Add filter_list processor to filter list elements

data-prepper-plugins/mutate-event-processors/README.md

@@ -808,6 +808,153 @@ After `add_entries`:
 
 ___
 
+## FilterListProcessor
+A processor that filters elements within an array field by evaluating a condition against each element, keeping only those where the condition is true. It supports arrays of objects and arrays of primitives (strings, numbers, booleans).
+
+### Basic Usage
+To get started, create the following `pipeline.yaml`.
+```yaml
+pipeline:
+  source:
+    file:
+      path: "/full/path/to/logs_json.log"
+      record_type: "event"
+      format: "json"
+  processor:
+    - filter_list:
+        iterate_on: "items"
+        keep_element_when: '/status == "active"'
+  sink:
+    - stdout:
+```
+
+Create the following file named `logs_json.log` and replace the `path` in the file source of your `pipeline.yaml` with the path of this file.
+
+```json
+{"items": [{"name": "item1", "status": "active"}, {"name": "item2", "status": "inactive"}, {"name": "item3", "status": "active"}]}
+```
+
+When run, the processor will filter the array in-place and produce the following output:
+
+```json
+{"items": [{"name": "item1", "status": "active"}, {"name": "item3", "status": "active"}]}
+```
+
+### Filtering to a different target
+
+You can write the filtered result to a different key, leaving the original array unchanged:
+
+```yaml
+  processor:
+    - filter_list:
+        iterate_on: "items"
+        target: "active_items"
+        keep_element_when: '/status == "active"'
+```
+
+With the same input, the output will be:
+
+```json
+{
+  "items": [{"name": "item1", "status": "active"}, {"name": "item2", "status": "inactive"}, {"name": "item3", "status": "active"}],
+  "active_items": [{"name": "item1", "status": "active"}, {"name": "item3", "status": "active"}]
+}
+```
+
+### Filtering primitive arrays
+
+For arrays of primitives (strings, numbers, booleans), each element is accessible via the `/value` key in the expression:
+
+```yaml
+  processor:
+    - filter_list:
+        iterate_on: "tags"
+        keep_element_when: '/value != ""'
+```
+
+With the following input:
+
+```json
+{"tags": ["important", "", "urgent", ""]}
+```
+
+The output will be:
+
+```json
+{"tags": ["important", "urgent"]}
+```
+
+Another example filtering numbers:
+
+```yaml
+  processor:
+    - filter_list:
+        iterate_on: "scores"
+        keep_element_when: '/value > 50'
+```
+
+With the following input:
+
+```json
+{"scores": [90, 30, 75, 10]}
+```
+
+The output will be:
+
+```json
+{"scores": [90, 75]}
+```
+
+### Using both conditions
+
+The `filter_list_when` condition controls whether the processor runs at all (evaluated against the root event), while `keep_element_when` controls which elements are kept (evaluated per element):
+
+```yaml
+  processor:
+    - filter_list:
+        iterate_on: "items"
+        keep_element_when: '/status == "active"'
+        filter_list_when: '/env == "production"'
+```
+
+With the following input:
+
+```json
+{"env": "production", "items": [{"name": "item1", "status": "active"}, {"name": "item2", "status": "inactive"}]}
+```
+
+Since `env` is `"production"`, the processor runs and filters by `status`, producing:
+
+```json
+{"env": "production", "items": [{"name": "item1", "status": "active"}]}
+```
+
+With a different event where `filter_list_when` evaluates to `false`:
+
+```json
+{"env": "staging", "items": [{"name": "item1", "status": "active"}, {"name": "item2", "status": "inactive"}]}
+```
+
+The processor is skipped entirely and the event passes through unchanged:
+
+```json
+{"env": "staging", "items": [{"name": "item1", "status": "active"}, {"name": "item2", "status": "inactive"}]}
+```
+
+### Configuration
+* `iterate_on` - (required) - The key of the array field to filter. Supports nested paths (e.g. `outer_key/inner_list`).
+* `target` - (optional) - The key to write the filtered array to. Defaults to the `iterate_on` key (in-place filtering). Supports nested paths.
+* `keep_element_when` - (required) - A [Data Prepper expression](https://opensearch.org/docs/latest/data-prepper/pipelines/expression-syntax/) evaluated per element. Elements where this expression evaluates to `true` are kept. For object elements, the expression is evaluated against the object's fields directly (e.g. `/status == "active"`). For primitive elements, the value is accessible via `/value` (e.g. `/value > 50`). When no elements match, the result is an empty list `[]`.
+* `filter_list_when` - (optional) - A [Data Prepper expression](https://opensearch.org/docs/latest/data-prepper/pipelines/expression-syntax/) evaluated against the root event. When provided, the processor only runs if this condition is `true`. By default, all events are processed.
+* `tags_on_failure` - (optional) - A list of tags to add to the event metadata when the processor fails to process the event.
+
+**Edge case behavior:**
+- If the `iterate_on` key does not exist or its value is `null`, the processor is a no-op and the event passes through unchanged.
+- If the `iterate_on` value is not a list (e.g. a string or number), the processor logs a warning and adds `tags_on_failure` if configured.
+- `null` elements within the list are evaluated normally. For example, with `keep_element_when: '/value != null'`, null elements are filtered out while non-null elements are kept.
+
+___
+
 ## Developer Guide
 This plugin is compatible with Java 11 and 17. Refer to the following developer guides for plugin development:
 - [Developer Guide](https://github.com/opensearch-project/data-prepper/blob/main/docs/developer_guide.md)

data-prepper-plugins/mutate-event-processors/src/main/java/org/opensearch/dataprepper/plugins/processor/mutateevent/FilterListProcessor.java

@@ -0,0 +1,159 @@
+/*
+ * Copyright OpenSearch Contributors
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * The OpenSearch Contributors require contributions made to
+ * this file be licensed under the Apache-2.0 license or a
+ * compatible open source license.
+ */
+
+package org.opensearch.dataprepper.plugins.processor.mutateevent;
+
+import org.opensearch.dataprepper.expression.ExpressionEvaluator;
+import org.opensearch.dataprepper.metrics.PluginMetrics;
+import org.opensearch.dataprepper.model.annotations.DataPrepperPlugin;
+import org.opensearch.dataprepper.model.annotations.DataPrepperPluginConstructor;
+import org.opensearch.dataprepper.model.event.Event;
+import org.opensearch.dataprepper.model.event.JacksonEvent;
+import org.opensearch.dataprepper.model.processor.AbstractProcessor;
+import org.opensearch.dataprepper.model.processor.Processor;
+import org.opensearch.dataprepper.model.record.Record;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.List;
+import java.util.Map;
+import java.util.Objects;
+
+import static org.opensearch.dataprepper.logging.DataPrepperMarkers.EVENT;
+import static org.opensearch.dataprepper.logging.DataPrepperMarkers.NOISY;
+import static org.opensearch.dataprepper.logging.DataPrepperMarkers.SENSITIVE;
+
+@DataPrepperPlugin(name = "filter_list", pluginType = Processor.class, pluginConfigurationType = FilterListProcessorConfig.class)
+public class FilterListProcessor extends AbstractProcessor<Record<Event>, Record<Event>> {
+
+    private static final Logger LOG = LoggerFactory.getLogger(FilterListProcessor.class);
+    private static final String FAILED_ELEMENTS_METADATA_KEY = "filter_list_processor_failed_elements";
+    private static final String FAILED_ELEMENTS_COUNT_METADATA_KEY = "filter_list_processor_failed_elements_count";
+    private final FilterListProcessorConfig config;
+    private final ExpressionEvaluator expressionEvaluator;
+
+    @DataPrepperPluginConstructor
+    public FilterListProcessor(final PluginMetrics pluginMetrics, final FilterListProcessorConfig config, final ExpressionEvaluator expressionEvaluator) {
+        super(pluginMetrics);
+        this.config = config;
+        this.expressionEvaluator = expressionEvaluator;
+
+        config.validateExpressions(expressionEvaluator);
+    }
+
+    @Override
+    public Collection<Record<Event>> doExecute(final Collection<Record<Event>> records) {
+        for (final Record<Event> record : records) {
+            final Event recordEvent = record.getData();
+
+            try {
+                if (Objects.nonNull(config.getFilterListWhen()) && !expressionEvaluator.evaluateConditional(config.getFilterListWhen(), recordEvent)) {
+                    continue;
+                }
+
+                final List<Object> sourceList;
+                try {
+                    sourceList = recordEvent.get(config.getIterateOn(), List.class);
+                } catch (final Exception e) {
+                    LOG.atWarn()
+                            .addMarker(EVENT)
+                            .addMarker(SENSITIVE)
+                            .setMessage("Given source path [{}] is not valid on record [{}]")
+                            .addArgument(config.getIterateOn())
+                            .addArgument(recordEvent)
+                            .setCause(e)
+                            .log();
+                    addTagsOnFailure(recordEvent);
+                    continue;
+                }
+
+                if (sourceList == null) {
+                    LOG.debug("Source list at path [{}] is null, skipping event", config.getIterateOn());
+                    continue;
+                }
+
+                final List<Object> filteredList = new ArrayList<>();
+                final List<Object> failedElements = new ArrayList<>();
+                int failedElementCount = 0;
+
+                for (final Object element : sourceList) {
+                    @SuppressWarnings("unchecked")
+                    final Map<String, Object> contextMap = element instanceof Map
+                            ? (Map<String, Object>) element
+                            : Collections.singletonMap("value", element);
+
+                    try {
+                        // TODO(#6609): Revisit this per-element Event construction when ExpressionEvaluator/JsonPointer
+                        // internals support a lighter evaluation path that avoids full tree conversion.
+                        final Event elementEvent = JacksonEvent.builder()
+                                .withEventType("event")
+                                .withData(contextMap)
+                                .build();
+
+                        if (expressionEvaluator.evaluateConditional(config.getKeepElementWhen(), elementEvent)) {
+                            filteredList.add(element);
+                        }
+                    } catch (final Exception e) {
+                        failedElementCount++;
+                        failedElements.add(element);
+                        LOG.atWarn()
+                                .addMarker(EVENT)
+                                .addMarker(SENSITIVE)
+                                .setMessage("Error evaluating keep_element_when expression [{}] for element in source list at path [{}]")
+                                .addArgument(config.getKeepElementWhen())
+                                .addArgument(config.getIterateOn())
+                                .setCause(e)
+                                .log();
+                    }
+                }
+
+                if (failedElementCount > 0) {
+                    addTagsOnFailure(recordEvent);
+                    recordEvent.getMetadata().setAttribute(FAILED_ELEMENTS_COUNT_METADATA_KEY, failedElementCount);
+                    recordEvent.getMetadata().setAttribute(FAILED_ELEMENTS_METADATA_KEY, failedElements);
+                }
+
+                recordEvent.put(config.getTarget(), filteredList);
+
+            } catch (final Exception e) {
+                LOG.atError()
+                        .addMarker(EVENT)
+                        .addMarker(NOISY)
+                        .setMessage("There was an exception while processing Event [{}]")
+                        .addArgument(recordEvent)
+                        .setCause(e)
+                        .log();
+                addTagsOnFailure(recordEvent);
+            }
+        }
+        return records;
+    }
+
+    private void addTagsOnFailure(final Event event) {
+        if (config.getTagsOnFailure() != null) {
+            event.getMetadata().addTags(config.getTagsOnFailure());
+        }
+    }
+
+    @Override
+    public void prepareForShutdown() {
+    }
+
+    @Override
+    public boolean isReadyForShutdown() {
+        return true;
+    }
+
+    @Override
+    public void shutdown() {
+    }
+}