[GitHub] [beam] ben-manes commented on a change in pull request #16130: [BEAM-13015] Start integrating a process wide cache.

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ben-manes commented on a change in pull request #16130:
URL: https://github.com/apache/beam/pull/16130#discussion_r765410980



##########
File path: sdks/java/harness/src/main/java/org/apache/beam/fn/harness/Caches.java
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@@ -0,0 +1,259 @@
+/*
+ * 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.beam.fn.harness;
+
+import java.util.Arrays;
+import java.util.Objects;
+import java.util.concurrent.TimeUnit;
+import java.util.function.Function;
+import org.apache.beam.sdk.options.PipelineOptions;
+import org.apache.beam.sdk.options.SdkHarnessOptions;
+import org.apache.beam.vendor.guava.v26_0_jre.com.google.common.annotations.VisibleForTesting;
+import org.apache.beam.vendor.guava.v26_0_jre.com.google.common.base.Preconditions;
+import org.apache.beam.vendor.guava.v26_0_jre.com.google.common.collect.Iterables;
+import org.apache.beam.vendor.guava.v26_0_jre.com.google.common.collect.Sets;
+import org.apache.beam.vendor.guava.v26_0_jre.com.google.common.util.concurrent.MoreExecutors;
+import org.cache2k.Cache2kBuilder;
+import org.cache2k.operation.Weigher;
+import org.github.jamm.MemoryMeter;
+
+/** Utility methods used to instantiate and operate over cache instances. */
+@SuppressWarnings("nullness")
+public final class Caches {
+
+  /** A cache that never stores any values. */
+  public static <K, V> Cache<K, V> noop() {
+    // We specifically use cache2k since it allows for recursive computeIfAbsent calls
+    // preventing deadlock from occurring when a loading function mutates the underlying cache

Review comment:
       fwiw, Caffeine handles this through `AsyncCache` where you do not need a separate thread to execute on. See FAQ's [example](https://github.com/ben-manes/caffeine/wiki/Faq#recursive-computations), which is similar to what Apache Solr does in their usage. It is a simple workaround for these rare cases and makes it more explicit about what is happening, as often recursive computes are surprising and can be error prone. I am not sure if one can expect per-key linearizability with recursive writes, so the plus/minus of this behavior depends on the use-case.
   
   One very minor topic to be aware of is that the `Cache#computeIfAbsent` used here does **not** follow the specification for Map's. In addition, until recently the Map's implementation was broken and caused the cache to be in an invalid state. See this [bug report](https://github.com/cache2k/cache2k/issues/174) where I found these issues for the simple single threaded cases. Since these types of mistakes are sadly common and only ironed by usage, you might want to spend a little extra time validating the behavior and avoiding regressions. Caffeine has had its share of bugs and it takes usage to wrangle them out, so early adopters should be a little more proactive to help the library authors catch their oversights.




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