Accessing Remote Data

Many modern web-based solutions make use of web services, hosted by web servers, to provide functionality for remote client applications. The operations that a web service exposes constitute a web API.

Client apps should be able to utilize the web API without knowing how the data or operations that the API exposes are implemented. This requires that the API abides by common standards that enable a client app and web service to agree on which data formats to use, and the structure of the data that is exchanged between client apps and the web service.

Introduction to Representational State Transfer

Representational State Transfer (REST) is an architectural style for building distributed systems based on hypermedia. A primary advantage of the REST model is that it's based on open standards and doesn't bind the implementation of the model or the client apps that access it to any specific implementation. Therefore, a REST web service could be implemented using Microsoft ASP.NET Core MVC, and client apps could be developing using any language and toolset that can generate HTTP requests and parse HTTP responses.

The REST model uses a navigational scheme to represent objects and services over a network, referred to as resources. Systems that implement REST typically use the HTTP protocol to transmit requests to access these resources. In such systems, a client app submits a request in the form of a URI that identifies a resource, and an HTTP method (such as GET, POST, PUT, or DELETE) that indicates the operation to be performed on that resource. The body of the HTTP request contains any data required to perform the operation.

The response from a REST request makes use of standard HTTP status codes. For example, a request that returns valid data should include the HTTP response code 200 (OK), while a request that fails to find or delete a specified resource should return a response that includes the HTTP status code 404 (Not Found).

A RESTful web API exposes a set of connected resources, and provides the core operations that enable an app to manipulate those resources and easily navigate between them. For this reason, the URIs that constitute a typical RESTful web API are oriented towards the data that it exposes, and use the facilities provided by HTTP to operate on this data.

The data included by a client app in an HTTP request, and the corresponding response messages from the web server, could be presented in a variety of formats, known as media types. When a client app sends a request that returns data in the body of a message, it can specify the media types it can handle in the Accept header of the request. If the web server supports this media type, it can reply with a response that includes the Content-Type header that specifies the format of the data in the body of the message. It's then the responsibility of the client app to parse the response message and interpret the results in the message body appropriately.

For more information about REST, see API design and API implementation.

Consuming RESTful APIs

The eShopOnContainers mobile app uses the Model-View-ViewModel (MVVM) pattern, and the model elements of the pattern represent the domain entities used in the app. The controller and repository classes in the eShopOnContainers reference application accept and return many of these model objects. Therefore, they are used as data transfer objects (DTOs) that hold all the data that is passed between the mobile app and the containerized microservices. The main benefit of using DTOs to pass data to and receive data from a web service is that by transmitting more data in a single remote call, the app can reduce the number of remote calls that need to be made.

Making Web Requests

The eShopOnContainers mobile app uses the HttpClient class to make requests over HTTP, with JSON being used as the media type. This class provides functionality for asynchronously sending HTTP requests and receiving HTTP responses from a URI identified resource. The HttpResponseMessage class represents an HTTP response message received from a REST API after an HTTP request has been made. It contains information about the response, including the status code, headers, and any body. The HttpContent class represents the HTTP body and content headers, such as Content-Type and Content-Encoding. The content can be read using any of the ReadAs methods, such as ReadAsStringAsync and ReadAsByteArrayAsync, depending on the format of the data.

Making a GET Request

The CatalogService class is used to manage the data retrieval process from the catalog microservice. In the RegisterDependencies method in the ViewModelLocator class, the CatalogService class is registered as a type mapping against the ICatalogService type with the Autofac dependency injection container. Then, when an instance of the CatalogViewModel class is created, its constructor accepts an ICatalogService type, which Autofac resolves, returning an instance of the CatalogService class. For more information about dependency injection, see Introduction to Dependency Injection.

Figure 10-1 shows the interaction of classes that read catalog data from the catalog microservice for displaying by the CatalogView.

Figure 10-1: Retrieving data from the catalog microservice

When the CatalogView is navigated to, the OnInitialize method in the CatalogViewModel class is called. This method retrieves catalog data from the catalog microservice, as demonstrated in the following code example:

public override async Task InitializeAsync(object navigationData)  
{  
    ...  
    Products = await _productsService.GetCatalogAsync();  
    ...  
}

This method calls the GetCatalogAsync method of the CatalogService instance that was injected into the CatalogViewModel by Autofac. The following code example shows the GetCatalogAsync method:

public async Task<ObservableCollection<CatalogItem>> GetCatalogAsync()  
{  
    UriBuilder builder = new UriBuilder(GlobalSetting.Instance.CatalogEndpoint);  
    builder.Path = "api/v1/catalog/items";  
    string uri = builder.ToString();  

    CatalogRoot catalog = await _requestProvider.GetAsync<CatalogRoot>(uri);  
    ...  
    return catalog?.Data.ToObservableCollection();            
}

This method builds the URI that identifies the resource the request will be sent to, and uses the RequestProvider class to invoke the GET HTTP method on the resource, before returning the results to the CatalogViewModel. The RequestProvider class contains functionality that submits a request in the form of a URI that identifies a resource, an HTTP method that indicates the operation to be performed on that resource, and a body that contains any data required to perform the operation. For information about how the RequestProvider class is injected into the CatalogService class, see Introduction to Dependency Injection.

The following code example shows the GetAsync method in the RequestProvider class:

public async Task<TResult> GetAsync<TResult>(string uri, string token = "")  
{  
    HttpClient httpClient = CreateHttpClient(token);  
    HttpResponseMessage response = await httpClient.GetAsync(uri);  

    await HandleResponse(response);  
    string serialized = await response.Content.ReadAsStringAsync();  

    TResult result = await Task.Run(() =>   
        JsonConvert.DeserializeObject<TResult>(serialized, _serializerSettings));  

    return result;  
}

This method calls the CreateHttpClient method, which returns an instance of the HttpClient class with the appropriate headers set. It then submits an asynchronous GET request to the resource identified by the URI, with the response being stored in the HttpResponseMessage instance. The HandleResponse method is then invoked, which throws an exception if the response doesn't include a success HTTP status code. Then the response is read as a string, converted from JSON to a CatalogRoot object, and returned to the CatalogService.

The CreateHttpClient method is shown in the following code example:

private HttpClient CreateHttpClient(string token = "")  
{  
    var httpClient = new HttpClient();  
    httpClient.DefaultRequestHeaders.Accept.Add(  
        new MediaTypeWithQualityHeaderValue("application/json"));  

    if (!string.IsNullOrEmpty(token))  
    {  
        httpClient.DefaultRequestHeaders.Authorization =   
            new AuthenticationHeaderValue("Bearer", token);  
    }  
    return httpClient;  
}

This method creates a new instance of the HttpClient class, and sets the Accept header of any requests made by the HttpClient instance to application/json, which indicates that it expects the content of any response to be formatted using JSON. Then, if an access token was passed as an argument to the CreateHttpClient method, it's added to the Authorization header of any requests made by the HttpClient instance, prefixed with the string Bearer. For more information about authorization, see Authorization.

When the GetAsync method in the RequestProvider class calls HttpClient.GetAsync, the Items method in the CatalogController class in the Catalog.API project is invoked, which is shown in the following code example:

[HttpGet]  
[Route("[action]")]  
public async Task<IActionResult> Items(  
    [FromQuery]int pageSize = 10, [FromQuery]int pageIndex = 0)  
{  
    var totalItems = await _catalogContext.CatalogItems  
        .LongCountAsync();  

    var itemsOnPage = await _catalogContext.CatalogItems  
        .OrderBy(c=>c.Name)  
        .Skip(pageSize * pageIndex)  
        .Take(pageSize)  
        .ToListAsync();  

    itemsOnPage = ComposePicUri(itemsOnPage);  
    var model = new PaginatedItemsViewModel<CatalogItem>(  
        pageIndex, pageSize, totalItems, itemsOnPage);             

    return Ok(model);  
}

This method retrieves the catalog data from the SQL database using EntityFramework, and returns it as a response message that includes a success HTTP status code, and a collection of JSON formatted CatalogItem instances.

Making a POST Request

The BasketService class is used to manage the data retrieval and update process with the basket microservice. In the RegisterDependencies method in the ViewModelLocator class, the BasketService class is registered as a type mapping against the IBasketService type with the Autofac dependency injection container. Then, when an instance of the BasketViewModel class is created, its constructor accepts an IBasketService type, which Autofac resolves, returning an instance of the BasketService class. For more information about dependency injection, see Introduction to Dependency Injection.

Figure 10-2 shows the interaction of classes that send the basket data displayed by the BasketView, to the basket microservice.

Figure 10-2: Sending data to the basket microservice

When an item is added to the shopping basket, the ReCalculateTotalAsync method in the BasketViewModel class is called. This method updates the total value of items in the basket, and sends the basket data to the basket microservice, as demonstrated in the following code example:

private async Task ReCalculateTotalAsync()  
{  
    ...  
    await _basketService.UpdateBasketAsync(new CustomerBasket  
    {  
        BuyerId = userInfo.UserId,   
        Items = BasketItems.ToList()  
    }, authToken);  
}

This method calls the UpdateBasketAsync method of the BasketService instance that was injected into the BasketViewModel by Autofac. The following method shows the UpdateBasketAsync method:

public async Task<CustomerBasket> UpdateBasketAsync(CustomerBasket customerBasket, string token)  
{  
    UriBuilder builder = new UriBuilder(GlobalSetting.Instance.BasketEndpoint);  
    string uri = builder.ToString();  
    var result = await _requestProvider.PostAsync(uri, customerBasket, token);  
    return result;  
}

This method builds the URI that identifies the resource the request will be sent to, and uses the RequestProvider class to invoke the POST HTTP method on the resource, before returning the results to the BasketViewModel. Note that an access token, obtained from IdentityServer during the authentication process, is required to authorize requests to the basket microservice. For more information about authorization, see Authorization.

The following code example shows one of the PostAsync methods in the RequestProvider class:

public async Task<TResult> PostAsync<TResult>(  
    string uri, TResult data, string token = "", string header = "")  
{  
    HttpClient httpClient = CreateHttpClient(token);  
    ...  
    var content = new StringContent(JsonConvert.SerializeObject(data));  
    content.Headers.ContentType = new MediaTypeHeaderValue("application/json");  
    HttpResponseMessage response = await httpClient.PostAsync(uri, content);  

    await HandleResponse(response);  
    string serialized = await response.Content.ReadAsStringAsync();  

    TResult result = await Task.Run(() =>  
        JsonConvert.DeserializeObject<TResult>(serialized, _serializerSettings));  

    return result;  
}

This method calls the CreateHttpClient method, which returns an instance of the HttpClient class with the appropriate headers set. It then submits an asynchronous POST request to the resource identified by the URI, with the serialized basket data being sent in JSON format, and the response being stored in the HttpResponseMessage instance. The HandleResponse method is then invoked, which throws an exception if the response doesn't include a success HTTP status code. Then, the response is read as a string, converted from JSON to a CustomerBasket object, and returned to the BasketService. For more information about the CreateHttpClient method, see Making a GET Request.

When the PostAsync method in the RequestProvider class calls HttpClient.PostAsync, the Post method in the BasketController class in the Basket.API project is invoked, which is shown in the following code example:

[HttpPost]  
public async Task<IActionResult> Post([FromBody]CustomerBasket value)  
{  
    var basket = await _repository.UpdateBasketAsync(value);  
    return Ok(basket);  
}

This method uses an instance of the RedisBasketRepository class to persist the basket data to the Redis cache, and returns it as a response message that includes a success HTTP status code, and a JSON formatted CustomerBasket instance.

Making a DELETE Request

Figure 10-3 shows the interactions of classes that delete basket data from the basket microservice, for the CheckoutView.

Figure 10-3: Deleting data from the basket microservice

When the checkout process is invoked, the CheckoutAsync method in the CheckoutViewModel class is called. This method creates a new order, before clearing the shopping basket, as demonstrated in the following code example:

private async Task CheckoutAsync()  
{  
    ...  
    await _basketService.ClearBasketAsync(_shippingAddress.Id.ToString(), authToken);  
    ...  
}

This method calls the ClearBasketAsync method of the BasketService instance that was injected into the CheckoutViewModel by Autofac. The following method shows the ClearBasketAsync method:

public async Task ClearBasketAsync(string guidUser, string token)  
{  
    UriBuilder builder = new UriBuilder(GlobalSetting.Instance.BasketEndpoint);  
    builder.Path = guidUser;  
    string uri = builder.ToString();  
    await _requestProvider.DeleteAsync(uri, token);  
}

This method builds the URI that identifies the resource that the request will be sent to, and uses the RequestProvider class to invoke the DELETE HTTP method on the resource. Note that an access token, obtained from IdentityServer during the authentication process, is required to authorize requests to the basket microservice. For more information about authorization, see Authorization.

The following code example shows the DeleteAsync method in the RequestProvider class:

public async Task DeleteAsync(string uri, string token = "")  
{  
    HttpClient httpClient = CreateHttpClient(token);  
    await httpClient.DeleteAsync(uri);  
}

This method calls the CreateHttpClient method, which returns an instance of the HttpClient class with the appropriate headers set. It then submits an asynchronous DELETE request to the resource identified by the URI. For more information about the CreateHttpClient method, see Making a GET Request.

When the DeleteAsync method in the RequestProvider class calls HttpClient.DeleteAsync, the Delete method in the BasketController class in the Basket.API project is invoked, which is shown in the following code example:

[HttpDelete("{id}")]  
public void Delete(string id)  
{  
    _repository.DeleteBasketAsync(id);  
}

This method uses an instance of the RedisBasketRepository class to delete the basket data from the Redis cache.

Caching Data

The performance of an app can be improved by caching frequently accessed data to fast storage that's located close to the app. If the fast storage is located closer to the app than the original source, then caching can significantly improve response times when retrieving data.

The most common form of caching is read-through caching, where an app retrieves data by referencing the cache. If the data isn't in the cache, it's retrieved from the data store and added to the cache. Apps can implement read-through caching with the cache-aside pattern. This pattern determines whether the item is currently in the cache. If the item isn't in the cache, it's read from the data store and added to the cache. For more information, see the Cache-Aside pattern.

Distributed applications, such as the eShopOnContainers reference application, should provide either or both of the following caches:

• A shared cache, which can be accessed by multiple processes or machines.
• A private cache, where data is held locally on the device running the app.

The eShopOnContainers mobile app uses a private cache, where data is held locally on the device that's running an instance of the app. For information about the cache used by the eShopOnContainers reference application, see .NET Microservices: Architecture for Containerized .NET Applications.

Managing Data Expiration

It's impractical to expect that cached data will always be consistent with the original data. Data in the original data store might change after it's been cached, causing the cached data to become stale. Therefore, apps should implement a strategy that helps to ensure that the data in the cache is as up-to-date as possible, but can also detect and handle situations that arise when the data in the cache has become stale. Most caching mechanisms enable the cache to be configured to expire data, and hence reduce the period for which data might be out of date.

When cached data expires, it should be removed from the cache, and the app must retrieve the data from the original data store and place it back into the cache.

It's also possible that a cache might fill up if data is allowed to remain for too long a period. Therefore, requests to add new items to the cache might be required to remove some items in a process known as eviction. Caching services typically evict data on a least-recently-used basis. However, there are other eviction policies, including most-recently-used, and first-in-first-out. For more information, see Caching Guidance.

Caching Images

The eShopOnContainers mobile app consumes remote product images that benefit from being cached. These images are displayed by the Image control, and the CachedImage control provided by the FFImageLoading library.

The Xamarin.Forms Image control supports caching of downloaded images. Caching is enabled by default, and will store the image locally for 24 hours. In addition, the expiration time can be configured with the CacheValidity property. For more information, see Downloaded Image Caching.

FFImageLoading's CachedImage control is a replacement for the Xamarin.Forms Image control, providing additional properties that enable supplementary functionality. Amongst this functionality, the control provides configurable caching, while supporting error and loading image placeholders. The following code example shows how the eShopOnContainers mobile app uses the CachedImage control in the ProductTemplate, which is the data template used by the ListView control in the CatalogView:

<ffimageloading:CachedImage
    Grid.Row="0"
    Source="{Binding PictureUri}"     
    Aspect="AspectFill">
    <ffimageloading:CachedImage.LoadingPlaceholder>
        <OnPlatform x:TypeArguments="ImageSource">
            <On Platform="iOS, Android" Value="default_campaign" />
            <On Platform="UWP" Value="Assets/default_campaign.png" />
        </OnPlatform>
    </ffimageloading:CachedImage.LoadingPlaceholder>
    <ffimageloading:CachedImage.ErrorPlaceholder>
        <OnPlatform x:TypeArguments="ImageSource">
            <On Platform="iOS, Android" Value="noimage" />
            <On Platform="UWP" Value="Assets/noimage.png" />
        </OnPlatform>
    </ffimageloading:CachedImage.ErrorPlaceholder>
</ffimageloading:CachedImage>

The CachedImage control sets the LoadingPlaceholder and ErrorPlaceholder properties to platform-specific images. The LoadingPlaceholder property specifies the image to be displayed while the image specified by the Source property is retrieved, and the ErrorPlaceholder property specifies the image to be displayed if an error occurs when attempting to retrieve the image specified by the Source property.

As the name implies, the CachedImage control caches remote images on the device for the time specified by the value of the CacheDuration property. When this property value isn't explicitly set, the default value of 30 days is applied.

Increasing Resilience

All apps that communicate with remote services and resources must be sensitive to transient faults. Transient faults include the momentary loss of network connectivity to services, the temporary unavailability of a service, or timeouts that arise when a service is busy. These faults are often self-correcting, and if the action is repeated after a suitable delay it's likely to succeed.

Transient faults can have a huge impact on the perceived quality of an app, even if it has been thoroughly tested under all foreseeable circumstances. To ensure that an app that communicates with remote services operates reliably, it must be able to do all of the following:

• Detect faults when they occur, and determine if the faults are likely to be transient.
• Retry the operation if it determines that the fault is likely to be transient and keep track of the number of times the operation was retried.
• Use an appropriate retry strategy, which specifies the number of retries, the delay between each attempt, and the actions to take after a failed attempt.

This transient fault handling can be achieved by wrapping all attempts to access a remote service in code that implements the retry pattern.

Retry Pattern

If an app detects a failure when it tries to send a request to a remote service, it can handle the failure in any of the following ways:

• Retrying the operation. The app could retry the failing request immediately.
• Retrying the operation after a delay. The app should wait for a suitable amount of time before retrying the request.
• Cancelling the operation. The application should cancel the operation and report an exception.

The retry strategy should be tuned to match the business requirements of the app. For example, it's important to optimize the retry count and retry interval to the operation being attempted. If the operation is part of a user interaction, the retry interval should be short and only a few retries attempted to avoid making users wait for a response. If the operation is part of a long running workflow, where cancelling or restarting the workflow is expensive or time-consuming, it's appropriate to wait longer between attempts and to retry more times.

If a request still fails after a number of retries, it's better for the app to prevent further requests going to the same resource and to report a failure. Then, after a set period, the app can make one or more requests to the resource to see if they're successful. For more information, see Circuit Breaker Pattern.

The eShopOnContainers mobile app does not currently implement the retry pattern when making RESTful web requests. However, the CachedImage control, provided by the FFImageLoading library supports transient fault handling by retrying image loading. If image loading fails, further attempts will be made. The number of attempts is specified by the RetryCount property, and retries will occur after a delay specified by the RetryDelay property. If these property values aren't explicitly set, their default values are applied – 3 for the RetryCount property, and 250ms for the RetryDelay property. For more information about the CachedImage control, see Caching Images.

The eShopOnContainers reference application does implement the retry pattern. For more information, including a discussion of how to combine the retry pattern with the HttpClient class, see .NET Microservices: Architecture for Containerized .NET Applications.

For more information about the retry pattern, see the Retry pattern.

Circuit Breaker Pattern

In some situations, faults can occur due to anticipated events that take longer to fix. These faults can range from a partial loss of connectivity to the complete failure of a service. In these situations, it's pointless for an app to retry an operation that's unlikely to succeed, and instead should accept that the operation has failed and handle this failure accordingly.

The circuit breaker pattern can prevent an app from repeatedly trying to execute an operation that's likely to fail, while also enabling the app to detect whether the fault has been resolved.

A circuit breaker acts as a proxy for operations that might fail. The proxy should monitor the number of recent failures that have occurred, and use this information to decide whether to allow the operation to proceed, or to return an exception immediately.

The eShopOnContainers mobile app does not currently implement the circuit breaker pattern. However, the eShopOnContainers does. For more information, see .NET Microservices: Architecture for Containerized .NET Applications.

For more information about the circuit breaker pattern, see the Circuit Breaker pattern.

Summary

Many modern web-based solutions make use of web services, hosted by web servers, to provide functionality for remote client applications. The operations that a web service exposes constitute a web API, and client apps should be able to utilize the web API without knowing how the data or operations that the API exposes are implemented.

The performance of an app can be improved by caching frequently accessed data to fast storage that's located close to the app. Apps can implement read-through caching with the cache-aside pattern. This pattern determines whether the item is currently in the cache. If the item isn't in the cache, it's read from the data store and added to the cache.

When communicating with web APIs, apps must be sensitive to transient faults. Transient faults include the momentary loss of network connectivity to services, the temporary unavailability of a service, or timeouts that arise when a service is busy. These faults are often self-correcting, and if the action is repeated after a suitable delay, then it's likely to succeed. Therefore, apps should wrap all attempts to access a web API in code that implements a transient fault handling mechanism.

Related Links

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• eShopOnContainers (GitHub) (sample)