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Interceptors

All communication from a client to a broker must first pass through a gateway. There they can be intercepted before being dispatched. Zeebe provides a way to load arbitrary interceptors into the gateway. Some typical examples of what you can accomplish with this include:

Implementing an interceptor​

For the communication between client and gateway, Zeebe uses the gRPC protocol. An interceptor is thus implemented as a gRPC ServerInterceptor.

An implementation must adhere to the following requirements:

  • It implements ServerInterceptor
  • It has public visibility
  • It has a public default constructor (i.e. no-arg constructor)

Let's consider an interceptor that provides logging of incoming calls as an example. Other ServerInterceptor examples can be found in the official grpc-java examples.

package io.camunda.zeebe.example;

import io.grpc.ForwardingServerCallListener.SimpleForwardingServerCallListener;
import io.grpc.Metadata;
import io.grpc.ServerCall;
import io.grpc.ServerCall.Listener;
import io.grpc.ServerCallHandler;
import io.grpc.ServerInterceptor;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
 * A simple interceptor that logs each incoming call. The class must be public
 * since we will load it via JAR into the gateway.
 */
public final class LoggingInterceptor implements ServerInterceptor {
  private static final Logger LOGGER =
      LoggerFactory.getLogger("LoggingInterceptor");

  @Override
  public <ReqT, RespT> Listener<ReqT> interceptCall(
      final ServerCall<ReqT, RespT> call,
      final Metadata headers,
      final ServerCallHandler<ReqT, RespT> next) {
    final var listener = next.startCall(call, headers);
    return new SimpleForwardingServerCallListener<>(listener) {
      @Override
      public void onMessage(final ReqT message) {
        LOGGER.trace("intercepted a call");
        super.onMessage(message);
      }
    };
  }
}

This example interceptor will log "intercepted a call" at TRACE level for each incoming call it intercepted. This specific interceptor always dispatches all incoming calls to the target broker, but it would also be possible to stop the message from interception by other interceptors and even to block it from dispatch to the broker.

Compiling your interceptor​

Our source code for the interceptor class can now be compiled. There are many ways to do this, but for simplicity we'll use javac directly.

When compiling your class, you need to make sure all compile-time dependencies are provided. In the example above, that means we need the grpc-api and slf4j-api libraries available when compiling.

Since the interceptor will be running inside the Zeebe gateway, the language level of the compiled code must be the same as Zeebe's (i.e. currently JDK 11) or lower. This example thus assumes you're using version 11 of javac.

# to compile LoggingInterceptor.java, we'll need to provide the api libraries
javac -classpath .:lib/grpc-api.jar:lib/slf4j-api.jar ./LoggingInterceptor.java

Packaging an interceptor​

Next, you need to package the interceptor class into a fat JAR. Such a JAR must contain all classes (i.e. including all classes your own classes depend upon at runtime).

Like compiling there are many ways to do this, but for simplicity we'll use jar directly. Note, that means we have to define a java manifest file by hand, in order to place the libraries' classes on the classpath.

Similar to your interceptor class, any libraries you package must be compiled for the same language level as Zeebe's (i.e. currently JDK 11) or lower.

# both runtime libraries and the manifest must be packaged together with the compiled classes
jar cvfm LoggingInterceptor.jar ./MANIFEST.MF ./*.class ./lib

# let's verify the contents of the JAR
jar tf ./LoggingInterceptor.jar
# META-INF/
# META-INF/MANIFEST.MF
# LoggingInterceptor.java
# LoggingInterceptor$1.class
# lib/
# lib/grpc-api.jar
# lib/grpc.jar
# lib/slf4j-api.jar
# lib/slf4j.jar

Loading an interceptor into a gateway​

An interceptor can be loaded into your gateway as a fat JAR. For each interceptor, you need to provide your gateway with:

  • An interception order index
  • An identifier to identify this specific interceptor
  • Where to find the JAR with the interceptor class
  • The fully qualified name of the interceptor class, e.g. com.acme.ExampleInterceptor

Let's continue with the LoggingInterceptor example. We can provide these configurations using a gateway config file, environment variables or a mix of both. We'll be using a config file here.

The following gateway config file configures our LoggingInterceptor so it can be loaded into the gateway at start-up.

zeebe:
  gateway:
    ...

    # allows specifying multiple interceptors
    interceptors:

      - # identifier, can be used for debugging
        id: logging-interceptor

        # name of our ServerInterceptor implementation
        # this must be the fully qualified name of the class
        className: io.camunda.zeebe.example.LoggingInterceptor

        # path to the fat JAR, can be absolute or relative
        jarPath: /tmp/LoggingInterceptor.jar

      # you can add additional interceptors by listing them
      - id: ...
        className: ...
        jarPath: ...

Note that multiple interceptors can be configured (i.e. zeebe.gateway.interceptors expects a list of interceptor configurations). The listing order determines the order in which a call is intercepted by the different interceptors. The first interceptor in the list wraps the second, etc. The first interceptor is thus the outermost interceptor. In other words, calls are intercepted first by the first listed interceptor, followed by the second listed interceptor, etc.

This configuration can also be provided using environment variables. You'll need to provide an index for the interceptor in the variable name, to distinguish the ordering of the different interceptors. For example, to configure the className of the first interceptor use: zeebe_gateway_interceptors_0_className. Likewise, a second interceptor's jarPath can be configured using zeebe_gateway_interceptors_1_jarPath.

About class loading​

Previously, we stated that you need to package the interceptor class into a fat JAR. Although good general advice, this is not entirely true. To understand why, let's discuss how the class loading of your interceptor works.

When your JAR is loaded into the gateway, Zeebe provides a special class loader for it. This class loader isolates your interceptor from the rest of Zeebe, but it also exposes our own code to your interceptor. When loading classes for your interceptor, it will always first look in this special class loader and only if it is not available it will look in Zeebe's main class loader. In other words, you can access any classes from Zeebe's main class loader when they are not provided by your JAR. For internal class loading, Zeebe will still only look in its main class loader.

This means you can reduce your JAR size by leaving out libraries that are already provided by Zeebe's class loader. In addition, if your interceptor depends on a different version of a class than the one provided by Zeebe, then you can provide your own version without having to worry about breaking Zeebe.

Troubleshooting​

Here we describe a few common errors. Hopefully, this will help you recognize these situations and provide an easy fix. Generally, the gateway will not be able to start up with a misconfigured interceptor.

Note that environment variables can overwrite your gateway configuration file. The gateway logs the configuration it uses during start-up. Please use that to verify your configuration.

java.lang.ClassNotFoundException Your ServerInterceptor implementation could not be found. Make sure you've configured the className correctly in the gateway configuration and that your JAR contains your class.

io.camunda.zeebe.gateway.interceptors.impl.InterceptorLoadException Something went wrong trying to load your interceptor. Make sure your JAR is packaged correctly, i.e. it contains all runtime dependencies and specifies them in the manifest file's classpath. The exception should provide a clear description, but generally we distinguish the following common cases:

io.camunda.zeebe.util.jar.ExternalJarLoadException: the JAR could not be loaded: make sure you've configured your interceptor correctly in the gateway configuration.

java.lang.UnsupportedClassVersionError Your interceptor has been compiled by a more recent version of the Java Runtime. Make sure your class is compiled with JDK 11.