diff --git a/Squirrel/Squirrel/DataModels/GuestDataModel.cs b/Squirrel/Squirrel/DataModels/GuestDataModel.cs
new file mode 100644
index 0000000..ffe18a0
--- /dev/null
+++ b/Squirrel/Squirrel/DataModels/GuestDataModel.cs
@@ -0,0 +1,40 @@
+using Squirrel.Exceptions;
+using Squirrel.Extensions;
+using Squirrel.Infrastructure;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Text.RegularExpressions;
+using System.Threading.Tasks;
+
+namespace Squirrel.DataModels;
+
+public class GuestDataModel(string id, string fio, string phoneNumber, double discountSize) : IValidation
+{
+    public string Id { get; private set; } = id;
+
+    public string FIO { get; private set; } = fio;
+
+    public string PhoneNumber { get; private set; } = phoneNumber;
+
+    public double DiscountSize { get; private set; } = discountSize;
+
+    public void Validate()
+    {
+        if (Id.IsEmpty())
+            throw new ValidationException("Field Id is empty");
+
+        if (!Id.IsGuid())
+            throw new ValidationException("The value in the field Id is not a unique identifier");
+
+        if (FIO.IsEmpty())
+            throw new ValidationException("Field FIO is empty");
+
+        if (PhoneNumber.IsEmpty())
+            throw new ValidationException("Field PhoneNumber is empty");
+
+        if (!Regex.IsMatch(PhoneNumber, @"^((8|\+7)[\- ]?)?(\(?\d{3}\)?[\- ]?)?[\d\- ]{7,10}$"))
+            throw new ValidationException("Field PhoneNumber is not a phone number");
+    }
+}
diff --git a/Squirrel/Squirrel/DataModels/PostDataModel.cs b/Squirrel/Squirrel/DataModels/PostDataModel.cs
new file mode 100644
index 0000000..0cfa8bd
--- /dev/null
+++ b/Squirrel/Squirrel/DataModels/PostDataModel.cs
@@ -0,0 +1,52 @@
+using Squirrel.Enums;
+using Squirrel.Exceptions;
+using Squirrel.Extensions;
+using Squirrel.Infrastructure;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Squirrel.DataModels;
+
+public class PostDataModel(string id, string postId, string postName, PostType postType, double salary, bool isActual, DateTime changeDate) : IValidation
+{
+    public string Id { get; private set; } = id;
+
+    public string PostId { get; private set; } = postId;
+
+    public string PostName { get; private set; } = postName;
+
+    public PostType PostType { get; private set; } = postType;
+
+    public double Salary { get; private set; } = salary;
+
+    public bool IsActual { get; private set; } = isActual;
+
+    public DateTime ChangeDate { get; private set; } = changeDate;
+
+    public void Validate()
+    {
+        if (Id.IsEmpty())
+            throw new ValidationException("Field Id is empty");
+
+        if (!Id.IsGuid())
+            throw new ValidationException("The value in the field Id is not a unique identifier");
+
+        if (PostId.IsEmpty())
+            throw new ValidationException("Field PostId is empty");
+
+        if (!PostId.IsGuid())
+            throw new ValidationException("The value in the field PostId is not a unique identifier");
+
+        if (PostName.IsEmpty())
+            throw new ValidationException("Field PostName is empty");
+
+        if (PostType == PostType.None)
+            throw new ValidationException("Field PostType is empty");
+
+        if (Salary <= 0)
+            throw new ValidationException("Field Salary is empty");
+    }
+}
diff --git a/Squirrel/Squirrel/DataModels/ProductDataModel.cs b/Squirrel/Squirrel/DataModels/ProductDataModel.cs
new file mode 100644
index 0000000..c13c0cc
--- /dev/null
+++ b/Squirrel/Squirrel/DataModels/ProductDataModel.cs
@@ -0,0 +1,42 @@
+using Squirrel.Enums;
+using Squirrel.Exceptions;
+using Squirrel.Extensions;
+using Squirrel.Infrastructure;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Squirrel.DataModels;
+
+public class ProductDataModel(string id, string productName, ProductType productType, double price, bool isDeleted) : IValidation
+{
+    public string Id { get; private set; } = id;
+
+    public string ProductName { get; private set; } = productName;
+
+    public ProductType ProductType { get; private set; } = productType;
+
+    public double Price { get; private set; } = price;
+
+    public bool IsDeleted { get; private set; } = isDeleted;
+
+    public void Validate()
+    {
+        if (Id.IsEmpty())
+            throw new ValidationException("Field Id is empty");
+
+        if (!Id.IsGuid())
+            throw new ValidationException("The value in the field Id is not a unique identifier");
+
+        if (ProductName.IsEmpty())
+            throw new ValidationException("Field ProductName is empty");
+
+        if (ProductType == ProductType.None)
+            throw new ValidationException("Field ProductType is empty");
+
+        if (Price <= 0)
+            throw new ValidationException("Field Price is less than or equal to 0");
+    }
+}
diff --git a/Squirrel/Squirrel/DataModels/ProductHistoryDataModel.cs b/Squirrel/Squirrel/DataModels/ProductHistoryDataModel.cs
new file mode 100644
index 0000000..c5bec22
--- /dev/null
+++ b/Squirrel/Squirrel/DataModels/ProductHistoryDataModel.cs
@@ -0,0 +1,31 @@
+using Squirrel.Exceptions;
+using Squirrel.Extensions;
+using Squirrel.Infrastructure;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Squirrel.DataModels;
+
+public class ProductHistoryDataModel(string productId, double oldPrice) : IValidation
+{
+    public string ProductId { get; private set; } = productId;
+
+    public double OldPrice { get; private set; } = oldPrice;
+
+    public DateTime ChangeDate { get; private set; } = DateTime.UtcNow;
+
+    public void Validate()
+    {
+        if (ProductId.IsEmpty())
+            throw new ValidationException("Field ProductId is empty");
+
+        if (!ProductId.IsGuid())
+            throw new ValidationException("The value in the field ProductId is not a unique identifier");
+
+        if (OldPrice <= 0)
+            throw new ValidationException("Field OldPrice is less than or equal to 0");
+    }
+}
diff --git a/Squirrel/Squirrel/DataModels/SalaryDataModel.cs b/Squirrel/Squirrel/DataModels/SalaryDataModel.cs
new file mode 100644
index 0000000..b316ef3
--- /dev/null
+++ b/Squirrel/Squirrel/DataModels/SalaryDataModel.cs
@@ -0,0 +1,31 @@
+using Squirrel.Exceptions;
+using Squirrel.Extensions;
+using Squirrel.Infrastructure;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Squirrel.DataModels;
+
+public class SalaryDataModel(string workerId, DateTime salaryDate, double workerSalary) : IValidation
+{
+    public string WorkerId { get; private set; } = workerId;
+
+    public DateTime SalaryDate { get; private set; } = salaryDate;
+
+    public double Salary { get; private set; } = workerSalary;
+
+    public void Validate()
+    {
+        if (WorkerId.IsEmpty())
+            throw new ValidationException("Field WorkerId is empty");
+
+        if (!WorkerId.IsGuid())
+            throw new ValidationException("The value in the field WorkerId is not a unique identifier");
+
+        if (Salary <= 0)
+            throw new ValidationException("Field Salary is less than or equal to 0");
+    }
+}
diff --git a/Squirrel/Squirrel/DataModels/SaleDataModel.cs b/Squirrel/Squirrel/DataModels/SaleDataModel.cs
new file mode 100644
index 0000000..f0e3685
--- /dev/null
+++ b/Squirrel/Squirrel/DataModels/SaleDataModel.cs
@@ -0,0 +1,56 @@
+using Squirrel.Enums;
+using Squirrel.Exceptions;
+using Squirrel.Extensions;
+using Squirrel.Infrastructure;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Squirrel.DataModels;
+
+public class SaleDataModel(string id, string workerId, string? guestId, double sum, DiscountType discountType, double discount, bool isCancel, List<SaleProductDataModel> products) : IValidation
+{
+    public string Id { get; private set; } = id;
+
+    public string WorkerId { get; private set; } = workerId;
+
+    public string? GuestId { get; private set; } = guestId;
+
+    public DateTime SaleDate { get; private set; } = DateTime.UtcNow;
+
+    public double Sum { get; private set; } = sum;
+
+    public DiscountType DiscountType { get; private set; } = discountType;
+
+    public double Discount { get; private set; } = discount;
+
+    public bool IsCancel { get; private set; } = isCancel;
+
+    public List<SaleProductDataModel> Products { get; private set; } = products;
+
+    public void Validate()
+    {
+        if (Id.IsEmpty())
+            throw new ValidationException("Field Id is empty");
+
+        if (!Id.IsGuid())
+            throw new ValidationException("The value in the field Id is not a unique identifier");
+
+        if (WorkerId.IsEmpty())
+            throw new ValidationException("Field WorkerId is empty");
+
+        if (!WorkerId.IsGuid())
+            throw new ValidationException("The value in the field WorkerId is not a unique identifier");
+
+        if (!GuestId?.IsGuid() ?? !GuestId?.IsEmpty() ?? false)
+            throw new ValidationException("The value in the field GuestId is not a unique identifier");
+
+        if (Sum <= 0)
+            throw new ValidationException("Field Sum is less than or equal to 0");
+
+        if ((Products?.Count ?? 0) == 0)
+            throw new ValidationException("The sale must include products");
+    }
+}
diff --git a/Squirrel/Squirrel/DataModels/SaleProductDataModel.cs b/Squirrel/Squirrel/DataModels/SaleProductDataModel.cs
new file mode 100644
index 0000000..3e08ea7
--- /dev/null
+++ b/Squirrel/Squirrel/DataModels/SaleProductDataModel.cs
@@ -0,0 +1,37 @@
+using Squirrel.Exceptions;
+using Squirrel.Extensions;
+using Squirrel.Infrastructure;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Squirrel.DataModels;
+
+public class SaleProductDataModel(string saleId, string productId, int count) : IValidation
+{
+    public string SaleId { get; private set; } = saleId;
+
+    public string ProductId { get; private set; } = productId;
+
+    public int Count { get; private set; } = count;
+
+    public void Validate()
+    {
+        if (SaleId.IsEmpty())
+            throw new ValidationException("Field SaleId is empty");
+
+        if (!SaleId.IsGuid())
+            throw new ValidationException("The value in the field SaleId is not a unique identifier");
+
+        if (ProductId.IsEmpty())
+            throw new ValidationException("Field ProductId is empty");
+
+        if (!ProductId.IsGuid())
+            throw new ValidationException("The value in the field ProductId is not a unique identifier");
+
+        if (Count <= 0)
+            throw new ValidationException("Field Count is less than or equal to 0");
+    }
+}
diff --git a/Squirrel/Squirrel/DataModels/WorkerDataModel.cs b/Squirrel/Squirrel/DataModels/WorkerDataModel.cs
new file mode 100644
index 0000000..974cb31
--- /dev/null
+++ b/Squirrel/Squirrel/DataModels/WorkerDataModel.cs
@@ -0,0 +1,52 @@
+using Squirrel.Exceptions;
+using Squirrel.Extensions;
+using Squirrel.Infrastructure;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Squirrel.DataModels;
+
+public class WorkerDataModel(string id, string fio, string postId, DateTime birthDate, DateTime employmentDate, bool isDeleted) : IValidation
+{
+    public string Id { get; private set; } = id;
+
+    public string FIO { get; private set; } = fio;
+
+    public string PostId { get; private set; } = postId;
+
+    public DateTime BirthDate { get; private set; } = birthDate;
+
+    public DateTime EmploymentDate { get; private set; } = employmentDate;
+
+    public bool IsDeleted { get; private set; } = isDeleted;
+
+    public void Validate()
+    {
+        if (Id.IsEmpty())
+            throw new ValidationException("Field Id is empty");
+
+        if (!Id.IsGuid())
+            throw new ValidationException("The value in the field Id is not a unique identifier");
+
+        if (FIO.IsEmpty())
+            throw new ValidationException("Field FIO is empty");
+
+        if (PostId.IsEmpty())
+            throw new ValidationException("Field PostId is empty");
+
+        if (!PostId.IsGuid())
+            throw new ValidationException("The value in the field PostId is not a unique identifier");
+
+        if (BirthDate.Date > DateTime.Now.AddYears(-18).Date)
+            throw new ValidationException($"Minors cannot be hired (BirthDate = {BirthDate.ToShortDateString()})");
+
+        if (EmploymentDate.Date < BirthDate.Date)
+            throw new ValidationException("The date of employment cannot be less than the date of birth");
+
+        if ((EmploymentDate - BirthDate).TotalDays / 365 < 18) // EmploymentDate.Year - BirthDate.Year
+            throw new ValidationException($"Minors cannot be hired (EmploymentDate - {EmploymentDate.ToShortDateString()}, BirthDate - {BirthDate.ToShortDateString()})");
+    }
+}
diff --git a/Squirrel/Squirrel/Enums/DiscountType.cs b/Squirrel/Squirrel/Enums/DiscountType.cs
new file mode 100644
index 0000000..8e9a712
--- /dev/null
+++ b/Squirrel/Squirrel/Enums/DiscountType.cs
@@ -0,0 +1,15 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Squirrel.Enums;
+
+public enum DiscountType
+{
+    None = 0,
+    OnSale = 1,
+    RegularCustomer = 2,
+    Coupon = 4
+}
\ No newline at end of file
diff --git a/Squirrel/Squirrel/Enums/PostType.cs b/Squirrel/Squirrel/Enums/PostType.cs
new file mode 100644
index 0000000..d040b03
--- /dev/null
+++ b/Squirrel/Squirrel/Enums/PostType.cs
@@ -0,0 +1,15 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Squirrel.Enums;
+
+public enum PostType
+{
+    None = 0,
+    Manager = 1,
+    Bartender = 2,
+    Waiter = 3
+}
\ No newline at end of file
diff --git a/Squirrel/Squirrel/Enums/ProductType.cs b/Squirrel/Squirrel/Enums/ProductType.cs
new file mode 100644
index 0000000..07c1aab
--- /dev/null
+++ b/Squirrel/Squirrel/Enums/ProductType.cs
@@ -0,0 +1,15 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Squirrel.Enums;
+
+public enum ProductType
+{
+    None = 0,
+    Drinks = 1,
+    Snacks = 2,
+    Desserts = 3
+}
\ No newline at end of file
diff --git a/Squirrel/Squirrel/Exceptions/ValidationException.cs b/Squirrel/Squirrel/Exceptions/ValidationException.cs
new file mode 100644
index 0000000..636984a
--- /dev/null
+++ b/Squirrel/Squirrel/Exceptions/ValidationException.cs
@@ -0,0 +1,11 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Squirrel.Exceptions;
+
+public class ValidationException(string message) : Exception(message)
+{
+}
diff --git a/Squirrel/Squirrel/Extensions/StringExtensions.cs b/Squirrel/Squirrel/Extensions/StringExtensions.cs
new file mode 100644
index 0000000..6411ff0
--- /dev/null
+++ b/Squirrel/Squirrel/Extensions/StringExtensions.cs
@@ -0,0 +1,20 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Squirrel.Extensions;
+
+public static class StringExtensions
+{
+    public static bool IsEmpty(this string str)
+    {
+        return string.IsNullOrWhiteSpace(str);
+    }
+    public static bool IsGuid(this string str)
+    {
+        return Guid.TryParse(str, out _);
+    }
+
+}
diff --git a/Squirrel/Squirrel/Infrastructure/IValidation.cs b/Squirrel/Squirrel/Infrastructure/IValidation.cs
new file mode 100644
index 0000000..1ec81e2
--- /dev/null
+++ b/Squirrel/Squirrel/Infrastructure/IValidation.cs
@@ -0,0 +1,12 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Squirrel.Infrastructure;
+
+internal interface IValidation
+{
+    void Validate();
+}