Introduction

What is ApiAggregator?

ApiAggregator is a .net utility to help combine multiple api requests to return a single aggregated response.
The framework allows fetching the whole of aggregated response or a partial response based on the list of configured apis included in the aggregator request.

When is ApiAggregator useful?

ApiAggregator is useful in many use cases. Few to list are:

• For creating Level 2 (functional or BFF) apis using Level 1 (core resource) apis.
• For easily extending an api without having to break existing consumers.
• For on demand retrieval of data using different subsets of configured apis to fetch varied datasets per request.
• and Many more.

Getting Started?

i. Installation

Install the latest version of ApiAggregator nuget package with command below.

NuGet\Install-Package ApiAggregator 

ii. Using ApiAggregator

Step 1. Create Aggregated Contract

Aggregated Contract is the resultant response from all the aggregated apis. To create aggregated contract derive the class from IContract interface.

Example.

 public class Customer : IContract
 {
        public int Id { get; set; }
        public string Code { get; set; }
        public string Name { get; set; }
        public Contacts Communication { get; set; }
        public Order[] Orders { get; set; }
}

Step 2. Create Api Aggregated

Api Aggregate is the composition of apis configured to obtain an aggregated data populated contract. To create an Api Aggregate derive from ApiAggregate<TContract> class where TContract is an implementation of IContract (ie. Agggregated Contract).

Example.

internal class CustomerAggregate : ApiAggregate<Customer>
{
        /// <summary>
        /// Constructs the api aggregate with web apis and result transformers to map data to aggregated contract.
        /// </summary>
        /// <returns>Mappings</returns>
        public override IEnumerable<Mapping<Customer, IApiResult>> Construct()
        {
            return CreateAggregate.For<Customer>()
                .Map<CustomerApi, CustomerTransform>(With.Name("customer"),
                 customer => customer.Dependents
                    .Map<CommunicationApi, CommunicationTransform>(With.Name("customer.communication"))
                    .Map<OrdersApi, OrdersTransform>(With.Name("customer.orders"),
                        customerOrders => customerOrders.Dependents
                            .Map<OrderItemsApi, OrderItemsTransform>(With.Name("customer.orders.items")))
                ).Create();
        }
}

Api Aggregate comprises of apis configured in hierarchical nested graph with each api having an associated result transformers. The result from the api is fed to the associated result transformer to map data to the aggregated contract.

2.1 Api & Transformer Pair

Every Api type in the ApiAggregate definition should have a complementing Transformer type.

• You need to assign a name to the api/transformer pair. See below rules for api naming convention.

Rules:

• You could nest api/transformer pairs in a parent/child hierarchy. In a given parent/child hierarchy, the output of the parent api will serve as the input to the nested api to resolve its api endpoint.
• The api/transformer mappings can be nested to 5 levels of dependency.
• By convention, The api name should be dot separated string with a dot for every nested level. The name should includes all parent names separated by a dot for a given hierarchy. ie. For a 3 level dependency api mapping the name should be like customer.orders.items

Example.

Below is the snippet from CustomerAggregate definition for parent/child relationship between Customer & Communication apis. The api response from CustomerApi is the input to CommunicationApi for resolving its endpoint url.

   .Map<CustomerApi, CustomerTransform>(With.Name("customer"), -- Parent mapping with name
           customer => customer.Dependents
              .Map<CommunicationApi, CommunicationTransform>(With.Name("customer.communication")) -- nested mapping with dot separated name

i. Web Api Class

The purpose of a api class is to execute the web api with api engine to fetch the response.

As mentioned previously, You can configure an api in Parent or Child (nested) mode in a hierarchical graph.

To create Web Api defined as parent or nested api, you need to implement from WebApi<TResult> class,
where TResult is IApiResult interface (or ApiResult base class) implementation and is the result that will be returned from executing the api.

Upon creating the web api class, you need to provide GetUrl() method implementation to return Uri instance.

• Implement the GetUrl(IRequestContext context, IApiResult parentApiResult) method to return the constructed endpoint using given parameters of the method.
• For Parent Api, only IRequestContext context parameter is passed to GetUrl() method to resolve the Url endpoint.
• For Nested Api, api result parameter (ie. IApiResult parentApiResult) from the parent api is additionally passed in to GetUrl() method along with IRequestContext context parameter.
• Optionally, override GetRequestHeaders() method to provide a dictionary of outgoing headers for the api request.
• Optionally, override GetResponseHeaders() method to provide any list of incoming headers from the api response.
• IApiResult implementation exposes Headers property for subscribed response headers received as part of the api response.

Important:

• The api endpoint needs to be resolved before executing the api with ApiEngine.
• IApiResult parentApiResult parameter is null for apis configured in parent mode.

Examples.

See example CustomerApi implemented to be configured and run in parent mode.

public class CustomerApi : WebApi<CustomerResult>
{
    public CustomerApi() : base(Endpoints.BaseAddress)
    {
    }

    // Override to construct the api endpoint.
    protected override Uri GetUrl(IRequestContext context, IApiResult parentApiResult)
    {
        // Executes as root or level 1 api. parentApiResult should be null.
        var customerContext = (CustomerContext)context;

        return new Uri(string.Format(Endpoints.Customer, customerContext.CustomerId));
    }

    // Override to pass custom outgoing headers with the api request.
    protected override IDictionary<string, string> GetRequestHeaders()
    {
        return new Dictionary<string, string>
        {
            { "x-meta-branch-code", "Geneva" }
        };
    }

    // Override to get custom incoming headers with the api response.
    protected override IEnumerable<string> GetResponseHeaders()
    {
        return new[] { "x-meta-branch-code" };
    }
}

See example CommunicationApi implemented to be configured and run as nested api to customer api below.

internal class CommunicationApi : WebApi<CommunicationResult>
    {
        public CommunicationApi() : base(Endpoints.BaseAddress)
        {
        }

        protected override Uri GetUrl(IRequestContext context, IApiResult parentApiResult)
        {
            var customer = (CustomerResult)parentApiResult;
            return new Uri(string.Format(Endpoints.Communication, customer.Id));
        }
    }

ii. Result Transformer Class

The purpose of the transformer class is to map the response fetched by the linked api to the aggregated contract.

To define a transformer class, you need to implement ResultTransformer<TResult, TContract> class.

• where TContract is Aggregated Contract implementing IContract. eg. Customer.
• where TResult is Api Result from associated Query. It is an implementation of IApiResult interface.

Example.

CustomerTransformer is implemented to map CustomerResult recevied from CustomerApi to Customer Aggregated Contract.

public class CustomerTransform : ResultTransformer<CustomerResult, Customer>
    {
        public override void Transform(CustomerResult apiResult, Customer contract)
        {
            var customer = contract ?? new Customer();
            customer.Id = apiResult.Id;
            customer.Name = apiResult.Name;
            customer.Code = apiResult.Code;
        }
    }

Step 3. ApiAggregator Setup

ApiAggregator needs to setup with required dependencies.

i. IoC Registrations

With ServiceCollection, you need to register the below dependencies.

    // Register core services
    services.AddTransient(typeof(IApiBuilder<>), typeof(ApiBuilder<>));
    services.AddTransient(typeof(IContractBuilder<>), typeof(ContractBuilder<>));
    services.AddTransient(typeof(IApiAggregator<>), typeof(ApiAggregator<>));
    services.AddTransient<IApiExecutor, ApiExecutor>();
     services.AddTransient<IApiEnginne, ApiEngine>();

    // Register instance of IApiNameMatcher.
    services.AddTransient(c => new StringColonSeparatedMatcher());

    // Enable logging
    services.AddLogging();

    // Enable HttpClient
     services.AddHttpClient();

    // Register api aggregate definitions. eg CustomerAggregate
    services.AddTransient<IApiAggregate<Customer>, CustomerAggregate>();

ii. With Fluent Registration Api

You could also acheieve the above registrations using fluent registration below.

    // Enable logging
    services.AddLogging();

    // Enable HttpClient
     services.AddHttpClient();

    // Fluent registration.
     services.UseApiAggregator()
             .AddApiAggregate<Customer>(new CustomerAggregate());

Note: Above examples to enable HttpClient is basic. However, you could additionally implement circuit breaking & retry policies.

Please see Circuit Breaker pattern for more details.

Step 4. Use IApiAggregator<TContract>

i. IApiAggrgator (DI)

To use Api aggregator, Inject IApiAggrgator where TContract is IContract, using constructor & property injection method or explicity Resolve using service provider

Example. IServiceProvider.GetService(typeof(IApiAggrgator<Customer>))

ii. Call Aggregator.GetData() method

You need to call the GetData() method with an instance of parameter class derived from IRequestContext interface.

• The IRequestContext provides a Names property which is a list of string to include all the api names to be included for the given request to fetch aggregated response.
• When no names are passed in the paramter then entire aggregated response for all configured apis is returned.
• When subset of apis are included using names then the returned aggregated response only includes api responses from included apis.
• When nested api with dot separated api name (eg. customer.orders.items) is included then all parent apis also get included for the dependency.

Example - Control Flow

Example execution flow for a nested api included in the GetData() parameter.