* **FATAL ERROR:** There is no way that the system can attempt to continue with the particular operation; it would be
dangerous to report success to the user.
You should use [user_error](http://www.php.net/user_error) to throw errors where appropriate. The more information we
have about what's not right in the system, the better we can make the application.
* **E_USER_WARNING:** Err on the side of over-reporting warnings. The more warnings we have, the less chance there is
of a developer leaving a bug. Throwing warnings provides a means of ensuring that developers know whow
* Deprecated functions / usage patterns
* Strange data formats
* Things that will prevent an internal function from continuing. Throw a warning and return null.
* **E_USER_ERROR:** Throwing one of these errors is going to take down the production site. So you should only throw
E_USER_ERROR if it's going to be **dangerous** or **impossible** to continue with the request.
Note that currently, the SilverStripe core doesn't follow these standards perfectly.
* Right now, **every** failed SQL statement throws a fatal error. Many 'select' queries could probably be reduced to
warnings.
* A lot of assertion checking in the system that throws errors when it should throw warnings.
## Friendly Website Errors
An HTTP 500 error will be sent when there has been a fatal error on either a test or production site. You can make this
friendlier - much like the 404 page, the error content can be edited within the CMS.
* Create a page of type `[api:ErrorPage]`
* Set the error code to 500
* Publish the page.
**HOW IT WORKS: **The publication script for ErrorPage will write the full HTML content, including the template styling,
to assets/error-500.html. The fatal error handler looks for the presence of this file, and if it exists, dumps the
content. This means that database access isn't required to provide a 500 error page.
## Filesystem Logs
### From SilverStripe
*Requires SilverStripe 2.3*
You can indicate a log file relative to the site root. The named file will have a terse log sent to it, and the full log
(an encoded file containing backtraces and things) will go to a file of a similar name, but with the suffix ".full"
added.
<mysite>/_config.php:
:::php
// log errors and warnings
SS_Log::add_writer(new SS_LogFileWriter('/my/logfile/path'), SS_Log::WARN, '<=');
// or just errors
SS_Log::add_writer(new SS_LogFileWriter('/my/logfile/path'), SS_Log::ERR);
#### Deprecated method (SS 2.3 ?)
<mysite>/_config.php:
:::php
Debug::log_errors_to("/my/logfile/path");
### From PHP
In addition to SilverStripe-integrated logging, it is adviseable to fall back to PHPs native logging functionality. A
script might terminate before it reaches the SilverStripe errorhandling, for example in the case of a fatal error.
<mysite>/_config.php:
:::php
ini_set("log_errors", "On");
ini_set("error_log", "/my/logfile/path");
## Email Logs
You can send both fatal errors and warnings in your code to a specified email-address.
<mysite>/_config.php:
:::php
// log errors and warnings
SS_Log::add_writer(new SS_LogEmailWriter('admin@domain.com'), SS_Log::WARN, '<=');
// or just errors
SS_Log::add_writer(new SS_LogEmailWriter('admin@domain.com'), SS_Log::ERR);
### Deprecated method (SS 2.3 ?)
:::php
Debug::send_errors_to("developer@example.org");
Debug::send_warnings_to("developer@example.org"); // Optional, implied by send_errors_to()