I am trying to build an e-commerce java web application, I am running into an issue for booking/purchasing items from the store. Each item can only be purchased once, my issue is that users on two different machines might be purchasing the same item, and for that I was thinking of synchronization. I don't quite understand synchronization and how the application can still run when no one is using it. Does the application run on server side? Can someone please give me some information on this?
One of the possible ways is to lock the field in the database. Locking in the database will prevent it to be used from any application that tries to access it. If you use lock, "synchronization" , from the server side app, you must be sure that only that app have access to the database or you could sell the same product twice. Some study about your database lock mechanism maybe can help you to prevent this issue.
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My employer has currently given me a project that has me scratching my head about synchronization.
I'm going to first talk about the situation I'm in:
I've been asked to create a pdf-report/quotation-tool that takes data (from csv-files; because the actual database the data is on is being used by old IBM software and they for reasons (unknown) don't want any direct access to this database (so instead of making copies of the data to other databases, they apparently found it incredibly fine to just create a folder on the server with loads and loads and loads of CSV-files.)), this piece of software is to load data into the application, query it, transform where needed, do calculations and then return with a pdf-file to the end-user.
The problem here is that getting, querying, and calculating things takes a fair amount of time, the other problem is: they want it to be a WebApp because the business team does not want to install any new software, they're mostly moving towards doing everything online (since the start of the pandemic), it being a WebApp means that every computation has to be done by the WebApp and getting the data likewise.
My question: Is each call to a servlet by a separate user treated as a separate servlet and should I only synchronize the methods on the business logic (getting and using the data); or should I write some code that puts itself in the middle of the servlet, receives a user-id (as reference), that then runs the business-logic in a synchronized-fashion, then receiving data and returning the pdf-file?
(I hope you get the gist of it...)
Everything will run on Apache Tomcat 8 if that helps. Build is Java 11lts.
Sorry, no code yet. But I've made some drawings.
With java web applications, the usual pattern is for the components to not have conversational state (meaning information specific to a specific user's request). If you need to keep state for a user on the server, you can use the http session. With a SPA or Ajax application it's often easier to keep a lot of that kind of state in the browser. The less state you keep on the server the easier things are as your application scales, you don't have to pin sessions to servers (messing up load balancing) or copy lots of session state across a cluster.
For simple (non-reactive) web apps that do blocking i/o, each request-response cycle gets its own dedicated thread from tomcat's pool. That thread delivers the http request to the servlet, handles the business logic and blocks while talking to the database, then carries the http response.
(Reactive webapps are going to be more complex to build, you will need a non-blocking database driver and you will have less choices for databases, so I would steer clear of those, at least for your first web application.)
The threadpool used by tomcat has to protect itself from concurrent access but that doesn't impact your code. Likewise there are 3rd party middletier caching libraries that have to deal with concurrency but you can avoid dealing with it directly. All of your logic is confined to one thread so it doesn't interfere with processing done by other threads unless there are shared mutable data structures. Those data structures would be the part of the application where synchronization might be one of several possible solutions.
Synchronization or other locking schemes are local to one instance of the application. If you want to stand up multiple instances of this application then you need to be aware each one would be locking separately from the others. So for some things it's better to do locking in the database, since that is shared across webapp instances.
If you can make use of a database to store your data, so that you can rely on the database for caching and indexing, then it seems likely your application should be able to avoid having doing a lot of locking.
If you want examples there are a lot of small examples for building web apps using spring at https://spring.io/guides. These are spring boot applications that are self hosted so you can put them together quickly and run them right away.
Going rogue with a database may not be the best course since databases need looking after by DBAs. My advice is put together two project plans, one for using a database, and one for using the flat files. The flat file one will have to allow for addressing issues like handling caching, indexing data, replication of data from the legacy database, and not having standard tools that generate pdfs from sql queries. The alternative plan using a database should have a lot less sorting out of infrastructure and a shorter time til you can get down to cranking out reports.
I have the following problem: I have Java application - Sprint boot, which uses Angular in the frontend. This application needs to store some data on the client side, however, this data is lost when the client changes their browser or opens an anonymous browser tab.
I need an alternative, other than linking data to the user in the database. Something that is implemented in Java itself.
Is there any way I can store data in Java - Even though I know they will be volatile, that is, we can assume that my application server will be up 100% of the time.
**edit
My server run a openshift plataform that have multiple pods, the load baancer of server are configured in a NON-Sticky sessions design. That's why we can assuming that my server will be 100% active.
This really depends on the design of your server. For example, why is it guaranteed to be up 100% of the time? Do you have multiple redundant instances? In that case you need to coordinate that "storage" between all instances; you may even want to deal with a quorum of instances keeping the state etc. Doesn't seem to be trivial. Or do you have just one single instance? But how do you guarantee 100% uptime?
I strongly recommend using some kind of data store or at least distributed cache.
I'm still rather new to java and I think I've started a project with a problem.
I created a job for a friend in which there are employees, shifts, sites and these needed to be loaded at the beginning.
I went looking for solutions and decided to use a Derby database. I've now programmed the Application and it works fine with the database. It loads all parameters and creates objects for handling,
Now I need to deploy my project to my friends computer so he can use it and I think I have the database set up wrong. I think I needed it to be embedded? so it goes with the application.
So my questions are what are my choices,
I read I can change the database to an 'embedded' one by making the database a class? I have no idea how to do this and maybe because I'm new to java, I'm finding all the write ups on this subject difficult to understand.
Alternatively I thought maybe I can install Derby separately and connect to that?
Or maybe I can drop the Derby idea and switch entirely to another database entirely,
I'm a bit confused over my choices here, basically I've built an application around an installation of Derby DB using this line to connect to it.
jdbc:derby://localhost:1527/SG_database
If someone can give me some 'Plain English' options here I would very much appreciate it.
To reconfigure your application to use Derby as an embedded database, all you have to do is change that JDBC Connection URL to jdbc:derby:SG_database, and change your CLASSPATH so that your program references derby.jar rather than derbyclient.jar. You should possibly add ;create=true to the end of that URL so that, the first time your friend runs your application, the database is created on their machine.
But yes, you have other choices, and without knowing a fair amount about your application it's hard to give you very detailed guidance.
When your friend is using the application, do you want you and your friend to be sharing the same set of data? Or is your application designed so that your data and your friend's data have nothing in common?
If you want to be sharing the data, then yes it will be important to have a single instance of the database, and both of you have to share it, in which case a client-server configuration can work quite well.
If you want to be two completely separate applications, with nothing shared, and each of you has your own copy of the data, then an embedded configuration can work quite well.
Perhaps you could simply try the embedded configuration, see how it behaves with your application, and then return here if you have a more specific question to ask?
I have a problem with a product that I am currently working on. Essentially, There is some very commonly used (and very seldomly updated) information that is retrieved from the database on server start up. We do not want to query the database every time this information is needed because it is very frequent. There is a way to update this information through the application (only by an admin). When this method is used, the data in the database is updated and the cached data in that single server (1 of 4) is updated. Unfortunately, if a user hits any of the other servers they will not see the updated information. Restarting the cluster remedies the problem however, that is not a feasible solution for our production environment. Now that I have explained the situation, I am open to suggestions. Thank you for your time.
For a simple solution, you can go to the cluster in the admin console and ripple start it. That stops/stars the nodes gracefully and one at a time. The only impact is a 25% reduction in capacity while it is working.
IBM WebSphere Application Server has a Dynamic Cache that you can use to store Java objects. The cache can be set up to use replication over a replication domain so it can be shared across a cluster.
Your code would use the DistributedMap interface to interact with the cache. All settings for the dynamic cache can be included with your application or it can be pre-configured. Examples are included in the javadoc link.
(Similar to Java EE Application-scoped variables in a clustered environment (Websphere)?)
That is, I think the standard answer would be a "Distributed Object Store". But a crude alternative (that we use) would be to configure a list of server:port combinations to contact to inform each cluster member to update their own copy of the data.
I am a newbie......I have a Java Swing application and it runs great on my machine. I want to access this application from a Server via Citrix. So when I click on the published icon, it would run the main method in the jar file and will enable me to access the application. This application will be able to access a DB in the DB Server.
But I want mutliple users to access the application at the same time and that is where my questions are:
I thought of creating n number of threads available for n number of users (i.e.) I can set a limit on concurrent access. But what will be the entry point.....I mean when I click on the published icon, it means each time the main method will be invoked.
I can think of seperating my User Interface from the Logic layer....but I have no idea how to do it. ofcourse I am following MVC model. My question is in terms of creating multiple instances of the gui each time the application is accessed.
And finally I want to use DB Connection pooling. So, would this mean that I have to create a separate java program that creates this pool and my application will use its datasource?
Can anyone please 'point' me in the right direction? I am not looking for specific ideas but just an idea on how to create this multi user application.
Typically, for a multi-user Swing application you would want to separate the "client" part of the application from the "server" application.
This works as follows:
Each user would get their own running instance of the client application. This can be on their own machine.
The server application is a single instance (or maybe a cluster) that accepts connections from multiple clients and talks to the database
The client applications talk to the server application when they need to access or change data. There are a wide variety of different communication methods you can use.
Optionally, the server application can send notifications to the client (e.g. in situations where data is updated by another client)
You can do without the server application if you are happy to let the clients connect to the same database. This is simpler to set up, but has some downsides:
You need to be much more careful about concurrent access to the database / potential corruption from different clients attempting to alter the same data at the same time.
You need to allow connections to your database, from clients that are potentially untrusted. This is a security risk.
Given that you already have a working application, the second option is probably easier for you to move to. Just be aware of the downsides: the first option is a much better architecture in general.