diff --git a/application/src/main/java/org/thingsboard/server/install/ThingsboardInstallService.java b/application/src/main/java/org/thingsboard/server/install/ThingsboardInstallService.java
index 661eab6aac..b3e9c26034 100644
--- a/application/src/main/java/org/thingsboard/server/install/ThingsboardInstallService.java
+++ b/application/src/main/java/org/thingsboard/server/install/ThingsboardInstallService.java
@@ -100,19 +100,24 @@ public class ThingsboardInstallService {
                     log.info("Migrating ThingsBoard latest timeseries data from cassandra to SQL database ...");
                     latestMigrateService.migrate();
                 } else {
+                    // TODO DON'T FORGET to update SUPPORTED_VERSIONS_FROM in DefaultDatabaseSchemaSettingsService
                     databaseSchemaVersionService.validateSchemaSettings();
-                    //TODO DON'T FORGET to update SUPPORTED_VERSIONS_FROM in DatabaseSchemaVersionService,
-                    // this list should include last version and can include previous versions without upgrade
                     String fromVersion = databaseSchemaVersionService.getDbSchemaVersion();
                     String toVersion = databaseSchemaVersionService.getPackageSchemaVersion();
                     log.info("Upgrading ThingsBoard from version {} to {} ...", fromVersion, toVersion);
                     cacheCleanupService.clearCache();
-                    entityDatabaseSchemaService.createDatabaseSchema(false);
+                    // Apply the schema_update.sql script. The script may include DDL statements to change structure
+                    // of *existing* tables and DML statements to manipulate the DB records.
                     databaseEntitiesUpgradeService.upgradeDatabase();
-                    dataUpdateService.updateData(); //TODO: update data should be cleaned after each release
-
+                    // All new tables that do not have any data will be automatically created here.
+                    entityDatabaseSchemaService.createDatabaseSchema(false);
+                    // Runs upgrade scripts that are not possible in plain SQL.
+                    // TODO: clear after each release
+                    dataUpdateService.updateData();
+                    // Re-create all views, functions.
                     entityDatabaseSchemaService.createOrUpdateViewsAndFunctions();
                     entityDatabaseSchemaService.createOrUpdateDeviceInfoView(persistToTelemetry);
+                    // Creates missing indexes.
                     entityDatabaseSchemaService.createDatabaseIndexes();
                     log.info("Updating system data...");
                     dataUpdateService.upgradeRuleNodes();
diff --git a/application/src/main/java/org/thingsboard/server/service/install/DatabaseSchemaSettingsService.java b/application/src/main/java/org/thingsboard/server/service/install/DatabaseSchemaSettingsService.java
index 49548a077d..5b037ae659 100644
--- a/application/src/main/java/org/thingsboard/server/service/install/DatabaseSchemaSettingsService.java
+++ b/application/src/main/java/org/thingsboard/server/service/install/DatabaseSchemaSettingsService.java
@@ -16,6 +16,7 @@
 package org.thingsboard.server.service.install;
 
 public interface DatabaseSchemaSettingsService {
+
     void validateSchemaSettings();
 
     void createSchemaSettings();
@@ -25,4 +26,5 @@ public interface DatabaseSchemaSettingsService {
     String getPackageSchemaVersion();
 
     String getDbSchemaVersion();
+
 }
diff --git a/application/src/main/java/org/thingsboard/server/service/install/DefaultDatabaseSchemaSettingsService.java b/application/src/main/java/org/thingsboard/server/service/install/DefaultDatabaseSchemaSettingsService.java
index e3cc1d9356..82a96eacdd 100644
--- a/application/src/main/java/org/thingsboard/server/service/install/DefaultDatabaseSchemaSettingsService.java
+++ b/application/src/main/java/org/thingsboard/server/service/install/DefaultDatabaseSchemaSettingsService.java
@@ -32,6 +32,8 @@ import java.util.List;
 public class DefaultDatabaseSchemaSettingsService implements DatabaseSchemaSettingsService {
 
     private static final String CURRENT_PRODUCT = "CE";
+    // This list should include all versions which are compatible for the upgrade.
+    // The compatibility cycle usually breaks when we have some scripts written in Java that may not work after new release.
     private static final List<String> SUPPORTED_VERSIONS_FOR_UPGRADE = List.of("3.8.0", "3.8.1");
 
     private final BuildProperties buildProperties;
diff --git a/application/src/main/java/org/thingsboard/server/service/install/SqlDatabaseUpgradeService.java b/application/src/main/java/org/thingsboard/server/service/install/SqlDatabaseUpgradeService.java
index 5780057340..5f20ddc98a 100644
--- a/application/src/main/java/org/thingsboard/server/service/install/SqlDatabaseUpgradeService.java
+++ b/application/src/main/java/org/thingsboard/server/service/install/SqlDatabaseUpgradeService.java
@@ -52,7 +52,7 @@ public class SqlDatabaseUpgradeService implements DatabaseEntitiesUpgradeService
 
     @Override
     public void upgradeDatabase() {
-        log.info("Updating schema ...");
+        log.info("Updating schema...");
         loadSql(getSchemaUpdateFile("basic"));
         log.info("Schema updated.");
     }
diff --git a/application/src/main/java/org/thingsboard/server/service/subscription/TbSubscription.java b/application/src/main/java/org/thingsboard/server/service/subscription/TbSubscription.java
index 95f5e932d2..01228eef18 100644
--- a/application/src/main/java/org/thingsboard/server/service/subscription/TbSubscription.java
+++ b/application/src/main/java/org/thingsboard/server/service/subscription/TbSubscription.java
@@ -15,9 +15,7 @@
  */
 package org.thingsboard.server.service.subscription;
 
-import lombok.AllArgsConstructor;
 import lombok.Data;
-import lombok.RequiredArgsConstructor;
 import org.thingsboard.server.common.data.id.EntityId;
 import org.thingsboard.server.common.data.id.TenantId;
 
@@ -25,7 +23,6 @@ import java.util.Objects;
 import java.util.function.BiConsumer;
 
 @Data
-@RequiredArgsConstructor
 public abstract class TbSubscription<T> {
 
     /** Cache the hash code */