feat: Add the file-based override source

lib/sdk/server/src/main/java/com/launchdarkly/sdk/server/integrations/DataSystemBuilder.java

@@ -134,6 +134,7 @@ public DataSystemBuilder persistentStore(ComponentConfigurer<DataStore> persiste
    *
    * @param overrideSource the override source configuration, or null for none
    * @return a reference to the builder
+   * @see FileOverrides
    * @since 7.18.0
    */
   public DataSystemBuilder overrides(ComponentConfigurer<OverrideSource> overrideSource) {

lib/sdk/server/src/main/java/com/launchdarkly/sdk/server/integrations/FileOverrideSourceBuilder.java

@@ -0,0 +1,147 @@
+package com.launchdarkly.sdk.server.integrations;
+
+import com.launchdarkly.logging.LDLogger;
+import com.launchdarkly.sdk.server.integrations.FileOverrides.ChangeDetection;
+import com.launchdarkly.sdk.server.subsystems.ClientContext;
+import com.launchdarkly.sdk.server.subsystems.ComponentConfigurer;
+import com.launchdarkly.sdk.server.subsystems.OverrideSource;
+
+import java.nio.file.InvalidPathException;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+import java.time.Duration;
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * Configures the file-based override source. Obtain an instance with {@link FileOverrides#source()},
+ * call {@link #filePaths(String...)} or {@link #filePaths(Path...)} to specify the files, adjust
+ * any other options, and pass the builder to
+ * {@link DataSystemBuilder#overrides(ComponentConfigurer)}.
+ * <p>
+ * For more details, see {@link FileOverrides}.
+ * <p>
+ * Flag overrides are currently experimental and subject to change.
+ *
+ * @since 7.18.0
+ */
+public final class FileOverrideSourceBuilder implements ComponentConfigurer<OverrideSource> {
+  private final List<Path> filePaths = new ArrayList<>();
+  private FileData.DuplicateKeysHandling duplicateKeysHandling = FileData.DuplicateKeysHandling.FAIL;
+  private ChangeDetection changeDetection = ChangeDetection.POLLING;
+  private Duration pollInterval = FileOverrides.DEFAULT_POLL_INTERVAL;
+
+  FileOverrideSourceBuilder() {}
+
+  /**
+   * Adds any number of files to load overrides from, specifying each file path as a string. The
+   * order is significant: it determines which file wins under the duplicate keys handling when the
+   * same key appears in more than one file.
+   * <p>
+   * Files are parsed as JSON if their first non-whitespace character is '{'. Otherwise, they are
+   * parsed as YAML.
+   *
+   * @param filePaths path(s) to the file(s); may be absolute or relative to the current working directory
+   * @return the same builder
+   * @throws InvalidPathException if one of the parameters is not a valid file path
+   */
+  public FileOverrideSourceBuilder filePaths(String... filePaths) throws InvalidPathException {
+    for (String p : filePaths) {
+      this.filePaths.add(Paths.get(p));
+    }
+    return this;
+  }
+
+  /**
+   * Adds any number of files to load overrides from, specifying each file path as a Path. The
+   * order is significant: it determines which file wins under the duplicate keys handling when the
+   * same key appears in more than one file.
+   *
+   * @param filePaths path(s) to the file(s); may be absolute or relative to the current working directory
+   * @return the same builder
+   */
+  public FileOverrideSourceBuilder filePaths(Path... filePaths) {
+    for (Path p : filePaths) {
+      this.filePaths.add(p);
+    }
+    return this;
+  }
+
+  /**
+   * Specifies how to handle the same key appearing in more than one file.
+   * <p>
+   * With {@link FileData.DuplicateKeysHandling#FAIL}, the default, a reload fails if the same flag or
+   * segment key appears in more than one file, and the previously loaded overrides stay in effect.
+   * With {@link FileData.DuplicateKeysHandling#IGNORE}, the entry from the first configured file that
+   * defines the key is kept and the others are discarded. A null value selects the default.
+   *
+   * @param duplicateKeysHandling specifies how to handle duplicate keys
+   * @return the same builder
+   */
+  public FileOverrideSourceBuilder duplicateKeysHandling(FileData.DuplicateKeysHandling duplicateKeysHandling) {
+    this.duplicateKeysHandling = duplicateKeysHandling == null
+        ? FileData.DuplicateKeysHandling.FAIL : duplicateKeysHandling;
+    return this;
+  }
+
+  /**
+   * Selects how the source detects file changes. The default is {@link ChangeDetection#POLLING}.
+   * The two modes are alternatives, so setting one replaces the other.
+   *
+   * @param changeDetection the change detection mode
+   * @return the same builder
+   */
+  public FileOverrideSourceBuilder changeDetection(ChangeDetection changeDetection) {
+    this.changeDetection = changeDetection;
+    return this;
+  }
+
+  /**
+   * Sets the interval between examinations of the files in {@link ChangeDetection#POLLING} mode.
+   * {@link ChangeDetection#WATCHING} mode ignores it. The default is
+   * {@link FileOverrides#DEFAULT_POLL_INTERVAL}. An interval below
+   * {@link FileOverrides#MINIMUM_POLL_INTERVAL} is raised to the minimum.
+   *
+   * @param pollInterval the polling interval
+   * @return the same builder
+   */
+  public FileOverrideSourceBuilder pollInterval(Duration pollInterval) {
+    this.pollInterval = pollInterval;
+    return this;
+  }
+
+  /**
+   * Called internally by the SDK to create the override source.
+   *
+   * @param context the client context
+   * @return the override source
+   * @throws IllegalArgumentException if no file paths were specified, or the change detection mode
+   *   or the poll interval is null
+   */
+  @Override
+  public OverrideSource build(ClientContext context) {
+    if (filePaths.isEmpty()) {
+      throw new IllegalArgumentException("no file paths were specified for the file-based override source");
+    }
+    if (changeDetection == null) {
+      throw new IllegalArgumentException("a change detection mode is required for the file-based override source");
+    }
+    if (pollInterval == null) {
+      throw new IllegalArgumentException("a poll interval is required for the file-based override source");
+    }
+    List<Path> absolutePaths = new ArrayList<>(filePaths.size());
+    for (Path p : filePaths) {
+      absolutePaths.add(p.toAbsolutePath().normalize());
+    }
+
+    LDLogger logger = context.getBaseLogger().subLogger("FileOverrideSource");
+
+    Duration effectivePollInterval = pollInterval;
+    if (changeDetection == ChangeDetection.POLLING && pollInterval.compareTo(FileOverrides.MINIMUM_POLL_INTERVAL) < 0) {
+      logger.warn("Poll interval {} is below the minimum; using {}", pollInterval, FileOverrides.MINIMUM_POLL_INTERVAL);
+      effectivePollInterval = FileOverrides.MINIMUM_POLL_INTERVAL;
+    }
+
+    return new FileOverrideSourceImpl(absolutePaths, duplicateKeysHandling, changeDetection, effectivePollInterval, logger);
+  }
+}

lib/sdk/server/src/main/java/com/launchdarkly/sdk/server/integrations/FileOverrideSourceImpl.java

@@ -0,0 +1,169 @@
+package com.launchdarkly.sdk.server.integrations;
+
+import com.launchdarkly.logging.LDLogger;
+import com.launchdarkly.sdk.server.integrations.FileDataSourceParsing.FileDataException;
+import com.launchdarkly.sdk.server.integrations.FileOverrides.ChangeDetection;
+import com.launchdarkly.sdk.server.integrations.OverrideFileLoader.FileSummary;
+import com.launchdarkly.sdk.server.integrations.OverrideFileLoader.LoadResult;
+import com.launchdarkly.sdk.server.subsystems.OverrideSink;
+import com.launchdarkly.sdk.server.subsystems.OverrideSource;
+
+import java.io.IOException;
+import java.nio.file.Path;
+import java.time.Duration;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.concurrent.atomic.AtomicBoolean;
+
+/**
+ * The file-based override source. It reads one or more files in the file data source document
+ * format, merges them in the configured order, and supplies the result to the override sink as
+ * one snapshot. It reloads when the files change, by polling or by watching.
+ * <p>
+ * A configured file that does not exist contributes no overrides. A file that exists but cannot
+ * be read or parsed fails that reload: the sink is not called, so the last good overrides stay in
+ * effect, and the reload is retried.
+ */
+final class FileOverrideSourceImpl implements OverrideSource {
+  private final List<Path> paths;
+  private final ChangeDetection changeDetection;
+  private final Duration pollInterval;
+  private final LDLogger logger;
+  private final OverrideFileLoader loader;
+
+  private volatile FileDataReloader reloader;
+  private volatile FileDataPoller poller;
+  private volatile FileDataWatcher watcher;
+  private final AtomicBoolean closed = new AtomicBoolean(false);
+
+  FileOverrideSourceImpl(
+      List<Path> paths,
+      FileData.DuplicateKeysHandling duplicateKeysHandling,
+      ChangeDetection changeDetection,
+      Duration pollInterval,
+      LDLogger logger
+  ) {
+    this.paths = new ArrayList<>(paths);
+    this.changeDetection = changeDetection;
+    this.pollInterval = pollInterval;
+    this.logger = logger;
+    this.loader = new OverrideFileLoader(paths, duplicateKeysHandling);
+  }
+
+  /**
+   * Performs the initial load synchronously, so overrides present in the files are in effect when
+   * the client constructor returns, then starts change detection. A file that does not exist yet
+   * contributes no overrides. A file that cannot be read or parsed is not fatal: the client runs
+   * with the last good overrides, the failure is logged, and the retry, plus the change signal,
+   * recovers once the file is readable.
+   */
+  @Override
+  public void start(OverrideSink sink) {
+    reloader = new FileDataReloader(
+        loader::load,
+        new FileDataReloader.Handler() {
+          @Override
+          public void apply(LoadResult result) {
+            sink.setOverrides(result.getData());
+            logOverridesInEffect(result);
+          }
+
+          @Override
+          public void onError(FileDataException e) {
+            // The reloader has logged the failure. The last good overrides stay in effect.
+          }
+        },
+        logger,
+        FileDataReloader.DEFAULT_DEBOUNCE_DELAY,
+        FileDataReloader.DEFAULT_RETRY_DELAY,
+        true
+    );
+    reloader.reloadNow();
+
+    if (closed.get()) {
+      return;
+    }
+    switch (changeDetection) {
+    case WATCHING:
+      try {
+        FileDataWatcher w = FileDataWatcher.create(paths, logger);
+        watcher = w;
+        w.start(reloader::trigger);
+      } catch (IOException e) {
+        // COVERAGE: constructing a watcher only fails under unusual OS conditions
+        logger.error("Unable to watch override files: {}", e.toString());
+      }
+      break;
+    case POLLING:
+    default:
+      poller = new FileDataPoller(paths, pollInterval, reloader::trigger);
+      break;
+    }
+  }
+
+  /**
+   * Returns the interval between file examinations in polling mode. Visible for tests.
+   *
+   * @return the effective poll interval
+   */
+  Duration getPollInterval() {
+    return pollInterval;
+  }
+
+  // Reports the overrides now in effect and the file each came from. The reloader applies a
+  // snapshot only when the content changed, so this logs each change once.
+  private void logOverridesInEffect(LoadResult result) {
+    List<String> details = new ArrayList<>(result.getFiles().size());
+    for (FileSummary file : result.getFiles()) {
+      if (!file.isPresent()) {
+        details.add(file.getPath() + ": absent");
+      } else if (file.getFlags() == 0 && file.getSegments() == 0) {
+        details.add(file.getPath() + ": no entries");
+      } else {
+        details.add(file.getPath() + ": " + countsText(file.getFlags(), file.getSegments()));
+      }
+    }
+    String fileDetails = String.join("; ", details);
+    if (result.getFlagCount() == 0 && result.getSegmentCount() == 0) {
+      logger.info("Flag overrides: none in effect ({})", fileDetails);
+      return;
+    }
+    logger.info("Flag overrides in effect: {} ({})", countsText(result.getFlagCount(), result.getSegmentCount()),
+        fileDetails);
+  }
+
+  // Formats flag and segment counts, for example "2 flags, 1 segment".
+  static String countsText(int flags, int segments) {
+    List<String> parts = new ArrayList<>(2);
+    if (flags > 0) {
+      parts.add(pluralize(flags, "flag"));
+    }
+    if (segments > 0) {
+      parts.add(pluralize(segments, "segment"));
+    }
+    return String.join(", ", parts);
+  }
+
+  private static String pluralize(int count, String noun) {
+    return count == 1 ? "1 " + noun : count + " " + noun + "s";
+  }
+
+  @Override
+  public void close() {
+    if (closed.getAndSet(true)) {
+      return;
+    }
+    FileDataWatcher w = watcher;
+    if (w != null) {
+      w.close();
+    }
+    FileDataPoller p = poller;
+    if (p != null) {
+      p.close();
+    }
+    FileDataReloader r = reloader;
+    if (r != null) {
+      r.close();
+    }
+  }
+}