feat: flagd file polling for offline mode (#614)

Signed-off-by: Kavindu Dodanduwa <kavindudodanduwa@gmail.com>

providers/flagd/src/main/java/dev/openfeature/contrib/providers/flagd/resolver/process/storage/connector/file/FileConnector.java

@@ -9,36 +9,73 @@
 import java.io.IOException;
 import java.nio.charset.StandardCharsets;
 import java.nio.file.Files;
+import java.nio.file.Path;
 import java.nio.file.Paths;
 import java.util.concurrent.BlockingQueue;
 import java.util.concurrent.LinkedBlockingQueue;
 
 /**
- * File connector reads flag configurations and expose the context through {@code Connector} contract.
+ * File connector reads flag configurations from a given file, polls for changes and expose the content through
+ * {@code Connector} contract.
  * The implementation is kept minimal and suites testing, local development needs.
  */
 @SuppressFBWarnings(value = {"EI_EXPOSE_REP", "PATH_TRAVERSAL_IN"},
         justification = "File connector read feature flag from a file source.")
 @Slf4j
 public class FileConnector implements Connector {
 
+    private static final int POLL_INTERVAL_MS = 5000;
+
     private final String flagSourcePath;
     private final BlockingQueue<StreamPayload> queue = new LinkedBlockingQueue<>(1);
+    private boolean shutdown = false;
 
     public FileConnector(final String flagSourcePath) {
         this.flagSourcePath = flagSourcePath;
     }
 
     /**
-     * Initialize file connector. Reads content of the provided source file and offer it through queue.
+     * Initialize file connector. Reads file content, poll for changes and offer content through the queue.
      */
     public void init() throws IOException {
-        final String flagData = new String(Files.readAllBytes(Paths.get(flagSourcePath)), StandardCharsets.UTF_8);
+        Thread watcherT = new Thread(() -> {
+            try {
+                final Path filePath = Paths.get(flagSourcePath);
+
+                // initial read
+                String flagData = new String(Files.readAllBytes(filePath), StandardCharsets.UTF_8);
+                if (!queue.offer(new StreamPayload(StreamPayloadType.DATA, flagData))) {
+                    log.warn("Unable to offer file content to queue: queue is full");
+                }
+
+                long lastTS = Files.getLastModifiedTime(filePath).toMillis();
+
+                // start polling for changes
+                while (!shutdown) {
+                    long currentTS = Files.getLastModifiedTime(filePath).toMillis();
+
+                    if (currentTS > lastTS) {
+                        lastTS = currentTS;
+                        flagData = new String(Files.readAllBytes(filePath), StandardCharsets.UTF_8);
+                        if (!queue.offer(new StreamPayload(StreamPayloadType.DATA, flagData))) {
+                            log.warn("Unable to offer file content to queue: queue is full");
+                        }
+                    }
+
+                    Thread.sleep(POLL_INTERVAL_MS);
+                }
 
-        if (!queue.offer(new StreamPayload(StreamPayloadType.DATA, flagData))) {
-            throw new RuntimeException("Unable to write to queue. Queue is full.");
-        }
+                log.info("Shutting down file connector.");
+            } catch (Throwable t) {
+                log.error("Error from file connector. File connector will exit", t);
+                if (!queue.offer(new StreamPayload(StreamPayloadType.ERROR, t.toString()))) {
+                    log.warn("Unable to offer file content to queue: queue is full");
+                }
+            }
+        });
 
+        watcherT.setDaemon(true);
+        watcherT.start();
         log.info(String.format("Using feature flag configurations from file %s", flagSourcePath));
     }
 
@@ -50,9 +87,9 @@ public BlockingQueue<StreamPayload> getStream() {
     }
 
     /**
-     * NO-OP shutdown.
+     * Shutdown file connector.
      */
     public void shutdown() throws InterruptedException {
-        // NO-OP nothing to do here
+        shutdown = true;
     }
 }

providers/flagd/src/test/java/dev/openfeature/contrib/providers/flagd/resolver/process/TestUtils.java

@@ -13,6 +13,7 @@ public class TestUtils {
     public static final String VALID_LONG = "flagConfigurations/valid-long.json";
     public static final String INVALID_FLAG = "flagConfigurations/invalid-flag.json";
     public static final String INVALID_CFG = "flagConfigurations/invalid-configuration.json";
+    public static final String UPDATABLE_FILE = "flagConfigurations/updatableFlags.json";
 
 
     public static String getFlagsFromResource(final String file) throws IOException {

providers/flagd/src/test/java/dev/openfeature/contrib/providers/flagd/resolver/process/storage/connector/file/FileConnectorTest.java

@@ -2,17 +2,22 @@
 
 import dev.openfeature.contrib.providers.flagd.resolver.process.storage.connector.StreamPayload;
 import dev.openfeature.contrib.providers.flagd.resolver.process.storage.connector.StreamPayloadType;
+import org.junit.jupiter.api.Disabled;
 import org.junit.jupiter.api.Test;
 
 import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+import java.nio.file.StandardOpenOption;
 import java.time.Duration;
 import java.util.concurrent.BlockingQueue;
 
+import static dev.openfeature.contrib.providers.flagd.resolver.process.TestUtils.UPDATABLE_FILE;
 import static dev.openfeature.contrib.providers.flagd.resolver.process.TestUtils.VALID_LONG;
 import static dev.openfeature.contrib.providers.flagd.resolver.process.TestUtils.getResourcePath;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertNotNull;
-import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.junit.jupiter.api.Assertions.assertTimeoutPreemptively;
 
 class FileConnectorTest {
@@ -39,12 +44,61 @@ void readAndExposeFeatureFlagsFromSource() throws IOException {
     }
 
     @Test
-    void throwsErrorIfInvalidFile(){
+    void emitErrorStateForInvalidPath() throws IOException {
         // given
         final FileConnector connector = new FileConnector("INVALID_PATH");
 
+        // when
+        connector.init();
+
+        // then
+        final BlockingQueue<StreamPayload> stream = connector.getStream();
+
+        // Must emit an error within considerable time
+        final StreamPayload[] payload = new StreamPayload[1];
+        assertTimeoutPreemptively(Duration.ofMillis(200), () -> {
+            payload[0] = stream.take();
+        });
+
+        assertNotNull(payload[0].getData());
+        assertEquals(StreamPayloadType.ERROR, payload[0].getType());
+    }
+
+    @Test
+    @Disabled("Disabled as unstable on GH Action. Useful for functionality validation")
+    void watchForFileUpdatesAndEmitThem() throws IOException {
+        final String initial = "{\"flags\":{\"myBoolFlag\":{\"state\":\"ENABLED\",\"variants\":{\"on\":true,\"off\":false},\"defaultVariant\":\"on\"}}}";
+        final String updatedFlags = "{\"flags\":{\"myBoolFlag\":{\"state\":\"ENABLED\",\"variants\":{\"on\":true,\"off\":false},\"defaultVariant\":\"off\"}}}";
+
+        // given
+        final Path updPath = Paths.get(getResourcePath(UPDATABLE_FILE));
+        Files.write(updPath, initial.getBytes(), StandardOpenOption.WRITE, StandardOpenOption.TRUNCATE_EXISTING);
+
+        final FileConnector connector = new FileConnector(updPath.toString());
+
+        // when
+        connector.init();
+
         // then
-        assertThrows(IOException.class, connector::init);
+        final BlockingQueue<StreamPayload> stream = connector.getStream();
+        final StreamPayload[] payload = new StreamPayload[1];
+
+        // first validate the initial payload
+        assertTimeoutPreemptively(Duration.ofMillis(200), () -> {
+            payload[0] = stream.take();
+        });
+
+        assertEquals(initial, payload[0].getData());
+
+        // then update the flags
+        Files.write(updPath, updatedFlags.getBytes(), StandardOpenOption.WRITE, StandardOpenOption.TRUNCATE_EXISTING);
+
+        // finally wait for updated payload
+        assertTimeoutPreemptively(Duration.ofSeconds(10), () -> {
+            payload[0] = stream.take();
+        });
+
+        assertEquals(updatedFlags, payload[0].getData());
     }
 
 }

providers/flagd/src/test/resources/flagConfigurations/updatableFlags.json

@@ -0,0 +1,12 @@
+{
+  "flags": {
+    "myBoolFlag": {
+      "state": "ENABLED",
+      "variants": {
+        "on": true,
+        "off": false
+      },
+      "defaultVariant": "on"
+    }
+  }
+}