PIP-193: Sink preprocessing Function

Pulsar IO connectors make it possible to connect Pulsar to an external system:

A Source reads continuously from an external system and writes to a Pulsar topic
A Sink reads continuously from a Pulsar topic and writes to an external system. Sources and Sinks are written in Java.

Pulsar also has a lightweight computing system named Pulsar Functions. A Pulsar Function reads from one or more topics, applies user logic written in Java, Python or Go and writes to an output topic.

When using Pulsar IO connectors, the format of what is read/written from/to the source/sink is defined by the connector code. But there are a lot of situations where a user wants to transform this data before using it. Currently the solution is to either :

write a custom connector that transforms the data the way we want but that means writing a lot of code without reuse, packaging and managing custom connectors and so on..
write a Function to transform the data after it was written to a topic by a Source or before it is read from a topic by a Sink. This is not very efficient as we have to use an intermediate topic, which means additional storage, IO, and latency.

Considering all this, it would be handy to be able to apply a Function on-the-fly to a connector without going through an intermediary topic.

Goal

This PIP defines the changes needed to be able to apply a preprocessing Function on-the-fly to a Sink. The preprocessing function can be a built-in function, a package function, or loaded through an http URL or a file path. Sources, Sinks and Functions are based on the same runtime process that:

reads from a Source. For Sinks and Functions this Source is a PulsarSource consuming from a Pulsar topic
applies a Function. For Sources and Sinks, this Function is IdentityFunction which returns the data it gets without modification.
writes to a Sink. For Sources and Functions, this Sink is a PulsarSink writing to a Pulsar topic.

This PIP reuses this and allows configuring a Function different from IdentityFunction to Sinks. Only Functions returning a Record will be authorized to ensure that the Function sets the Schema explicitly.

Out of the scope of this PIP, for future work:

Applying a post-processing Function to a Source
Loading the Function jar through the Sink CLI

API Changes

Admin CLI

The following options will be added to the pulsar-admin sinks CLI create, update and localrun:

preprocess-function: the preprocess function applied before the Sink. Starts by builtin:// for built-in functions, function:// for package function, http:// or file://
preprocess-function-classname: the preprocess function class name (optional if the function is a NAR)
preprocess-function-config: the configuration of the preprocess function in the same format as the user-config parameter of the functions create CLI command.

The corresponding fields will be added to SinkConfig:

    private String preprocessFunction;
    private String preprocessFunctionClassName;
    private String preprocessFunctionConfig;

Function definition

The field extraFunctionPackageLocation to the protobuf structure FunctionMetaData will be added. This field will be filled with the location of the extra function to apply when registering a sink and used in the Runtime to load the function code.

message FunctionMetaData {
    ...
    PackageLocationMetaData extraFunctionPackageLocation = 7;
}

Runtime

The parameters extraFunctionFile and originalExtraFunctionFileName will be added to RuntimeFactory::createContainer

   Runtime createContainer(
            InstanceConfig instanceConfig, String codeFile, String originalCodeFileName,
            String extraFunctionFile, String originalExtraFunctionFileName,
            Long expectedHealthCheckInterval) throws Exception;

Instance function cache

A field extraFunctionId to InstanceConfig that will hold the UUID cache key of the extra function will be added.

public class InstanceConfig {
    private int instanceId;
    private String functionId;
    private String extraFunctionId;

JavaInstanceStarter

The following parameters will be added to JavaInstanceStarter:

--extra_function_jar: the path to the extra function jar
--extra_function_id: the extra function UUID cache key

These parameters are then used by the ThreadRuntime to load the function from the FunctionCacheManager or create it there if needed.

Download the extra function

The statefulset spawned in KubernetesRuntime needs to be able to download the extra functions code via API. An extra-function query param will be added to the download function HTTP endpoint

   @Path("/{tenant}/{namespace}/{functionName}/download")
    public StreamingOutput downloadFunction(
            @ApiParam(value = "The tenant of functions")
            final @PathParam("tenant") String tenant,
            @ApiParam(value = "The namespace of functions")
            final @PathParam("namespace") String namespace,
            @ApiParam(value = "The name of functions")
            final @PathParam("functionName") String functionName) {
            final @PathParam("functionName") String functionName,
            @ApiParam(value = "Whether to download the extra-function")
            final @QueryParam("extra-function") boolean extraFunction) {

If extraFunction is true then the extra function will be returned instead of the sink.

The Java admin SDK will have the following methods added:

   /**
     * Download Function Code.
     *
     * @param destinationFile
     *           file where data should be downloaded to
     * @param tenant
     *            Tenant name
     * @param namespace
     *            Namespace name
     * @param function
     *            Function name
     * @param extraFunction
     *            Whether to download the extra-function (for sources and sinks)
     * @throws PulsarAdminException
     */
    void downloadFunction(String destinationFile, String tenant, String namespace, String function,
                          boolean extraFunction) throws PulsarAdminException;

    /**
     * Download Function Code asynchronously.
     *
     * @param destinationFile
     *           file where data should be downloaded to
     * @param tenant
     *            Tenant name
     * @param namespace
     *            Namespace name
     * @param function
     *            Function name
     * @param extraFunction
     *            Whether to download the extra-function (for sources and sinks)
     */
    CompletableFuture<Void> downloadFunctionAsync(
            String destinationFile, String tenant, String namespace, String function, boolean extraFunction);

The parameter --extra-function will be added to the admin CLI command functions download

Implementation

Pulsar-admin

Add the admin CLI options when creating/updating/localrunning the sink (see API changes)

Pulsar broker

On the broker API, in registerSink/updateSink, if a preprocessing function is present in the Sink config, we:
- validate the function
  - get the function classloader (from builtin or download a package file)
  - load the function
  - inspect the function types and set the first arg as Sink type. Also verify that the second arg is of type Record.
- use the function classloader instead of the sink classloader to verify if custom schemas, serdes, crypto key readers can be loaded and are conform.
- get the function package location and fill the protobuf extraFunctionPackageLocation field with it. A name for this preprocessing function is generated from the sink name so it can be referenced when stored in BookKeeper or in package management. The name of the preprocessing function is {sink name}__sink-function.
- set the functionDetails with the preprocessing function config (function class name and function userConfig)
The --extra-function query parameter is added to the functions download CLI command, admin SDK and HTTP API (see API changes).

Function worker

When the InstanceConfig is created, an UUID is set to the extraFunctionId field. This field will serve as a cache key for the extra function (see API changes).
When the FunctionActioner starts the function, if extraFunctionPackageLocation is present, the same is done for the extra function as what is done for the connector:
- if the runtime is not externally managed, the extra function code is downloaded from the extraFunctionPackageLocation and the Runtime is created with the extra package file path and original name (see API changes to RuntimeFactory::createContainer)
- if the runtime is externally managed, the Runtime is created with the extraFunctionPackageLocation and original name.
Depending on the configured runtime, if there’s an extra function file:
- For the ThreadRuntime, the extra function classloader is obtained with the instance extraFunctionId cache key, then this classloader is passed to the JavaInstanceRunnable. The JavaInstanceRunnable then switches between the connector classloader and the extra function classloader accordingly..
- For the ProcessRuntime, the path to the extra function jar is added to the --extra_function_jar parameter in the JavaInstanceStarter command. The JavaInstanceStarter then uses it when creating its ThreadRuntime.
- For the KubernetesRuntime, a command is added in the statefulset exec command to download the extra function using the –extra-function flag of the functions download command. And the path to this downloaded jar is added to the --extra_function_jar parameter of the JavaInstanceStarter command.

LocalRunner

If sinkConfig has a preprocessFunction, the LocalRunner will use the same methods as in the broker to get the function file and functionDetails and use them when spawning the Runtime.

Reject Alternatives

N/A