I have created a java application that is inserting data to a mysql database. Under some conditions, i need to post some of these data via email, using a java application that I will write as well.
My problem is that i am not sure how i should implement this.
From what I understand, I could use a UDF inside MySql to execute a java application, for which there are many against opinions into using it. Let alone that both the database and the mail client application will reside in a VM that i dont have admin access, and dont want to install anything that neither me nor the admin knows.
My other alternative, that I can think of, is to set up the mail client (or some other application), to run every minute just to check for newly inserted data. Is this a better aproach? Isn't it going to use resources for doing almost nothing. At the moment the VM might not be heavily loaded, but i have no idea how many applications there might end up running on the same machine.
Is there any other alternative i should consider using?
You also need to consider the speed of internet, database server load, system resources. If you have enough memory and less load to insert data in databases or database load is not so much then you can approach this by cron setup. For linux call a script for every 5 minutes. The script perform the following-
1. Fetch unread Emails as files
3. Perfrom shell script to read needed data.
3. write data to mysql
4. Delete the email
If you have heavy loaded system then wise you need to do this once or twice in an hour or may vary.
Related
Production Environment: Tomcat 9 on CentOS 7 x64, mysql/mariadb 5.5x
Testing Environment: Tomcat 9 in Eclipse on Windows 7 x64, mysql 5.5x
I'm a Tomcat newbie looking for the best method to have server-wide variables readable/writable from all Webapps for things like MaintenanceMode(on/off) and MaintenanceMessage, etc.
These are the variable properties I'm looking for:
Readable/writable from all instances of all java servlets on all webapps.
Value persists after OS, Tomcat, or Webapp restart.
I must be able to change the value of it from one webapp and then all other webapps recognize the change quickly, ideally without restarting.
Ideally I wouldn't want to read the variable from disk on each server request. In case server is getting DDOSed or something.
Ideally the solution is OS independent.
If it's a disk file solution please recommend a place for me to store the file.
I'm new to Tomcat so some detail in any answers would be appreciated or links to detail. I'll probably be using a servlet on it's own 'admin' webapp that's only accessible through SSH-tunneling, etc, to set the variables. Then the public webapps would respond to any changes, like showing a maintenance message while I backup databases. I could also possibly change the variables using linux commands if needed.
If I stored the server variables in a database that could be fine but I wouldn't want to read the DB on every single request most likely, and when I change a variable I'd have to once again notify every webapp/servlet that something was changed and to re-read the DB.
Thanks for any help.
I implemented this recently in the form of "system messages", some of which are for maintenance. But the effect is the same. We had some additional "requirements" which helped us form the solution. These may or may not match up to your expectations/desires:
Multiple-server coordination
Immediate synchronization was not necessary
We used our relational database for the actual data storage. A single table with "system messages" and a few other fields such as when the messages became effective (not_before DATETIME) and when the messages became ineffective (not_after DATETIME).
On startup, the application reads the system messages table to find the currently-valid messages and stores them in application scope, including their validity dates. Whenever we want to show them on the screen, we check each item in memory and evict any that have expired. This is pretty fast.
Every X minutes (e.g. from cron), we make a request to a special servlet (on each server) that re-loads the system messages from the database. We protect that servlet by only allowing requests from certain IPs, so DOS is not an issue.
Not only can we add a system message from any server in the cluster, but we can also add one by writing directly to the database. That may be advantageous if you don't always want to use your application to generate these events.
You can change the value for X to anything as low as 1 (minute) if you are using cron. If you use some other kind of system, you can probably get updated even more often. I would seriously reconsider your requirement of "immediate" recognition because that requires a system that has much worse performance.
If you can guarantee that only your application can generate these changes, and you have a list of all other servers in the cluster somewhere, you could theoretically ping them all with the new message (or notify them that a new message exists and it's time to update their message-list), but that kind of thing is better-done with an orchestration tool such as Kubernetes, and we are getting a little out of scope IMO.
I'm using the H2 Database Engine for java to have access to a database in my java programs. I developed many java programs which use the same database. The problem is that whenever I start such a program while another is already running it can't access the database because it is opened by the other program.
Is there a way to let both programs have a connection to the database? Whenever one program has to query the database the database should execute the query. In case it is executing the query of the other program the query should be executed directly after the query of the other program. Since my queries don't take long time the user wouldn't recognize that his program has to wait for a moment and everything would be fine.
H2 server mode is what you want.
You need to have at least started the server this way for example:
org.h2.tools.Server.createTcpServer().start();
Then replace all the jdbc url with jdbc:h2://yourhost/yourdb, keeping in mind yourdb.h2.db will be located where the server was started. I strongly advise not to use absolute path in your jdbc url, as it will give away the database path in case of hacking.
Last but not least: using the server mode for all has a performance penalty. You might want to use mixed mode so that the 1st client will have almost embedded performances.
To do this, just replace for this client the url with jdbc:h2:yourdb;AUTO_SERVER=TRUE. You could decide to use the same url for all clients as well: the 1st one to connect will be using embedded mode, the other clients will be using tcp performance.
Note that if you're using H2 > 1.4.*, you need to give absolute or relative path like this: jdbc:h2:./yourdb;AUTO_SERVER=TRUE. Note the ./
I have a program that I've created that is meant to poll an html internal page with different IPs that update and then will run a telnet session to those IPs to see if the device still has a connection... I'm attempting to challenge myself in creating something further with a dynamic webpage instead of my program spitting out console output...
My Issue:
I dont know what technologies / libraries Java has to execute such things
I want:
A Local Server, to upload a page LOCALLY only (no security is needed as this will be strictly intranet)
My program to implement: A database of sorts to save "logs" essentially that a certain IP / device has had successful connections
in the past....maybe stored to an external file is fine i presume (my
program currently has to re-poll everytime i run it.. i want some kind
of "remembering"..
Is it possible this can all be done in one file? so if i want my computer to run this as soon as it starts up... it will run... grab
its current state of the database of IPs... poll them (periodically)
and then persist and save and update the HTML page dynamically....
I hope i'm being as descriptive as possible... Its a bit of an abstract.. I really just want some introduction to different libraries ... a friend recommended stuff like MongoDB or something but I want to stay strictly to Java programming
I have a Java application that sends user score to the mysql table. When the user is done, Java app accesses the .php file on server and the .php server performs a query on the database (inserting score).
I am concerned about the (in)security of this method. I mean, if someone finds out the direct url to the .php on a server, they can produce a lot of mess in the dabase. Can you advise how I could prevent the .php from executing the query other than accessed by the Java app?
edit: The problem is that Java application is NOT run on the server, it's run on the user computer using Java Web Launcher platform. So it's not an applet...
The problem is conceptual. You should never be sure that users can't find out the real address (security by obscurity). You could use SSL, still this is no means against a good guess.
Since the Java program is run on the client side, a .htaccess restricting access to a certain IP is also not an option.
My suggestion is to create a separate user in mysql, grant this user access only to necessary tables and perform the database queries on behalf of this user directly in Java. This way all data is encrypted (see http://dev.mysql.com/doc/refman/5.5/en/ssl-connections.html) and no URL/access point is exposed. Of course it means your MySQL server must be reachable from outside which poses a risk, too. You should have a good root password!
I want to create java desktop application, which stores it's data offline in a database (not just some config files). The application should work fine when the user is offline. When the user becomes online, the offline database should be able to sync with the online master.
Any ideas which technologies can be used to achieve this?
This has been discussed on stackoverflow a lot and it usually boils down to: don't roll out your own solution - This is a very specialized field - Look up SymmetricDS. It does what you want.
One of my fav discussions is Strategy for Offline/Online data synchronization
Use one of the available pure java DB implementations as a local DB. Use any other DB as a remote one.
Implement logic that tries to connect to remote DB and fall backs to local one on failure. If it connects successfully to remote DB implement the data synchronization.
When the local application operates, it should not only change database, but also log the changes. That changes are sent to the server when the connection is available. Also, the application receives logged changes stored on the server (from other application instances).
The main problem is how to merge changes made by different instances. There can be 3 variants:
1) Each application instance can modify only its private part of the whole database. Your are lucky, no merging needed, and server can store only logs and not run the whole database.
2) modifications always can be merged automatically (for example, application can add a value to a common variable, but cannot set it directly). The server runs the whole database, accepts partial logs from clients, generates its own log and sends it to clients.
3) Clients are allowed to do arbitrary modifications. This leads to potential conflicts. In case of conflicts, one of conflicting changes should be rejected. That means, that if a client made local modifications, that modifications can be rejected later by the server. The user interface must reflect this issue. In the rest, this is similar to the variant 2.