REST APIs and restaurants [Take II*]

[Level C3]

 - "... and sauteed with the finest aromatic saffron - we import directly from our suppliers in Tibet - and garnished with the finest quality ocra from the heart of Morocco and ..."

- : ".. hmmm, bu..."

- : " and Sir, the King Shrimps day fresh from our suppliers in Belgium mildly cooked with a hint of lime zest and aromatic Thai basil snuggled with a swirl of Cornish Creme Fraiche and blossoms of Burmese jasmine and ..."

- : "... I mean, ..."

- : " and I should remind you that our head chef has used a special blend of exotic spices, marinated for 42 minutes exact, to create a sensual oriental effect of ... "

- : "I REALLY DON'T CARE! This stuff is awful!"

⚑  ⚑  ⚑

In more than one way a REST API is similar to a restaurant. They both hide a very complex process of accomplishing a service (serving food or data) behind a nice, warm and welcoming presentation. And we need to bear in mind that presentation is the key: no matter what goes on behind the scene in the kitchen, the food and its presentation is the key. And your API is the menu, the door to the services provided by the server. Whatever goes on in the kitchen (your server's internal domain) is not the concern of your customers as long as the output and presentation is good.

Commonly, the perspective in which we look at our services is wrong. They are server-centric. [I have talked before about the definition of server (and client) and their interaction] In brief, when we talk about the server, this is what comes in mind (as per CSDS):

This view is useful when building a server. It highlights inherent complexity of the server and how it should hide its complexity behind its API. But it is way too easy to get attached to the bird's eye view of the server where all internals of the server is visible.

And it is too easy to forget that this is how the client sees the server:

Client perspective of a server, where it sees a black box (here it's blue!) and
the gateway to the server is through its API.

When talking of the domain in an API, we are very likely to mean the server's private domain - its internals and complex process. Reality is server's internal domain does not surface beyond the API layer. API Layer is responsible to define resources which in turn define a coherent public domain composed of API and its messages - abstracted away from server's internal domain. If the server is an online retail API, I can create, edit or cancel orders. The fact that this is achieved through contact to many suppliers and their own custom processes is not a concern of the client.

In fact, in the same way that kitchen of a restaurant and its dining area are completely abstracted away, server's public domain is an abstraction on the top of server's internal domain. This is good for the chef and the customer. It is true the food that is served comes out of the kitchen but the whole process of preparation is irrelevant for the customer. 

Why talking of server's public domain is important? Because we commonly look down on the public domain. When we talk of the DTO messages passed to and fro, we regard them as View Models so they are not even proper models, they are just DTOs, no behaviour just some property bag. We use a mapper to strip down the sensitive data from our real models and spit out the view models to the client. And when we talk about DDD, we only mean server - we think view models do not deserve the love of care of DDD.

But this results in a brittle API that has to be changed by every change of the server. It becomes a cut down version of the server's internal API. We all know examples of good and bad API but too many changes in an API is a sign of bad design usually from Server Chauvinism. So always do a separate DDD for your server's public domain. It might sound strange since I am in favour of no-business-logic-in-api-layer. Well, it is true - we do not implement any business logic in the API Layer but the biggest responsibility of API Layer is talking HTTP. This translates to defining resources in REST.

So practice DDD in your API Layer. Define its aggregate roots, entities and value types - it is simplistic to think such concepts can be entirely hidden from the clients. Specify the commands and queries - with commands usually being the payload of the PUT, DELETE and POST and queries usually expressed in the URL itself to make them cache friendly.

What we see in the above picture define server's public domain: it is API (set of resources), input messages (commands and queries) and output messages (entities and value types). The case of queries - as we said - is a bit special since it is normally expressed in URL. This has the benefit of cacheability in contract with sending a payload (using other methods) which not suitable for caching. As an example, here is twitter's API for getting status timelines:

Get /1.1/statuses/user_timeline.json?screen_name=twitterapi&count=2

Defining parameters as query strings is useful as they can also imply optionality. One downside to this approach is the fact that ordering of parameters cannot be expressed in query string and while any combination can be valid, semantically equal combinations will be considered different resources and cached separately. For example, screen_name=twitterapi&count=2 and screen_name=twitterapi&count=2 are semantically equal while they are considered different resources. This reduces effectiveness of caching for GET requests. For this reason, URL segments separated by / are superior although in a big API can result in prohibitive complexity of routing.

Items returned in this API are Entities. it is evident by having an ID (id_str) which is returned as part of data.

Looking at what each timeline contains, we can see list of urls, where each url returned is a Value Type. It has three properties: url, indices and expanded_url.

In the case of commands, we have a similar situation. Looking at an example in twitter, tweeting (sending a tweet) is achieved using this command:

POST /1.1/statuses/update.json

status=Maybe%20he%27ll%20finally%20find%20his%20keys.%20%23peterfalk

Perhaps a better implementation would have been to omit the update.json totally - as this naming has an RPC smell. Using singular or plural is mainly down to taste - although I prefer singular. The command can be expressed by the simple class below:

Not all commands have to have a payload. Deleting a tweet can get send an ID in the URL with the DELETE command to /1.1/statuses/update.json although actual twitter API uses a POST instead.

Resources are basically all that can be accessed through the API although a more useful definition is the grouping around identities. For example Flickr API defines cameras, people, photos, photo comments, etc. It is true that Flickr is not considered a good example of a REST API when it comes to HTTP semantics, however, it does a good job in defining a coherent domain represented by resources while hiding its internals. A simple example is how it sends you previous and next photo instead of letting the client guess what these are based on the internal implementations.

In brief, it is responsibility of the API to create a coherent set of resources that can layout a cohesive picture in front of the client.

Common Anti-Patterns

As we said, the main danger is to have a server-centric perspective and neglect the public domain of your API.

While performing appropriate-sized DDD exercise is important, we need to be careful not expose too much of the server. One common anti-pattern is exposing server's internal to outside world. OData is a RESTful protocol which defines semantics for querying (mainly tabular) data store.

GET /api/products?$filter=name eq 'Gizmo 3'&$orderby=price desc&$top=1

While OData itself is not necessarily prescribing exposing server's internals, most of the implementations are actually built straight on the top of an RDBMS exposing its internal schema and and potentially allowing ad-hoc changes using SQL-like commands rather than domain-rich commands. Netflix who initially warmed up to the idea of exposing its films database to the public, reached the conclusion this is not such a RESTful idea and retired its OData service.

Another issue is the server-centric design: server assuming how its services will be used. I believe (Sorry Roy!) HATEOAS touches the subjects discussed here. When we talk of Hypermedia As The Engine OF Application State, which application do we mean? OK, let's consider Twitter's API. How can you think same hypermedia can equally drive the state of a twitter client, a game that posts scores on twitter, 4Square app that declares you a mayor on twitter or a twitter realtime map showing location of tweets using the firehose data pipe? The relationship from clients to servers are not one-to-one any more and effectiveness of HATEOAS is questionable. Application is defined by the client and not the server, as such server's hypermedia cannot be the engine of the application.

At the end of the day, none of us want to sound like the pompous chef who bores the customers with self-flattery. Server serves, so design it for servant-hood.


* this post was originally posted slightly differently under the name: "Server Chauvinism - API's public domain"

CacheCow update - Moving to MemoryCache for in-memory stores on both Client and Server

As you might know, CacheCow is a framework that implements HTTP Caching for ASP.NET Web API both for the client and server. If you are familiar with it, feel free to jump to the section "What's the update?".

HTTP Caching defined by HTTP Specification (currently at version 1.1 according to RFC 2616 although the work HTTP 2.0 is very close to finish) is an important feature of HTTP resulting in scalability of web as we know it.

It is important to remember that resources are cached on the client, and not on the server. In ASP.NET we can use Output Caching and System.Web.Caching.Cache to cache items on the Server but HTTP caching is different in the fact that resources get stored on the client. A resource is

• either not cacheable (identified by no-store value in the Cache-Control header)
• or can be only cached by the client (identified by private value in the Cache-Control header)
• or can be cached by the client and all intermediaries (identified by public value in the Cache-Control header)

Currently CacheCow.Client looks after the storage of the items in an implementation of ICacheSore interface. Currently these implementations exist:

• In-memory
• Memcached
• Redis
• SQL Server
• File-based

On the other hand, server also needs to store cache metadata. This is normally information such as Last-Modified and ETag of the resources. You configure CacheCow.Server to use one of several implementations of IEntityTagStore on the server. Currently these implementations exist:

• In-memory
• Memcached
• RavenDB
• MongoDB
• SQL Server

So what is the change?

In the latest release of CacheCow which stands at 0.4.12, in-memory implementation of ICacheStore and IEntityTagStore have been changed from ConcurrentDictionary<TKey, TValue> based to MemoryCache based. The problem with dictionary-based implementation is that the store just grows and the items will never be freed. MemoryCache, on the other hand, is designed to be able to keep its memory usage to a threshold and expel old or least frequently used items out of the cache. 

To use in-memory stores, all you have to do is to use the CachingHandler with default constructor which results in-memory stores to be used, as they are default. The good thing with these implementations are ability to set a maximum memory limit. This is achieved using app.config to web.config of your application:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <system.runtime.caching>
    <memoryCache>
      <namedCaches>
        <add name="TheName" cacheMemoryLimitMegabytes="40" pollingInterval="00:05:00" />
      </namedCaches>
    </memoryCache>
  </system.runtime.caching>
</configuration>

Above configuration will set the memory usage limit to 40MB and this limit will be checked every 5 minutes. CacheCow uses these names for its MemoryCache objects:

• CacheCow.Client: "###InMemoryCacheStore_###"
• CacheCow.Server uses 2 MemoryCache objects:
• Storing ETag and LastModified: "###_InMemoryEntityTagStore_ETag_###"
• Storing route patterns: "###_InMemoryEntityTagStore_RoutePattern_###"

You can use the configuration to limit the memory used by these MemoryCache objects. Since MemoryCache is in-memory and is in the same AppDomain as the application, it provides the fastest and most efficient storage.

Use in-memory storage wherever you can and surely it provides better performance compared to the likes of SQL Server.

The only caveat with using these implementations is that even though you may set a memory limit, memory can increase above your threshold. This is not an issue and is related to garbage collection and using GC.Collect() the memory will return back to the actual usage.

API Layer and its responsibilities - a REST viewpoint

[Level C4] With the emergence and popularity of REST APIs, there is a need to define a layer dedicated to the performing API-related functions. This article tries to define these responsibilities and separate it from the responsibility of other layers.

TLDR; If you are putting business logic of your domain in your API Layer (which is the presentation layer of your service), you are doing it wrong.

Layered (Tiered or N-Tiered) Architecture is a software design pattern that implements a software system in a stack of layers. Each layer has a distinct responsibility and has a dependency upon the deeper layer - with the deepest layer normally accessing a relational database.

This pattern, also known as the Layers pattern, was formalised in the book Pattern-Oriented Software Architecture: Layers pattern "helps to structure applications that can be
decomposed into groups of subtasks in which each group of subtasks is at a particular level of abstraction." This pattern emerged as a replacement to the Client-Server architecture and became very popular in late 90s and early 2000s.

Service Oriented Architecture (SOA) did not change the popularity and importance of tiered architecture since each service contains the tiers inside it (Figure 1)

Figure 1 - Layered Architecture in SOA

Layers are usually domain-agnostic and can be uniquely identified across different implementations and systems. The layers themselves are not necessarily physical, i.e. deployed on different machines; they can be simply logical implementations in different libraries - or even in the same library. What identifies a layer is its responsibility not its physical location.

There are different approaches in identifying and naming layers across a system stack. Historically, three tiers have been identified: Presentation Layer, Business Layer and Data Access Layer (Figure 2).

Figure 2 - Traditional Layers

This traditional layering is still prevalent and popular in the industry. As the names imply:

• Presentation Layer deals with user interactions
• Business Layer contains the business logic of the domain
• Data Access Layer is concerned with the persistence of the domain objects

Sometimes a Service Façade Layer is identified between Business Layer and Presentation Layer which is responsible to organise the business services as a coarsely granular set of services. This layer is usually very thin.

However, Eric Evans in his Domain Driven Design book introduced an alternative layering which was a better fit for DDD: Presentation Layer, Application Layer, Domain Layer and Infrastructure Layer. (Figure 3)

Figure 3 - Layers according to DDD (two alternative interpretations)

In this model, responsibilities of each layer is defined as below:

• Presentation Layer has the same responsibility as the traditional model: user interaction (user could equally be a machine or human)
• Application Layer which I prefer to call Workflow Layer is the thin layer that connects and coordinates coarse granular business tasks. This layer can have state regarding the state of a business activity. 
• Domain Layer is the heart of the system which contains all the business logic
• Infrastructure Layer is responsible for persistence, passing messages to buses and providing a framework and utility across other layers. As such, all layers could have access to this layer. 

API Layer

In none of the above we saw any mention of an API Layer, so what is an API Layer? API Layer is the outermost layer of a service and is concerned with presentation of a bounded context services through the API. In other words, in an SOA, we can call the Presentation Layer of an SOA Services its API Layer.

This layer was very thin or non-existent in services exposed through SOAP but it is an important layer when it comes to REST. The reason for its lack of importance in SOAP services is the fact that an RPC service can be easily exposed through SOAP with some configuration and little or no coding - using various tools and frameworks that make this possible with a click of a button. WCF is an example of this in the Microsoft stack while IBM Websphere is a popular tool in the Java world capable of achieving this.

API Layer Responsibilities

With REST, however, API requires a distinct layer that is responsible for translating the HTTP semantics to and from the code world. It also responsible for cross-cutting concerns such as monitoring, logging, identity, etc. We will be looking into each and expanding the meaning and examples.

Table 1 - API Layer responsibilities (in REST)

Bear in mind, none of the above involves any domain-related business logic. In other words, API Layer can use a common framework that can be equally used among different services since it is mainly domain-agnostic and does not implement any.business logic. As such, this IBM's online article (Figure 4) turns into How Not To Do API Layer since it puts Domain classes in the API Layer.

Figure 4 - This is IBM's view of an API Layer according to this article.
However, I will call it How Not To Design An API LAYER.
(Source: http://www.ibm.com/developerworks/library/j-ts3/layers.gif)

So let's look into each responsibility in depth. But before we do that it is important to define what we mean by no-business-logic. Since API Layer has an access to underlying layers, it is bound to touch the higher level abstractions of the domain. However it does not implement any of the business logic and simply directs the calls to the layers below.

HTTP Semantics

Talking HTTP is the most important responsibility of a REST API. HTTP provides different axis of sending intent or data which cannot be easily mapped to an RPC method's parameters. This problem is known as the impedance mismatch between RPC and HTTP.

Figure 5 - Impedance mismatch between HTTP/REST and RPC.
Challenge of shaping HTTP requests and responses from RPC methods
is the most important responsibility of the API Layer. (From the book Pro ASP.NET Web API)

Different frameworks provide different solutions to this problem but it is important to bear in mind that exposing a REST endpoint for your service means you need to a lot more to take advantage of all the flexibilities of HTTP messages.

Table 2 lists important HTTP semantics that are responsibility of a REST API Layer.

Table 2 - HTTP Semantics that are responsibility of a REST API Layer

A few points are important to consider. While, none of above are truly dependent on a domain's business logic, it is possible that it could require business decision in some form of configuration. For example, cache expiry interval is usually a concern that the business stakeholders need to understand and happy with. Or Hypermedia exposes the relationship of resources in your API but the API Layer should really just take the natural relationship defined in your domain and expose in HTTP.

Monitoring

If you have an API that you do not monitor its health, performance and usage, then this is not a serious API. Bear in mind that Quality of Service (QoS) is one of the mainstays of an API which can be translated into SLA. Regardless of whether your API is public or private, you have to monitor your API against its baseline.

Identity and Authentication

Very rarely, if ever, an API allows anonymous clients. Even if your API is free, you should build an identity mechanism such as an API Key. API Layer is responsible for turning identity/authentication mechanisms (Basic Authentication, OAuth, OAuth 2.0, Windows Authentication) into rich claim-based identity.

Please note that we did not mention Authorization. Authorization is a concern of deeper layers of the system and fully ingrained with Business Logic - as such is not suited for implementation in the API Layer.

Logging, tracing and documentation

Developing against an API is not an easy task and you should make it very easy for the clients of your API to be able to build, debug and deploy. This is most neglected feature of an API that can endanger success of your public API or private API within a big enterprise.

Monetisation and Capping?

Similar to Authorization, I am on the belief that monetisation and usage capping of an API is heavily dependent on business logic and information that are available in deeper layers. As such, I believe that monetisation and usage capping of an API must be implemented in deeper layers and not in the API Layer.

What about the relationship of the API with Server-side generated views?

A common question that crops up in the stackoverflow is how to lay out the architecture of web applications that historically were based around Server-side MVC Frameworks. For example, should a RoR web application use a Ruby-based API layer? Or if you are in the Microsoft world, should an ASP.NET MVC application use ASP.NET Web API services?

Well it depends. API Layer and Server-side MVC Frameworks both sit at the Presentation Layer. So you should not really add the networking and serialisation overhead of accessing an API Layer from Server-side MVC Frameworks if they are part of the same application. As we said, API Layer is a thin layer that is only responsible with the API centric responsibilities. In most cases, MVC Frameworks should directly use deeper layers of the system and sit side-by-side with the API Layer. However, if API Layer is the presentation layer of an SOA service and the MVC Framework uses multiple APIs to build the views, then it is OK to separate it from the API Layer.

Beware of undisposed or unconsumed HttpResponseMessage

[Level T2] This is just a short post to bring something important to the reader's attention. I guess if you are well into ASP.NET Web API, you are bound to see it - especially if you are consuming APIs using HttpClient.

Let's look at this innocent looking delegating handler (useful on the client side):

public class InnocentLookingDelegatingHandler : DelegatingHandler
{

    protected override Task SendAsync(HttpRequestMessage request, 
        CancellationToken cancellationToken)
    {
        return base.SendAsync(request, cancellationToken)
            .ContinueWith(t =>
                {
                    var response = t.Result;
                    if (response.StatusCode == HttpStatusCode.NotModified)
                    {
                        var cachedResponse = request.CreateResponse(HttpStatusCode.OK);
                        cachedResponse.Content = new StringContent("Just for display purposes");
                        return cachedResponse;
                    }
                    else
                    return response;
                });
    }   
}

There isn't really a lot happening. This delegating handler sniffs the incoming responses and if it intercepts a 304 response, it replaces this with a cached response - of course the implementation is a minimal one just for display purposes.

Now let's use it with a resource which returns 304:

class Program
{
    static void Main(string[] args)
    {
        var client = new HttpClient(new InnocentLookingDelegatingHandler()
                        {
                            InnerHandler = new HttpClientHandler()
                        });
        for (int i = 0; i < 1000 * 1000; i++)
        {
            var request = new HttpRequestMessage(HttpMethod.Get, 
                "http://ajax.googleapis.com/ajax/libs/angularjs/1.0.7/angular.min.js");
            request.Headers.IfModifiedSince = DateTimeOffset.Now;
            var response = client.SendAsync(request).Result;
            response.Dispose();
            Console.Write("\r" + i);
        }
            
    }
}

You can find a ready project to test this on GitHub. Please note that System.Net's maxConnection needs to be set up in the app.config.

So what do we see?

1. Application starts to leak memory. 
2. After a while you get an error telling you have run out of sockets.

We can see the effect using SciTech tool:

Well, reason is the server response in the Task continuation is not getting consumed. What does it mean? Well, adding a line to consume the content is enough:

var cachedResponse = request.CreateResponse(HttpStatusCode.OK);
cachedResponse.Content = new StringContent("Just for display purposes");
response.Content.ReadAsByteArrayAsync().Result; // just read the content which is empty!!
return cachedResponse;

Or simply dispose the response:

var cachedResponse = request.CreateResponse(HttpStatusCode.OK);
cachedResponse.Content = new StringContent("Just for display purposes");
response.Dispose(); // Dispose the response
return cachedResponse;

This should be enough to solve the problem. Now one would think that we have got the response and 304 response actually would never have a content so it is silly to read the content - but well, until you hit this bug.

Thanks to Carl Duguay who reported the issue on CacheCow, the resultant memory leak on 304 responses is fixed now. I think it is very likely that you might run into similar problem so beware when getting a response - always consume or dispose it.

This brings the question that should we always dispose HttpRequestMessage and HttpResponseMessage? Recently, there has many classes that implement IDisposable but they do not require use of dispose pattern. Examples include HttpClient, Task or MemoryStream. On the other hand, you cannot find a single sample that either request or response is used in a Dispose pattern - and the fact that the API is fully async makes use of Dispose pattern very difficult.

In any case, ideas are welcome. But as for the bug we had in CacheCow, it is fixed now.