SOLID Is Dead, Long Live SOLID

Software development is constantly evolving, and so are the principles we follow to write clean, maintainable code. For years, SOLID principles have been the go-to guide for developers, but are they still as useful as we think? Or have they become a set of "rules" that limit our creativity and adaptability?

In this talk, we'll take a fresh look at SOLID. We'll unpack what these principles were meant to achieve, where they help, and where they hold us back. Instead of blindly following "best practices," we'll explore how modern development approaches — like event-driven systems, functional programming, and AI-driven development — push us to think differently.

But this isn’t a call to throw SOLID away. It’s about rethinking it. You’ll leave with new ways to approach design, keeping what's useful from SOLID and letting go of what no longer serves us. It’s not about following rules — it’s about making better choices for real-world projects.

What You’ll Learn:

* Why following SOLID "by the book" can lead to over-engineering.
* How modern development techniques challenge old design principles.
* Simple, practical ways to design flexible and maintainable code.

Join us for an honest conversation about the realities of software design today. It’s time to rethink what “good code” really means.

Transcript

1. SOLID DEAD IS SOLID LONG LIVE LEMİ ORHAN ERGİN co-founder, craftgate THE NEW PERSPECTIVE THE OLD LEGACY
2. I know SOLID principles Really? How you apply? S for Single responsibility. I forgot the others. Of course we know SOLID. We think we use it all the time while coding and during code reviews. And of course it is a good thing. I've never heard the opposite. EVERYONE KNOWS SOLID PRINCIPLES, KNOWS THEM WELL!
3. Preexisting knowledge impedes one’s ability to reach an optimal solution Every a tt empt referring to past solutions adds a new bracket to the wall of illusion of competence Agile, Devops, Clean Coding, SOLID, else? THE EINSTELLUNG EFFECT means mindset, attitude you past experience gives strong opinion about what it really is WE ALREADY KNOW IT, WHY BOTHERS? you never doubt the truth of what you know
5. UNLEARN forget what you already know try to look from other perspectives re-learn from different views WHAT IF ALL WE KNOW IS WRONG, OR INCOMPLETE, OR MISSING years can change how things look, like how paintings change. real mona lisa seems different, isn’t it ?
6. GROWS SOFTWARE HOW LET’S START FROM A NEW PERSPECTIVE!
7. Payment API after a payment are completed, we save the data to database every design starts with the simpliest solution IMPLEMENT PAYMENT OPERATIONS 1 Save payment Retrieve payment GROWS SOFTWARE HOW DB
8. Payment API We need to search existing payments FEATURE REQUEST where do we need to add the search operation ? It is a payment operation, so add it to same service ? Save payment Retrieve payment DB
9. Payment API it’s a simple as querying the database with a search criteria and returning the results we add it to payment component as a new responsibility, a new flow 2 ADD SEARCH TO EXISTING FLOW Save payment Retrieve payment Search payment start with the simplest solution DB
10. Payment API Search service requires additional models, introduces new dependencies and differentiates from core payment flow payment service starts to be huge and unmaintainable (new dependencies for search operation are not used and relevant with core payment flow) search flow has different business needs and triggered by different roles a change in the repository for search improvements directly triggers change in core payment flow Save payment Retrieve payment Search payment DB
11. Payment API separate the search db operation from other db operations SPLIT INTO BOUNDED CONTEXTS 3 Save payment Retrieve payment Search payment two bounded contexts DB
12. Search results are expected to be returned as files in csv and excel formats FEATURE REQUEST Payment API Save payment Retrieve payment Search payment DB
13. Payment API generate report simply searches the data and fills it in a file with the given extension ADD NEW FLOW TO SEARCH MODULE 4 Save payment Retrieve payment Search payment Generate report we put generate report to the context of search DB
14. Payment API Improvements in report generation triggers deployments on search, which is used excessively deploying the whole api is required whenever something added to report logic, that increases the risc on payment Save payment Retrieve payment Search payment Generate report DB the user of report generation is different than the users of payment and search operations
15. Payment API Report API it’s time to introduce a new reporting api with generate report capabilities MOVE TO A NEW MICRO SERVICE 5 Save payment Retrieve payment Generate report Search payment DB
16. Report generation takes time and users wait too long, timeout errors start to occur Payment API Report API things go even worse when multiple users trigger reports at the same time Save payment Retrieve payment Generate report Search payment DB
17. Payment API Report API QUEUE each demand is queued, the throttlling logic can be managed Save payment Retrieve payment Generate report Search payment Demand report QUEUE DEMANDS AND PROCESS 6 two bounded contexts DB
18. Payment API Report API QUEUE Report generation affects performance of search operations since the data source is shared, the load of reporting api has direct impact on payment operations Generate report Demand report Save payment Retrieve payment Search payment DB
19. Payment API Report API QUEUE ELASTIC separate the read model of report generation from the model of payment operations Generate report Demand report Save payment Retrieve payment Search payment SEPARATE THE DATASOURCE 7 DB
20. Payment API Report API ELASTIC Inform the user when the report is generated FEATURE REQUEST Save payment Retrieve payment Search payment QUEUE Generate report Demand report Report API DB
21. Payment API ELASTIC Notification API email notification api directly sends emails and slack messages that provided by the report api INTRODUCE NOTIFICATION SERVICE 8 Save payment Retrieve payment Search payment Report API QUEUE Generate report Demand report Report API Send Slack message Send email DB
22. Payment API ELASTIC Notification API email Report needs to know notification api and how to trigger its functionalities If notification api stops working, we lose messages Save payment Retrieve payment Search payment Report API QUEUE Generate report Demand report Report API Send Slack message Send email DB
23. Payment API ELASTIC email QUEUE keep messages in queue and manage the error logic with retries QUEUE NOTIFICATION MESSAGES 9 Save payment Retrieve payment Search payment Report API QUEUE Generate report Demand report Report API Notification API Send Slack message Send email DB
24. Payment API ELASTIC email QUEUE Whenever queue stops responding, the whole flow stops and messages are lost we lose messages when the queue is down :( Save payment Retrieve payment Search payment Report API QUEUE Generate report Demand report Report API Notification API Send Slack message Send email we lose demands when the queue is down :( DB
25. Payment API ELASTIC email Outbox Inbox make messaging transactional and be sure you create messages MAKE MESSAGING TRANSACTIONAL 10 Save payment Retrieve payment Search payment Report API Generate report Demand report Report API Notification API Send Slack message Send email Generate report Send notification DB
26. Payment API ELASTIC email Outbox Inbox We design a kind-a maintainable, expandable, robust system at the end make messaging transactional and be sure you create messages Save payment Retrieve payment Search payment Report API Generate report Demand report Report API Notification API Send Slack message Send email DB Generate report Send notification
27. Extract to method, to classes, to functions Centralize decision making (behavior) Make responsible of one behavior Apply design patters Clearify boundaries of contexts Extract functionality as a new microservice Seperate by behaviors, not models or packages Avoid layered arc, build with hexagonal arch Avoid wrappers around CRUD operations Do not depend on same datasource Avoid distributed monoliths Avoid centralized common modules Do not depend on implementations Let the control of flow inverted (IoC) Prefer duplication to make independent Introduce abstrations to eliminate down times WE MANAGED COMPLEXITY BY DECREASING COUPLING AND INCREASING COHESION WHILE REFACTORING AND ADDING NEW CODE
28. https://twitter.com/KentBeck/status/10668695983 THE LAWS OF PHYSICS OF SOFTWARE DESIGN
29. COUPLING In highly coupled systems, one change triggers a change in somewhere else The measure of the strength of interconnection, measure of independence If one system waits, changes, works, fails, starts, stops due to another systems action, these systems are highly coupled with the butterfly effect, every change thiggers something bigger Controller Service Service Service Repository Repository Utility Rest Client
30. COHESION The code that changes together (i.e. in the same feature), stays together (only coupled to each other) Belonging together, relatedness, sharing same purpose/responsibility Cohesiveness exists inside boundaries, that makes it a module, a component, an element, a team, an organization
31. Coupling & Cohesion perfectly fit to many other levels of abstractions of the world ONE TEAM TEAM A TEAM B when do we decide to split a team / an api / a module into two ? like team topologies
32. EPENDENCY INVERSION PRINCIPLE INGLE RESPONSIBILITY PRINCIPLE PEN-CLOSED PRINCIPLE ISKOV SUBSTITUTION PRINCIPLE NTERFACE SEGREGATION PRINCIPLE S O L I D
33. Solid is not about good or simple design, it's about be tt er design. Solid is not about understanding OOP be tt er, it is about understanding the code (i.e. the design) be tt er. Solid is not the goal, it is a guideline for evaluating the way you refactor so ft ware into a be tt er design. IS NOT ABOUT... SOLID
34. IS ABOUT MANAGING COMPLEXITY BY TAKING THE CONTROL OF COST BY COUPLING & COHESION SOLID SOLID is about limiting the impact of change by making easy to change the cost of change is the cost of software the impact of dependencies on easiness of change CODE PLANS PRIORITIES DECISIONS TEAMS APIS/SERVICES INFRASTRUCTURE
35. is it about not writing too much code in one place ? SINGLE RESPONSIBILITY PRINCIPLE SRP
36. SINGLE RESPONSIBILITY PRINCIPLE A class should only have a one single responsibility. There should never be more than one reason to change. A component should do only one thing, and do it right. This principle is about people. WHAT WE HAVE LEARNED SO FAR SRP
37. SINGLE RESPONSIBILITY PRINCIPLE what is responsibility ? from what perspective ? many small classes ? you mean "high cohesion" ? what about fixing bugs and adding features ? what about cross cutting concerns ? converters ? facades ? patterns ? that's not always true A class should only have a one single responsibility. There should never be more than one reason to change. A component should do only one thing, and do it right. This principle is about people. multi layered architecture is not about SRP Programmers stated that needs clarifications WHAT WE HAVE LEARNED SO FAR SHOULD BE CHALLENGED SRP
38. SEPARATED CONCERN PRINCIPLE SRP is all about separation of concerns & contexts and increasing cohesion in an optimum level. A component should belong to one concern based on business responsibilities. Without context, every module can violate SPR to some extend. There is no such a thing as "do one thing” or “single”. There exists di ff erent levels of abstractions. It triggers how we split things into pieces. PRODUCT TEAMS FINE-GRAINED TASKS TEAM TOPOLOGIES BOUNDED CONTEXTS ENCAPSULATION INFIRMATION HIDING COGNITIVE FIT AGGREGATES MICROSERVICES SINGLE RESPONSIBILITY PRINCIPLE SRP keep cohesion as high as possible
39. is it about small interfaces on top of huge codebase ? INTERFACE SEGREGATION PRINCIPLE ISP
40. Clients should not be forced to depend on methods they do not use. Many client specific interfaces are be tt er than one general purpose interface. Interfaces should be small. INTERFACE SEGREGATION PRINCIPLE ISP WHAT WE HAVE LEARNED SO FAR
41. Clients should not be forced to depend on methods they do not use. Many client specific interfaces are be tt er than one general purpose interface. Interfaces should be small. Programmers stated that needs clarifications is having interfaces a must ? should every class have an interface ? interface soup ? single method interfaces ? then aggregate again ? INTERFACE SEGREGATION PRINCIPLE WHAT WE HAVE LEARNED SO FAR SHOULD BE CHALLENGED ISP WHAT WE HAVE LEARNED SO FAR SHOULD BE CHALLENGED
42. Limit coupling surface, minimize number of methods in the contract, limiting points to communicate. Keep abstractions highly cohesive so that users don’t end up depending on things they don’t need. Avoid depending on things that you do not use. INTERFACE SEGREGATION PRINCIPLE GATEWAY INFO HIDING ISOLATION COMPOSITION LIMITED CONTACT SURFACE PRINCIPLE ISP LIMIT DEPENDENCIES RICH DOMAIN MODEL lower the number the touching points of coupling
43. is it about subtypes that I've never used ? LISKOV SUBSTITUTION PRINCIPLE LSP
44. LISKOV SUBSTITUTION PRINCIPLE LSP If an object y has all the properties of object x, then we can safely use y anywhere we use x and that y is a subtype of x. If y does not have all the properties of x, it is not a subtype of x. Every derived class should be substitutable for their base class. WHAT WE HAVE LEARNED SO FAR
45. If an object y has all the properties of object x, then we can safely use y anywhere we use x and that y is a subtype of x. If y does not have all the properties of x, it is not a subtype of x. Every derived class should be substitutable for their base class. isn't it all OOP related ? what if we do not use subtypes at all ? Programmers stated that needs clarifications LISKOV SUBSTITUTION PRINCIPLE LSP is inheritance a must ? WHAT WE HAVE LEARNED SO FAR SHOULD BE CHALLENGED WHAT WE HAVE LEARNED SO FAR SHOULD BE CHALLENGED
46. A class/module/team should do what it sounds like it does. Any behavioral change breaking this that should be a di ff erent dependency. LSP is not about inheritance, it is about behaviour of code units. Build so ft ware systems from interchangeable parts, those parts must adhere to a contract that allows those parts to be substituted one for another. LISKOV SUBSTITUTION PRINCIPLE LSP GATEWAY INFO HIDING ISOLATION COMPOSITION TEST DOUBLES INTER-CHANGEABLE DEPENDENCIES PRINCIPLE prefer un-fixed dependencies
47. is it dependency injection in reality ? DEPENDENCY INVERSION PRINCIPLE DIP
48. DEPENDENCY INVERSION PRINCIPLE DIP High-level modules should not depend on low-level modules. from code that is stable separate code that changes frequently business code domain logic integration code client specific code framework dependent code delay technological decisions immune to technical evolution test domain in isolation knows about needs WHAT WE HAVE LEARNED SO FAR
49. DEPENDENCY INVERSION PRINCIPLE DIP Abstractions should not depend on details. Details should depend on abstractions. interfaces ports implementations adapters the depending class has no knowledge about the concrete class that it is going to use WHAT WE HAVE LEARNED SO FAR
50. DEPENDENCY INVERSION PRINCIPLE DIP High-level modules should not depend on low-level modules. Abstractions should not depend on details Details should depend on abstractions. WHAT WE HAVE LEARNED SO FAR SHOULD BE CHALLENGED WHAT WE HAVE LEARNED SO FAR SHOULD BE CHALLENGED Programmers stated that needs clarifications where is the inversion ? isn’t it too abstract ? concrete classes are depent on interfaces in nature, so what ?
51. DIP is about making the business independent of external systems by making coupling cheap and so ft . Decouple your behavioral models from your data models. DIP decouples the construction of an object from its use. DEPENDENCY INVERSION PRINCIPLE DIP PORTS & ADAPTERS HOLLYWOOD PRINCIPLE EVENT DRIVEN DESIGN INVERSION OF CONTROL SELF ORGANIZATION MODULAR DESIGN PLUGINS & ADD-ONS soften the impact of coupling and improve cohesiveness DECOUPLED DEPENDENCIES PRINCIPLE
52. OPEN CLOSED PRINCIPLE OCP am I not allowed to change existing code anymore ?
53. OPEN CLOSED PRINCIPLE OCP So ft ware entities should be open for extension, but closed for modification. Developers must support new functionality without editing the source code of the existing modules. The source code of such a [class] is inviolate. No one is allowed to make source code changes to it. You can use interfaces as extension points to make your code truly adaptable. WHAT WE HAVE LEARNED SO FAR
54. OPEN CLOSED PRINCIPLE OCP So ft ware entities should be open for extension, but closed for modification. Developers must support new functionality without editing the source code of the existing modules. The source code of such a [class] is inviolate. No one is allowed to make source code changes to it. You can use interfaces as extension points to make your code truly adaptable. Programmers stated that needs clarifications source code can’t be changed ? who can write perfect code ? abuse of inheritance ? change requires refactoring WHAT WE HAVE LEARNED SO FAR SHOULD BE CHALLENGED WHAT WE HAVE LEARNED SO FAR SHOULD BE CHALLENGED
55. Expose potential extension points. For protection against change to the existing code, identify points of evolution points, create a stable interface around them. We need just enough adaptability. You should be explicit in what you allow and disallow to be extended. PROTECTED VARIATION PRINCIPLE ROOT OF ALL PATTERNS OPEN CLOSED PRINCIPLE OCP information hiding, PV and OCP are all same principle with a different name and view PROTECTED VARIATION PRINCIPLE manage the growth of codebase by adding coupling in control EVOLUTIONARY DESIGN TEAM STANDARDS CONTINUOUS REFACTORING CONWAY’S LAW
56. SRP Separated Concerns keep cohesion as high as possible ISP Limited Contact Surface lower the number the touching points of coupling prefer un-fixed dependencies Interchangeable Dependencies LSP DIP manage the growth of codebase by adding coupling in control Decoupled Dependencies OCP Protected Variation so ft en the impact of coupling and improve cohesiveness Cohesion Coupling SOLID PRINCIPLES
57. One-Stop-Shop Payment Orchestration talent@craftgate.io we are hiring! co-founder, craftgate https://speakerdeck.com/lemiorhan LEMİ ORHAN ERGİN