In my application, when a user is created, an Event called UserCreatedEvent is thrown.
This user must have all his description data retrieved from Facebook (with his authorization of course) and should be treated by some custom process.
I chose to apply eventual consistency (using Akka and his eventStream feature to handle events) between the fact of saving the user aggregate and the fact to retrieve all his description data and apply them, since it involves some call network to Facebook.
The point is I also allowed any newly created user to override some of the description (job, age etc.) through the REST api PUT /api/users/:id.
1) Should the process of handling UserCreatedEvent (basically an Akka actor) call this api (causing an indirection) to assign default user description (from Facebook) or should it retrieve and update user itself?
2) Should the process of handling UserCreatedEvent and the api aiming to manage User (CRUD) be part of the same bounded context? (in that case, no recommendation to involve the indirection).
I'm not sure I would call this eventual consistency, since consistency (making sure that an entity respects invariants) is not at stake. This is more like out-of-band asynchronous data completion.
What hoops this side process has to go through again basically depends on your context :
REST API if there's important stuff there you want to reapply (logging, security, whatever)
Application Service / Command handler layer if you want that data completion to be treated as a full blown modification with proper validation (e.g. did Facebook return data that would be unsupported in your application ?)
In the domain layer, you could model it either as just any other modification of the User, or as a specific, maybe more intention-revealing operation : User.completeDescriptionData(...)
I wouldn't necessarily place this in a different Bounded Context since it doesn't introduce a new definition of User. It's important that the part that knows the nitty gritty details of talking to Facebook is in a separate non domain layer though.
Related
Background
We have very strict auditing requirements and want to be able to correlate every action our system takes on behalf of the user to a specific authentication operation (sign-on). In addition to these strict auditing requirements, we also have some complex authorization requirements unsolvable by simple claims based authorization.
Considering both of these together led me to wondering the feasibility of using an 'envelope' type design, where messages sent stemming from a user request are wrapped in an envelope containing the necessary information, such as their auth token and info about the sending machine. Now, it would be fairly simple enough to add a property for this token to every message, but that seems tacky and since its a rather cross-cutting concern, I would rather it not pollute every protocol in the system, which is why I'm thinking the envelope idea is worth considering. This approach would also require the cooperation of every actor in the system and my goal is to have this be transparent to actors who don't need any of this information, but also make the information available in case actors do need it. In the case of actors needing it, it's also OK if they just accept the envelope type directly.
Imagined Solution
Overview
Wrap each Tell operation in an envelope used to transport required contextual information
Perhaps implemented w/ a custom actor ref provider and actor ref wrapping the ones configured
Unwrap envelope, if exists, on each receive operation
Custom mailbox
Would also handle sending a message to the auditing service
How to make the contextual information available to the actor?
Can we add to the actor's Context object somehow?
Also acceptable for actor to accept the envelope type/not use custom mailbox in this case
Discussion
In order to make this all transparent, my initial thinking is to 'intercept' the send/receive operations. I understand enough akka.net to implement a custom mailbox and I think this would be the way to go for this kind of approach, but I'm open to other ideas. The mailbox would perform the unwrap and make the contextual information available to the actor in case it's required (99% of the time it's not, likely better to just accept the envelope directly when it is required to be explicit). The mailbox would also handle fulfilling the auditing requirement by sending a message to the auditing service w/ the required information, which not only includes the contextual information from the request, but also local machine information to know where/who did the processing.
The part I'm second guessing myself on is intercepting the send operation (Tell). Since IActorRef instances are created via a configured IActorRefProvider and since this guy handles the Tell operation (via it's created IActorRef instances), I think it makes sense to write a custom IActorRefProvider and a custom IActorRef. Both would wrap the implementations that are configured (decorator pattern), and the custom IActorRef would provide the required behavior in it's Tell method. For webapi apps (only entry point for users), it would pull the required contextual information from HttpContext (one custom ref provider) and for backend apps (another custom ref provider), it would pull the required contextual information from the current message's context. I'm not sure how to add data to the actor's Context property, but I'm assuming it is possible.
With these two pieces in place, the contextual information would effectively be passed along, from actor to actor, and service to service. So even if a message is 20 actors down the line, if it was initially instigated by a user via the REST API, it would still have that contextual information, thereby allowing a full and complete audit and tracing of each action our system takes back to a specific sign on.
What I'm Hoping For
The primary thing I'm hoping for with this post is validation that this is a reasonable approach to take, and if not, why not and alternate suggestions for achieving the desired behavior. Also very welcomed would be any code samples for custom mailboxes/actor ref/actor ref providers and extra cookies if they're doing something similar to what I'm trying to accomplish here. Another welcomed tidbit is how to do the mailbox configuration so I don't need to manually update all of my Props with the custom mailbox implementation. Akka.net configuration is definitely a weaker point of mine, particularly the deployments section, so any core knowledge/articles/advice here is greatly appreciated!
Thanks for taking the time to read this! Any and all help is much appreciated!
Other StackOverflow Issues:
The answers provided in these issues require the cooperation of every actor. Ideally this is all transparent and actors that don't need to use this contextual information can be written as if it didn't exist.
Passing Contextual Information
How to elegantly log contextual information along with every message
There were a couple others I viewed [can't find them right now for some reason], but they all either required cooperation or global shared state [isn't that what akka avoids? :p]
Phobos, a proprietary observability library for Akka.NET, wraps all messages inside a distributed tracing context - which can be aggregated back together again in an off-the-shelf tracing system that supports OpenTracing, such as Jaeger / Zipkin / Azure Application Insights.
You can append custom data to each of the traces that are captured inside your actors via the Context.GetInstrumentation() method inside any of your actors' - custom data can include tags that might include a unique userId, a transaction Id, and so on. That's all part of the OpenTracing specification.
Disclosure: I work for Petabridge, the makers of Phobos. It's proprietary and costs money to use, but it's purpose built to offer this type of decentralized, but complete tracing out of the box.
Alternatively, if you didn't want to use Phobos you might be able to accomplish this using a custom messaging protocol for context propagation and structured logging with the Akka.Logger.Serilog library.
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Can we say Like Rest API we have SOAP, XML RPC, JSON RPC, GRAPH QL I mean Rest is just a type of API?
Is Rest is just a mechanism to share data between applications by using HTTP methods?
REST can share data between applications only with HTTP there is no alternative?
What is the relation between REST & CRUD exactly? we are saying HTTP: POST is CRUD: CREATE my question is HTTP: POST will just post the data to server and it's business logic's headache to CREATE A NEW RESOURCE in the server side but how we all are saying HTTP: POST is nothing but CRUD: CREATE here HTTP: POST is just helping to share the data only right then how it is related to CRUD: CREATE? If that is the case we can create a new resource with HTTP: GET by writing business logic right then why so many sites are saying REST is a mechanism to do CRUD operations... But it should be REST is a mechanism to help in Sharing the data between applications right?!!! (I have doubt similarly with HTTP: DELETE, GET, PUT aswell)
Last but not least... what exactly Representational state transfer mean? could you please answer this with a very low level general answer instead of definition.
Can we say Like Rest API we have SOAP, XML RPC, JSON RPC, GRAPH QL I mean Rest is just a type of API?
REST stands for representation state transfer and is just an architectural style, not a technology or protocol. According to Robert C. "Uncle Bob" Martin an architecture is about intent and the intention behind REST is the decoupling of servers and clients in a distributed system.
REST basically just defines a set of constraints that when followed correctly allow servers to change at any time without breaking clients as clients will just depend on the data given by the server and not on any external data or documentation. REST can be regarded as Web surfing for applications. The main premise should always be that a server teaches a client on how certain things can be achieved.
On the Web a server can i.e. teach a client on the supported properties of a resource through the help of HTML forms. Not only does a client learn that way what a server expects as input, it also learns what HTTP operation should be used to send the data to the server, the endpoint URI to send the request to as well as, usually implicitly given, also the media type to convert the input to, which is application/x-www-form-urlencoded usually by default, which transforms an input like the HTML example below for first and lastname to something like this:
fName=Roman&lName=Vottner
Is Rest is just a mechanism to share data between applications by using HTTP methods?
REST itself is protocol agnostic meaning that it is not tide to HTTP itself and could just work on other transport protocols as well. Though the common perception many developers have is that it is based on HTTP. After all, as Jim Webber put it, HTTP is just a transport layer whose domain is the transfer of files or data over the Web. All HTTP does is to send one document from one machine to the next and any business rules we conclude from sending/receiving a request are just a side effect of the actual document management. It is therefore always better to think of a request as a whole document and the HTTP operation define how the document should be stored on the current machine, especially when such a document is already available, instead of thinking of a service method invocations. The latter one is a typical RPC view.
REST can share data between applications only with HTTP there is no alternative?
HTTP is just a transport layer used in a REST architecture. The architecture cares more on the interaction model between client and servers than on the technical nuances of HTTP. As REST itself is transport protocol agnostic it could be used with other, maybe proprietary protocols as well.
What is the relation between REST & CRUD exactly? we are saying HTTP: POST is CRUD: CREATE my question is HTTP: POST will just post the data to server and it's business logic's headache to CREATE A NEW RESOURCE in the server side but how we all are saying HTTP: POST is nothing but CRUD: CREATE here HTTP: POST is just helping to share the data only right then how it is related to CRUD: CREATE? If that is the case we can create a new resource with HTTP: GET by writing business logic right then why so many sites are saying REST is a mechanism to do CRUD operations... But it should be REST is a mechanism to help in Sharing the data between applications right?!!! (I have doubt similarly with HTTP: DELETE, GET, PUT aswell)
Fielding's thesis on the REST architecture style does not contain the term CRUD at all. The term REST nowadays is heavily misused as people probably didn't bother to actually read the thesis, which admittingly is a bit abstract, and just follow what some people thought may be REST but was actually RPC. Nowadays, if a typical stakeholder talks about REST they usually think of a JSON-based HTTP CRUD-API whose supported endpoints are defined in some Web documentation (Swagger, OpenAPI, ...) and where the HTTP operations for POST (= Create), GET (= Read), PUT (= Update), Delete (= Delete) are supported by default. However, this is unfortunately far from the truth. Though people are just to accustomed with their (wrong) definition and don't see or don't care about the actual problem in their misusage. They don't care about a long-lasting service as in 2-5 years the next-gen technology is here that allows to reduce the number of lines of codes even more and if a new "version" of a service is needed, this usually goes hand in hand with a technology switch also, to justify the "cost of change" somehow.
Last but not least... what exactly Representational state transfer mean? could you please answer this with a very low level general answer instead of definition.
Probably the easiest way to grasp how the interaction in a REST architecture should be is by analyzing typical interaction on the Web, the big cousin of REST. You, as a user, usually start by opening your browser (= client) and typing in some URL in the search bar. Next a Web page is rendered on your screen. Behind the curtain a couple of things happened. Besides the whole connection management and any eventual TLS handshake your browser sent at least one GET request to the target server. On sending the GET request, the client included information on his capabilities, i.e. through the Accept HTTP header. This header is used on the server side to decide which representation format to generate and send to the user. On the Web this might be something like text/html or application/xhtml+xml or if some report is generated might be something like application/pdf or application/vnd.ms-excel or the like, depending which representation format fit the data best.
The representation format itself is now a concrete instance of a document following a certain media type specification. I.e. the HTML forms specification defines the supported elements within a <form>...</form> tag pair as well as describes the semantics of each of the elements. The concrete instance may now define a form as such:
<form action="/action_page.php" method="get">
<label for="fname">First name:</label>
<input type="text" id="fname" name="fname"><br><br>
<label for="lname">Last name:</label>
<input type="text" id="lname" name="lname"><br><br>
<input type="submit" value="Submit">
</form>
which should only use elements and attributes defined within the specification else a receiver of that document might not be able to process it correctly.
This process of telling the server which document types a client supports and allowing the server to chose a fitting representation is called content type negotiation and allows the exchange of arbitrary, type-less documents. Of course, both parties need to support and understand at least one common media type to be able to interact with each other. This is similar to a Frenchmen who does not understand a word of Chinese and a Chinese one that does not understand any word of French who need to communicate (for whatever reason), if both speak English they will be able to communicate.
There are loads of different media types already available that all server different purposes. Depending on your needs an all-purpose one, such as text/html, might be sufficient, others such as application/json or even application/hal+json might though lack support for certain needed elements. Existing media types might not support needed elements. In such a case extending such media types and registering those is probably easier than creating a whole media type from scratch.
REST assumes that a resource (i.e. a remote document) contains some internal data, its state. This state can be represented in many different ways. Think of some monthly sails figures. You might ship the data either in a HTML table, as CSV data, as Excel file, as PDF or yet a different representation format. Regardless of the chosen media type to marshal the data in, the actual data at least should express the same. Instead of questioning which media type you want to support, you should better ask how many different ones you want to support as this just increases the likelihood that other clients may interact with your server also.
edit:
I got all the points except 4th point... Could you please elaborate a bit.. So is it just a mechanism that helps in sharing the data between applications by using HTTP methods? we can say like that?
CRUD is a typical term in the context of persistence, especially with databases. REST or more formally the REST architecture itself treats persistence as internal detail. A typical user usually does not care whether some data is persisted into a DB, a local file system or is kept just in memory. All s/he cares is that the server can process it or for storage services also return the same data that was sent to the server.
In regards to the mapping of CRUD to the HTTP operations, if you take a look at HTML you might see that it only supports POST and GET operations. So anything related to C, U or D are performed with POST which is defined to process the enclosed representation according to the resource's own semantic. With POST you are basically allowed to do anything, even retrieving data if you like.
However, HTTP defines certain properties for the respective operations:
safe
idempotent
cacheable
The first property is a promise to clients that a well-behaved server should not alter the state of a resource upon requesting. The second one is a promise in regards to automatic retry attempts caused by i.e. temporary connection issues. And the latter one allows clients to store responses locally and reuse these instead of requesting the same resource again, if the cached content is "fresh-enough".
GET and HEAD are both safe, idempotent and cacheable, meaning that a client can request resources with such an operation without being hold accountable for any eventual changes. Think of a Web spider that is invoking arbitrary URIs all the time to learn new pages over time. If a GET request on a URI would trigger an order of a Pizza or the like, it is basically the server's fault and not the clients one if a crawler would order Pizzas every time such URIs are called.
PUT and DELETE are only idempotent, which basically allows a client to automatically resend a request in case of a network issue as the outcome of the operation leads to the same result regardless whether the request was processed once or multiple times in a row. Note that this property does not consider changes done by other clients to that resource between a resend. Such data would of course be overwritten.
The remaining operations (POST, PATCH, CONNECT, TRACE) are neither of these.
While technically it is sufficient to only use POST for each request, the above mentioned properties should trigger an inner intention to use them, when appropriate. However, as before mentioned, not the client should chose which operation to perform but the server should tell a client which HTTP operation it should use.
In regards to POST vs. PUT, both operations should behave similar on creating a resource. Both need to add a Location header within the response that teaches a client about the location of the new resource. PUT however, in contrast to POST, replaces the current representation of the requested resource with the one provided in the request body. So it already targets the respective resource while for a POST request the server defines where the resource is created. It is allowed to perform certain sanity checks and also to transform the representation to fit the representation format of the current one. It is also allowed to have side effects, i.e. think of Git where a commit creates a new entry on top of the current branch and moves the HEAD to the new commit.
PUT is probably considered as update operation as the replacement of the document more or less has the effect of an update. If no representation was yet available this just has the effect of the creation (including the location header). In the past, unfortunately, many developers used PUT incorrectly by performing a partial update instead of really replacing the whole document. While the spec states that a partial update could be achieved by overlapping resources (i.e. share parts of the same data in multiple resources), the usage of PATCH, which also is used incorrectly most of the time, may be better from a performance standpoint on larger resources.
Due to POSTs definition, one can do anything with it, though historically a document upload in HTML was triggered through this operation that is basically a resource creation on the server side. That POST is used for many other things as well though is not that important for that CRUD paradigm.
In regards to your concerns about the right terminology, most people, according to my experience, simply do not care. They just want to get the job done ASAP and move on. As roughly 90% of the users seem to understand a pretty similar concept when talking about REST (even though this view is flawed) which usually resolves around JSON, HTTP, CRUD, Swagger/OpenAPI, ... they usually only look for quick-win-solutions and more or less agreement on their thought process.
As HTTP (0.9-1.1) is a plain text protocol sending a GET request is not much different from a POST or PUT request, so technically you can create resources with GET request or support payloads on GET requests (semantics of the payload is undefined according to the spec). That's why I mentioned well-behaved client/servers above. In such a case, however, due to the safe property of GET, if you as a server maintainer violate the HTTP protocol you are the one to blame in case something "unexpected" is happening (crawler is ordering 500 Pizzas).
I'm designing a restful web service and I was wondering what should I name my DTOs. Can I use suffixes like Request and Response for them? for example for addUser service, there will be 2 DTOs named: AddUserRequest and AddUserResponse.
Does your organization already have a schema that describes a canonical user that you pass in? If that's what you're using, of course you would use the name from that schema. Otherwise, describe them just as you would any class or schema element.
Note that since a DTO doesn't contain its own methods, you probably would not give it a name with an action verb.
However, consider calling them AddUserRequest and AddUserResponse, especially if the method requires more info than just your regular user DTO. This fits with the Interface Segregation Principle in that your interface parameters should be specifically tailored to the request itself (it shouldn't require elements that are unrelated to the request; and you shouldn't have function-type parameters that change the request, those should be extracted into their own calls.) The AddUserRequest might then contain an element called User that holds the user-specific data, and another element holding the set of other associated data on the request, perhaps groups or access permissions, that sort of thing.
DTOs (Data Transfer Object) are like POJOs(Plain Old Java Objects). It should only have getters and setters and not any business logic.
From Wikepedia:
A data transfer object is an object that carries data between
processes. The motivation for its use is that communication between
processes is usually done resorting to remote interfaces (e.g., web
services), where each call is an expensive operation. Because the
majority of the cost of each call is related to the round-trip time
between the client and the server, one way of reducing the number of
calls is to use an object (the DTO) that aggregates the data that
would have been transferred by the several calls, but that is served
by one call only.
The difference between data transfer objects and business objects or
data access objects is that a DTO does not have any behavior except
for storage and retrieval of its own data (mutators and accessors).
DTOs are simple objects that should not contain any business logic
that would require testing.
This pattern is often incorrectly used outside of remote interfaces.
This has triggered a response from its author[3] where he reiterates
that the whole purpose of DTOs is to shift data in expensive remote
calls.
So ideally for those actions you should create some helpers or you can add those as controllers.
Since it is a RESTful service, ideally the user addition/creation request should send back 201 created HTTP status code , with userId in location header and no response body. For the request, you could name it like UserDetails or UserData or simply User. Refer https://pontus.ullgren.com/view/Return_Location_header_after_resource_creation
This is a generic design question, but where should the responsibility fall in this situation? Should it be the caller's responsibility to check if a record already exists and then call Update? Or should it be the responsibility of the API to make that decision?
In the first scenario, the problem is that the caller is burdened with the business logic, but in the second scenario, the logic pollutes the API and creates hybrid behavior, violating the separation of concerns principle.
Implementing a CreateOrUpdate endpoint will be breaking some REST principles, but may be convenient for the application developer. You are thinking in terms for a remote function call rather than in terms of resource-oriented API.
Consider this: the API URL identifies the resource.
If the URL points to a collection (i.e. /customers/) then the Create action (typically mapped to the POST method) certainly makes sense. The Update function might makes sense, if you want allow update to multiple resources at one. The POST should return code 201 and an identifier to a newly created resource (i.e. /customers/1); or if the create failed due to resource already existing it should return code 409; 400 if some other constraints like data validation are not met.
If the URL points to an existing resource (i.e. /customers/id/1) then Create action does not make sense and should result in code 400. The update is typically mapped to the PUT method (or sometiemes PATCH, if partial resource update) and generally would return 200 if the update was successful or 4xx series if not.
If you choose to create a /CreateOrUpdate endpoint, which takes POST requests you will have to design your own protocol around it because its behavior and return values will be different depending on circumstance.
#Evert the PUT can be used for create but only when you require client to formulate the endpoint URI with the identifier i.e.
PUT /users/myusername
Problems with that are:
the client must discover an available one,
if a natural identifier is used, there may also exist a natural reason for changing it, which depending on the implementation may be problematic
The main point I am making is to avoid creating REST API endpoints, which represent an action (function). Instead use HTTP methods to effect respective actions on persisted resources.
We have set up an architecture based on DDD and CQRS. Additionally we have a restful API with an OAUTH implementation for our clients to connect to.
Our clients connect to our API and perform operations on behalf of their clients. Their clients are represented by profiles on our side.
We don't have a good solution for the following problem. Clients are able to create a profile by calling a method on our API. The problem is that we need to guarantee the uniqueness of the profiles. So what we currently do is check for an existing profile in the read model, create a command if it doesn't exist and return the profile ID back to the client so they can perform other API calls.
When a client performs multiple calls in rapid succession, a profile is created twice instead of once due to an out of date read model. We don't want that, but how do we resolve this issue?
We have thought about creating a saga to prevent more than one profile being created in the domain, but that is still problematic because we need to return the same profile ID to the client if their request is the same.
Any thoughts?
Commands are not supposed to return results.
What you can do is create a command that includes the ID of the new profile, if it is a GUID. If you are using a seeded identity column of some sort, of course this won't work.
But say that your ID is a GUID. Then you can pass a GUID in the command to the back end. The back end will create the new profile only if the GUID doesn't already exist -- and you have guaranteed unicity.
From what I understand from the CQRS pattern, the command layer should not make use of the read model to take any decision. The command layer do its processing based on the domain it self. Not based ont he read model. Validation is always made ont the domain data.
You profil creation command handler should check for the pre existence of the profil in the domain and not in the read model.
That's correct. Command should not rely on ReadModel, because of Eventually Consistent principle of ReadModel.
Just use your Domain in commands to make decision based on it.
Usually CQRS + EventSourcing repositories have very few methods, but on of them is GetById(Guid id). You can use it to check if such entity is already present in domain.