In my previous article, I did a detailed discussion about the Bounded Context and learn that to tackle the complexity of a Domain it is the best way to divide the domains into several subdomains and mapped them with different bounded contexts where each business entity/Value object has certain meaning on that context, so every stakeholder of the business like Product owner, Developers, Architect, sponsorers are understood the context and referring entity with the proper name, there should be no confusion of the naming when we discuss the terminology basis on a context. A ubiquitous language which creates a unified language among business stakeholder.
By Bounded context, we properly define a business model, create a different context based on the business domains, but always a functionality spans over multiple business entities , and those entities lie in the different Bounded contexts/domains, so it is utmost important to understand the relationship between the Bounded contexts, to architecting the business solution Context Map is a technique by which we can visualize the relationship between different contexts and Integration architect choose the best integration patterns to communicate to other contexts.
Why Context Map is so important while designing solution?
By UML diagram architect cam understand how different parts will communicate with other parts, It gives the architect a view on the communication between different contexts, that is fine but context map stepped in before UML diagram, it helps to visualize the nature of the relationship and based on the nature , architects can decide what kind of technical solution would be adopted.
The best part of Context Map visualization is, it talks about nature of the relationship, It not only tell is that relationship is upstream or downstream, Publisher or subscriber, it also tells how different team dependent on another team their motifs, their politics everything. so counting all of that now architect in a position to decide optimal solution to minimize the risk while integrating with another context.
In the era of Microservices, Context Map is the key player, because before design the holistic Microservice architecture where every team owns a Microservice, It is important to understand how one team is dependent on other, which teams are in commendable position, which team only seeks help then you can architect the solution best possible way .
Think of our Student online Tutor app, as a full grown app say it has more than fifty microservices are deployed in production , so here fifty teams are coming into play and for developing a functionality say Student registrations , multiple contexts will be affected, so we can say to implement that feature multiple teams will be involved so what would be their relationship, while designing this feature who is the pivot service whose data is most needed obviously that service is in commendable position as if that team is not ready other teams can’t do anything, so all teams should align with that team , they have to sync there product backlog with that team, so here the internal politics comes to the picture , if the data of the service comes from an External team not with in the organization then the solution is way more complex as you can’t force them so only way is to request them and wait for there changes, so based on these diffrent scenarios, politics context map has diffrent solutions. I will cover most important solutions here
 1. Shared Kernel : Shared kernel talks about a partnership relation where two or many teams shared common data model/ value object, it reduces the code duplication as different context use that common model, but that common model/value object is very sensitive, any changes major/minor should be agreed upon all the parties unless it would break other parties code, so more communication and synchronization needs among those teams say one team need a change in the common model but another team is not ready so the priro team has to wait for other oartner to be ready or otherhand othar partner has to change there code unnecessery although that is not heliping them, but to be in sync with other partners, diffrent shades of problems woven while maintaining the pertnership relation ship so choose that pattern  when your common model remains constant or changes once in an era.
In our example, say we developed an analytical module which analysis which courses are most chosen by the students, which students are chosen more than five courses etc, so that module works with Student model, course model so I can say our Analytics module can share Registration modules student model, and they also agreed upon any changes on student model.
Customer/ Supplier: Generally this is the common relationship between two contexts, where a context consumers or depend on data from another context, the context which produces data marked as upstream and the context which consumes data called downstream. When we visualize this relationship in terms of politics we can visualize the power distribution has many shades.
like following
Upstream as the leader: In this type of relationship, the upstream team is in a commendable position, that team does not care about the downstream team, as they producing the data and downstream team is on the mercy of that upstream team they are always changing their model based on the data structure produces by upstream.
In our Student Registration app relationship between Payment app and Notification, app is kind of upstream and downstream where Payment app decide what information in which structure they provide and Notification module consumes that data structure.
Downstream as the Leader: In some cases, the relationship is revert although upstream is produces data, it must have to follow the rule, the data structure for downstream, in this scenario downstream is in a commendable position.
say in our Studen registration system we need to submit Form 16 to government as a tax payee so our payment module has to submit form 16 data to Government exposed API but government API has certain rules and data structure for submitting form 16 data, so although government API is downstream it has total control, our Payment module should communicate with down steam in such a way so it can fulfil downstream rules.
The customer-supplier relation works best when both the parties upstream and downstream are aligned with the work both party agreed upon the interfaces and change in the structure, in case of any changes in the contract both parties will do a discussion synchronize their priority backlogs and agreed upon the changes, If one party does nor care upon another party then every time contract will be broken and it is tough to maintain a customer-supplier relationship.
Conformist: Sometimes, there is a relation between two parties in such a way that downstream team always dependent on an upstream team and they can’t do a mutual agreement with upstream about requirements.  The upstream is not aligned with downstream and does not care they are free to change there published endpoint or contract any time and not taking any request from downstream. It is happens when Upstream team is an external system or under a different management hierarchy , and many downstream systems are registered with it so it can’t give a priority to any downstream, rather then all downstream system must be aligned with upstream contact and data structures if upstream contracts or data structures change it ps downstream responsibility to changes accordingly.
Say, In our online Student registration app we have a free tutorial module where all students or other application can consume our free tutorials and embed them in their application, so here out free tutorial module acts as upstream and independent of any other third party app who consumes our free tutorials , we can;t give any priority to them and we don’t have any contract with them if we change the contracts or data structures it is other third parties duty to change their application accordingly to consume our free tutorials. Other parties are acting as a conformist.
Anti Corruption Layer: When two system interacts if we consume the data directly from upstream we pollute our downstream system as upstream data structure leak through the downstream so if the upstream become polluted our downstream too as it imitates the upstream data while consuming. So it is a good idea while consume data from third party or from a legacy application always use a translation layer where the upstream data translate to downstream data structure before fed in to downstream in that way we can resist the data leakage from upstream, if upstream contract changes it does not pollute downstream internal system only Translation layer has to be changed in order to  adopt new data structure from upstream and convert it into downstream data structure, this technique is called Anti-corruption layer. Anti-corruption layers save the downstream system from upstream changes.
In our application Notification module can implement an ACL while consuming data from payment module so if payment module data structure changes only ACL layers affected.
Open Host : In some cases, your Domain API needs to be accessed by many other services like our Free Tutorial Publisher module, Many external or internal domains want to consume this service, so as Upstream it should be hosted as a service and maintains a protocol and service contract like REST and JSON structure so another system can consume the data.
Published Language: Often two or more system receive and send messages among themselves, in that case, a common language will be needed for the transformation of the data from one system to another like XML, JSON we call that structure as Published language.
The holistic  view of our Student online registration app in terms of context Map
 Conclusion: Context Map is a very important exercise to realize how one domain communicate with other, It gives a proper view of the organization structure, how different domains are distributed, how domain owners are dependent on each other? What is the relationship between team structure? Can they be aligned while developing a feature, based on all the parameters Integration architect can adopt a suitable integration pattern to integrate domains? Prior to designing the integration solution, always architect has to define context map to understand the relationship and structure of the teams based on that Architect can choose the best possible solution.

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DDD: Thinking in terms of Context Map

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