Models

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Overview

Models in Taxi define specific data structures used by services or applications. Unlike semantic types which define meaning, models describe how data is organized.

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Basic syntax

model Person {
    id: PersonId
    firstName: FirstName
    lastName: LastName
    email: EmailAddress?  // Optional field
    friends: Person[]     // Array of Person
}

Key features:

• Fields can use semantic types
• ? marks optional (nullable) fields
• Arrays are supported using [] syntax
• Documentation can be added using [[ ]]

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Parameter and closed models

Taxi lets you control how models can be constructed and returned using the parameter and closed modifiers.

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Parameter models

Parameter models explicitly indicate that an object can be constructed as input to a service:

parameter model CreateCustomerRequest {
    firstName: FirstName
    lastName: LastName
    email: EmailAddress
}

service CustomerService {
    operation createCustomer(CreateCustomerRequest): Customer
}

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Closed models

Closed models can only be returned from services - they cannot be constructed by combining fields from other sources:

closed model CustomerRecord {
    id: CustomerId
    internalId: InternalId        // System-generated
    lastModified: Timestamp       // System-managed
}

service CustomerService {
    operation getCustomer(CustomerId): CustomerRecord
}

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Parameter and closed combined

Some models need to be both parameter and closed. This is common with database tables, where the model:

• Can be used as input for insert/update operations (parameter)
• Can be returned from queries (closed)
• Should not be constructed by combining fields from other sources (closed)

parameter closed model Customer {
    id: CustomerId
    name: CustomerName
    email: EmailAddress
}

service CustomerDatabase {
    // Can be used as input
    operation insertCustomer(Customer): CustomerId
    
    // Can be returned from queries
    operation findCustomer(CustomerId): Customer
    
    // Declared as a table
    table customers: Customer[]
}

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Field types

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Basic fields

model Customer {
    // Required fields
    id: CustomerId
    name: CustomerName
    
    // Optional field
    nickname: Nickname?
    
    // Array field
    orders: Order[]
    
    // Map field
    preferences: Map<PreferenceKey, PreferenceValue>
}

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Nested objects

model Order {
    orderId: OrderId
    
    // Inline object definition
    address: {
        street: StreetAddress
        city: City
        country: CountryCode
    }
    
    // Alternative: reference a defined model
    shippingAddress: Address
}

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Inheritance

While possible, model inheritance should be used sparingly:

model PersonBase {
    id: PersonId
    firstName: FirstName
    lastName: LastName
}

model Employee inherits PersonBase {
    employeeId: EmployeeId
    department: DepartmentName
}

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Best practices

Models are typically defined by producing systems - such as REST APIs, databases, or message queues.

The best practices here focus on describing those models using Taxi, rather than the practices of designing good APIs or event payloads.

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Use semantic types

// Good - uses semantic types
model Customer {
    id: CustomerId
    email: EmailAddress
}

// Bad - uses primitive types
model Customer {
    id: String      // What kind of ID?
    email: String   // Is this really an email?
}

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Document nullability

model User {
    // Required fields - core identity
    id: UserId
    email: EmailAddress
    
    // Optional fields - additional information
    middleName: MiddleName?
    phoneNumber: PhoneNumber?
}

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Organization

// Service-specific models together
model CreateOrderRequest {
    customerId: CustomerId
    items: OrderItem[]
}

model CreateOrderResponse {
    orderId: OrderId
    status: OrderStatus
}

// Service definition with its models
service OrderService {
    operation createOrder(CreateOrderRequest): CreateOrderResponse
}

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Common patterns

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Request/response models

parameter model SearchRequest {
    query: SearchQuery
    maxResults: MaxResults?
    offset: Offset?
}

closed model SearchResponse {
    results: SearchResult[]
    totalCount: TotalCount
    hasMore: Boolean
}

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Event models

closed model CustomerEvent {
    eventId: EventId
    timestamp: EventTimestamp
    customerId: CustomerId
    type: EventType
    payload: CustomerEventPayload
}

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View models

closed model CustomerSummary {
    id: CustomerId
    name: CustomerName
    status: CustomerStatus
    // Computed field
    isActive: Boolean by (this.status == CustomerStatus.ACTIVE)
}