richardstartin commented on a change in pull request #7820: URL: https://github.com/apache/pinot/pull/7820#discussion_r762942133
########## File path: pinot-segment-spi/src/main/java/org/apache/pinot/segment/spi/evaluator/json/JsonPathEvaluators.java ########## @@ -0,0 +1,150 @@ +/** + * Licensed to the Apache Software Foundation (ASF) under one + * or more contributor license agreements. See the NOTICE file + * distributed with this work for additional information + * regarding copyright ownership. The ASF licenses this file + * to you under the Apache License, Version 2.0 (the + * "License"); you may not use this file except in compliance + * with the License. You may obtain a copy of the License at + * + * http://www.apache.org/licenses/LICENSE-2.0 + * + * Unless required by applicable law or agreed to in writing, + * software distributed under the License is distributed on an + * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY + * KIND, either express or implied. See the License for the + * specific language governing permissions and limitations + * under the License. + */ +package org.apache.pinot.segment.spi.evaluator.json; + +import com.google.common.base.Preconditions; +import java.lang.invoke.MethodHandle; +import java.lang.invoke.MethodHandles; +import java.lang.invoke.MethodType; +import java.util.concurrent.atomic.AtomicReferenceFieldUpdater; +import org.slf4j.Logger; +import org.slf4j.LoggerFactory; + + +/** + * Allows registration of a custom {@see JsonPathEvaluator} which can handle custom storage + * functionality also present in a plugin. A default evaluator which can handle all default + * storage types will be provided to delegate to when standard storage types are encountered. + * + * This is an evolving SPI and subject to change. + */ +public final class JsonPathEvaluators { + + private static final Logger LOGGER = LoggerFactory.getLogger(JsonPathEvaluators.class); + + private static final AtomicReferenceFieldUpdater<JsonPathEvaluators, JsonPathEvaluatorProvider> UPDATER = + AtomicReferenceFieldUpdater.newUpdater(JsonPathEvaluators.class, JsonPathEvaluatorProvider.class, "_provider"); + private static final JsonPathEvaluators INSTANCE = new JsonPathEvaluators(); + private static final DefaultProvider DEFAULT_PROVIDER = new DefaultProvider(); + private volatile JsonPathEvaluatorProvider _provider; + + /** + * Registration point to override how JSON paths are evaluated. This should be used + * when a Pinot plugin has special storage capabilities. For instance, imagine a + * plugin with a raw forward index which stores JSON in a binary format which + * pinot-core is unaware of and cannot evaluate JSON paths against (pinot-core only + * understands true JSON documents). Whenever JSON paths are evaluated against this + * custom storage, different storage access operations may be required, and the provided + * {@see JsonPathEvaluator} can inspect the provided {@see ForwardIndexReader} to + * determine whether it is the custom implementation and evaluate the JSON path against + * the binary JSON managed by the custom reader. If it is not the custom implementation, + * then the evaluation should be delegated to the provided delegate. + * + * This prevents the interface {@see ForwardIndexReader} from needing to be able to model + * any plugin storage format, which creates flexibility for the kinds of data structure + * plugins can employ. + * + * @param provider provides {@see JsonPathEvaluator} + * @return true if registration is successful, false otherwise + */ + public static boolean registerProvider(JsonPathEvaluatorProvider provider) { + Preconditions.checkArgument(provider != null, ""); + if (!UPDATER.compareAndSet(INSTANCE, null, provider)) { + LOGGER.warn("failed to register {} - {} already registered", provider, INSTANCE._provider); + return false; + } + return true; + } + + /** + * pinot-core must construct {@see JsonPathEvaluator} via this method to ensure it uses + * the registered implementation. Using the registered implementation allows pinot-core + * to evaluate JSON paths against data structures it doesn't understand or model. + * @param jsonPath the JSON path + * @param defaultValue the default value + * @return a JSON path evaluator which must understand all possible storage representations of JSON. + */ + public static JsonPathEvaluator create(String jsonPath, Object defaultValue) { + // plugins compose and delegate to the default implementation. + JsonPathEvaluator defaultEvaluator = DEFAULT_PROVIDER.create(jsonPath, defaultValue); + return Holder.PROVIDER.create(defaultEvaluator, jsonPath, defaultValue); + } + + /** + * Storing the registered evaluator in this holder and initialising it during + * the class load gives the best of both worlds: plugins have until the first + * JSON path evaluation to register an evaluator via + * {@see JsonPathEvaluators#registerProvider}, but once this class is loaded, + * the provider is constant and calls may be optimise aggressively by the JVM + * in ways which are impossible with a volatile reference. + */ + private static final class Holder { + static final JsonPathEvaluatorProvider PROVIDER; + + static { + JsonPathEvaluatorProvider provider = JsonPathEvaluators.INSTANCE._provider; + if (provider == null) { + provider = DEFAULT_PROVIDER; + if (!UPDATER.compareAndSet(INSTANCE, null, provider)) { + provider = JsonPathEvaluators.INSTANCE._provider; + } + } + PROVIDER = provider; + } + } + + private static class DefaultProvider implements JsonPathEvaluatorProvider { + + // default implementation uses MethodHandles to avoid pulling lots of implementation details into the SPI layer + + private static final MethodHandle FACTORY; + + static { + String className = "org.apache.pinot.core.common.evaluators.DefaultJsonPathEvaluator"; + MethodHandle factory = null; + try { + Class<?> clazz = Class.forName(className, false, JsonPathEvaluators.class.getClassLoader()); + factory = MethodHandles.publicLookup() + .findStatic(clazz, "create", MethodType.methodType(JsonPathEvaluator.class, String.class, Object.class)); + } catch (Throwable implausible) { + LOGGER.error("could not construct MethodHandle for {}", className, + implausible); + } + FACTORY = factory; + } + + public JsonPathEvaluator create(String jsonPath, Object defaultValue) { + return create(null, jsonPath, defaultValue); + } + + @Override + public JsonPathEvaluator create(JsonPathEvaluator delegate, String jsonPath, Object defaultValue) { Review comment: The idea here is there is a chain of responsibility. There is the standard way of evaluating JSON paths (the result of the refactoring here), but a plugin could be registered which adds special storage as well as an evaluator which can interpret that storage. There could be a mix of segments using the default storage and the plugin storage. On a case by case basis, the registered plugin either handles the encountered storage or delegates if it doesn't know how. I'm not sure what I was thinking when I wrote this particular line though - `DefaultJsonPathEvaluator` is the end of the chain and will never delegate, so its delegate will always be null. -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. 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