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Filters

danger

Filters are only applied to the REST API of the gateway, and do not affect any gRPC calls.

For gRPC-related middleware, review the gateway interceptors documentation.

All communication from a client to a broker must first pass through a gateway. There, it can be filtered before being dispatched.

Zeebe provides a way to load arbitrary REST API filters into the gateway. Some typical examples of what you can accomplish with this include:

  • Enforcing custom authorization rules on incoming calls
  • Monitoring and logging incoming calls

Implementing a filter​

To communicate between client and gateway, Zeebe uses REST.

A filter is then implemented as a Jakarta servlet filter.

An implementation must adhere to the following requirements:

  • It implements a filter
  • It has public visibility
  • It has a public default constructor (for example, no-arg constructor)

Consider a filter logging incoming calls as an example:

package io.camunda.zeebe.example;

import jakarta.servlet.Filter;
import jakarta.servlet.FilterChain;
import jakarta.servlet.ServletException;
import jakarta.servlet.ServletRequest;
import jakarta.servlet.ServletResponse;
import java.io.IOException;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

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

  @Override
  public void doFilter(
          final ServletRequest servletRequest,
          final ServletResponse servletResponse,
          final FilterChain filterChain)
          throws IOException, ServletException {
    LOGGER.trace("filtered a call");
    filterChain.doFilter(servletRequest, servletResponse);
  }
}

This example filter will log filtered a call at TRACE level for each incoming call it filters. This specific filter always dispatches all incoming calls to the target broker, but it is possible to stop the message from being filtered by other filters, and even block it from dispatching to the broker.

note

Camunda 8 relies on Identity for tenant management and, as of 8.5.0, the REST API component of the Zeebe Gateway does not support custom tenant-providing filters.

Compiling your filter​

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

When compiling your class, ensure all compile-time dependencies are provided. In the example above, you'll need the jakarta.servlet-api and slf4j-api libraries available when compiling.

Since the filter 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 21) or lower. This example thus assumes you're using version 21 of javac.

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

Packaging a filter​

Next, package the filter class into a fat JAR. Such a JAR must contain all classes (for example, all classes your own classes depend upon at runtime).

Like compiling, there are many ways to do this, but we'll use jar directly. This means we must define a Java manifest file by hand to place the libraries' classes on the classpath.

Similar to your filter class, any libraries you package must be compiled for the same language level as Zeebe's (currently JDK 21) or lower.

note

The file path for jar should match the package name. For example, if your package name is com.example, you should package jar as jar cvfm LoggingFilter.jar ./MANIFEST.MF ./com/example/*.class ./lib.

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

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

Loading a filter into a gateway​

A filter can be loaded into your gateway as a fat JAR. For each filter, provide the gateway with:

  • A filter order index
  • An identifier to identify this specific filter
  • Where to find the JAR with the filter class
  • The fully qualified name of the filter class. For example, com.acme.ExampleFilter.

Continuing with the LoggingFilter example, provide these configurations using a gateway config file, environment variables, or a mix of both. We'll be using a config file in this example.

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

zeebe:
  gateway:
    ...

    # allows specifying multiple filters
    filters:

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

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

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

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

Multiple filters can be configured (for example, zeebe.gateway.filters expects a list of filter configurations). The listing order determines the order in which a call is filtered by the different filters. The first filter in the list wraps the second, etc. The first filter is thus the outermost filter. In other words, calls are filtered first by the first listed filter, followed by the second listed filter, etc.

This configuration can also be provided using environment variables. Provide an index for the filter in the variable name to distinguish the ordering of the different filters. For example, to configure the className of the first filter use zeebe_gateway_filters_0_className. Likewise, a second filter's jarPath can be configured using zeebe_gateway_filters_1_jarPath.

Class loading​

Previously, we stated that you need to package the filter 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 filter works.

When your JAR is loaded into the gateway, Zeebe provides a special class loader for it. This class loader isolates your filter from the rest of Zeebe, but it also exposes our own code to your filter.

When loading classes for your filter, it will always first look in this special class loader. If it is not available, it will look in Zeebe's main class loader. Therefore, 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.

You can reduce your JAR size by leaving out libraries already provided by Zeebe's class loader. Additionally, if your filter depends on a different version of a class than the one provided by Zeebe, you can provide your own version without having to worry about breaking Zeebe.

Troubleshooting​

Here we describe a few common errors to help you recognize these situations and provide a solution. Generally, the gateway will not be able to start up with a misconfigured filter.

note

Environment variables can overwrite your gateway configuration file. The gateway logs the configuration it uses during start-up; use this to verify your configuration.

java.lang.ClassNotFoundException​

Your Filter implementation could not be found. Ensure you've configured the className correctly in the gateway configuration and your JAR contains your class.

io.camunda.zeebe.gateway.rest.impl.filters.FilterLoadException​

Something went wrong trying to load your filter. Ensure your JAR is packaged correctly. For example, ensure 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:

*Unable to instantiate your class

Ensure your class adheres to the requirements described above.

The JAR could not be loaded

Ensure you've configured your filter correctly in the gateway configuration.

io.camunda.zeebe.util.jar.ExternalJarLoadException​

The JAR could not be loaded. Ensure you've configured your filter correctly in the gateway configuration.

java.lang.UnsupportedClassVersionError​

Your filter has been compiled by a more recent version of the Java Runtime. Ensure your class is compiled with JDK 21.