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These configs are for localizing the framework resources in a YARN environment. If using a different execution environment, then it will be necessary to specify localization configs specific to that environment for the framework API and framework infrastructure resources. Other environments may have a different way for specifying the resource locations.

Existing JAR management

Currently, Samza infrastructure code and dependencies are included in the tarball with the Samza application. This means that conflicting dependencies between the application and Samza are resolved at build time before the tarball is created, which can cause a certain version of a dependency to be excluded. All JARs in the tarball are installed into a single directory for classpath generation and execution.

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Generating the Samza API whitelist

In order to load the Samza API classes from the API classloader, we need to tell cytodynamics what those classes are. We can do this by providing a whitelist of packages/classes when building the cytodynamics classloader. All public interfaces/classes inside of samza-api should be considered an API class. One way to generate this whitelist is to use a Gradle task to find all the classes from samza-api and put that list in a file. Then, that file can be read by Samza when constructing the cytodynamics classloader. The Gradle task should also include classes from samza-kv.

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A consequence of this structure is that there are "multiple" application classloaders on the job coordinator: one in this describe flow and the one described above at "Application" classloader. Therefore, any classes loaded by one of the application classloaders cannot be used by the classes of the other application classloader. An example of when this could happen is in the low-level API. The application's TaskFactory implementation will be loaded by the application classloader described above, but the Kafka events deserialized into Avro objects will be loaded by the other application classloader. Even though the Avro objects are the same class (even associated with the same binary), the TaskFactory implementation won't be able to use the Avro objects since a different classloader instance was used. We can solve this by serializing the components specified through the descriptor and deserializing those components using the classloader that is used for the rest of the AM. This is consistent with the strategy to be able to serialize the whole job description. The interfaces have already been marked as Serializable.

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When making a request to YARN, clients are allowed to pass a map of resources to localize on the container. Currently, the "yarn.package.path" config is used to localize the application package, and this includes the Samza infrastructure code. Applications will need to add other framework resources using "yarn.resources.*.path" configs.

1. Continue to use "yarn.package.path" for the application package.
2. Set "yarn.resources.__samzaFrameworkApi.path" to the path for the API package.
3. Set "yarn.resources.__samzaFrameworkInfrastructure.path" to the path for the infrastructure package.

Samza will look in specific locations on the file system for the JARs for setting up the classpaths for the different classloaders. The framework API classpath will come from "${user.dir}/__samzaFrameworkApi", the framework infrastructure classpath will come from "${user.dir}/__samzaFrameworkInfrastructure", and the application classpath will come from "${user.dir}/__package". When using the above 3 configs, YARN will place the resources into the desired locations.

In non-YARN execution environments, the "yarn" localization configurations won't apply. Other environments will have their own localization flows. If those other environments are unable to localize the resources into the desired file locations, then we can add a generic way (e.g. configuration or environment variables) to specify the file locations to get the classpath resources. The file location variables would apply to any Samza job; only the environment-specific localization flows would be different.

Generating classpaths for the JARs

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