diff --git a/helfi_api_base.services.yml b/helfi_api_base.services.yml
index 5afde275..b367294f 100644
--- a/helfi_api_base.services.yml
+++ b/helfi_api_base.services.yml
@@ -150,6 +150,7 @@ services:
       - '@datetime.time'
       - '@helfi_api_base.pubsub_settings'
 
+  Drupal\helfi_api_base\Cache\CacheTagInvalidatorInterface: '@helfi_api_base.cache_tag_invalidator'
   Drupal\helfi_api_base\Cache\CacheTagInvalidator: '@helfi_api_base.cache_tag_invalidator'
   helfi_api_base.cache_tag_invalidator:
     class: Drupal\helfi_api_base\Cache\CacheTagInvalidator
diff --git a/src/Cache/CacheTagInvalidator.php b/src/Cache/CacheTagInvalidator.php
index c6987386..ad96c433 100644
--- a/src/Cache/CacheTagInvalidator.php
+++ b/src/Cache/CacheTagInvalidator.php
@@ -10,7 +10,7 @@
 /**
  * A service to invalidate cache tags on all instances.
  */
-final class CacheTagInvalidator {
+final class CacheTagInvalidator implements CacheTagInvalidatorInterface {
 
   /**
    * Constructs a new instance.
@@ -24,17 +24,9 @@ public function __construct(
   }
 
   /**
-   * Invalidates given cache tags.
-   *
-   * @param array $tags
-   *   An array of cache tags.
-   * @param array $instances
-   *   The instances to flush caches from.
-   *
-   * @return $this
-   *   The self.
+   * {@inheritdoc}
    */
-  public function invalidateTags(array $tags, array $instances = []) : self {
+  public function invalidateTags(array $tags, array $instances = []) : void {
     try {
       $this->pubSubManager->sendMessage([
         'tags' => $tags,
@@ -43,7 +35,6 @@ public function invalidateTags(array $tags, array $instances = []) : self {
     }
     catch (ConnectionException) {
     }
-    return $this;
   }
 
 }
diff --git a/src/Cache/CacheTagInvalidatorInterface.php b/src/Cache/CacheTagInvalidatorInterface.php
new file mode 100644
index 00000000..840db4a7
--- /dev/null
+++ b/src/Cache/CacheTagInvalidatorInterface.php
@@ -0,0 +1,25 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\helfi_api_base\Cache;
+
+/**
+ * A service to invalidate cache tags on all instances.
+ */
+interface CacheTagInvalidatorInterface {
+
+  /**
+   * Invalidates given cache tags.
+   *
+   * @param array $tags
+   *   An array of cache tags.
+   * @param array $instances
+   *   The instances to flush caches from.
+   */
+  public function invalidateTags(
+    array $tags,
+    array $instances = []
+  ): void;
+
+}