I have been recently looking into event sourcing and have some questions about the interactions with clients.
So event-sourcing sounds great. decoupling all your microservices, keeping your information in immutable events and formulating a stored states off of that to fit your needs is really handy. Having event propagate through your system/services and reacting to events in their own way is all fine.
The issue i am having lies with understanding the client interaction.
So you want clients to interact with the system, but they need to do this now by events. They can not longer submit a state to mutate your existing one.
So the question is how do clients fire off specific event and interact with (not only an event based system) but a system based on event sourcing.
My understanding is that you no longer use the rest api as resources (which you can get, update, delete, etc.. handling them as a resource), but you instead post to an endpoint as an event.
So how do these endpoint work?
my second question is how does the user get responses back?
for instance lets say we have an event to place an order.
your going to fire off an event an its going to do its thing. Again my understanding is that you dont now validate the request, e.g. checking if the user ordering the order has enough money, but instead fire it to be place and it will be handled in the system.
e.g. it will not be
- order placed
- this will be picked up by the pricing service and it will either fire an reserved money or money exceeded event based on if the user can afford it.
- The order service will then listen for those and then mark the order as denied or not enough credit.
So because this is a async process and the user has fired and forgotten, how do you then show the user it has either failed or succeeded? do you show them an order confirmation page with the order status as it is (even if its pending)
or do you poll it until it changes (web sockets or something).
I'm sorry if a lot of this is all nonsense, I am still learning about this architecture and am very much in the mindset of a monolith with REST responses.
Any help would be appreciated.
The issue i am having lies with understanding the client interaction.
Some of the issue may be understanding, but I promise you a fair share of the issue is that the literature sucks.
In particular, the word "Event" gets re-used a lot of different ways. If you aren't paying very careful attention to which meaning is being used, you are going to get knotted.
Event Sourcing is really about persistence - how does a micro-server store its private copy of state for later re-use? Instead of destructively overwriting our previous state, we write new information that links back to the previous state. If you imagine each microservice storing each change of state as a commit in its own git repository, you are in the right ballpark.
That's a different animal from using Event Messages to communicate information between one microservice and another.
There's some obvious overlap, of course, because the one message that you are likely to share with other microservices is "I just changed state".
So how do these endpoint work?
The same way that web forms do. I send you a representation of a form, the client displays the form to you. You fill in your data and submit the form, the client processes the contents of the form, and sends back to me an HTTP request with a "FormSubmitted" event in the message body.
You can achieve similar results by sending new representations of the state, but its a bit error prone to strip away the semantic intent and then try to guess it again on the server. So you are more likely to instead see task based user interfaces, or protocols that clearly identify the semantics of the change.
When the outside world is the authority for some piece of data (a shopper's shipping address, for example), you are more likely to see the more traditional "just edit the existing representation" approach.
So because this is a async process and the user has fired and forgotten, how do you then show the user it has either failed or succeeded?
Fire and forget really doesn't work for a distributed protocol on an unreliable network. In most cases, at-least-once delivery is important, so Fire until verified is the more common option. The initial acknowledgement of the message might be something like 202 Accepted -- "We received your message, we wrote it down, here's our current progress, here are some links you can fetch for progress reports".
It doesnt seem to me that event-sourcing fits with the traditional REST model where you CRUD a resource.
Jim Webber's 2011 talk may help to prune away the noise. A REST API is a disguise that your domain model wears; you exchange messages about manipulating resources, and as a side effect your domain model does useful work.
One way you could do this that would look more "traditional" is to work with representations of the event stream. I do a GET /08ff2ec9-a9ad-4be2-9793-18e232dbe615 and it returns me a representation of a list of events. I append a new event onto the end of that list, and PUT /08ff2ec9-a9ad-4be2-9793-18e232dbe615, and interesting side effects happen. Or perhaps I instead create a patch document that describes my change, and PATCH /08ff2ec9-a9ad-4be2-9793-18e232dbe615.
But more likely, I would do something else -- instead of GET /08ff2ec9-a9ad-4be2-9793-18e232dbe615 to fetch a representation of the list of events, I'd probably GET /08ff2ec9-a9ad-4be2-9793-18e232dbe615 to fetch a representation of available protocols - which is to say, a document filled with hyper links. From there, I might GET /08ff2ec9-a9ad-4be2-9793-18e232dbe615/603766ac-92af-47f3-8265-16f003ce5a09 to obtain a representation of the data collection form. I fill in the details of my event, submit the form, and POST /08ff2ec9-a9ad-4be2-9793-18e232dbe615 the form data to the server.
You can, of course, use any spelling you like for the URI.
In the first case, we need something like an HTTP capable document editor; the second case uses something more like a web browser.
If there were lots of different kinds of events, then the second case might well have lots of different form resources, all submitting POST /08ff2ec9-a9ad-4be2-9793-18e232dbe615 requests.
(You don't have to have all of the forms submitting to the same URI, but there are advantages to consider).
In a non event sourcing pattern I guess that would be first put into the database, then the event gets risen.
Even when you aren't event sourcing, there may still be some advantages to committing events to your durable store before emitting them. See Pat Helland: Data on the Outside versus Data on the Inside.
So you want clients to interact with the system, but they need to do this now by events.
Clients don't have to. Client may even not be aware of the underlying event store.
There are a number of trade-offs to consider and decisions to take when implementing an event-sourced system. To start with you can try to name a few pre computer era examples of event-sourced systems and look at their non-functional characteristics.
So the question is how do clients fire off specific event
Clients don't send events. They rather should express an intent (a command). Then it is the responsibility of the event-sourced system to validate the intent and either reject it or accept and store the corresponding event. It would mean that an intent to change the system's state was accepted and the stored event confirms the change.
My understanding is that you no longer use the rest api as resources
REST is one of the options. You just consider different things as resources. A command can be a REST resource. An event-sourced entity can be a resource, to which you POST a command. If you like it async - you can later GET the command to check its status. You can GET an entity to know its current state. You cant GET events from a class of entities as a means of subscription.
If we are talking about an end user, then most likely it doesn't deal with the event store directly. There is some third tier in between, which does CQRS. From a user client perspective it can be provided with REST, GraphQL, SOAP, gRPC or event e-mail. Whatever transport solution you find suitable. Command-processing part from CQRS is what specifically domain-driven. It decides which intent to accept and which to reject.
Event store itself is responsible for the data consistency. I.e. it should not allow two concurrent event leading to invalid state be published. This is what pre-computer event-sourced systems are good at. You usually have some physical object as an entity, so you lock for update by just getting hand of it.
Then an end-user client usually reads from some prepared read model. The responsibility of a read (R in CQRS) component is to prepare read-optimised data for clients. This data may come from multiple event-sourced of the same or different classes. Again, client may interact with a read model with whatever transport is suitable.
While an event-store is consistent and consistent immediately, a read model is eventually consistent. But it's up to you to tune this eventuality.
Just try to throw REST out of the architecture for a while. Consider it a one of available transport options - that may help to look at the root.
Related
I know the use of http verbs is based on standard specification. But my question if I use "GET" for update operations and write a code logic to update, does it create issues in any scenario? Apart from the standard, what else could be the reason to use these verbs for a specific purpose only?
my question if I use "GET" for update operations and write a code logic to update, does it create issues in any scenario?
Yes.
A simple example - suppose the network between the client and the server is unreliable; specifically, for a time, HTTP responses are being lost. A general purpose component (like a web proxy) might time out, and then, noticing that the method token of the request is GET, resend the request a second/third/fourth time, with your server performing its update on every GET request.
Let us further assume that these multiple update operations lead to an undesirable outcome; where do we properly affix blame?
Second example: you send someone a copy of the link to the update operation, so that they can send you a request at the appropriate time. But suppose you send that link to them in an email, and the email client recognizes the uri and (as a performance optimization) pre-fetches the link, triggering your update operation too early. Where do we properly affix the blame?
HTTP does not attempt to require the results of a GET to be safe. What it does is require that the semantics of the operation be safe, and therefore it is a fault of the implementation, not the interface or the user of that interface, if anything happens as a result that causes loss of property -- Fielding, 2002
In these, and other examples, blame is correctly affixed to your server, because GET has a standardized meaning which include the constraint that the semantics of the request are safe.
That's not to say that you can't have side effects when handling a GET request; "hit counters" are almost as old as the web itself. You have a lot of freedom in your implementation; so long as you respect the uniform interface, there won't be too much trouble.
Experience report: one of our internal tools uses GET requests to trigger scheduling; in our carefully controlled context (which is not web scale), we get away with it, and have for a very long time.
To borrow your language, there are certainly scenarios that would give us problems; but given our controls we manage to avoid them.
I wouldn't like our chances, though, if requests started coming in from outside of our carefully controlled context.
I think it's a decent question. You're asking a hypothetical: is there any value to doing the right other than that's we agree to use GET for fetching? e.g.: is there value beyond the fact that it's 'semantically nice'. A similar question in HTML might be: "Is it ok to use a <div> with an onclick instead of a <button>? (the answer is no).
There certainly is. Clients, servers and intermediates all change their behavior depending on what method is used. Even if your server can process GET for updates, and you build a client that uses this, your browser might still get confused.
If you are interested in this subject, don't ask on a forum; read the spec. The HTTP specification tells you what clients, servers and proxies should do when they encounter certain methods, statuses and headers.
Start at RFC7231
Still pretty young at ES and CQRS, I understand that they are tightly related to eventual consistency of data.
Eventual consistency can be problematic when we should perform validation before writing to the store, like checking that an email address isn't already used by an existing user. The only way to do that in a strongly consistent way would be to stop accepting new events, finish processing the remaining events against our view and then querying the view. We obviously don't want to go that far and Greg Young actually recommends to embrace eventual consistency and deal with (rare) cases where we break constraints.
Pushing this approach to the limits, my understanding is that this would mean, when developing a web API for example, to respond 'OK' to every request because it is impossible, at the time of the request, to validate it... Am I on the right track, or missing something here?
As hinted in my comment above, a RESTful API can return 202 Accepted.
This provides a way for a client to poll for status updates if that's necessary.
The client can monitor for state if that's desirable, but alternatively, it can also simply fire and forget, assuming that if it gets any sort of 200-range response, the command will eventually be applied. This can be a good alternative if you have an alternative channel on which you can propagate errors. For example, if you know which user submitted the command, and you have that user's email address, you can send an email in the event of a failure to apply the command.
One of the points of a CQRS architecture is that the edge of the system should do whatever it can to validate the correctness of a Command before it accepts it. Based on the known state of the system (as exposed by the Query side), the system can make a strong effort to validate that a given Command is acceptable. If it does that, the only permanent error that should happen if you accept a Command is a concurrency conflict. Depending on how fast your system approaches consistent states, such concurrency conflicts may be so few that e.g. sending the user an email is an appropriate error-handling strategy.
Let's say we have a User, Wallet REST microservices and an API gateway that glues things together. When Bob registers on our website, our API gateway needs to create a user through the User microservice and a wallet through the Wallet microservice.
Now here are a few scenarios where things could go wrong:
User Bob creation fails: that's OK, we just return an error message to the Bob. We're using SQL transactions so no one ever saw Bob in the system. Everything's good :)
User Bob is created but before our Wallet can be created, our API gateway hard crashes. We now have a User with no wallet (inconsistent data).
User Bob is created and as we are creating the Wallet, the HTTP connection drops. The wallet creation might have succeeded or it might have not.
What solutions are available to prevent this kind of data inconsistency from happening? Are there patterns that allow transactions to span multiple REST requests? I've read the Wikipedia page on Two-phase commit which seems to touch on this issue but I'm not sure how to apply it in practice. This Atomic Distributed Transactions: a RESTful design paper also seems interesting although I haven't read it yet.
Alternatively, I know REST might just not be suited for this use case. Would perhaps the correct way to handle this situation to drop REST entirely and use a different communication protocol like a message queue system? Or should I enforce consistency in my application code (for example, by having a background job that detects inconsistencies and fixes them or by having a "state" attribute on my User model with "creating", "created" values, etc.)?
What doesn't make sense:
distributed transactions with REST services. REST services by definition are stateless, so they should not be participants in a transactional boundary that spans more than one service. Your user registration use case scenario makes sense, but the design with REST microservices to create User and Wallet data is not good.
What will give you headaches:
EJBs with distributed transactions. It's one of those things that work in theory but not in practice. Right now I'm trying to make a distributed transaction work for remote EJBs across JBoss EAP 6.3 instances. We've been talking to RedHat support for weeks, and it didn't work yet.
Two-phase commit solutions in general. I think the 2PC protocol is a great algorithm (many years ago I implemented it in C with RPC). It requires comprehensive fail recovery mechanisms, with retries, state repository, etc. All the complexity is hidden within the transaction framework (ex.: JBoss Arjuna). However, 2PC is not fail proof. There are situations the transaction simply can't complete. Then you need to identify and fix database inconsistencies manually. It may happen once in a million transactions if you're lucky, but it may happen once in every 100 transactions depending on your platform and scenario.
Sagas (Compensating transactions). There's the implementation overhead of creating the compensating operations, and the coordination mechanism to activate compensation at the end. But compensation is not fail proof either. You may still end up with inconsistencies (= some headache).
What's probably the best alternative:
Eventual consistency. Neither ACID-like distributed transactions nor compensating transactions are fail proof, and both may lead to inconsistencies. Eventual consistency is often better than "occasional inconsistency". There are different design solutions, such as:
You may create a more robust solution using asynchronous communication. In your scenario, when Bob registers, the API gateway could send a message to a NewUser queue, and right-away reply to the user saying "You'll receive an email to confirm the account creation." A queue consumer service could process the message, perform the database changes in a single transaction, and send the email to Bob to notify the account creation.
The User microservice creates the user record and a wallet record in the same database. In this case, the wallet store in the User microservice is a replica of the master wallet store only visible to the Wallet microservice. There's a data synchronization mechanism that is trigger-based or kicks in periodically to send data changes (e.g., new wallets) from the replica to the master, and vice-versa.
But what if you need synchronous responses?
Remodel the microservices. If the solution with the queue doesn't work because the service consumer needs a response right away, then I'd rather remodel the User and Wallet functionality to be collocated in the same service (or at least in the same VM to avoid distributed transactions). Yes, it's a step farther from microservices and closer to a monolith, but will save you from some headache.
This is a classic question I was asked during an interview recently How to call multiple web services and still preserve some kind of error handling in the middle of the task. Today, in high performance computing, we avoid two phase commits. I read a paper many years ago about what was called the "Starbuck model" for transactions: Think about the process of ordering, paying, preparing and receiving the coffee you order at Starbuck... I oversimplify things but a two phase commit model would suggest that the whole process would be a single wrapping transaction for all the steps involved until you receive your coffee. However, with this model, all employees would wait and stop working until you get your coffee. You see the picture ?
Instead, the "Starbuck model" is more productive by following the "best effort" model and compensating for errors in the process. First, they make sure that you pay! Then, there are message queues with your order attached to the cup. If something goes wrong in the process, like you did not get your coffee, it is not what you ordered, etc, we enter into the compensation process and we make sure you get what you want or refund you, This is the most efficient model for increased productivity.
Sometimes, starbuck is wasting a coffee but the overall process is efficient. There are other tricks to think when you build your web services like designing them in a way that they can be called any number of times and still provide the same end result. So, my recommendation is:
Don't be too fine when defining your web services (I am not convinced about the micro-service hype happening these days: too many risks of going too far);
Async increases performance so prefer being async, send notifications by email whenever possible.
Build more intelligent services to make them "recallable" any number of times, processing with an uid or taskid that will follow the order bottom-top until the end, validating business rules in each step;
Use message queues (JMS or others) and divert to error handling processors that will apply operations to "rollback" by applying opposite operations, by the way, working with async order will require some sort of queue to validate the current state of the process, so consider that;
In last resort, (since it may not happen often), put it in a queue for manual processing of errors.
Let's go back with the initial problem that was posted. Create an account and create a wallet and make sure everything was done.
Let's say a web service is called to orchestrate the whole operation.
Pseudo code of the web service would look like this:
Call Account creation microservice, pass it some information and a some unique task id 1.1 Account creation microservice will first check if that account was already created. A task id is associated with the account's record. The microservice detects that the account does not exist so it creates it and stores the task id. NOTE: this service can be called 2000 times, it will always perform the same result. The service answers with a "receipt that contains minimal information to perform an undo operation if required".
Call Wallet creation, giving it the account ID and task id. Let's say a condition is not valid and the wallet creation cannot be performed. The call returns with an error but nothing was created.
The orchestrator is informed of the error. It knows it needs to abort the Account creation but it will not do it itself. It will ask the wallet service to do it by passing its "minimal undo receipt" received at the end of step 1.
The Account service reads the undo receipt and knows how to undo the operation; the undo receipt may even include information about another microservice it could have called itself to do part of the job. In this situation, the undo receipt could contain the Account ID and possibly some extra information required to perform the opposite operation. In our case, to simplify things, let's say is simply delete the account using its account id.
Now, let's say the web service never received the success or failure (in this case) that the Account creation's undo was performed. It will simply call the Account's undo service again. And this service should normaly never fail because its goal is for the account to no longer exist. So it checks if it exists and sees nothing can be done to undo it. So it returns that the operation is a success.
The web service returns to the user that the account could not be created.
This is a synchronous example. We could have managed it in a different way and put the case into a message queue targeted to the help desk if we don't want the system to completly recover the error". I've seen this being performed in a company where not enough hooks could be provided to the back end system to correct situations. The help desk received messages containing what was performed successfully and had enough information to fix things just like our undo receipt could be used for in a fully automated way.
I have performed a search and the microsoft web site has a pattern description for this approach. It is called the compensating transaction pattern:
Compensating transaction pattern
All distributed systems have trouble with transactional consistency. The best way to do this is like you said, have a two-phase commit. Have the wallet and the user be created in a pending state. After it is created, make a separate call to activate the user.
This last call should be safely repeatable (in case your connection drops).
This will necessitate that the last call know about both tables (so that it can be done in a single JDBC transaction).
Alternatively, you might want to think about why you are so worried about a user without a wallet. Do you believe this will cause a problem? If so, maybe having those as separate rest calls are a bad idea. If a user shouldn't exist without a wallet, then you should probably add the wallet to the user (in the original POST call to create the user).
IMHO one of the key aspects of microservices architecture is that the transaction is confined to the individual microservice (Single responsibility principle).
In the current example, the User creation would be an own transaction. User creation would push a USER_CREATED event into an event queue. Wallet service would subscribe to the USER_CREATED event and do the Wallet creation.
If my wallet was just another bunch of records in the same sql database as the user then I would probably place the user and wallet creation code in the same service and handle that using the normal database transaction facilities.
It sounds to me you are asking about what happens when the wallet creation code requires you touch another other system or systems? Id say it all depends on how complex and or risky the creation process is.
If it's just a matter of touching another reliable datastore (say one that can't participate in your sql transactions), then depending on the overall system parameters, I might be willing to risk the vanishingly small chance that second write won't happen. I might do nothing, but raise an exception and deal with the inconsistent data via a compensating transaction or even some ad-hoc method. As I always tell my developers: "if this sort of thing is happening in the app, it won't go unnoticed".
As the complexity and risk of wallet creation increases you must take steps to ameliorate the risks involved. Let's say some of the steps require calling multiple partner apis.
At this point you might introduce a message queue along with the notion of partially constructed users and/or wallets.
A simple and effective strategy for making sure your entities eventually get constructed properly is to have the jobs retry until they succeed, but a lot depends on the use cases for your application.
I would also think long and hard about why I had a failure prone step in my provisioning process.
One simple Solution is you create user using the User Service and use a messaging bus where user service emits its events , and Wallet Service registers on the messaging bus, listens on User Created event and create Wallet for the User. In the mean time , if user goes on Wallet UI to see his Wallet, check if user was just created and show your wallet creation is in progress, please check in some time
What solutions are available to prevent this kind of data inconsistency from happening?
Traditionally, distributed transaction managers are used. A few years ago in the Java EE world you might have created these services as EJBs which were deployed to different nodes and your API gateway would have made remote calls to those EJBs. The application server (if configured correctly) automatically ensures, using two phase commit, that the transaction is either committed or rolled back on each node, so that consistency is guaranteed. But that requires that all the services be deployed on the same type of application server (so that they are compatible) and in reality only ever worked with services deployed by a single company.
Are there patterns that allow transactions to span multiple REST requests?
For SOAP (ok, not REST), there is the WS-AT specification but no service that I have ever had to integrate has support that. For REST, JBoss has something in the pipeline. Otherwise, the "pattern" is to either find a product which you can plug into your architecture, or build your own solution (not recommended).
I have published such a product for Java EE: https://github.com/maxant/genericconnector
According to the paper you reference, there is also the Try-Cancel/Confirm pattern and associated Product from Atomikos.
BPEL Engines handle consistency between remotely deployed services using compensation.
Alternatively, I know REST might just not be suited for this use case. Would perhaps the correct way to handle this situation to drop REST entirely and use a different communication protocol like a message queue system?
There are many ways of "binding" non-transactional resources into a transaction:
As you suggest, you could use a transactional message queue, but it will be asynchronous, so if you depend on the response it becomes messy.
You could write the fact that you need to call the back end services into your database, and then call the back end services using a batch. Again, async, so can get messy.
You could use a business process engine as your API gateway to orchestrate the back end microservices.
You could use remote EJB, as mentioned at the start, since that supports distributed transactions out of the box.
Or should I enforce consistency in my application code (for example, by having a background job that detects inconsistencies and fixes them or by having a "state" attribute on my User model with "creating", "created" values, etc.)?
Playing devils advocate: why build something like that, when there are products which do that for you (see above), and probably do it better than you can, because they are tried and tested?
In micro-services world the communication between services should be either through rest client or messaging queue. There can be two ways to handle the transactions across services depending on how are you communicating between the services. I will personally prefer message driven architecture so that a long transaction should be a non blocking operation for a user.
Lets take you example to explain it :
Create user BOB with event CREATE USER and push the message to a message bus.
Wallet service subscribed to this event can create a wallet corresponding to the user.
The one thing which you have to take care is to select a robust reliable message backbone which can persists the state in case of failure. You can use kafka or rabbitmq for messaging backbone. There will be a delay in execution because of eventual consistency but that can be easily updated through socket notification. A notifications service/task manager framework can be a service which update the state of the transactions through asynchronous mechanism like sockets and can help UI to update show the proper progress.
Personally I like the idea of Micro Services, modules defined by the use cases, but as your question mentions, they have adaptation problems for the classical businesses like banks, insurance, telecom, etc...
Distributed transactions, as many mentioned, is not a good choice, people now going more for eventually consistent systems but I am not sure this will work for banks, insurance, etc....
I wrote a blog about my proposed solution, may be this can help you....
https://mehmetsalgar.wordpress.com/2016/11/05/micro-services-fan-out-transaction-problems-and-solutions-with-spring-bootjboss-and-netflix-eureka/
Eventual consistency is the key here.
One of the services is chosen to become primary handler of the event.
This service will handle the original event with single commit.
Primary handler will take responsibility for asynchronously communicating the secondary effects to other services.
The primary handler will do the orchestration of other services calls.
The commander is in charge of the distributed transaction and takes control. It knows the instruction to be executed and will coordinate executing them. In most scenarios there will just be two instructions, but it can handle multiple instructions.
The commander takes responsibility of guaranteeing the execution of all instructions, and that means retires.
When the commander tries to effect the remote update and doesn’t get a response, it has no retry.
This way the system can be configured to be less prone to failure and it heals itself.
As we have retries we have idempotence.
Idempotence is the property of being able to do something twice such a way that the end results be the same as if it had been done once only.
We need idempotence at the remote service or data source so that, in the case where it receives the instruction more than once, it only processes it once.
Eventual consistency
This solves most of distributed transaction challenges, however we need to consider couple of points here.
Every failed transaction will be followed by a retry, the amount of attempted retries depends on the context.
Consistency is eventual i.e., while the system is out of consistent state during a retry, for example if a customer has ordered a book, and made a payment and then updates the stock quantity. If the stock update operations fail and assuming that was the last stock available, the book will still be available till the retry operation for the stock updating has succeeded. After the retry is successful your system will be consistent.
Why not use API Management (APIM) platform that supports scripting/programming? So, you will be able to build composite service in the APIM without disturbing micro services. I have designed using APIGEE for this purpose.
Is it totally forbidden (or just inappropriate) for the domain model in CQRS to return state to the client? e.g. if live updated information exists in the domain model. I'm thinking of a command causing events and changes to the domain model and returning info to the client.
The alternatives seem to be:
the client determines the updated state itself (duplication of code), which should hopefully match the eventual update of the read model,
the client waits for the read model to be updated (eventual consistency) and somehow knows when to read from there to update its state, or
the whole command is done synchronously and the read model somehow pushes the information to the client, e.g. with some sort of web socket.
I guess the latter relates to the desire to build reactive applications incorporating CQRS. The notion of the client polling for updates seem unsatisfactory (and similarly for the read model polling for updates from the domain model).
Is it totally forbidden (or just inappropriate) for the domain model in CQRS to return state to the client?
Assuming you mean DDD/CQRS, I think the dogmatic answer is "so inappropriate it may as well be forbidden". Sure, it's not going to hard-break anything on its own, but your architecture has taken a step down a very slippery slope that (at its bottom) is a big-ball-of-mud.
You want your Domain model to be isolated from all the issues of what people want to see afterwards.
returning info to the client
Consider that for any given Command sent to the Domain model, there can be multiple versions of "info" the client might want, depending on how the command is being invoked!
Imagine a button called "Send Invoice" that appears both on the "regular flow" page and the "bulk modify" page. Those might invoke the same Command yet have drastically different requirements for what the user sees as "the result" of what they did.
The alternatives seem to be:
I'd look at the Application Layer for this, or whatever layer would know "where" the user is when they are doing things. (Perhaps the same place where POST data is becoming an instance of a Command object.) That layer is the one that has enough context to determine what read-model to look at for the eventual results.
Whether you want the server to block (polling the DB), or the client do its own polling, or asynchronous websocket-stuff... That's another story.
I'm wondering how you'd implement the following use-case in REST. Is it even possible to do without compromising the conceptual model?
Read or update multiple resources within the scope of a single transaction. For example, transfer $100 from Bob's bank account into John's account.
As far as I can tell, the only way to implement this is by cheating. You could POST to the resource associated with either John or Bob and carry out the entire operation using a single transaction. As far as I'm concerned this breaks the REST architecture because you're essentially tunneling an RPC call through POST instead of really operating on individual resources.
Consider a RESTful shopping basket scenario. The shopping basket is conceptually your transaction wrapper. In the same way that you can add multiple items to a shopping basket and then submit that basket to process the order, you can add Bob's account entry to the transaction wrapper and then Bill's account entry to the wrapper. When all the pieces are in place then you can POST/PUT the transaction wrapper with all the component pieces.
There are a few important cases that aren't answered by this question, which I think is too bad, because it has a high ranking on Google for the search terms :-)
Specifically, a nice propertly would be: If you POST twice (because some cache hiccupped in the intermediate) you should not transfer the amount twice.
To get to this, you create a transaction as an object. This could contain all the data you know already, and put the transaction in a pending state.
POST /transfer/txn
{"source":"john's account", "destination":"bob's account", "amount":10}
{"id":"/transfer/txn/12345", "state":"pending", "source":...}
Once you have this transaction, you can commit it, something like:
PUT /transfer/txn/12345
{"id":"/transfer/txn/12345", "state":"committed", ...}
{"id":"/transfer/txn/12345", "state":"committed", ...}
Note that multiple puts don't matter at this point; even a GET on the txn would return the current state. Specifically, the second PUT would detect that the first was already in the appropriate state, and just return it -- or, if you try to put it into the "rolledback" state after it's already in "committed" state, you would get an error, and the actual committed transaction back.
As long as you talk to a single database, or a database with an integrated transaction monitor, this mechanism will actually work just fine. You might additionally introduce time-outs for transactions, which you could even express using Expires headers if you wanted to.
In REST terms, resources are nouns that can be acted on with CRUD (create/read/update/delete) verbs. Since there is no "transfer money" verb, we need to define a "transaction" resource that can be acted upon with CRUD. Here's an example in HTTP+POX. First step is to CREATE (HTTP POST method) a new empty transaction:
POST /transaction
This returns a transaction ID, e.g. "1234" and according URL "/transaction/1234". Note that firing this POST multiple times will not create the same transaction with multiple IDs and also avoids introduction of a "pending" state. Also, POST can't always be idempotent (a REST requirement), so it's generally good practice to minimize data in POSTs.
You could leave the generation of a transaction ID up to the client. In this case, you would POST /transaction/1234 to create transaction "1234" and the server would return an error if it already existed. In the error response, the server could return a currently unused ID with an appropriate URL. It's not a good idea to query the server for a new ID with a GET method, since GET should never alter server state, and creating/reserving a new ID would alter server state.
Next up, we UPDATE (PUT HTTP method) the transaction with all data, implicitly committing it:
PUT /transaction/1234
<transaction>
<from>/account/john</from>
<to>/account/bob</to>
<amount>100</amount>
</transaction>
If a transaction with ID "1234" has been PUT before, the server gives an error response, otherwise an OK response and a URL to view the completed transaction.
NB: in /account/john , "john" should really be John's unique account number.
Great question, REST is mostly explained with database-like examples, where something is stored, updated, retrieved, deleted. There are few examples like this one, where the server is supposed to process the data in some way. I don't think Roy Fielding included any in his thesis, which was based on http after all.
But he does talk about "representational state transfer" as a state machine, with links moving to the next state. In this way, the documents (the representations) keep track of the client state, instead of the server having to do it. In this way, there is no client state, only state in terms of which link you are on.
I've been thinking about this, and it seems to me reasonable that to get the server to process something for you, when you upload, the server would automatically create related resources, and give you the links to them (in fact, it wouldn't need to automatically create them: it could just tell you the links, and it only create them when and if you follow them - lazy creation). And to also give you links to create new related resources - a related resource has the same URI but is longer (adds a suffix). For example:
You upload (POST) the representation of the concept of a transaction with all the information. This looks just like a RPC call, but it's really creating the "proposed transaction resource". e.g URI: /transaction
Glitches will cause multiple such resources to be created, each with a different URI.
The server's response states the created resource's URI, its representation - this includes the link (URI) to create the related resource of a new "committed transaction resource". Other related resources are the link to delete the proposed transaction. These are states in the state-machine, which the client can follow. Logically, these are part of the resource that has been created on the server, beyond the information the client supplied. e.g URIs: /transaction/1234/proposed, /transaction/1234/committed
You POST to the link to create the "committed transaction resource", which creates that resource, changing the state of the server (the balances of the two accounts)**. By its nature, this resource can only be created once, and can't be updated. Therefore, glitches committing many transactions can't occur.
You can GET those two resources, to see what their state is. Assuming that a POST can change other resources, the proposal would now be flagged as "committed" (or perhaps, not available at all).
This is similar to how webpages operate, with the final webpage saying "are you sure you want to do this?" That final webpage is itself a representation of the state of the transaction, which includes a link to go to the next state. Not just financial transactions; also (eg) preview then commit on wikipedia. I guess the distinction in REST is that each stage in the sequence of states has an explicit name (its URI).
In real-life transactions/sales, there are often different physical documents for different stages of a transaction (proposal, purchase order, receipt etc). Even more for buying a house, with settlement etc.
OTOH This feels like playing with semantics to me; I'm uncomfortable with the nominalization of converting verbs into nouns to make it RESTful, "because it uses nouns (URIs) instead of verbs (RPC calls)". i.e. the noun "committed transaction resource" instead of the verb "commit this transaction". I guess one advantage of nominalization is you can refer to the resource by name, instead of needing to specify it in some other way (such as maintaining session state, so you know what "this" transaction is...)
But the important question is: What are the benefits of this approach? i.e. In what way is this REST-style better than RPC-style? Is a technique that's great for webpages also helpful for processing information, beyond store/retrieve/update/delete? I think that the key benefit of REST is scalability; one aspect of that is not needing to maintain client state explicitly (but making it implicit in the URI of the resource, and the next states as links in its representation). In that sense it helps. Perhaps this helps in layering/pipelining too? OTOH only the one user will look at their specific transaction, so there's no advantage in caching it so others can read it, the big win for http.
I've drifted away from this topic for 10 years. Coming back, I can't believe the religion masquerading as science that you wade into when you google rest+reliable. The confusion is mythic.
I would divide this broad question into three:
Downstream services. Any web service you develop will have downstream services that you use, and whose transaction syntax you have no choice but to follow. You should try and hide all this from users of your service, and make sure all parts of your operation succeed or fail as a group, then return this result to your users.
Your services. Clients want unambiguous outcomes to web-service calls, and the usual REST pattern of making POST, PUT or DELETE requests directly on substantive resources strikes me as a poor, and easily improved, way of providing this certainty. If you care about reliability, you need to identify action requests. This id can be a guid created on the client, or a seed value from a relational DB on the server, it doesn't matter. For server generated ID's, use a 'preflight' request-response to exchange the id of the action. If this request fails or half succeeds, no problem, the client just repeats the request. Unused ids do no harm.This is important because it lets all subsequent requests be fully idempotent, in the sense that if they are repeated n times they return the same result and cause nothing further to happen. The server stores all responses against the action id, and if it sees the same request, it replays the same response. A fuller treatment of the pattern is in this google doc. The doc suggests an implementation that, I believe(!), broadly follows REST principals. Experts will surely tell me how it violates others. This pattern can be usefully employed for any unsafe call to your web-service, whether or not there are downstream transactions involved.
Integration of your service into "transactions" controlled by upstream services. In the context of web-services, full ACID transactions are considered as usually not worth the effort, but you can greatly help consumers of your service by providing cancel and/or confirm links in your confirmation response, and thus achieve transactions by compensation.
Your requirement is a fundamental one. Don't let people tell you your solution is not kosher. Judge their architectures in the light of how well, and how simply, they address your problem.
If you stand back to summarize the discussion here, it's pretty clear that REST is not appropriate for many APIs, particularly when the client-server interaction is inherently stateful, as it is with non-trivial transactions. Why jump through all the hoops suggested, for client and server both, in order to pedantically follow some principle that doesn't fit the problem? A better principle is to give the client the easiest, most natural, productive way to compose with the application.
In summary, if you're really doing a lot of transactions (types, not instances) in your application, you really shouldn't be creating a RESTful API.
You'd have to roll your own "transaction id" type of tx management. So it would be 4 calls:
http://service/transaction (some sort of tx request)
http://service/bankaccount/bob (give tx id)
http://service/bankaccount/john (give tx id)
http://service/transaction (request to commit)
You'd have to handle the storing of the actions in a DB (if load balanced) or in memory or such, then handling commit, rollback, timeout.
Not really a RESTful day in the park.
First of all transferring money is nothing that you can not do in a single resource call. The action you want to do is sending money. So you add a money transfer resource to the account of the sender.
POST: accounts/alice, new Transfer {target:"BOB", abmount:100, currency:"CHF"}.
Done. You do not need to know that this is a transaction that must be atomic etc. You just transfer money aka. send money from A to B.
But for the rare cases here a general solution:
If you want to do something very complex involving many resources in a defined context with a lot of restrictions that actually cross the what vs. why barrier (business vs. implementation knowledge) you need to transfer state. Since REST should be stateless you as a client need to transfer the state around.
If you transfer state you need to hide the information inside from the client. The client should not know internal information only needed by the implementation but does not carry information relevant in terms of business. If those information have no business value the state should be encrypted and a metaphor like token, pass or something need to be used.
This way one can pass internal state around and using encryption and signing the system can be still be secure and sound. Finding the right abstraction for the client why he passes around state information is something that is up to the design and architecture.
The real solution:
Remember REST is talking HTTP and HTTP comes with the concept of using cookies. Those cookies are often forgotten when people talk about REST API and workflows and interactions spanning multiple resources or requests.
Remember what is written in the Wikipedia about HTTP cookies:
Cookies were designed to be a reliable mechanism for websites to remember stateful information (such as items in a shopping cart) or to record the user's browsing activity (including clicking particular buttons, logging in, or recording which pages were visited by the user as far back as months or years ago).
So basically if you need to pass on state, use a cookie. It is designed for exactly the very same reason, it is HTTP and therefore it is compatible to REST by design :).
The better solution:
If you talk about a client performing a workflow involving multiple requests you usually talk about protocol. Every form of protocol comes with a set of preconditions for each potential step like perform step A before you can do B.
This is natural but exposing protocol to clients makes everything more complex. In order to avoid it just think what we do when we have to do complex interactions and things in the real world... . We use an Agent.
Using the Agent metaphor you can provide a resource that can perform all necessary steps for you and store the actual assignment / instructions it is acting upon in its list (so we can use POST on the agent or an 'agency').
A complex example:
Buying a house:
You need to prove your credibility (like providing your police record entries), you need to ensure financial details, you need to buy the actual house using a lawyer and a trusted third party storing the funds, verify that the house now belongs to you and add the buying stuff to your tax records etc. (just as an example, some steps may be wrong or whatever).
These steps might take several days to be completed, some can be done in parallel etc.
In order to do this, you just give the agent the task buy house like:
POST: agency.com/ { task: "buy house", target:"link:toHouse", credibilities:"IamMe"}.
Done. The agency sends you back a reference to you that you can use to see and track the status of this job and the rest is done automatically by the agents of the agency.
Think about a bug tracker for instance. Basically you report the bug and can use the bug id to check whats going on. You can even use a service to listen to changes of this resource. Mission Done.
You must not use server side transactions in REST.
One of the REST contraints:
Stateless
The client–server communication is further constrained by no client context being stored on the server between requests. Each request from any client contains all of the information necessary to service the request, and any session state is held in the client.
The only RESTful way is to create a transaction redo log and put it into the client state. With the requests the client sends the redo log and the server redoes the transaction and
rolls the transaction back but provides a new transaction redo log (one step further)
or finally complete the transaction.
But maybe it's simpler to use a server session based technology which supports server side transactions.
I think that in this case it is totally acceptable to break the pure theory of REST in this situation. In any case, I don't think there is anything actually in REST that says you can't touch dependent objects in business cases that require it.
I really think it's not worth the extra hoops you would jump through to create a custom transaction manager, when you could just leverage the database to do it.
In the simple case (without distributed resources), you could consider the transaction as a resource, where the act of creating it attains the end objective.
So, to transfer between <url-base>/account/a and <url-base>/account/b, you could post the following to <url-base>/transfer.
<transfer>
<from><url-base>/account/a</from>
<to><url-base>/account/b</to>
<amount>50</amount>
</transfer>
This would create a new transfer resource and return the new url of the transfer - for example <url-base>/transfer/256.
At the moment of successful post, then, the 'real' transaction is carried out on the server, and the amount removed from one account and added to another.
This, however, doesn't cover a distributed transaction (if, say 'a' is held at one bank behind one service, and 'b' is held at another bank behind another service) - other than to say "try to phrase all operations in ways that don't require distributed transactions".
I believe that would be the case of using a unique identifier generated on the client to ensure that the connection hiccup not imply in an duplicity saved by the API.
I think using a client generated GUID field along with the transfer object and ensuring that the same GUID was not reinserted again would be a simpler solution to the bank transfer matter.
Do not know about more complex scenarios, such as multiple airline ticket booking or micro architectures.
I found a paper about the subject, relating the experiences of dealing with the transaction atomicity in RESTful services.
I guess you could include the TAN in the URL/resource:
PUT /transaction to get the ID (e.g. "1")
[PUT, GET, POST, whatever] /1/account/bob
[PUT, GET, POST, whatever] /1/account/bill
DELETE /transaction with ID 1
Just an idea.