Users can extend Axon Server functionality by defining interceptors and hooks. Interceptors intercept requests that client applications send and perform actions before and/or after the request is handled. Hooks can perform actions on messages, but are not able to change the content of the messages.
Axon Server supports interceptors/hooks for the following types:
- Command
- Query (scatter/gather and point-to-point)
- Subscription Query
- Event
- Snapshot
Users can define multiple interceptors for the same point, these interceptors will be executed based on the value of the order operation in the interceptor instance.
Each interceptor operation has an ExecutionContext instance as its last parameter. This contains information about the caller of the request, the Axon Server context and allows implementors of interceptors to pass data in the interceptor chain. For instance, for a command the ExecutionContext for the request interceptors is the same instance as the one provided in the response interceptors.
Commands can be intercepted before they are sent to a command handler and CommandResults are intercepted after the command handler has handled the command.
Axon Server executes all registered CommandRequestInterceptor instances before sending the command to the handler. If one of the interceptor instances throws an exception Axon Server will not send the command to the handler, and it will return a CommandResponse with an error to the client that sent the command.
Once the command handler has handled the command and returned the reply to Axon Server, Axon Server executes all registered CommandResponseInterceptor instances. The response interceptor will only receive the CommandResponse object and the ExecutionContext, if it needs any information of the Command request, this has to be added to the ExecutionContext by a CommandRequestInterceptor. The response interceptor is also executed when the command failed in the command handler. In this case the CommandResponse object contains an error code and error message.
Similar interceptors to commands (QueryRequestInterceptor and QueryResponseInterceptor). Note that a query may return multiple responses, in case of a scatter/gather query. In this case Axon Server executes the response interceptors for each response.
The subscription query interceptors intercept SubscriptionQueryRequest and SubscriptionQueryResponse messages. These messages are complex messages that contain different types of messages.
Each SubscriptionQueryRequest contains one of the following types:
- Subscribe, to subscribe to the updates for the subscription query
- Unsubscribe, to unsubscribe from updates
- GetInitialResult, to request the initial result
- FlowControl, to grant permits to the updates providers to send more updates
A SubscriptionQueryResponse message contains one of the following types:
- InitialResult, the initial result for the subscription query
- Update, an update for the subscription query
- Complete, an indicator that the query is complete and no more updates will be sent
- CompleteExceptionally, an indicator that the query is completed exceptionally and no more updates will be sent
For events there are interceptors around the storing of events and interceptors for reading events.
Storing events
When storing events, a client sends a stream of events to Axon Server, for each event that reaches Axon Server, Axon Server executes the AppendEventInterceptor instances. These interceptors can manipulate the content of the event, and if one of the interceptors throws an error the transaction fails.
When the client closes the stream, to commit the events, Axon Server executes the EventsPreCommitHook instances. This interceptor receives the list of events in the transaction and the ExecutionContext.
Once Axon Server has stored the events in the event store, and before it returns the confirmation to the client, it executes any EventsPostCommitHook instances.
If the one of the AppendEventInterceptor or EventsPreCommitHook makes changes in an external system, that you want to have undone if the transaction was cancelled, the interceptor can register an onFailure action in the _ ExecutionContext_. If the transaction fails for any reason in Axon Server, or because one of the subsequent interceptors throws an exception, all the registered onFailure actions are executed. The actions are executed in reverse order (the last registered action is executed first).
Reading events
Axon Server executes the ReadEventInterceptor instances for each event read from the event store and sent to a client application. The interceptor may change the content of the event.
For snapshots there are interceptors around the storing of snapshot and interceptors for reading snapshots. Before storing a snapshot, Axon Server executes all AppendSnapshotInterceptor instances. Once the snapshot is stored in AxonServer, it executes all PostCommitSnapshotHook instances.
Similar as for events, you can also register onFailure actions from interceptors from the AppendSnapshotInterceptor interceptors.
On sending snapshots to a client, Axon Server executes all SnapshotReadInterceptor instances.
To add plugins in Axon Server, create an OSGi module that contains the plugin implementations. You can add all plugins in a single module, or you can define multiple modules, each containing some interceptors.
The implementation of the OSGi module must contain an BundleActivator class, that makes the interceptors available in the OSGi container.
package org.example.interceptor;
import io.axoniq.axonserver.plugin.ReadEventInterceptor;
import org.example.interceptor.impl.FirstEventReadInterceptor;
import org.osgi.framework.BundleActivator;
import org.osgi.framework.BundleContext;
import org.osgi.framework.ServiceRegistration;
import java.util.HashSet;
import java.util.Set;
public class Activator implements BundleActivator {
private Set<ServiceRegistration<?>> registration = new HashSet<>();
public void start(BundleContext bundleContext) {
registration.add(bundleContext.registerService(ReadEventInterceptor.class,
new FirstEventReadInterceptor(),
null));
}
public void stop(BundleContext bundleContext) {
registration.forEach(ServiceRegistration::unregister);
}
}To bundle the interceptors in a jar file you can use the following maven template:
<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
<groupId>org.sample</groupId>
<version>1.0.1-SNAPSHOT</version>
<artifactId>custom-interceptors</artifactId>
<modelVersion>4.0.0</modelVersion>
<!-- Set to bundle to create an OSGi bundle jar file -->
<packaging>bundle</packaging>
<dependencies>
<!-- Dependency on the axonserver-plugin-api as this contains all the plugin interfaces,
provided as it should not be included in the output bundle -->
<dependency>
<groupId>io.axoniq</groupId>
<artifactId>axonserver-plugin-api</artifactId>
<version>4.7.0</version>
<scope>provided</scope>
</dependency>
<!-- Access to the OSGI classes,
provided as it should not be included in the output bundle -->
<dependency>
<groupId>org.osgi</groupId>
<artifactId>org.osgi.core</artifactId>
<version>6.0.0</version>
<scope>provided</scope>
</dependency>
</dependencies>
<build>
<plugins>
<plugin>
<groupId>org.apache.felix</groupId>
<artifactId>maven-bundle-plugin</artifactId>
<extensions>true</extensions>
<configuration>
<instructions>
<Embed-Dependency>*;scope=compile|runtime</Embed-Dependency>
<!-- import the packages exposed by the plugin api and the Axon Server message types -->
<Import-Package>
io.axoniq.axonserver.plugin.*;io.axoniq.axonserver.grpc.*;org.osgi.framework;com.google.protobuf.*
</Import-Package>
<!-- internal packages for the bundle -->
<Private-Package>org.example.interceptor.*</Private-Package>
<!-- references the BundleActivator class -->
<Bundle-Activator>org.example.interceptor.Activator</Bundle-Activator>
</instructions>
</configuration>
</plugin>
</plugins>
</build>
</project>You can change the contents of a request inside an interceptor. As the request passed to the interceptor is immutable, you need to create a new version of the request. The recommended way is to initialize a builder with the current request and set the changed values in the builder. The following example adds a meta-data field in an event:
@Override
public Event appendEvent(Event event,ExecutionContext executionContext) {
return Event.newBuilder(event)
.putMetaData("createdBy",
MetaDataValue.newBuilder()
.setTextValue(pluginUoW.principal()==null?
"[anonymous]":
pluginUoW.principal())
.build())
.build();
}
Plugins can require configuration you don't want to hard-code in the package. To define these configurable properties, you can implement a class implementing the ConfigurationListener interface, define bind this to the bundle context and pass the class to the interceptor. The configuration is set in Axon Server per context.
Here's an example of a configuration listener class:
package org.sample.impl;
import io.axoniq.axonserver.plugin.AttributeType;
import io.axoniq.axonserver.plugin.Configuration;
import io.axoniq.axonserver.plugin.ConfigurationListener;
import io.axoniq.axonserver.plugin.PluginPropertyDefinition;
import java.util.Collections;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import static java.util.Arrays.asList;
public class SampleConfigurator extends DefaultConfigurationListener {
public SampleConfigurator() {
super("demo", asList(
PluginPropertyDefinition.newBuilder("hostname", "Hostname")
.defaultValue("127.0.0.1")
.description("The hostname")
.build(),
PluginPropertyDefinition.newBuilder("port", "Port")
.type(AttributeType.INTEGER)
.defaultValue(654)
.build(),
PluginPropertyDefinition.newBuilder("username", "Username")
.defaultValue("guest")
.build(),
PluginPropertyDefinition.newBuilder("password", "Password")
.type(AttributeType.PASSWORD)
.build()
));
}
}The constructor sets up a list of configurable properties. For each property you can define the type, a default value, the cardinality and a list of options. Axon Server uses the configuration operation to retrieve the information about the configurable properties. When a plugin is started for a context, or when the properties are updated through Axon Server, Axon Server calls the updated operation.
The following example shows how the configuration listener is registered in the bundle context and passed to an interceptor:
package org.sample;
import io.axoniq.axonserver.pluginAppendEventInterceptor;
import io.axoniq.axonserver.pluginConfigurationListener;
import org.osgi.framework.BundleActivator;
import org.osgi.framework.BundleContext;
import org.osgi.framework.ServiceRegistration;
import org.sample.impl.EventInterceptor;
import org.sample.impl.SampleConfigurator;
import java.util.ArrayList;
import java.util.List;
public class ConfigActivator implements BundleActivator {
private final List<ServiceRegistration<?>> registrations = new ArrayList<>();
@Override
public void start(BundleContext context) {
SampleConfigurator configurationListener = new SampleConfigurator();
// make the SampleConfiguration available to Axon Server by registering it as a ConfigurationListener
registrations.add(context.registerService(ConfigurationListener.class, configurationListener, null));
// register the interceptor, the EventInterceptor gets the SampleConfiguration as constructor parameter
registrations.add(context.registerService(AppendEventInterceptor.class,
new EventInterceptor(configurationListener),
null));
}
@Override
public void stop(BundleContext context) {
registrations.forEach(ServiceRegistration::unregister);
}
}Plugins run in their own environment, therefore the logging is not integrated with Axon Server's own logging. To do logging from your own plugin you have to add a logging library to the plugin. To use sl4j in combination with logback you can add the following dependencies in the project:
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-api</artifactId>
<version>1.7.30</version>
</dependency>
<dependency>
<groupId>ch.qos.logback</groupId>
<artifactId>logback-classic</artifactId>
<version>1.2.3</version>
</dependency>Note, that although Axon Server is using the same logging classes, you are responsible for configuring the logging in the plugin. To configure the logging with logback, add a logback.xml file in the plugin jar.
It is possible to log to the same file as Axon Server is logging to (if you have set up Axon Server to log to a file). The log messages will be mixed inside the file.