To develop a new Java filter for Logstash, you write a new Java class that conforms to the Logstash Java Filters API, package it, and install it with the logstash-plugin utility. We’ll go through each of those steps.
Start by copying the example filter plugin. The plugin API is currently part of the
Logstash codebase so you must have a local copy of that available. You can
obtain a copy of the Logstash codebase with the following git
command:
git clone --branch <branch_name> --single-branch https://github.com/elastic/logstash.git <target_folder>
The branch_name
should correspond to the version of Logstash containing the
preferred revision of the Java plugin API.
The GA version of the Java plugin API is available in the 7.2
and later branches of the Logstash codebase.
Specify the target_folder
for your local copy of the Logstash codebase. If you
do not specify target_folder
, it defaults to a new folder called logstash
under your current folder.
After you have obtained a copy of the appropriate revision of the Logstash
codebase, you need to compile it to generate the .jar file containing the Java
plugin API. From the root directory of your Logstash codebase ($LS_HOME), you
can compile it with ./gradlew assemble
(or gradlew.bat assemble
if you’re
running on Windows). This should produce the
$LS_HOME/logstash-core/build/libs/logstash-core-x.y.z.jar
where x
, y
, and
z
refer to the version of Logstash.
After you have successfully compiled Logstash, you need to tell your Java plugin
where to find the logstash-core-x.y.z.jar
file. Create a new file named
gradle.properties
in the root folder of your plugin project. That file should
have a single line:
LOGSTASH_CORE_PATH=<target_folder>/logstash-core
where target_folder
is the root folder of your local copy of the Logstash codebase.
The example filter plugin allows one to configure a field in each event that
will be reversed. For example, if the filter were configured to reverse the
day_of_week
field, an event with day_of_week: "Monday"
would be transformed
to day_of_week: "yadnoM"
. Let’s look at the main class in that example filter:
@LogstashPlugin(name = "java_filter_example") public class JavaFilterExample implements Filter { public static final PluginConfigSpec<String> SOURCE_CONFIG = PluginConfigSpec.stringSetting("source", "message"); private String id; private String sourceField; public JavaFilterExample(String id, Configuration config, Context context) { this.id = id; this.sourceField = config.get(SOURCE_CONFIG); } @Override public Collection<Event> filter(Collection<Event> events, FilterMatchListener matchListener) { for (Event e : events) { Object f = e.getField(sourceField); if (f instanceof String) { e.setField(sourceField, StringUtils.reverse((String)f)); matchListener.filterMatched(e); } } return events; } @Override public Collection<PluginConfigSpec<?>> configSchema() { return Collections.singletonList(SOURCE_CONFIG); } @Override public String getId() { return this.id; } @Override public void close() { this.sourceField = null; return; } }
Let’s step through and examine each part of that class.
@LogstashPlugin(name = "java_filter_example") public class JavaFilterExample implements Filter {
Notes about the class declaration:
-
All Java plugins must be annotated with the
@LogstashPlugin
annotation. Additionally:-
The
name
property of the annotation must be supplied and defines the name of the plugin as it will be used in the Logstash pipeline definition. For example, this filter would be referenced in the filter section of the Logstash pipeline defintion asfilter { java_filter_example => { .... } }
-
The value of the
name
property must match the name of the class excluding casing and underscores.
-
The
-
The class must implement the
co.elastic.logstash.api.Filter
interface. -
Java plugins may not be created in the
org.logstash
orco.elastic.logstash
packages to prevent potential clashes with classes in Logstash itself.
The snippet below contains both the setting definition and the method referencing it:
public static final PluginConfigSpec<String> SOURCE_CONFIG = PluginConfigSpec.stringSetting("source", "message"); @Override public Collection<PluginConfigSpec<?>> configSchema() { return Collections.singletonList(SOURCE_CONFIG); }
The PluginConfigSpec
class allows developers to specify the settings that a plugin supports complete with setting
name, data type, deprecation status, required status, and default value. In this example, the source
setting defines
the name of the field in each event that will be reversed. It is not a required setting and if it is not explicitly
set, its default value will be message
.
The configSchema
method must return a list of all settings that the plugin supports. In a future phase of the
Java plugin project, the Logstash execution engine will validate that all required settings are present and that
no unsupported settings are present.
private String id; private String sourceField; public JavaFilterExample(String id, Configuration config, Context context) { this.id = id; this.sourceField = config.get(SOURCE_CONFIG); }
All Java filter plugins must have a constructor taking a String
id and a
Configuration
and Context
argument. This is the constructor that will be
used to instantiate them at runtime. The retrieval and validation of all plugin
settings should occur in this constructor. In this example, the name of the
field to be reversed in each event is retrieved from its setting and stored in
a local variable so that it can be used later in the filter
method.
Any additional initialization may occur in the constructor as well. If there are any unrecoverable errors encountered in the configuration or initialization of the filter plugin, a descriptive exception should be thrown. The exception will be logged and will prevent Logstash from starting.
@Override public Collection<Event> filter(Collection<Event> events, FilterMatchListener matchListener) { for (Event e : events) { Object f = e.getField(sourceField); if (f instanceof String) { e.setField(sourceField, StringUtils.reverse((String)f)); matchListener.filterMatched(e); } } return events;
Finally, we come to the filter
method that is invoked by the Logstash
execution engine on batches of events as they flow through the event processing
pipeline. The events to be filtered are supplied in the events
argument and
the method should return a collection of filtered events. Filters may perform a
variety of actions on events as they flow through the pipeline including:
-
Mutation — Fields in events may be added, removed, or changed by a filter. This
is the most common scenario for filters that perform various kinds of
enrichment on events. In this scenario, the incoming
events
collection may be returned unmodified since the events in the collection are mutated in place. - Deletion — Events may be removed from the event pipeline by a filter so that subsequent filters and outputs do not receive them. In this scenario, the events to be deleted must be removed from the collection of filtered events before it is returned.
- Creation — A filter may insert new events into the event pipeline that will be seen only by subsequent filters and outputs. In this scenario, the new events must be added to the collection of filtered events before it is returned.
-
Observation — Events may pass unchanged by a filter through the event pipeline.
This may be useful in scenarios where a filter performs external actions (e.g.,
updating an external cache) based on the events observed in the event pipeline.
In this scenario, the incoming
events
collection may be returned unmodified since no changes were made.
In the example above, the value of the source
field is retrieved from each
event and reversed if it is a string value. Because each event is mutated in
place, the incoming events
collection can be returned.
The matchListener
is the mechanism by which filters indicate which events
"match". The common actions for filters such as add_field
and add_tag
are
applied only to events that are designated as "matching". Some filters such as
the grok
filter
have a clear definition for what constitutes a matching event and will notify
the listener only for matching events. Other filters such as the
UUID
filter
have no specific match criteria and should notify the listener for every event
filtered. In this example, the filter notifies the match listener for any event
that had a String
value in its source
field and was therefore able to be
reversed.
@Override public String getId() { return id; }
For filter plugins, the getId
method should always return the id that was provided to the plugin through its
constructor at instantiation time.
@Override public void close() { // shutdown a resource that was instantiated during the filter initialization phase. this.sourceField = null; return; }
Filter plugins can use additional resources to perform operations, such as creating new database connections.
Implementing the close
method will allow the plugins to free up those resources when shutting down the pipeline.
Lastly, but certainly not least importantly, unit tests are strongly encouraged. The example filter plugin includes an example unit test that you can use as a template for your own.
Java plugins are packaged as Ruby gems for dependency management and
interoperability with Ruby plugins. Once they are packaged as gems, they may
be installed with the logstash-plugin
utility just as Ruby plugins are.
Because no knowledge of Ruby or its toolchain should be required for Java
plugin development, the procedure for packaging Java plugins as Ruby gems
has been automated through a custom task in the Gradle build file provided
with the example Java plugins. The following sections describe how to
configure and execute that packaging task as well as how to install the
packaged Java plugin in Logstash.
The following section appears near the top of the build.gradle
file supplied
with the example Java plugins:
// =========================================================================== // plugin info // =========================================================================== group 'org.logstashplugins' // must match the package of the main plugin class version "${file("VERSION").text.trim()}" // read from required VERSION file description = "Example Java filter implementation" pluginInfo.licenses = ['Apache-2.0'] // list of SPDX license IDs pluginInfo.longDescription = "This gem is a Logstash plugin required to be installed on top of the Logstash core pipeline using \$LS_HOME/bin/logstash-plugin install gemname. This gem is not a stand-alone program" pluginInfo.authors = ['Elasticsearch'] pluginInfo.email = ['info@elastic.co'] pluginInfo.homepage = "http://www.elastic.co/guide/en/logstash/current/index.html" pluginInfo.pluginType = "filter" pluginInfo.pluginClass = "JavaFilterExample" pluginInfo.pluginName = "java_filter_example" // ===========================================================================
You should configure the values above for your plugin.
-
The
version
value will be automatically read from theVERSION
file in the root of your plugin’s codebase. -
pluginInfo.pluginType
should be set to one ofinput
,filter
,codec
, oroutput
. -
pluginInfo.pluginName
must match the name specified on the@LogstashPlugin
annotation on the main plugin class. The Gradle packaging task will validate that and return an error if they do not match.
Several Ruby source files along with a gemspec
file and a Gemfile
are
required to package the plugin as a Ruby gem. These Ruby files are used only
for defining the Ruby gem structure or at Logstash startup time to register
the Java plugin. They are not used during runtime event processing. The
Gradle packaging task automatically generates all of these files based on
the values configured in the section above.
You run the Gradle packaging task with the following command:
./gradlew gem
For Windows platforms: Substitute gradlew.bat
for ./gradlew
as appropriate in the command.
That task will produce a gem file in the root directory of your
plugin’s codebase with the name logstash-{plugintype}-<pluginName>-<version>.gem
After you have packaged your Java plugin as a Ruby gem, you can install it in Logstash with this command:
bin/logstash-plugin install --no-verify --local /path/to/javaPlugin.gem
For Windows platforms: Substitute backslashes for forward slashes as appropriate in the command.
The following is a minimal Logstash configuration that can be used to test that the Java filter plugin is correctly installed and functioning.
input { generator { message => "Hello world!" count => 1 } } filter { java_filter_example {} } output { stdout { codec => rubydebug } }
Copy the above Logstash configuration to a file such as java_filter.conf
.
Start Logstash with:
bin/logstash -f /path/to/java_filter.conf
The expected Logstash output (excluding initialization) with the configuration above is:
{ "sequence" => 0, "@version" => "1", "message" => "!dlrow olleH", "@timestamp" => yyyy-MM-ddThh:mm:ss.SSSZ, "host" => "<yourHostName>" }
If you have any feedback on Java plugin support in Logstash, please comment on our main Github issue or post in the Logstash forum.