diff --git a/src/main/java/jp/azisaba/lgw/kdstatus/api/IDatabase.java b/src/main/java/jp/azisaba/lgw/kdstatus/api/IDatabase.java
new file mode 100644
index 0000000..4810234
--- /dev/null
+++ b/src/main/java/jp/azisaba/lgw/kdstatus/api/IDatabase.java
@@ -0,0 +1,86 @@
+package jp.azisaba.lgw.kdstatus.api;
+
+import jp.azisaba.lgw.kdstatus.sql.KDUserData;
+import jp.azisaba.lgw.kdstatus.sql.KillRankingData;
+import jp.azisaba.lgw.kdstatus.utils.TimeUnit;
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
+
+import java.util.List;
+import java.util.UUID;
+
+/**
+ * The interface of database.
+ */
+public interface IDatabase {
+    /**
+     * Init database connection
+     *
+     * @return is succeeded
+     */
+    boolean connect();
+
+    /**
+     * Close database connection.
+     *
+     * @return is succeeded
+     */
+    boolean close();
+
+    /**
+     * Is alive database connection.
+     *
+     * @return is succeeded
+     */
+    boolean isAlive();
+
+    /**
+     * Is database have userdata which contains this uuid.
+     *
+     * @param uuid uuid of player
+     * @return If it has, returns true. Else returns false.
+     */
+    boolean exist(UUID uuid);
+
+    /**
+     * Set the new userdata.
+     *
+     * @param uuid uuid of player
+     * @param userData the new userdata.
+     * @return is succeeded
+     */
+    boolean set(UUID uuid, @NotNull KDUserData userData);
+
+    /**
+     * Get the specific userdata.
+     *
+     * @param uuid uuid of player
+     * @return returns userdata. If failed, returns null.
+     */
+    @Nullable
+    KDUserData getUserData(UUID uuid);
+
+    /**
+     * @param uuid uuid of player
+     * @param unit ranking timeunit
+     * @return ranking order. If failed, returns -1.
+     */
+    int getRank(UUID uuid, @NotNull TimeUnit unit);
+
+    /**
+     * @param unit ranking timeunit
+     * @param maxCount maximum list size
+     * @return List of RankingData. If failed, returns empty list.
+     */
+    List<KillRankingData> getTopKillRankingData(TimeUnit unit, int maxCount);
+
+    // === wrapper methods ===
+
+    /**
+     * reconnect to database
+     */
+    default boolean reconnect() {
+        if(!close()) return false;
+        return connect();
+    }
+}