I am just trying to make a project in which there might be many employee and doctors may access. it will contain a database and interface and i want to do it in java.But now i am facing a problem -i am not sure how to make the database be able to update at a time ,i mean when an employee update the database the other can access the updated database.i have previously use database in the local host where user is one.
You can use synchronized blocks or monitorfunctions with semaphors in Java.
Put this around the method calling the database and you can control number of simultaneous access to the database.
Related
My aim is to built a trusted synchronization between both local DB and server DB with NO conflictions or deadlock.
I suppose that a user may request his transaction using DML (insert, update and delete) from his local DB where there is NO access to the Interent and when he wants he asks for sync and send all the transaction with the data i need to the server via RESTful API.
My problem when tow sync operations has performed in a different time and different user which may cause a conflict (ex. the first user update a row and the second user tries to update the same row but another column while they were both offline before sync operations).
In the local DB i track every transaction on another table for every type of transaction (inserted_table , updated_table and deleted_table) every one of them contain table_id,row_id and time.
My question when i send these information to the server for the first user when he asks for sync and then after while the second user makes sync too how i can know the information for the first sync process for the first user(like time of the transaction in the local for the first user) to make rollback if i need and how to mange the conflict resolution?
The whole synchronization logic is up to you to decide how it should work!
One solution to your problem is to add columns STATUS(varchar) and LASTMODIFICATION(datetime). When syncing the databases, you can choose the most recent by comparing the LASTMODIFICATION field and overwrite the data with the newest update.
The problem I have right now deals with the SQL UPDATE and DELETE statements concurrently. If the program is only called one after the other then there is no problems, however, if two people decide to run the program it might fail.
What my program does:
A program about food which all has a description and a date of when that description was made. As people enter the description of the food it gets entered into a database where you can quickly retrieve the description. If the description is lets say 7 days old then we delete it cause its outdated. However, if a user enters a food already in the database with a different description then we update it and change the date. The deletion happens after the update/insertion (those that dont need updating will be inserted and then the program checks for outdated things in the database and deletes them).
The problem:
Two people run the program and right as one person is trying to update a food, the other clears it out with the deletion cause it just finished. The update will not happen, and the program will continue with the rest of the updates (<- I read that this is because my driver doesn't stop. Some drivers stop updating if there is an error).
What I want to do:
I want my program to stop at the bad update or grab that food position and restart the process/thread. The restarting will include sorting out which foods needs to be updated or inserted. Therefore, the bad record will be moved into the inserting method and not the update. The update will continue where it left off. And all's well.
I know this is not the only way, so different methods on how to solve this problem is welcome. I have looked up that you can use an upsert statement, but that also has race conditions. (Question about the upsert statement: If I make the upsert method synchronized will it not have race conditions?)
Thanks
There are different pratical solutions to your problem depending on jout jdbc connection management.
If the application is a client server one and it uses a dedicated persistent connection (i.e. it opens a jdbc connection at program startup and it closes when the program shutdowns) for each client you can use a select for update statement.
You must issue a select for update when displaying records to the user and when the user does its action you do what is needed and commit.
This approach serializes the dabatabase operations and if you show and lock multiple records it may not be feasible.
A second approach is usable when you have a web application with a connection pool or when you don't have a dedicated connection you can use for the read and update/delete operation. In this case you have this scenario
user 1 selects its data with jdbc connection 1
user 2 selects its data (the same as user 1) with jdbc connection 2
user 2 submit data causing some deletions with jdbc connection 3
user 1 submit data and lot an update beacuse the data was deleted with jdbc connection 2
Since you cannot realy on the same jdbc connection to lock the data you read, you can issue a select for update before updating the data and check if there are data. If you have the data you can update them (and they will not be deleted by other sessions since every delete command on the same data is waiting your select for update to terminate); if you don't have the data because they where deleted during user display you must reinsert them. You delete statement must have a filter on the date column that represent the last update.
You can use other approaches and avoid the select for update using for example an
update food-table set last_update=? where id=? and last_update=<the last update you have in java program>
and you must check that the update statement did update a row (in jdbc executeUpdate returns the number of rows modified, but you did not specifiy if you are using "plain" JDBC or some sort of framework) and if it did not update a row you must isse the insert statement.
Set transaction level to serializable in java code. Then your statements should look like:
update food_table set update_time = ? where ....
delete from food_table where update_time < ?
You may get an serializable exception in either case. In the case of the update you will need to reinsert the entry. In the second case, just ignore and run again.
We have a Oracle DB with around 4K tables in it. And around 30 different applications accessing the data. We have an issue where one of the application is deleting a row in one of our tables. We do not know which app and why. I'm trying to figure out, but, first thought I got is to use Triggers when ever something is deleted and log it, but, is there a way to find out which application has deleted it in oracle?
Thanks
If you didnt want to go down the autiting or logging route and the application is not distinct from v$session as it stands, you could set the name of the application by calling
dbms_application_info.set_client_info('my_application');
This sets v$session.client_info for the session, which you can read via
dbms_application_info.read_client_info(client_info out varchar2);.
You could then use triggers and to record this value.
I'm creating an application using Java RMI. In my case, more than 10 users are adding data to the database. My application has a class GenerateID and it's on server side. For every user's id generation process is going through this class. It has a method named generateID() I need to know, if more than one users are inserting data to the database at same time, What will be happening?
Thanks in Advance!
i am going to call a function which will retrieve some data value from database. but before that i am sending those data. i am just checking whether those data properly inserted or not with this function call. but inserting data taking some time to insert into the database but my function calling starts before it actually inserts the data into the database. As because of that it is finding that no data is inserted in the database. Can any one tell how do i resolve this issue. How to synchronize this. whether i should get the proper result after the proper insertion into the database. i cant use here runnable interface or thread class. the think that i have to do is to call the data access function after certain time so that data gets enough time to get inserted into the database. please help me out.
Don't know what language you are using, but maybe the function has a parameter which causes it to wait until the query finishes before returning? Something mentioning the word "synchronous?"
Use the Database Driver
I'm not familiar with JDBC so I'm not sure which tools are/aren't available to you, but it seems like you're doing more work than you need to.
Typically the database driver will inform you whether the query was executed successfully, so you should not need to have your application query the data afterward to verify that the data is there. Instead, ask the driver for errors to see whether there was a problem with the query.
If you're inserting a large about of data and your database supports it, you may want to use a transaction to perform your insert. This will pass all of the data into the db, attempt the insert, and warn you of any problems.
If there are problems with the transaction, you can roll back, and the database state will be the same as when you started (obviously you will need to handle the errors to save your data). If there are no problems, you can finish committing the transaction, and rest assured that the database state matches the application state.
Alternatives
If for some reason the above methods won't work, you can try to resolve the race condition using an event pattern. In simple terms, you want to raise an event when the data is done inserting to alert the validator that it can start reading data. The validator will listen for that event and trigger when it hears it.