diff --git a/scripts/util/mainapp.js b/scripts/util/mainapp.js
index 8112af2..0d141f9 100644
--- a/scripts/util/mainapp.js
+++ b/scripts/util/mainapp.js
@@ -24,7 +24,7 @@ function startMainApp(env) {
   return new Promise((resolve, reject) => {
     const task = spawn('node', [initPath], {
       stdio: ['inherit', 'pipe', 'inherit'],
-      env: { ...process.env, PORT: 0 /* random port */ }
+      env: { ...process.env, PORT: 0 /* random port */, ...env }
     })
 
     task.on('exit', () => reject(new Error('task terminated too early')))
diff --git a/src/database/config.ts b/src/database/config.ts
index c4580ec..1563102 100644
--- a/src/database/config.ts
+++ b/src/database/config.ts
@@ -47,5 +47,6 @@ export const createConfigModel = (sequelize: Sequelize.Sequelize): ConfigModelSt
 
 export const configItemIds = {
   statusMessage: 'status_message',
-  selfTestData: 'self_test_data'
+  selfTestData: 'self_test_data',
+  secondSelfTestData: 'self_test_data_two'
 }
diff --git a/src/database/index.ts b/src/database/index.ts
index 945c412..d1e5813 100644
--- a/src/database/index.ts
+++ b/src/database/index.ts
@@ -1,6 +1,6 @@
 /*
  * server component for the TimeLimit App
- * Copyright (C) 2019 - 2020 Jonas Lochmann
+ * Copyright (C) 2019 - 2021 Jonas Lochmann
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU Affero General Public License as
@@ -15,130 +15,6 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
-import * as Sequelize from 'sequelize'
-import { generateIdWithinFamily } from '../util/token'
-import { AddDeviceTokenModelStatic, createAddDeviceTokenModel } from './adddevicetoken'
-import { AppModelStatic, createAppModel } from './app'
-import { AppActivityModelStatic, createAppActivityModel } from './appactivity'
-import { AuthTokenModelStatic, createAuthtokenModel } from './authtoken'
-import { CategoryModelStatic, createCategoryModel } from './category'
-import { CategoryAppModelStatic, createCategoryAppModel } from './categoryapp'
-import { CategoryNetworkIdModelStatic, createCategoryNetworkIdModel } from './categorynetworkid'
-import { ChildTaskModelStatic, createChildTaskModel } from './childtask'
-import { configItemIds, ConfigModelStatic, createConfigModel } from './config'
-import { createDeviceModel, DeviceModelStatic } from './device'
-import { createFamilyModel, FamilyModelStatic } from './family'
-import { createMailLoginTokenModel, MailLoginTokenModelStatic } from './maillogintoken'
-import { createUmzug } from './migration/umzug'
-import { createOldDeviceModel, OldDeviceModelStatic } from './olddevice'
-import { createPurchaseModel, PurchaseModelStatic } from './purchase'
-import { createSessionDurationModel, SessionDurationModelStatic } from './sessionduration'
-import { createTimelimitRuleModel, TimelimitRuleModelStatic } from './timelimitrule'
-import { createUsedTimeModel, UsedTimeModelStatic } from './usedtime'
-import { createUserModel, UserModelStatic } from './user'
-import { createUserLimitLoginCategoryModel, UserLimitLoginCategoryModelStatic } from './userlimitlogincategory'
-
-export type Transaction = Sequelize.Transaction
-
-export interface Database {
-  addDeviceToken: AddDeviceTokenModelStatic
-  authtoken: AuthTokenModelStatic
-  app: AppModelStatic
-  appActivity: AppActivityModelStatic
-  category: CategoryModelStatic
-  categoryApp: CategoryAppModelStatic
-  categoryNetworkId: CategoryNetworkIdModelStatic
-  childTask: ChildTaskModelStatic
-  config: ConfigModelStatic
-  device: DeviceModelStatic
-  family: FamilyModelStatic
-  mailLoginToken: MailLoginTokenModelStatic
-  oldDevice: OldDeviceModelStatic
-  purchase: PurchaseModelStatic
-  sessionDuration: SessionDurationModelStatic
-  timelimitRule: TimelimitRuleModelStatic
-  usedTime: UsedTimeModelStatic
-  user: UserModelStatic
-  userLimitLoginCategory: UserLimitLoginCategoryModelStatic
-  transaction: <T> (autoCallback: (t: Transaction) => Promise<T>, options?: { transaction: Transaction }) => Promise<T>
-  dialect: string
-}
-
-const createDatabase = (sequelize: Sequelize.Sequelize): Database => ({
-  addDeviceToken: createAddDeviceTokenModel(sequelize),
-  authtoken: createAuthtokenModel(sequelize),
-  app: createAppModel(sequelize),
-  appActivity: createAppActivityModel(sequelize),
-  category: createCategoryModel(sequelize),
-  categoryApp: createCategoryAppModel(sequelize),
-  childTask: createChildTaskModel(sequelize),
-  categoryNetworkId: createCategoryNetworkIdModel(sequelize),
-  config: createConfigModel(sequelize),
-  device: createDeviceModel(sequelize),
-  family: createFamilyModel(sequelize),
-  mailLoginToken: createMailLoginTokenModel(sequelize),
-  oldDevice: createOldDeviceModel(sequelize),
-  purchase: createPurchaseModel(sequelize),
-  sessionDuration: createSessionDurationModel(sequelize),
-  timelimitRule: createTimelimitRuleModel(sequelize),
-  usedTime: createUsedTimeModel(sequelize),
-  user: createUserModel(sequelize),
-  userLimitLoginCategory: createUserLimitLoginCategoryModel(sequelize),
-  transaction: <T> (autoCallback: (transaction: Transaction) => Promise<T>, options?: { transaction: Transaction }) => (sequelize.transaction({
-    isolationLevel: Sequelize.Transaction.ISOLATION_LEVELS.READ_COMMITTED,
-    transaction: options?.transaction
-  }, autoCallback) as any) as Promise<T>,
-  dialect: sequelize.getDialect()
-})
-
-export const sequelize = new Sequelize.Sequelize(process.env.DATABASE_URL || 'sqlite://test.db', {
-  define: {
-    timestamps: false
-  },
-  logging: false
-})
-
-export const defaultDatabase = createDatabase(sequelize)
-export const defaultUmzug = createUmzug(sequelize)
-
-class NestedTransactionTestException extends Error {}
-class TestRollbackException extends NestedTransactionTestException {}
-class NestedTransactionsNotWorkingException extends NestedTransactionTestException { constructor () { super('NestedTransactionsNotWorkingException') } }
-class IllegalStateException extends NestedTransactionTestException {}
-
-export async function assertNestedTransactionsAreWorking (database: Database) {
-  const testValue = generateIdWithinFamily()
-
-  // clean up just for the case
-  await database.config.destroy({ where: { id: configItemIds.selfTestData } })
-
-  await database.transaction(async (transaction) => {
-    const readOne = await database.config.findOne({ where: { id: configItemIds.selfTestData }, transaction })
-
-    if (readOne) throw new IllegalStateException()
-
-    await database.transaction(async (transaction) => {
-      await database.config.create({ id: configItemIds.selfTestData, value: testValue }, { transaction })
-
-      const readTwo = await database.config.findOne({ where: { id: configItemIds.selfTestData }, transaction })
-
-      if (readTwo?.value !== testValue) throw new IllegalStateException()
-
-      try {
-        await database.transaction(async (transaction) => {
-          await database.config.destroy({ where: { id: configItemIds.selfTestData }, transaction })
-
-          throw new TestRollbackException()
-        }, { transaction })
-      } catch (ex) {
-        if (!(ex instanceof TestRollbackException)) throw ex
-      }
-
-      const readThree = await database.config.findOne({ where: { id: configItemIds.selfTestData }, transaction })
-
-      if (readThree?.value !== testValue) throw new NestedTransactionsNotWorkingException()
-
-      await database.config.destroy({ where: { id: configItemIds.selfTestData }, transaction })
-    }, { transaction })
-  })
-}
+export { Transaction, Database, defaultDatabase, defaultUmzug } from './main'
+export { assertNestedTransactionsAreWorking } from './utils/nested-transactions'
+export { assertSerializeableTransactionsAreWorking, shouldRetryWithException } from './utils/serialized'
diff --git a/src/database/main.ts b/src/database/main.ts
new file mode 100644
index 0000000..7dd6f93
--- /dev/null
+++ b/src/database/main.ts
@@ -0,0 +1,101 @@
+/*
+ * server component for the TimeLimit App
+ * Copyright (C) 2019 - 2021 Jonas Lochmann
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, version 3 of the License.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+import * as Sequelize from 'sequelize'
+import { AddDeviceTokenModelStatic, createAddDeviceTokenModel } from './adddevicetoken'
+import { AppModelStatic, createAppModel } from './app'
+import { AppActivityModelStatic, createAppActivityModel } from './appactivity'
+import { AuthTokenModelStatic, createAuthtokenModel } from './authtoken'
+import { CategoryModelStatic, createCategoryModel } from './category'
+import { CategoryAppModelStatic, createCategoryAppModel } from './categoryapp'
+import { CategoryNetworkIdModelStatic, createCategoryNetworkIdModel } from './categorynetworkid'
+import { ChildTaskModelStatic, createChildTaskModel } from './childtask'
+import { ConfigModelStatic, createConfigModel } from './config'
+import { createDeviceModel, DeviceModelStatic } from './device'
+import { createFamilyModel, FamilyModelStatic } from './family'
+import { createMailLoginTokenModel, MailLoginTokenModelStatic } from './maillogintoken'
+import { createUmzug } from './migration/umzug'
+import { createOldDeviceModel, OldDeviceModelStatic } from './olddevice'
+import { createPurchaseModel, PurchaseModelStatic } from './purchase'
+import { createSessionDurationModel, SessionDurationModelStatic } from './sessionduration'
+import { createTimelimitRuleModel, TimelimitRuleModelStatic } from './timelimitrule'
+import { createUsedTimeModel, UsedTimeModelStatic } from './usedtime'
+import { createUserModel, UserModelStatic } from './user'
+import { createUserLimitLoginCategoryModel, UserLimitLoginCategoryModelStatic } from './userlimitlogincategory'
+
+export type Transaction = Sequelize.Transaction
+
+export interface Database {
+  addDeviceToken: AddDeviceTokenModelStatic
+  authtoken: AuthTokenModelStatic
+  app: AppModelStatic
+  appActivity: AppActivityModelStatic
+  category: CategoryModelStatic
+  categoryApp: CategoryAppModelStatic
+  categoryNetworkId: CategoryNetworkIdModelStatic
+  childTask: ChildTaskModelStatic
+  config: ConfigModelStatic
+  device: DeviceModelStatic
+  family: FamilyModelStatic
+  mailLoginToken: MailLoginTokenModelStatic
+  oldDevice: OldDeviceModelStatic
+  purchase: PurchaseModelStatic
+  sessionDuration: SessionDurationModelStatic
+  timelimitRule: TimelimitRuleModelStatic
+  usedTime: UsedTimeModelStatic
+  user: UserModelStatic
+  userLimitLoginCategory: UserLimitLoginCategoryModelStatic
+  transaction: <T> (autoCallback: (t: Transaction) => Promise<T>, options?: { transaction: Transaction }) => Promise<T>
+  dialect: string
+}
+
+const createDatabase = (sequelize: Sequelize.Sequelize): Database => ({
+  addDeviceToken: createAddDeviceTokenModel(sequelize),
+  authtoken: createAuthtokenModel(sequelize),
+  app: createAppModel(sequelize),
+  appActivity: createAppActivityModel(sequelize),
+  category: createCategoryModel(sequelize),
+  categoryApp: createCategoryAppModel(sequelize),
+  childTask: createChildTaskModel(sequelize),
+  categoryNetworkId: createCategoryNetworkIdModel(sequelize),
+  config: createConfigModel(sequelize),
+  device: createDeviceModel(sequelize),
+  family: createFamilyModel(sequelize),
+  mailLoginToken: createMailLoginTokenModel(sequelize),
+  oldDevice: createOldDeviceModel(sequelize),
+  purchase: createPurchaseModel(sequelize),
+  sessionDuration: createSessionDurationModel(sequelize),
+  timelimitRule: createTimelimitRuleModel(sequelize),
+  usedTime: createUsedTimeModel(sequelize),
+  user: createUserModel(sequelize),
+  userLimitLoginCategory: createUserLimitLoginCategoryModel(sequelize),
+  transaction: <T> (autoCallback: (transaction: Transaction) => Promise<T>, options?: { transaction: Transaction }) => (sequelize.transaction({
+    isolationLevel: Sequelize.Transaction.ISOLATION_LEVELS.SERIALIZABLE,
+    transaction: options?.transaction
+  }, autoCallback) as any) as Promise<T>,
+  dialect: sequelize.getDialect()
+})
+
+export const sequelize = new Sequelize.Sequelize(process.env.DATABASE_URL || 'sqlite://test.db', {
+  define: {
+    timestamps: false
+  },
+  logging: false
+})
+
+export const defaultDatabase = createDatabase(sequelize)
+export const defaultUmzug = createUmzug(sequelize)
diff --git a/src/database/utils/nested-transactions.ts b/src/database/utils/nested-transactions.ts
new file mode 100644
index 0000000..a6b5239
--- /dev/null
+++ b/src/database/utils/nested-transactions.ts
@@ -0,0 +1,62 @@
+/*
+ * server component for the TimeLimit App
+ * Copyright (C) 2019 - 2021 Jonas Lochmann
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, version 3 of the License.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+import { generateIdWithinFamily } from '../../util/token'
+import { configItemIds } from '../config'
+import { Database } from '../main'
+
+class NestedTransactionTestException extends Error {}
+class TestRollbackException extends NestedTransactionTestException {}
+class NestedTransactionsNotWorkingException extends NestedTransactionTestException { constructor () { super('NestedTransactionsNotWorkingException') } }
+class IllegalStateException extends NestedTransactionTestException {}
+
+export async function assertNestedTransactionsAreWorking (database: Database) {
+  const testValue = generateIdWithinFamily()
+
+  // clean up just for the case
+  await database.config.destroy({ where: { id: configItemIds.selfTestData } })
+
+  await database.transaction(async (transaction) => {
+    const readOne = await database.config.findOne({ where: { id: configItemIds.selfTestData }, transaction })
+
+    if (readOne) throw new IllegalStateException()
+
+    await database.transaction(async (transaction) => {
+      await database.config.create({ id: configItemIds.selfTestData, value: testValue }, { transaction })
+
+      const readTwo = await database.config.findOne({ where: { id: configItemIds.selfTestData }, transaction })
+
+      if (readTwo?.value !== testValue) throw new IllegalStateException()
+
+      try {
+        await database.transaction(async (transaction) => {
+          await database.config.destroy({ where: { id: configItemIds.selfTestData }, transaction })
+
+          throw new TestRollbackException()
+        }, { transaction })
+      } catch (ex) {
+        if (!(ex instanceof TestRollbackException)) throw ex
+      }
+
+      const readThree = await database.config.findOne({ where: { id: configItemIds.selfTestData }, transaction })
+
+      if (readThree?.value !== testValue) throw new NestedTransactionsNotWorkingException()
+
+      await database.config.destroy({ where: { id: configItemIds.selfTestData }, transaction })
+    }, { transaction })
+  })
+}
diff --git a/src/database/utils/serialized.ts b/src/database/utils/serialized.ts
new file mode 100644
index 0000000..7afa283
--- /dev/null
+++ b/src/database/utils/serialized.ts
@@ -0,0 +1,106 @@
+/*
+ * server component for the TimeLimit App
+ * Copyright (C) 2019 - 2021 Jonas Lochmann
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, version 3 of the License.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+import * as Sequelize from 'sequelize'
+import { configItemIds } from '../config'
+import { Database } from '../main'
+
+export class SerializationFeatureCheckException extends Error {}
+
+export function shouldRetryWithException(database: Database, e: any): boolean {
+  if (e instanceof Sequelize.TimeoutError) return true
+
+  if (!(e instanceof Sequelize.DatabaseError)) return false
+
+  const parent = e.parent
+
+  if (typeof parent !== 'object') return false
+
+  if (database.dialect === 'sqlite') {
+    if (parent.message.startsWith('SQLITE_BUSY:')) return true
+  } else if (database.dialect === 'postgres') {
+    // 40001 = serialization_failure
+    if ((parent as any).code === '40001') return true
+    // 40P01 = deadlock detected
+    if ((parent as any).code === '40P01') return true
+  } else if (database.dialect === 'mariadb') {
+    const errno = (parent as any).errno
+
+    // ER_LOCK_DEADLOCK
+    // Deadlock found when trying to get lock; try restarting transaction
+    if (errno === 1213) return true
+  }
+
+  return false
+}
+
+export async function assertSerializeableTransactionsAreWorking(database: Database) {
+  // clean up just for the case
+  await database.config.destroy({
+    where: {
+      id: {
+        [Sequelize.Op.in]: [ configItemIds.selfTestData, configItemIds.secondSelfTestData ]
+      }
+    }
+  })
+
+  // insert specific data
+  await database.config.bulkCreate([
+    {
+      id: configItemIds.selfTestData,
+      value: '123'
+    },
+    {
+      id: configItemIds.secondSelfTestData,
+      value: '456'
+    }
+  ])
+
+  try {
+    // use two parallel transactions
+    await database.transaction(async (transactionOne) => {
+      await database.transaction(async (transactionTwo) => {
+        await database.config.findAll({ transaction: transactionOne })
+        await database.config.findAll({ transaction: transactionTwo })
+
+        await Promise.all([
+          (async () => {
+            await database.config.update({ value: 'c' }, { where: { id: configItemIds.selfTestData }, transaction: transactionOne })
+          })(),
+          (async () => {
+            await database.config.update({ value: 'd' }, { where: { id: configItemIds.secondSelfTestData }, transaction: transactionTwo })
+          })(),
+        ])
+      })
+    })
+
+    throw new SerializationFeatureCheckException()
+  } catch (ex) {
+    if (!shouldRetryWithException(database, ex)) {
+      throw new SerializationFeatureCheckException()
+    }
+  }
+
+  // finish clean up
+  await database.config.destroy({
+    where: {
+      id: {
+        [Sequelize.Op.in]: [ configItemIds.selfTestData, configItemIds.secondSelfTestData ]
+      }
+    }
+  })
+}
diff --git a/src/function/sync/apply-actions/index.ts b/src/function/sync/apply-actions/index.ts
index c83c902..e9b5ff8 100644
--- a/src/function/sync/apply-actions/index.ts
+++ b/src/function/sync/apply-actions/index.ts
@@ -18,7 +18,7 @@
 import { BadRequest } from 'http-errors'
 import { ClientPushChangesRequest } from '../../../api/schema'
 import { VisibleConnectedDevicesManager } from '../../../connected-devices'
-import { Database } from '../../../database'
+import { Database, shouldRetryWithException } from '../../../database'
 import { EventHandler } from '../../../monitoring/eventhandler'
 import { WebsocketApi } from '../../../websocket'
 import { notifyClientsAboutChangesDelayed } from '../../websocket'
@@ -104,7 +104,11 @@ export const applyActionsFromDevice = async ({ database, request, websocket, con
           }
         })
       } catch (ex) {
-        if (ex instanceof ApplyActionException) {
+        if (shouldRetryWithException(ex)) {
+          eventHandler.countEvent('applyActionsFromDevice got exception which should cause retry')
+
+          throw ex
+        } else if (ex instanceof ApplyActionException) {
           eventHandler.countEvent('applyActionsFromDevice errorDispatchingAction:' + ex.staticMessage)
         } else {
           const stack = ex instanceof Error && ex.stack ? ex.stack.substring(0, 4096) : 'no stack'
diff --git a/src/index.ts b/src/index.ts
index 260df4d..5d8530c 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -1,6 +1,6 @@
 /*
  * server component for the TimeLimit App
- * Copyright (C) 2019 - 2020 Jonas Lochmann
+ * Copyright (C) 2019 - 2021 Jonas Lochmann
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU Affero General Public License as
@@ -19,7 +19,7 @@ import { Server } from 'http'
 import { createApi } from './api'
 import { config } from './config'
 import { VisibleConnectedDevicesManager } from './connected-devices'
-import { assertNestedTransactionsAreWorking, defaultDatabase, defaultUmzug } from './database'
+import { assertNestedTransactionsAreWorking, assertSerializeableTransactionsAreWorking, defaultDatabase, defaultUmzug } from './database'
 import { EventHandler } from './monitoring/eventhandler'
 import { InMemoryEventHandler } from './monitoring/inmemoryeventhandler'
 import { createWebsocketHandler } from './websocket'
@@ -31,6 +31,7 @@ async function main () {
   const eventHandler: EventHandler = new InMemoryEventHandler()
 
   await assertNestedTransactionsAreWorking(database)
+  await assertSerializeableTransactionsAreWorking(database)
 
   const connectedDevicesManager = new VisibleConnectedDevicesManager({
     database