diff --git a/helfi_api_base.services.yml b/helfi_api_base.services.yml
index 7afcd4c5..38fa3459 100644
--- a/helfi_api_base.services.yml
+++ b/helfi_api_base.services.yml
@@ -73,10 +73,15 @@ services:
     tags:
       - { name: event_subscriber }
 
+  helfi_api_base.logger_filesystem:
+    class: Symfony\Component\Filesystem\Filesystem
+    public: false
+
   helfi_api_base.logger_json:
     class: Drupal\helfi_api_base\Logger\JsonLog
     arguments:
       - '@logger.log_message_parser'
+      - '@helfi_api_base.logger_filesystem'
       - '%helfi_api_base.json_logger_path%'
       - '%helfi_api_base.logger_enabled%'
     tags:
diff --git a/src/Logger/JsonLog.php b/src/Logger/JsonLog.php
index 23992776..667d67df 100644
--- a/src/Logger/JsonLog.php
+++ b/src/Logger/JsonLog.php
@@ -9,6 +9,7 @@
 use Drupal\Core\Logger\RfcLogLevel;
 use Drupal\Core\StringTranslation\TranslatableMarkup;
 use Psr\Log\LoggerInterface;
+use Symfony\Component\Filesystem\Filesystem;
 
 /**
  * This class allows logging to stdout and stderr.
@@ -22,28 +23,27 @@ final class JsonLog implements LoggerInterface {
    *
    * @param \Drupal\Core\Logger\LogMessageParserInterface $parser
    *   The parser to use when extracting message variables.
+   * @param \Symfony\Component\Filesystem\Filesystem $filesystem
+   *   The file system.
    * @param string $stream
    *   The output path.
    * @param bool $loggerEnabled
    *   Whether logger is enabled or not.
    */
   public function __construct(
-    private LogMessageParserInterface $parser,
-    private string $stream,
-    private bool $loggerEnabled = TRUE
+    private readonly LogMessageParserInterface $parser,
+    private readonly Filesystem $filesystem,
+    private readonly string $stream,
+    private readonly bool $loggerEnabled = TRUE
   ) {
   }
 
   /**
-   * Outputs the given entry.
-   *
-   * @param array $entry
-   *   The log entry.
+   * Outputs the messages.
    */
-  private function output(array $entry) : void {
-    $stream = fopen($this->stream, 'a');
-    fwrite($stream, json_encode(['message' => $entry]) . PHP_EOL);
-    fclose($stream);
+  private function output(array $message) : void {
+    $this->filesystem
+      ->appendToFile($this->stream, json_encode(['message' => $message]) . PHP_EOL);
   }
 
   /**