I have two bounded contexts which lead into two micro services
PersonalManagement
DocumentStorage
I keep the entity model simple here.
PersonalManagement:
Entity/Table Person:
#id - int
tenantId - int
name - string
...
DocumentStorage
Entity/Table Document:
#id - int
tenantId - int
personId - int
dateIssued - string
...
You need to know that before the application is started - a company (tenant) is choosen to define the company context.
I want to store a new document by using REST/JSON.
This is a POST to /tenants/1/persons/5/documents
with the body
{
"dateIssued" : "2018-06-11"
}
On the backend side - I validate the input body.
One validation might be "if the person specified exists and really belongs to given tenant".
Since this info is stored in the PersonalManagement-MicroService, I need to provide an operation like this:
"Does exists (personId=5,tenantId=1)"
in PersonalManagement to ensure consistence since caller might be evil.
Or in general:
What is best practise to check "ownership" of entities cross database in micro services
It might also be an option that if a new person is created (tenantId,personId) this information is stored additionally(!) in DocumentStorage but wanna avoid this redundancy.
I'm not going to extend this answer into whether your bounded contexts and service endpoints are well defined since your question seems to be simplifying the issue to keep a well defined scope, but regarding your specific question:
What is best practise to check "ownership" of entities cross database in micro services
Microservice architectures use strive for a "share nothing" principle. And that usually extends from code base to data base. So you're right to assume you're checking for this constraint "cross-DB" in your scenario.
You have a few options on this particular case, each with their set of drawbacks:
1) Your proposed "Does exists (personId=5,tenantId=1)" call from the DocumentContext to the PersonContext is not wrong on itself, but you will generate a straight dependency between these two microservices, so you must ask yourself whether it seems ok for you not to accept new documents if the PersonManagement microservice is offline.
In specific situations, such dependencies might be acceptable but the more of these you have, the less your microservice architecture will behave as one and more like a "distributed monolith" which on itself it pretty much an anti-pattern.
2) The other main option you have is that you should recognize that the DocumentContext is a very much interested in some information/behavior relating to People so it should be ok with modelling the Person Entity inside its boundaries.
That means, you can have the DocumentContext subscribe for changes in the PersonContext to be aware of which People currently exist and what their characteristics are and thus being able to keep a local copy of such information.
That way, your validation will be kept entirely inside the DocumentContext which will have its operation unhindered by eventual issues with the PersonContext and you will find out your modelling of the document related entities will be much cleaner than before.
But in the end, you will also discover that a "share nothing" principle usually will cost you in what seems to be redundancy, but it's actually independence of contexts.
just for the tenancy check , this can be done using the JWT token (token which can store tenancy information and other metadata).
Let me provide another example of the same scenario which can't be solved with JWT.
Assume one Customer wants to create a Order and our system wants to check whether the customer exist or not while creating the order.
As Order and Customer service are separate, and we want minimal dependencies between them, there are multiple sol. to above problems:
create Order in "validating state" and on OrderCreated event check for customer validity and update customer state to "Valid"
another one before creating order check for the customer (which is not the right way as it creates dependency, untill and unless very critical do not do it)
last way is the let the order be created , somebody who will final check the order for delivery will verify customer will remove
Related
If an aggregate is "metadata driven", and such metadata needs to be managed centrally together with metadata of other aggregates, is this anti pattern?
For instance, there are metadata for product, user, action etc so product type, user group or action type don't get hardcoded. There is also a single metadata microservice that manages all metadata (large amount of versioned data).
How should a product microservice get its metadata for things like command validation?
Any suggestions on how to model this in Axon?
I am not overly convinced it makes sense to construct a separate service just for MetaData management. Then again, I am not a domain expert of the entire world, so I might be completely off with that assumption.
I feel that using the term MetaData sounds overloaded if you're also speaking about making it a central service. Maybe there's another term that makes more sense?
At any rate, I think it's valuable to know that Axon Framework supports the notion of MetaData for every type of message. Hence, Commands, Events, and Queries will always carry a MetaData object. This MetaData is, in all honesty, just a Map of String to ?, which can store whatever metadata you have.
In most cases, this includes fields like a userId, traceId, correlationId, some form of security information. Furthermore, in most cases this does not include first-class citizen data like productTypes, as far as I know.
To come back to your original question:
If an aggregate is "metadata driven", and such metadata needs to be managed centrally together with metadata of other aggregates, is this anti pattern?
I can't say for sure that it's an anti-pattern to model a service to manage all metadata. However, it does feel like unnecessary complexity to me. Be sure to update your question with more information to clarify your idea, Bing. I'll be sure to update my response once you've done that.
I'm new to Domain Driven Desing and completely stucked with modelling problem. Here is a screenshot of design level event storming of simple feature with only one invariant:
Let's assume that eventual consistency is not allowed for certificate's condition and it is absolutely crucial to fulfill this condition immediately to process an order. In this case to check my certificate invariant I need an access to:
season of the order
supplier's country
season from which certificates are checked for specific country
supplier's certificates and if they are accepted and up to date
Initially I tried to split my application into multiple bounded contexts and for me those informations belong to different bounded contexts. Should I then have all these necassary data (list above) in one aggregate in order to ensure consistency and have its quite large or maybe there is any other solution? Maybe my whole reasoning is wrong and it doesn't make much sense? Maybe the bounded contexts boundaries are wrong due to necesity to reach for data to other contexts (not being autonomous)? Maybe domain service could be a solution? I would be really grateful for any help because I'm quite confused with those ddd topics :/
before answering your question I suggest you keep in mind the next statements:
Consolidate your business invariants(rules) inside Aggregate.
Design small and autonomous aggregates, if it is possible.
Reference other Aggregates by using their id.
Update other Aggregates using eventual consistency.
now let me share my opinion and give you advice (I've used C# syntax in below code snippets)
I recommend to you use 2 BC - Order and Supplier, and each of those is a good candidate for the individual aggregate. Certificate is not an aggregate and we have access to it via Supplier aggregate.
Preface: We have 2 services, - Order application service which injects repository interfaces to retrieve specific data. Also, we have an Order domain service where we're working with our aggregates.
Now if you want to check the certificate invariant, to do the next steps -
We should arrange all required data in the order application service by pulling data from repositories
Now it's time to validate
(supplier's certificates and if they are accepted and up to date) certificate invariant.
Let's switch to the Order domain service to decide about order processing where we have the next method public void AddOrder(Order newOrder, Supplier supplier).
But let's start with adding the concrete business rule class like that
public class SupplierCertificatesAreValidRule{
public SupplierCertificatesAreValidRule(IList<Certificate> supplierCertificates){
...
}
public bool IsValid() {...}
}
Get from the Supplier entity collection of its certificates.
Let's define the season of the order
Then get the supplier's country
Fetch the season from which certificates are checked for specific country
Add the Add method in the Order entity itself, where at beginning of this method we should call our business rule like that :
public static Order Add(Order newOrder, Supplier supplier){
CheckRule(new SupplierCertificatesAreValidRule(supplier.Certificates));
return new Order();
}
If the rule does not meet, then we should throw some business rule validation exception and explain the reason there.
If our rule is met, then it means that we can process our order
This question seemed fool at first sight for me, but then I realized that I don't have a proper answer yet, and interestingly also didn't find good explanation about it in my searches.
I'm new to Domain Driven Design concepts, so, even if the question is basic, feel free to add any considerations to it.
I'm designing in Rest API to configure Server Instances, and I came up with a Aggregate called Instance that contains a List of Configurations, only one specific Configuration will be active at a given time.
To add a Configuration, one would call an endpoint POST /instances/{id}/configurations with the body on the desired configuration. In response, if all okay, it would receive a HTTP 204 with a Header Location containing the new Configuration ID.
I'm planning to have only one Controller, InstanceController, that would call InstanceService that would manipulate the Instance Aggregate and then store to the Repo.
Since the ID's are generated by the repository, If I call Instance.addConfiguration and then InstanceRepository.store, how would I get the ID of the newly created configuration? I mean, it's a List, so It's not trivial as calling Instance.configuration.identity
A option would implement a method in Instance like, getLastAddedConfiguration, but this seems really brittle.
What is the general approach in this situation?
the ID's are generated by the repository
You could remove this extra complexity. Since Configuration is an entity of the Instance aggregate, its Id only needs to be unique inside the aggregate, not across the whole application. Therefore, the easiest is that the Aggregate assigns the ConfigurationId in the Instance.addConfiguration method (as the aggregate can easily ensure the uniqueness of the new Id). This method can return the new ConfigurationId (or the whole object with the Id if necessary).
What is the general approach in this situation?
I'm not sure about the general approach, but in my opinion, the sooner you create the Ids the better. For Aggregates, you'd create the Id before storing it (maybe a GUID), for entities, the Aggregate can create it the moment of creating/adding the entity. This allows you to perform other actions (eg publishing an event) using these Ids without having to store and retrieve the Ids from the DB, which will necessarily have an impact on how you implement and use your repositories and this is not ideal.
Your backend has two Models:
One Company to Many Employees.
You want to accomplish the following:
Get all Companies
Get a Company by ID
Get all Employees for a Company
Get all Employees
Get a Employee by ID
What is the best practice for handling the REST URLs when your models have 1:M relationships? This is what I have thought of so far:
/companies/
/companies/<company_id>/
/companies/<company_id>/employees/
/employees/
/employees/id/<employee_id>/
Now let's pretend One Company has Many Models. What is the best name to use for "Adding an employee to a Company" ? I can think of several alternatives:
Using GET:
/companies/<company_id>/add-employee/<employee_id>/
/employees/<employee_id/add-company/<company_id>/
Using POST:
/companies/add-employee/
/employees/add-company/
The URIs look fine to me, except maybe the last one, that does not need an additional "id" in the path. Also, I prefer singular forms of words, but that is just me perhaps:
/company/
/company/<company_id>/
/company/<company_id>/employee/
/employee/
/employee/<employee_id>/
The URIs do not matter that much actually, and can be changed at any point later in time when done properly. That is, all the URIs are linked to, instead of hardcoded into the client.
As far as adding an employee, I would perhaps use the same URIs defined above, and the PUT method:
PUT /employee/123
With some representation of an employee. I would prefer the PUT because it is idempotent. This means, if the operation seems to fail (timeout, network error occurs, whatever) the operation can be repeated without checking whether the previous one "really" failed on the server or not. The PUT requires some additional work on the server side, and some additional work to properly link to (such as forms), but offers a more robust design.
As an alternative you can use
POST /employee
With the employee representation as body. This does not offer any guarantees, but it is easier to implement.
Do not use GET to add an employee (or anything for that matter). This would go against the HTTP Specification for the GET method, which states that it should be a pure information retrieval method.
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When designing resource hierarchies, when should one use sub-resources?
I used to believe that when a resource could not exist without another, it should be represented as its sub-resource. I recently ran across this counter-example:
An employee is uniquely identifiable across all companies.
An employee's access control and life-cycle depend on the company.
I modeled this as: /companies/{companyName}/employee/{employeeId}
Notice, I don't need to look up the company in order to locate the employee, so should I? If I do, I'm paying a price to look up information I don't need. If I don't, this URL mistakenly returns HTTP 200:
/companies/{nonExistingName}/employee/{existingId}
How should I represent the fact that a resource to belongs to another?
How should I represent the fact that a resource cannot be identified without another?
What relationships are sub-resources meant and not meant to model?
A year later, I ended with the following compromise (for database rows that contain a unique identifier):
Assign all resources a canonical URI at the root (e.g. /companies/{id} and /employees/{id}).
If a resource cannot exist without another, it should be represented as its sub-resource; however, treat the operation as a search engine query. Meaning, instead of carrying out the operation immediately, simply return HTTP 307 ("Temporary redirect") pointing at the canonical URI. This will cause clients to repeat the operation against the canonical URI.
Your specification document should only expose root resources that match your conceptual model (not dependent on implementation details). Implementation details might change (your rows might no longer be unique identifiable) but your conceptual model will remain intact. In the above example, you'd tell clients about /companies but not /employees.
This approach has the following benefits:
It eliminates the need to do unnecessary database look-ups.
It reduces the number of sanity-checks to one per request. At most, I have to check whether an employee belongs to a company, but I no longer have to do two validation checks for /companies/{companyId}/employees/{employeeId}/computers/{computerId}.
It has a mixed impact on database scalability. On the one hand you are reducing lock contention by locking less tables, for a shorter period of time. But on the other hand, you are increasing the possibility of deadlocks because each root resource must use a different locking order. I have no idea whether this is a net gain or loss but I take comfort in the fact that database deadlocks cannot be prevented anyway and the resulting locking rules are simpler to understand and implement. When in doubt, opt for simplicity.
Our conceptual model remains intact. By ensuring that the specification document only exposes our conceptual model, we are free to drop URIs containing implementation details in the future without breaking existing clients. Remember, nothing prevents you from exposing implementation details in intermediate URIs so long as your specification declares their structure as undefined.
This is problematic because it's no longer obvious that a user belongs
to a particular company.
Sometimes this may highlight a problem with your domain model. Why does a user belong to a company? If I change companies, am I whole new person? What if I work for two companies? Am I two different people?
If the answer is yes, then why not take some company-unique identifier to access a user?
e.g. username:
company/foo/user/bar
(where bar is my username that is unique within that specific company namespace)
If the answer is no, then why am I not a user (person) by myself, and the company/users collection merely points to me: <link rel="user" uri="/user/1" /> (note: employee seems to be more appropriate)
Now outside of your specific example, I think that resource-subresource relationships are more appropriate when it comes to use rather than ownership (and that's why you're struggling with the redundancy of identifying a company for a user that implicitly identifies a company).
What I mean by this is that users is actually a sub-resource of a company resource, because the use is to define the relationship between a company and its employees - another way of saying that is: you have to define a company before you can start hiring employees. Likewise, a user (person) has to be defined (born) before you can recruit them.
Your rule to decide if a resource should be modeled as sub resource is valid. Your problem does not arise from a wrong conceptual model but you let leak your database model into your REST model.
From a conceptual view an employee if it can only exist within a company relationship is modeled as a composition. The employee could be thus only identified via the company. Now databases come into play and all employee rows get a unique identifier.
My advice is don't let the database model leak in your conceptional model because you're exposing infrastructure concerns to your API. For example what happens when you decide to switch to a document oriented database like MongoDB where you could model your employees as part of the company document and no longer has this artificial unique id? Would you want to change your API?
To answer your extra questions
How should I represent the fact that a resource to belongs to another?
Composition via sub resources, other associations via URL links.
How should I represent the fact that a resource cannot be identified without another?
Use both id values in your resource URL and make sure not to let your database leak into your API by checking if the "combination" exists.
What relationships are sub-resources meant and not meant to model?
Sub resources are well suited for compositions but more generally spoken to model that a resource cannot exist without the parent resource and always belongs to one parent resource. Your rule when a resource could not exist without another, it should be represented as its sub-resource is a good guidance for this decision.
if a subresource is uniquely identifiable without its owning entity, it is no subresource and should have its own namespace (i.e. /users/{user} rather than /companies/{*}/users/{user}).
Most importantly: never ever ever everer uses your entity's database primary key as the resource identifier. that's the most common mistake where implementation details leak to the outside world. you should always have a natural business key (like username or company-number, rather than user-id or company-id). the uniqueness of such a key can be enforced by a unique constraint, if you wish, but the primary key of an entity should never ever everer leave the persistence-layer of your application, or at least it should never be an argument to any service method. If you go by this rule, you shouldn't have any trouble distinguishing between compositions (/companies/{company}/users/{user}) and associations (/users/{user}), because if your subresource doesn't have a natural business key, that identifies it in a global context, you can be very certain it really is a depending subresource (or you must first create a business key to make it globally identifiable).
This is one way you can resolve this situation:
/companies/{companyName}/employee/{employeeId} -> returns data about an employee, should also include the person's data
/person/{peopleId} -> returns data about the person
Talking about employee makes no sense without also talking about the company, but talking about the person does make sense even without a company and even if he's hired by multiple companies. A person's existence is independent of whether he's hired by any companies, but an employment's existence does depend on the company.
The issue seems to be when there is no specific company but an employee technically belongs to some company or organization otherwise they could be called bums or politicians. Being an employee implies a company/organization relationship somewhere but not a specific one. Also employees can work for more than one company/organization. When the specific company context is required then your original works /companies/{companyName}/users/{id}
Lets say you want to know the EmployerContribution for your ira/rsp/pension you'd use:
/companies/enron/users/fred/EmployerContribution
You get the specific amount contributed by enron (or $0).
What if you want the EmployerContributions from any or all companies fred works(ed) for? You don't need a concrete company for it to make sense. /companies/any/employee/fred/EmployerContribution
Where "any" is obviously an abstraction or placeholder when the employee's company doesn't matter but being an employee does. You need to intercept the 'company" handler to prevent a db lookup (although not sure why company wouldn't be cached? how many can there be?)
You can even change the abstraction to represent something like all companies for which Fred was employed over the last 10 years.
/companies/last10years/employee/fred/EmployerContribution