title: Logging and Error Handling
summary: Trap, fire and report diagnostic logs, user exceptions, warnings and errors.

# Logging and Error Handling

SilverStripe uses Monolog for both error handling and logging. It comes with two default configurations: one for
development environments, and another for test or live environments. On development environments, SilverStripe will
deal harshly with any warnings or errors: a full call-stack is shown and execution stops for anything, giving you early
warning of a potential issue to handle.

## Raising errors and logging diagnostic information.

For informational and debug logs, you can use the Logger directly. The Logger is a PSR-3 compatible LoggerInterface and
can be accessed via the `Injector`:

	:::php
	Injector::inst()->get(LoggerInterface::class)->info('User has logged in: ID #' . Member::currentUserID());
	Injector::inst()->get(LoggerInterface::class)->debug('Query executed: ' . $sql);

Although you can raise more important levels of alerts in this way, we recommend using PHP's native error systems for
these instead.

For notice-level and warning-level issues, you should use [user_error](http://www.php.net/user_error) to throw errors
where appropriate. These will not halt execution but will send a message to the 

	:::php
	function delete() {
		if($this->alreadyDelete) {
			user_error("Delete called on already deleted object", E_USER_NOTICE);
			return;
		}
		...
	}
	
	function getRelatedObject() {
		if(!$this->RelatedObjectID) {
			user_error("Can't find a related object", E_USER_WARNING);
			return null;
		}
		...
	}

For errors that should halt execution, you should use Exceptions. Normally, Exceptions will halt the flow of executuion,
but they can be caught with a try/catch clause.

	:::php
	throw new \LogicException("Query failed: " . $sql);

### Accessing the logger via dependency injection.

It can quite verbose to call `Injector::inst()->get(LoggerInterface::class)` all the time. More importantly,
it also means that you're coupling your code to global state, which is a bad design practise. A better
approach is to use depedency injection to pass the logger in for you. The [Injector](../extending/Injector)
can help with this. The most straightforward is to specify a `dependencies` config setting, like this:

	:::php
	class MyController {

		private static $dependencies = array(
			'logger' => '%$Psr\Log\LoggerInterface',
		);

		// This will be set automatically, as long as MyController is instantiated via Injector
		public $logger;

		function init() {
			$this->logger->debug("MyController::init() called");
			parent::init();
		}

	}

In other contexts, such as testing or batch processing, logger can be set to a different value by the code calling
MyController.

### Error Levels

*  **E_USER_WARNING:** Err on the side of over-reporting warnings. Throwing warnings provides a means of ensuring that 
developers know:
    * 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.

## Configuring error logging

You can configure your logging using Monolog handlers. The handlers should be provided int the `Logger.handlers`
configuration setting. Below we have a couple of common examples, but Monolog comes with [many different handlers](https://github.com/Seldaek/monolog/blob/master/doc/02-handlers-formatters-processors.md#handlers)
for you to try.

### Sending emails

To send emails, you can use Monolog's [NativeMailerHandler](https://github.com/Seldaek/monolog/blob/master/src/Monolog/Handler/NativeMailerHandler.php#L74), like this:

	SilverStripe\Core\Injector\Injector:
	  Psr\Log\LoggerInterface: 
	    calls:
	      MailHandler: [ pushHandler, [ %$MailHandler ] ]
	  MailHandler:
	      class: Monolog\Handler\NativeMailerHandler
	      constructor:
	        - me@example.com
	        - There was an error on your test site
	        - me@example.com
	        - error
	      properties:
	        ContentType: text/html
	        Formatter: %$SilverStripe\Logging\DetailedErrorFormatter

The first section 4 lines passes a new handler to `Logger::pushHandler()` from the named service `MailHandler`. The
next 10 lines define what the service is.

The calls key, `MailHandler`, can be anything you like: its main purpose is to let other configuration disable it
(see below).

### Logging to a file

To log to a file, you can use Monolog's [StreamHandler](https://github.com/Seldaek/monolog/blob/master/src/Monolog/Handler/StreamHandler.php#L74), like this:

	SilverStripe\Core\Injector\Injector:
	  Psr\Log\LoggerInterface: 
	    calls:
	      LogFileHandler: [ pushHandler, [ %$LogFileHandler ] ]
	  LogFileHandler:
	    class: Monolog\Handler\StreamHandler
	    constructor:
	      - "../silverstripe.log"
	      - "info"

The log file will be relative to the framework/ path, so "../silverstripe.log" will create a file in your project root.

### Disabling the default handler

You can disable a handler by removing its pushHandlers call from the calls option of the Logger service definition.
The handler key of the default handler is `DisplayErrorHandler`, so you can disable it like this:

	SilverStripe\Core\Injector\Injector:
	  Psr\Log\LoggerInterface:
	    calls:
	      DisplayErrorHandler:	%%remove%%

### Setting a different configuration for dev

In order to set different logging configuration on different environment types, we rely on the environment-specific
configuration features that the config system proviers. For example, here we have different configuration for dev and
non-dev.

	---
	Name: dev-errors
	Only:
	  environment: dev
	---
	SilverStripe\Core\Injector\Injector:
	  Psr\Log\LoggerInterface:
	    calls:
	      - [ pushHandler, [ %$DisplayErrorHandler ]] 
	  DisplayErrorHandler:
	    class: SilverStripe\Logging\HTTPOutputHandler
	    constructor:
	      - "notice"
	    properties:
	      Formatter: %$SilverStripe\Logging\DetailedErrorFormatter
          CLIFormatter: %$SilverStripe\Logging\DetailedErrorFormatter
	---
	Name: live-errors
	Except:
	  environment: dev
	---
	SilverStripe\Core\Injector\Injector:
	  Psr\Log\LoggerInterface:
	    calls:
	      - [ pushHandler, [ %$LogFileHandler ]] 
	      - [ pushHandler, [ %$DisplayErrorHandler ]] 
	  LogFileHander:
	    class: Monolog\Handler\StreamHandler
	    constructor:
	      - "../silverstripe.log"
	      - "notice"
	    properties:
	      Formatter: %$Monolog\Formatter\HtmlFormatter
	      ContentType: text/html
	  DisplayErrorHandler:
	    class: SilverStripe\Logging\HTTPOutputHandler
	    constructor:
	      - "error"
	    properties:
	      Formatter: %$SilverStripe\Logging\DebugViewFriendlyErrorFormatter
	  SilverStripe\Logging\DebugViewFriendlyErrorFormatter:
	    class: SilverStripe\Logging\DebugViewFriendlyErrorFormatter
	    properties:
	      Title: "There has been an error"
	      Body: "The website server has not been able to respond to your request"

<div class="info" markdown="1">
In addition to SilverStripe-integrated logging, it is advisable to fall back to PHPs native logging functionality. A
script might terminate before it reaches the SilverStripe error handling, for example in the case of a fatal error. Make
sure `log_errors` and `error_log` in your PHP ini file are configured.
</div>

## Email Logs

You can send both fatal errors and warnings in your code to a specified email-address.

**mysite/_config.php**

	:::php
	if(!Director::isDev()) {
		// log errors and warnings
		SS_Log::add_writer(new SS_LogFileWriter('../silverstripe-errors-warnings.log'), SS_Log::WARN, '<=');

		// or just errors
		SS_Log::add_writer(new SS_LogEmailWriter('admin@domain.com'), SS_Log::ERR);
	}


## Replacing default implementations

For most application, Monolog and its default error handler should be fine, as you can get a lot of flexibility simply
by changing that handlers that are used. However, some situations will call for replacing the default components with
others.

### Replacing the logger

Monolog comes by default with SilverStripe, but you may use another PSR-3 compliant logger, if you wish. To do this,
set the `Injector.Logger` configuration parameter, providing a new injector definition. For example:

	SilverStripe\Core\Injector\Injector:
	  ErrorHandler:
	    class: Logging\Logger
	    constructor:
	     - 'alternative-logger'

If you do this, you will need to supply your own handlers, and the `Logger.handlers` configuration parameter will
be ignored.

### Replacing the error handler

The Injector service `ErrorHandler` is responsible for initialising the error handler. By default it 

 * Create a `SilverStripe\Logging\MonologErrorHandler` object.
 * Attach the registered service `Psr\Log\LoggerInterface` to it, to start the error handler.
 
Core.php will call `start()` on this method, to start the error handler.

This error handler is flexible enough to work with any PSR-3 logging implementation, but sometimes you will want to use
another. To replace this, you should registered a new service, `ErrorHandlerLoader`.  For example:

	SilverStripe\Core\Injector\Injector:
	  ErrorHandler: 
	    class: MyApp\CustomErrorHandlerLoader

You should register something with a `start()` method.

## Differences from SilverStripe 3

In SilverStripe 3, logging was based on the Zend Log module. Customisations were added using `SS_Log::add_writer()`.
This function no longer works, and any Zend Log writers will need to be replaced with Monolog handlers. Fortunately,
a range of handlers are available, both in the core package and in add-ons. See the
[Monolog documentation](https://github.com/Seldaek/monolog/blob/master/doc/01-usage.md) for more information.