# Chapter 6: Filter chains

Have lots of ideas and throw away the bad ones. You aren't going to have good ideas unless you have lots of ideas and some sort of principle of selection.

—LINUS PAULING

As we have seen, logback has several built-in ways for filtering log requests, including the context-wide filter, logger-level selection rule and appender filters. These provide high performance filtering for the most commonly encountered cases. These filters are largely inspired from Linux ipchains or iptables as they are called in more recent Linux kernels. Logback filters are based on ternary logic allowing them to be assembled or chained together to compose an arbitrarily complex filtering policy.

There are two main types of filters, namely `Filter` and `TurboFilter`.

## Logback Classic

`Filter` objects all extend the `Filter` abstract class. The `decide(Object event)` method is passed a newly created `LoggingEvent` object.

### Filter chains

This abstract class assumes that filters are organized in a linear chain. Its member field next points to the next filter in the chain, or `null` if there are no further filters in the chain. Figure 6.1 depicts a sample filter chain consisting of three filters.

Filters are based on ternary logic. The `decide(Object event)` method of each filter is called in sequence. This method returns one of the `FilterReply` enumeration values, i.e. one of `FilterReply.DENY`, `FilterReply.NEUTRAL` or `FilterReply.ACCEPT`. If the returned value is `FilterReply.DENY`, then the log event is dropped immediately without consulting the remaining filters. If the value returned is `FilterReply.NEUTRAL`, then the next filter in the chain is consulted. If there are no further filters to consult, then the logging event is processed normally. If the returned value is `FilterReply.ACCEPT`, then the logging event is processed immediately skipping the remaining filters.

In logback-classic `Filter` objects can only be added to `Appender` instances. By adding filters to an appender you can filter events by various criteria, such as the contents of the log message, the contents of the MDC, the time of day or any other part of the logging event.

### Implementing your own Filter

Creating your own filter is not difficult. All you have to do is extend the `Filter` abstract class. The only method that you will have to implement is the `decide()` method, allowing you to contentrate only on the behaviour of your filter.

The next class is all it takes to implement one's own filter. All it does is accept logging events who's message contains the String sample. The filter will give a neutral response to any logging event who's message does not contain this String.

Example 6.1: Basic custom filter (logback-examples/src/main/java/chapter6/SampleFilter.java)

    package chapter6;

    import ch.qos.logback.classic.spi.LoggingEvent;
    import ch.qos.logback.core.filter.Filter;
    import ch.qos.logback.core.spi.FilterReply;

    public class SampleFilter extends Filter {

      @Override
      public FilterReply decide(Object eventObject) {
        LoggingEvent event = (LoggingEvent)eventObject;

        if (event.getMessage().contains("sample")) {
          return FilterReply.ACCEPT;
        } else {
          return FilterReply.NEUTRAL;
        }
      }
    }

What is shown above might be the simplest filter. Like any filter, it can be attached to any appender using the <Filter> element, as shown below:

Example 6.2: SampleFilter configuration (logback-examples/src/main/java/chapter6/SampleFilterConfig.xml)

    <configuration>
      <appender name="STDOUT"
        class="ch.qos.logback.core.ConsoleAppender">
        <Filter class="chapter6.SampleFilter" />

        <layout class="ch.qos.logback.classic.PatternLayout">
          <pattern>
            %-4relative [%thread] %-5level %logger - %msg%n
          </pattern>
        </layout>
      </appender>

      <root>
        <appender-ref ref="STDOUT" />
      </root>
    </configuration>

Thanks to Joran, logback's powerful configuration framework, adding an option to such a filter is very easy. Just add the corresponding getter and setter methods in the class, and you can specify the option's value in an xml element, nested within the filter element.

In case you want to implement a filter that provides different behaviour depending on the result of its test (say, a filter that would accept or deny an event depending on the content of its message), you can extend the `AbstractMatcherFilter` class. It will provide your filter with two attribute: OnMatch and OnMismatch, that can be configured like any other option.

### Logback Filters

At the moment, there are two filters that ship with logback. `LevelFilter` provides event filtering based on a `Level` value. It the event's level is equal to the configured level, the filter accepts of denies the event, depending on its configuration. It allows you to choose the behaviour of logback for a precise given level. Here is a sample configuration that uses `LevelFilter`.

Example 6.3: Sample LevelFilter configuration (logback-examples/src/main/java/chapter6/LevelFilterConfig.xml)

    <configuration>
      <appender name="CONSOLE"
        class="ch.qos.logback.core.ConsoleAppender">
        <filter class="ch.qos.logback.classic.filter.LevelFilter">
          <level>INFO</level>
          <onMatch>ACCEPT</onMatch>
          <onMismatch>DENY</onMismatch>
        </filter>
        <layout class="ch.qos.logback.classic.PatternLayout">
          <pattern>
            %-4relative [%thread] %-5level %logger{30} - %msg%n
          </pattern>
        </layout>
      </appender>
      <root>
        <level value="DEBUG" />
        <appender-ref ref="CONSOLE" />
      </root>
    </configuration>

The second filter that ships with logback is `ThresholdFilter`. It is also based on level value, but acts as a threshold to deny any request whose level is not equal or greater to the configured level. A sample use of the `ThresholdFilter` is shown below.

Example 6.4: Sample ThresholdFilter configuration (logback-examples/src/main/java/chapter6/ThresholdFilterConfig.xml)

    <configuration>
      <appender name="CONSOLE"
        class="ch.qos.logback.core.ConsoleAppender">
        <filter class="ch.qos.logback.classic.filter.ThresholdFilter">
          <level>INFO</level>
        </filter>
        <layout class="ch.qos.logback.classic.PatternLayout">
          <pattern>
            %-4relative [%thread] %-5level %logger{30} - %msg%n
          </pattern>
        </layout>
      </appender>
      <root>
        <level value="DEBUG" />
        <appender-ref ref="CONSOLE" />
      </root>
    </configuration>

### Evaluator Filters taking Java Expressions

A special category of filters is implemented by the `EvaluatorFilter` class. These filters use an `EventEvaluator` object to decide wether to accept or deny a request. This allows unprecedented flexibility in the way that you can affect filtering of logging events.

As a user, you do not need to worry about the actual plumbing. All you need to do is to give a name to the evaluator and to specify an evaluation expression, that is a boolean expression in regular Java syntax. These evaluation expressions are compiled on-the-fly during the interpretation of the configration file. It is the users reponsibility to ensure that the expression is boolean, that it evaluates to true or false. In evaluation expressions, logback implicitly exposes various fields of a logging event as variables. The list of these implicit variables is given below. The scope of evaluation expressions is limited to the logging event.

Name | Type | Description
--- | --- | ---
event | `LoggingEvent` | The raw logging event associated with the logging request. All of the following variables are also available from the event. For example, `event.getMessage()` returns the same String value as the message variable described next.
message | `String` | The message of the logging request.
logger | `LoggerRemoteView` | This object is a proxy for the logger object where the logging request was issued. It can be viewed as a regular logger object. However, it has some nice performance properties, especially with respect to serialization.
level | `int` | The int value corresponding to the level. To help create easily expressions involving levels, the default value DEBUG, INFO, WARN and ERROR are also available. Thus, using level > INFO is a correct expression.
timeStamp | `long` | The timestamp corresponding to the logging event's creation.
marker | `Marker` | The `Marker` object associated with the logging request.
mdc | `Map` | A map containing all the MDC values at the time of the creation of the logging event. A value can be access by using the following expression: mdc.get("myKey").
throwable | `Throwable` | The exception associated with the logging event

The behaviour of the `EvaluatorFilter` is also affected by its OnMatch and OnMismatch options taking values of type `FilterReply`, i.e. DENY, ACCEPT, NEUTRAL.

Here is a concrete example.

Example 6.5: Basic event evaluator usage (logback-examples/src/main/java/chapter6/basicEventEvaluator.xml)

    <configuration>

      <appender name="STDOUT"
        class="ch.qos.logback.core.ConsoleAppender">
        <filter class="ch.qos.logback.core.filter.EvaluatorFilter">
          <evaluator name="myEval">
            <expression>message.contains("billing")</expression>
          </evaluator>
          <OnMismatch>NEUTRAL</OnMismatch>
          <OnMatch>DENY</OnMatch>
        </filter>
        <layout class="ch.qos.logback.classic.PatternLayout">
          <pattern>
            %-4relative [%thread] %-5level %logger - %msg%n
          </pattern>
        </layout>
      </appender>

      <root>
        <level value="INFO" />
        <appender-ref ref="STDOUT" />
      </root>
    </configuration>

The bold part in the previous configuration adds an `EvaluatorFilter` to a `ConsoleAppender`. An `EventEvaluator` is then injected into the `EvaluatorFilter`. The expression element corresponds to the evaluation expression described previously. The expression `message.contains("billing")` returns a boolean value. Notice that the message variable is defined implicitly.

This evalutor filter will drop all logging events whose message contains the string "billing".

The `FilterEvents` class issues ten logging requests, numbered from 0 to 9. Let us rist run `FilterEvents` class without any filters:

    java chapter6.FilterEvents src/main/java/chapter6/basicConfiguration.xml

All requests will be displayed, as shown below:

    0    [main] INFO  chapter6.FilterEvents - logging statement 0
    0    [main] INFO  chapter6.FilterEvents - logging statement 1
    0    [main] INFO  chapter6.FilterEvents - logging statement 2
    0    [main] DEBUG chapter6.FilterEvents - logging statement 3
    0    [main] INFO  chapter6.FilterEvents - logging statement 4
    0    [main] INFO  chapter6.FilterEvents - logging statement 5
    0    [main] ERROR chapter6.FilterEvents - billing statement 6
    0    [main] INFO  chapter6.FilterEvents - logging statement 7
    0    [main] INFO  chapter6.FilterEvents - logging statement 8
    0    [main] INFO  chapter6.FilterEvents - logging statement 9

Suppose that we want to get rid of the billing information. The basicEventEvaluator.xml configuration file just desribed, does exactly what we want.

Running with filters:

java chapter6.FilterEvents src/main/java/chapter6/basicEventEvaluator.xml

we obtain:

    0    [main] INFO  chapter6.FilterEvents - logging statement 0
    0    [main] INFO  chapter6.FilterEvents - logging statement 1
    0    [main] INFO  chapter6.FilterEvents - logging statement 2
    0    [main] DEBUG chapter6.FilterEvents - logging statement 3
    0    [main] INFO  chapter6.FilterEvents - logging statement 4
    0    [main] INFO  chapter6.FilterEvents - logging statement 5
    0    [main] INFO  chapter6.FilterEvents - logging statement 7
    0    [main] INFO  chapter6.FilterEvents - logging statement 8
    0    [main] INFO  chapter6.FilterEvents - logging statement 9

### TurboFilters

`TurboFilter` objects all extend the `TurboFilter` abstract class. Like the regular filters, they use ternary logic to return their evaluation of the logging event.

Overall, they work much like the previously mentionned filters. However, there are two main differences between `Filter` and `TurboFilter` objects.

`TurboFilter` objects are tied to the logging context. Hence, they are called not only when a given appender is used, but each and every time a logging request is issued. Their scope is wider than appender-attached filters.

More importantly, they are called before the `LoggingEvent` object creation. `TurboFilter` objects do not require the instantiation of a logging event to filter a logging request. As such, turbo filters are intended for high performance filtering of logging event, even before they are created.

### Implementing your own TurboFilter

To create your own `TurboFilter` component, just extend the `TurboFilter` abstract class. As previously, when implementing a custumized filter object, developing a custom `TurboFilter` only ask that one implement the `decide()` method. In the next example, we create a slightly more complex filter:

Example 6.6: Basic custom `TurboFilter` (logback-examples/src/main/java/chapter6/SampleTurboFilter.java)

    package chapter6;

    import org.slf4j.Marker;
    import org.slf4j.MarkerFactory;

    import ch.qos.logback.classic.Level;
    import ch.qos.logback.classic.Logger;
    import ch.qos.logback.classic.turbo.TurboFilter;
    import ch.qos.logback.core.spi.FilterReply;

    public class SampleTurboFilter extends TurboFilter {

      String marker;
      Marker markerToAccept;

      @Override
      public FilterReply decide(Marker marker, Logger logger, Level level,
          String format, Object[] params, Throwable t) {

        if (!isStarted()) {
          return FilterReply.NEUTRAL;
        }

        if ((markerToAccept.equals(marker))) {
          return FilterReply.ACCEPT;
        } else {
          return FilterReply.NEUTRAL;
        }
      }

      public String getMarker() {
        return marker;
      }

      public void setMarker(String markerStr) {
        this.marker = markerStr;
      }

      @Override
      public void start() {
        if (marker != null && marker.trim().length() > 0) {
          markerToAccept = MarkerFactory.getMarker(marker);
          super.start();
        }
      }
    }

The `TurboFilter` above accepts events that contain a specific marker. If said marker is not found, then the filter passes the responsability to the next filter in the chain.

To allow more flexibility, the marker that will be tested can be specified in the configuration file. Hence the getter and setter methods. We also implemented the `start()` method, to check that the option has been specified during the configuration process.

Here is a sample configuration that makes use of our newly created `TurboFilter`.

Example 6.7: Basic custom `TurboFilter` configuration (logback-examples/src/main/java/chapter6/sampleTurboFilterConfig.xml)

    <configuration>
      <turboFilter class="chapter6.SampleTurboFilter">
        <Marker>sample</Marker>
      </turboFilter>

      <appender name="STDOUT"
        class="ch.qos.logback.core.ConsoleAppender">
        <layout class="ch.qos.logback.classic.PatternLayout">
          <pattern>
            %-4relative [%thread] %-5level %logger - %msg%n
          </pattern>
        </layout>
      </appender>

      <root>
        <appender-ref ref="STDOUT" />
      </root>
    </configuration>

Logback classic ships with several `TurboFilter` classes ready for use. The `MDCFilter` check the presence of a given value in the MDC whereas `DynamicThresholdFilter` allows filtering based on MDC key/level thresold associations. On the other hand, `MarkerFilter` checks for the presence of a specific marker associated with the logging request.

Here is a sample configuration, using both `MDCFilter` and `MarkerFilter`.

Example 6.8: `MDCFilter` and `MarkerFilter` configuration (logback-examples/src/main/java/chapter6/turboFilters.xml)

    <configuration>

      <turboFilter class="ch.qos.logback.classic.turbo.MDCFilter">
        <MDCKey>username</MDCKey>
        <Value>sebastien</Value>
        <OnMatch>ACCEPT</OnMatch>
      </turboFilter>

      <turboFilter class="ch.qos.logback.classic.turbo.MarkerFilter">
        <Marker>billing</Marker>
        <OnMatch>DENY</OnMatch>
      </turboFilter>

      <appender name="console" class="ch.qos.logback.core.ConsoleAppender">
        <layout class="ch.qos.logback.classic.PatternLayout">
          <Pattern>%date [%thread] %-5level %logger - %msg%n</Pattern>
      </layout>
      </appender>

      <root>
        <level value="info"/>
        <appender-ref ref="console" />
      </root>
    </configuration>

You can see this configuration in action by issuing the following command:

    java chapter6.FilterEvents src/main/java/chapter6/turboFilters.xml

As we've seen previously, the `FilterEvents` class creates 10 logging requests, each with its number from 0 to 9. All of the requests are of level INFO, just like the configured overall level, except for two requests. The 3rd request, is a DEBUG level corresponding to the key username. This obviously satisfies the first `TurboFilter` declared in the previous configuration file. The 6th request, a ERROR level request, which is issued along with the billing marker, matches the second `TurboFilter`.

Here is the output of the previous command:

    2006-12-04 15:17:22,859 [main] INFO  chapter6.FilterEvents - logging statement 0
    2006-12-04 15:17:22,875 [main] INFO  chapter6.FilterEvents - logging statement 1
    2006-12-04 15:17:22,875 [main] INFO  chapter6.FilterEvents - logging statement 2
    2006-12-04 15:17:22,875 [main] DEBUG chapter6.FilterEvents - logging statement 3
    2006-12-04 15:17:22,875 [main] INFO  chapter6.FilterEvents - logging statement 4
    2006-12-04 15:17:22,875 [main] INFO  chapter6.FilterEvents - logging statement 5
    2006-12-04 15:17:22,875 [main] INFO  chapter6.FilterEvents - logging statement 7
    2006-12-04 15:17:22,875 [main] INFO  chapter6.FilterEvents - logging statement 8
    2006-12-04 15:17:22,875 [main] INFO  chapter6.FilterEvents - logging statement 9

One can see that the 3rd request, who should not be displayed if we only followed the overall INFO level, appears anyway, because it matched the first `TurboFilter` requirements and was accepted.

On the other hand, the 6th request, that is a ERROR level request should have been displayed. But it satisfied the second `TurboFilter` whose OnMatch option is set to DENY. Thus, the 6th request was not displayed.

## Logback-access

Logback-access offers most of the features available with logback-classic. `Filter` objects are available and work in the same way as their logback-classic counterparts. They handle access' implementation of logging events: `AccessEvent`. Thus, a customized filter for logback access follows strictly the same rules as those for logback-classic, except for the event type recieved as parameter. On the other hand, `TurboFilter` objects are supported by logback-access.

### Filters

`EvaluatorFilter` objects with java expressions supplied in in `evaluator` configuration elements are supported by logback-access. However, list implicit variables available for constructing an expression are different. Only the `AccessEvent` object can be used by inserting the event variable in the expression. Nevertheless the access `evaluator` is just as powerfull. All the request and response components are reachable from the event variable.

Here is a sample configuration that will ensure that any 404 error will be logged:

Example 6.9: Access Evaluator (logback-examples/src/main/java/chapter6/accessEventEvaluator.xml)

    <configuration>

      <appender name="STDOUT"
        class="ch.qos.logback.core.ConsoleAppender">
        <filter class="ch.qos.logback.core.filter.EvaluatorFilter">
          <evaluator name="myEval">
            <expression>event.getStatusCode() == 404</expression>
          </evaluator>
          <OnMismatch>NEUTRAL</OnMismatch>
          <OnMatch>ACCEPT</OnMatch>
        </filter>
        <layout class="ch.qos.logback.access.PatternLayout">
          <pattern>
            %h %l %u %t %r %s %b
          </pattern>
        </layout>
      </appender>

      <appender-ref ref="STDOUT" />
    </configuration>

We might imagine a slightly more complex use of filters to ensure logging of 404 errors, except those returned on access to CSS resources. Here is what such a configuration would look like:

Example 6.10: Access Evaluator (logback-examples/src/main/java/chapter6/accessEventEvaluator2.xml)

    <configuration>

      <appender name="STDOUT"
        class="ch.qos.logback.core.ConsoleAppender">
        <filter class="ch.qos.logback.core.filter.EvaluatorFilter">
          <evaluator name="Eval404">
            <expression>event.getStatusCode() == 404</expression>
          </evaluator>
          <OnMismatch>NEUTRAL</OnMismatch>
          <OnMatch>ACCEPT</OnMatch>
        </filter>
        <filter class="ch.qos.logback.core.filter.EvaluatorFilter">
          <evaluator name="EvalCSS">
            <expression>event.getRequestURI().contains("css")</expression>
          </evaluator>
          <OnMismatch>NEUTRAL</OnMismatch>
          <OnMatch>DENY</OnMatch>
        </filter>
        <layout class="ch.qos.logback.access.PatternLayout">
          <pattern>
            %h %l %u %t %r %s %b
          </pattern>
        </layout>
      </appender>

      <appender-ref ref="STDOUT" />
    </configuration>