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Backend developers mostly deal with data, business logic, and the efficient retrieval of data from variety of sources. The list doesn't end here. There are a number of things that a good backend developer is expected to be adept at. This <a href="roadmap.sh/backend" target="_blank">roadmap</a> tried to cover some of them. This post will try to list down the resources instead.
Internet and basic frontend knowledge
Mozilla Developer Network(MDN) docs is a great start, if you are trying to starting to take the web development path. I've given some important links below but you can explore the entirety, there is a ton of information that is up to the point and well explained.
Speaking the browser language(HTML-CSS-Javascript)
Although the below resources are not for backend developers, some of you might be interested in some frontend frameworks that guys in your company are using.
OS and basic machine usage skills
Programming languages and frameworks
Although it is not necessary that you learn every language in the world, it is good to have skills in at least one compiled, scripting language and choose one for a paradigm. Languages are essential tools for programmers and they deserve a section in backend developer roadmap and resources list.
Version control
You won't write clean code on your first iteration most of the time. Also, if you happen to be in a situation where you lost your code due to variety of reasons, then version control systems got you covered. Git and Mercurial are among the most popular version control systems out there. You should definitely learn Git. Below are some resources where you can start.
Databases(relational) and ORM
As we discussed already, backend developers mostly deal with data. Hence, it is essential that you learn several ways to work with the same. Relational databases were the goto for every use case. But, that has changed over the years, and several purpose built databases took its place. Initially you should be able to interact with the relational databases using SQL, or an ORM. But, as you get more comfortable, you should explore the database internals like how indexes work, how joins will effect performance, transactions, isolation levels, and designing for availability and consistency etc.
MySQL architecture and History
Rick Houlihan's talk on matching the workload to DB
Databases(NoSQL)
NoSQL databases came into picture to solve several problems of relational databases like schema rigidity, extensibilty, scalability etc. There are several databases in this space. I will list down some of them and resources to learn about them.
Advanced design patterns for DynamoDB(or any NoSQL in general)
Single table design - DynamoDB
[Why Dynamo scales where relational databases don't] (alexdebrie.com/posts/dynamodb-no-bad-queries)
Different take on NoSQL vs SQL
Databases(Graph)
There is another class of purpose built databases which puts relations in the data, forefront. These databases exploit the related nature of the data and help answer complex queries easily. These databases are the goto when you want to answer queries like friends of friends, what do my friends like, efficiently. Below are some resources where you can learn about these.
Graph databases for beginners series
Big data(optional)
Everything you need to know about Big Data
APIs and authentication
More and more companies are finding value in GraphQL recently. You can learn about the same below. GraphQL
Web security fundamentals
Web application security should not be an after thought. Many developers focus much on the application logic, since the code is generally audited by a security researcher for an initial release. But, developers should write code keeping security in perspective always. The first link below is more relevant to the Rails ecosystem, but it will cover some general security best practices. Be sure to check it out.
Virtual private clouds explained
Cross Origin Resource Sharing(CORS)
If you are working with node.js, you can take advantage of npm audit to find out and fix known vulnerabilities in npm modules.