Develop Reference

How would I implement an embedded SFTP Server on Openshift

Background Context:
Due to enterprise limitations, an uncooperative 3rd party vendor, and a lack of internal tools, this approach has been deemed most desirable. I am fully aware that there are easier ways to do this, but that decision is a couple of pay grades away from my hands, and I'm not about to fund new development efforts out of my own pocket.
Problem:
We need to send an internal file to an external vendor. The team responsible for these types of files only transfers with SFTP, while our vendor only accepts files via REST API calls. The idea we came up with (considering the above constraints) was to use our OpenShift environment to host a "middle-man" SFTP server (running from a jar file) that will hit the vendor's API after our team sends it the file.
I have learned that if we want to get SFTP to work with OpenShift we need to set up of our cluster and pods with an ingress/external IP. This looks promising, but due to enterprise bureaucracy, I'm waiting for the OpenShift admins to make the required changes before I can see if this works, and I'm running out of time.
Questions:
Is this approach even possible with the technologies involved? Am I on the right track?
Are there other configuration options I should be using instead of what I explained above?
Are there any clever ways in which an SFTP client can send a file via HTTP request? So instead of running an embedded SFTP server, we could just set up a web service instead (this is what our infrastructure supports and prefers).
References:
https://docs.openshift.com/container-platform/4.5/networking/configuring_ingress_cluster_traffic/configuring-externalip.html
https://docs.openshift.com/container-platform/4.5/networking/configuring_ingress_cluster_traffic/configuring-ingress-cluster-traffic-service-external-ip.html#configuring-ingress-cluster-traffic-service-external-ip

That's totally possible, I have done it in the past as well with OpenShift 3.10. The approach to use externalIPs is the right way.

Design considerations for J2EE webapp on Tomcat in Amazon WebServices

My project is looking to deploy a new j2ee application to Amazon's cloud. ElasticBeanstalk supports Tomcat apps, which seems perfect. Are there any particular design considerations to keep in mind when writing said app that might differ from just a standalone tomcat on a server?
For example, I understand that the server is meant to scale automatically. Is this like a cluster? Our application framework tends to like to stick state in the HttpSession, is that a problem? Or when it says it scales automatically, does that just mean memory and CPU?

Automatic scaling on AWS is done via adding more servers, not adding more CPU/RAM. You can add more CPU/RAM manually, but it requires shutting down the server for a minute to make the change, and then configuring any software running on the server to take advantage of the added RAM, so that's not the way automatic scaling is done.
Elastic Beanstalk is basically a management interface for Amazon EC2 servers, Elastic Load Balancers and Auto Scaling Groups. It sets all that up for you and provides a convenient way of deploying new versions of your application easily. Elastic Beanstalk will create EC2 servers behind an Elastic Load Balancer and use an Auto Scaling configuration to add more servers as your application load increases. It handles adding the servers to the load balancer when they are ready to receive traffic, and removing them from the load balancer and deleting the extra servers when they are no longer needed.
For your Java application running on Tomcat you have a few options to handle horizontal scaling well. You can enable sticky sessions on the Load Balancer so that all requests from a specific user will go to the same server, thus keeping the HttpSession tied to the user. The main problem with this is that if a server is removed from the pool you may lose some HttpSessions and cause any users that were "stuck" to that server to be logged out of your application. The solution to this is to configure your Tomcat instances to store sessions in a shared location. There are Tomcat session store implementations out there that work with AWS services like ElastiCache (Redis) and DynamoDB. I would recommend using one of those, probably the Redis implementation if you aren't already familiar with DynamoDB.
Another consideration for moving a Java application to AWS is that you cannot use any tools or libraries that rely on multi-cast. You may not be using multi-cast for anything, but in my experience every Java app I've had to migrate to AWS relied on multi-cast for clustering and I had to modify it to use a different clustering method.
Also, for a successful migration to AWS I suggest you read up a bit on VPCs, private IP versus public IP, and Security Groups. A solid understanding of those topics is key to setting up your network so that your web servers can communicate with your DB and cache servers in a secure and performant manner.

How to load existing http sessions from an existing server to another server?

I would like to know if Servlet specifications provides a way to load http sessions into my web application.
The idea is simple : every time a new http client is connected, a new session is created... and I will send this session and its values into a database (for the time being this step is easy to do).
If this "master server" dies, another machine will take its IP address, so http clients will now send their requests to this new machine (lets call it "slave server").
Here I would like my slave server retrieve sessions from the old server... but I don't know which method from Servlet specifications can "add" session ! Is there a way to do it ?
PS: it's for an university project, so I cannot use already existing modules like Tomcat's mod_jk for this homemade load-balancer.
EDIT:
I think that a lot of people think I am crazy to not use already existing tools. It's an university project, and I have to make it with my bare hands in order to show to my professors the low level mecanisms that I have used. I already know it would be crazy to use what I am doing in production, when this project will be finished, it will be thrown in the trash.
For the moment, I didn't find a "standard way" to make it with the Servlet specifications, but I can maybe do it with Manager and Session from Tomcat native classes... How can I get the instances for those interfaces ?

This isn't exactly a new idea and is called session replication. There are a couple of ways to do this. The easiest ones imho are (in ascending order of preference):
Jetty's Session clustering with a database
Tomcat's Session clustering. I personally prefer the BackupManager, which makes sure that a session lives on 2 servers in a cluster at any given point in time and forwards clients accordingly. This reduces the network traffic for session replication to a bare minimum.
Session replication with a distributed cache like hazelnuts or ehcache. There are plugins for both jetty and Tomcat to do this. Since most often a cache is used anyway, this is to be the best solution for me. What I tend to do is to put 2 round robin balanced varnish servers in front of such a cluster, which serve the dual purpose role of load balancing the cluster and serving static content from in memory cache.
As for your university project, I'd turn in an embedded jetty with automatic session replication which connects to other servers via broadcast using hazelcast. Useful, not overcomplicated (iirc, you need to implement 2 relatively simple interfaces), yet powerful. Put a varnish in front of your test machines and you should be good to go.

This feature is supported by all major Java EE application server vendors out of the box, so you shouldn't implement anything by yourself. As Markus wrote it is referred as session replication or session persistence. You can take a look at WebSphere Liberty which is available for free for development. It supports it out of the box, without need to implement anything. You just need to:
install Liberty Download just the Liberty profile runtime
configure session replication Configuring session persistence for the Liberty profile
install and configure IBM Http Server for load balancing Configuring a web server plug-in for the Liberty profile

Log, database and mail support in Java desktop apps

I'm a Java EE developer and we typically use Weblogic to deploy our apps. Now I'm faced with a new desktop application which requires logging, database connectivity and mail.
After some investigation I'm realizing that desktop apps are a completely new world to me and I'm not sure if I'm choosing the right libraries to support my app.
These are my questions:
In our Weblogic projects we used Log4j and I want to use it again in my desktop app. Is it a bad idea? Should I use a better logging framework?
In Weblogic we retrieve database connections with JNDI but now it seems impossible to do the same. How do I perform the same action in a desktop application so I can connect with a remote database? Is the combination c3p0 + database driver a good approach for this?
Is there any framework/JAR which provides all this stuff (log + ddbb + mail) as an integrated solution? Workmates told me Spring could help. I also found Warework.

In our Weblogic projects we used Log4j and I want to use it again in
my desktop app. Is it a bad idea? Should I use a better logging
framework?
No, it is not a bad idea and perfectly works. Personally, I'd go with java.util.logging as it does the job fairly well and it reduces your applications' footprint (storage). Although, it's configuration is a bit tricky.
In Weblogic we retrieve database connections with JNDI but now it
seems impossible to do the same. How do I perform the same action in a
desktop application so I can connect with a remote database? Is the
combination c3p0 + database driver a good approach for this?
You can directly connect to your database using pure java.sql JDBC API (tons of examples available in the internet), but always have to distribute the proprietary database drivers as part of your application (mySQL, Oracle, DB2, etc.). Furthermore it's possible to directly use connection pools provided with those drivers by using their proprietary APIs (fairly easy to encapsulate). Nevertheless, there are a number of issues:
latency; database protocols are fairly sensitive when it comes to latency (distance between client and database server). Having a database in the UK and desktop clients in US is probably not a good idea.
security 1; you have to distribute database user credentials to each and every desktop client. Be aware of that.
security 2; your database security requirements may demand for transport security (packet encryption).
change management; applying non-backward compatible updates to your database requires you to update all desktop clients (believe me - it's not fun).
network; depending on your environment, certain ports and/or protocols may be blocked.
Is there any framework/JAR which provides all this stuff (log + ddbb +
mail) as an integrated solution? Workmates told me Spring could help.
I also found Warework.
Logging and database access are not an issue and work fairly well without any third-party framework. Of course, those frameworks might provide value regarding other aspects (abstraction, DI, JDBC abstraction, etc.), but this is a topic of detailed software design. Sending emails directly from a desktop application might become an issue, regardless of the framework in use. Just some things to keep in mind:
which SMTP relay server do you want to use?
in case of an enterprise environment, your IT operations teams might not allow you to use their SMTP server from each desktop (keep spam in mind).
Conclusion: In desktop scenarios an application server is not a bad idea either. You should have your desktop application to communicate with an application server only by using e.g. JSON, XML, SOAP over HTTP/HTTPS or RMI, etc. The application should be responsible for the complex tasks like database access, transaction management, fine grained security, email, etc.

What are the specific uses of Java Application Server that cannot be done with web servers?

I am a little confused about the roles of a java application server and its differences from a web server.
I found many sites explaining the same difference between the two but not to my satisfaction.
So please explain me about the two following cases:-
1)App. Server and its difference with web server:
From these two links:
Difference between an application server and a servlet container?
What is the difference between application server and web server?
web server: It handles everything through http protocol by accepting requests from clients and sending
responses to them with the help of its servlet container(e.g Apache Tomcat)
App. Server: An application server supports the whole of JavaEE like JMS,JPA,RPC etc.
Now what I am confused with is that how can I use a lot of JavaEE APIs like JMS,JPA etc. with my Tomcat
by adding their jar files in my web application ?
Does that mean that if I use an appliation server I don't have to add those jar files?(I don't think so)
2)The roles of an appl. server (This is very important to me)
From Wikipedia
http://en.wikipedia.org/wiki/Application_Server
An application server provides services such as security,transaction support etc.
"The term is often used for web servers which support the JavaEE" -- It sounds like if we add the required jar files of JavaEE APIs a web server becomes an appl. server.What about it.
Now my question is how an application server performs the tasks of security control or transaction management by itself ?
E.g. in my web application using Spring framework I am providing security by using spring-security and transaction management by using #Transactional annotation and all those things you know.
So does the appl. server have anything to do with my security or transaction management or it has its own ways ?
Forgive my ignorance.

Using Spring, you're in fact embedding some kind of Java EE container inside your application. But even when using Spring, if you need JTA support (because you need distributed XA transactions), you'll need to use an additional transaction manager. If you need JMS, you'll need to install an additional JMS broker. If you need connection pooling, you'll need to use an additional connection pool. Sometimes it's as simple as adding additional jars to the classpath and properties or XML files. Sometimes it's harder.
A Java EE app server comes with everything bundled. You have less flexibility, but you don't need to install, configure and make everything work by yourself.

When you use the Java EE framework, that is a specification. So the application server, if it is Java EE compliant, needs to implement this. So once it is implemented the specification, then it will address Security,transaction etc because it is mentioned in the spec. So it is a contract. Whereas, in a web server, it will just pull out your static resource. There is no need for handling other stuff.
In case of the Spring framework, the framework knows how to handle transaction, security etc. So particularly the developer need not look into these aspects which are implemented by the Application Server in the other scenario.
how an application server performs the tasks of security control or transaction management by itself
It is rather the specification that address these issues, not the application server. So, the duty of the app server is to implement these.
So, if your application is Java EE compliant, then these areas will be addressed and the implementation would have been done by the app server.

May be this is oversimplification,
A web server is basically a HTTP server serving contents over http protocol. So a web server is simply about serving the contents over http protocol. A typical example would be Apache web server. This is simply a file server.
Now the question is where does the web server gets the contents from ? Possible sources are
Static contents (the contents like images/css etc) which are not generated on request but statically served.
Dynamic contents: Simply put, the contents to be served are generated upon the user request.
For the static contents, the web server does not need anything as it simply reads the file and serves it.
For dynamic contents, the web server might need help of additional components which will generate the contents to be served.
Here the Application Server comes into picture.
Now these additional components referred earlier, might interact with database or some other system etc.
In a web environment where your website is exposed to huge number of users (intended/unintended), you need typical services like transaction/security/concurrency etc. so that the user get expected responses and do not see inconsistencies in the behavior of the application.
An application server has inbuilt abilities to manage transaction/security/concurrency/resource management. generally these are referred as Managed services and environment offered by them is called Managed Environment where these basic services are managed by the application server and programmer does not have be bother for them.
Application Server needs web servers or we can say Web servers use Application server's services to generate dynamic contents.
For example, JBoss uses Tomcat as inbuilt web server. Whereas web logic has its own web server. Tomcat again can be called as application server (in principle) as it also offers managed environment for servlets (it manages concurrency and instance pool of servlets/JSPs ).
Coming your your example of Spring:
An Application server will come inbuilt with transaction/security etc whether you need it or not. The Spring offers a very nice way handling this. Spring has all these things BUT you use what you need. Not just these, but just a Java Web Sever like Tomcat is sufficient to build a full fledged services that needs an application server.