In an n-layered (5-layer, let's say) application, if there are options available for a certain operation to bypass one of the layers and communicate with the next layer directly, can it still be called an "n-layer" architecture, or does it turn into an (n-1)-layered (4-layer) architecture?
And should the layer in question, which you can bypass, be considered as a "layer" at all?
EDIT: I'm trying to implement an application with following structure -
Presentation layer (contains WPF grids)
Application layer (contains application logic and workflow as application services, extracts display model objects from domain model objects, which are then bound to the UI grids)
Domain layer (contains domain model objects only)
Repository (stores data fetched from the database, isolates the lower layers from the upper layer)
Data mapping layer (maps domain model objects to data model objects)
Data access layer (contains data model objects, and stores and retrieves data to and from the database)
-each one above is implemented as separate project and the domain layer is referenced by the application layer, repository and data mapping layer. Now the thing is, the application layer is directly communicating with the repository, not through the domain layer and the domain layer (if I can call it a layer at all) is acting just like a cross-cutting reference. So that's where my question comes, should I call it domain "layer"? I think NOT. But in domain-driven Design there exists a domain layer, right? There must be something wrong in my architecture? Where and what is it?
You could have as many layers as you want and call it an n-layered system...whether they are used properly or loosely coupled is another question.
The fact that you talk about bypassing a layer may mean you've over engineered a solution or you have implemented a layer in an unhelpful/incorrect way...You'd need to be providing some samples of usage to really help out more here...
Related
I understand about the power of layers for additive operations, ie extending a diagram by showing more options/more detail etc.
However what I want to understand is what is the way in which layers can be used to fundamentally change the objects in the base diagram.
for instance, if I want a business process whereby the connections between entities no long go from A to B to C but go from A to D to E, I want to be able to remove the connectors from the base layer within my FMO layer and add the new connectors acordingly.
Is there a clever way of achieving this using layers and lucidchart?
You can achieve that by having 3 layers: one for the elements that don't change (your "base layer"), one for the connectors (or elements) that you want to hide and finally one layer for the new elements. You can create a button with actions to toggle the second and third layers so that they are not visible at the same time and easily see your model evolution.
In an n-tier application, in the business layer, I need to read database entities multiple times for validation purposes, so I decide to add caching layer to reduce the round trips to the database to fetch the data that will rarely change,
my question is where the caching layer should be added, or how the architecture should be,
to add caching layer without changing the whole architecture ( n-tier )
The layers:
API layer
||
Business Layer
||
Database Layer
Any recommendations or examples I can follow?
I was wondering how Microservices in the Streaming Pipeline based on Event Driven Architecture can be truly decoupled from the data model perspective. We have implemented a data processing pipeline using Event-Driven Architecture where the data model is very critical. Although all the Microservices are decoupled from the business perspective, they are not truly decoupled as the data model is shared across all the services.
In the ingestion pipeline, we have collected data from multiple sources where they have a different data model. Hence, a normalizer microservice is required to normalize those data models to a common data model that can be used by downstream consumers. The challenge is Data Model can change for any reason and we should be able to easily manage the change here. However, that level of change can break the consumer applications and can easily introduce a cascade of modification to all the Microservices.
Is there any solution or technology that can truly decouple microservices in this scenario?
This problem is solved by carefully designing the data model to ensure backward and forward compatibility. Such design is important for independent evolution of services, rolling upgrades etc. A data model is said to be backward compatible if a new client (using new model) can read / write the data written by another client (using old model). Similarly, forward compatibility means a client (using old data model) can read / write the data written by another client (using new data model).
Let's say in a Person object is shared across services in a JSON encoded format. Now a one of the services introduces a new field alternateContact. A service consuming this data and using the old data model can simply ignore this new field and continue its operation. If you're using Jackson library, you'd use #JsonIgnoreProperties(ignoreUnknown = true). Thus the consuming service is designed for forward compatibility.
Problem arises when the service (using old data model) deserializes a Person data written with the new model, updates one or more field values and writes the data back. Since the unknown properties are ignored, the write will result in data loss.
Fortunately, binary encoding format such as Protocol Buffer 3.5 and later versions preserve unknown fields during deserialization using old model. Thus when you serialize the data back, the new fields remain as is.
There maybe other data model evolutions hou need to deal with like field removal, field rename etc. The nasic idea is you need to be aware of and plan for these possibilities early on in the design phase. The common data encoding formats are JSON, Apache Thrift, Protocol Buffer, Avro etc.
I'm a bit confused in which situations these patterns should be used, because in some sense, they seem similar to me?
I understand that Layered is used when system is complex, and can be divided by its hierarchy, so each layer has a function on different level of hierarchy, and uses the functions on the lower level, while in the same time exposes its function to higher level.
On the other hand, Pipe-and-Filter is based on independent components that process data, and can be connected by pipes so they make a whole that executes the complete algorithm.
But if the hierarchy does not exist, it all comes to question if order of the modules can be changed?
And an example that confuses me is compiler. It is an example of pipe-and-filter architecture, but the order of some modules is relevant, if I'm not wrong?
Some example to clarify things would be nice, to remove my confusion. Thanks in advance...
Maybe it is too late to answer but I will try anyway.
The main difference between the two architectural styles are the flow of data.
On one hand, for Pipe-and-Filter, the data are pushed from the first filter to the last one.
And they WILL be pushed, otherwise, the process will not be deem success.
For example, in car manufacturing factory, each station is placed after one another.
The car will be assembled from the first station to the last.
If nothing goes wrong, you will get a complete car at the end.
And this is also true for compiler example. You get the binary code after from the last compiling process.
On the other hand, Layered architecture dictates that the components are grouped in so-called layers.
Typically, the client (the user or component that accesses the system) can access the system only from the top-most layer. He also does not care how many layers the system has. He cares only about the outcome from the layer that he is accessing (which is the top-most one).
This is not the same as Pipe-and-Filter where the output comes from the last filter.
Also, as you said, the components in the same layer are using "services" from the lower layers.
However, not all services from the lower layer must be accessed.
Nor that the upper layer must access the lower layer at all.
As long as the client gets what he wants, the system is said to work.
Like TCP/IP architecture, the user is using a web browser from application layer without any knowledge how the web browser or any underlying protocols work.
To your question, the "hierarchy" in layered architecture is just a logical model.
You can just say they are packages or some groups of components accessing each other in chain.
The key point here is that the results must be returned in chain from the last component back to the first one (where the client is accessing) too.
(In contrast to Pipe-and-Filter where the client gets the result from the last component.)
1.) Layered Architecture is hierarchical architecture, it views the entire system as -
hierarchy of structures
The software system is decomposed into logical modules at different levels of hierarchy.
where as
2.) Pipe and Filter is a Data-Flow architecture, it views the entire system as -
series of transformations on successive sets of data
where data and operations on it are independent of each other.
Greetings ye ol whimsical denizens of truthful knowledge,
Got a quickie for ya'll:
I'm wondering if the Application Layer is analogus with the UI Layer, generally ?
I'm reading Evans DDD book and he keeps referring to the Application Layer, but doesn't
mention the UI explicitly, and so I'm left to wonder.
Could someone please help me make this distinction ? Thanks.
The application layer contains the application behavior, i.e. what happens when the user clicks somewhere. In front of the application layer there is often a presentation layer which defines the look-and-feel of the application and specific GUI widgets used. Together, these form the UI.
domain <- application <- presentation
DDD is mostly concerned with the domain layer and a forming a ubiquitous model/language. It is usually not concerned with how layers are defined, except that non-domain concepts are kept out of the domain layer and in other layers such as the application layer.