I have a scenario where i have a database which will be updated at any time by end user... my java application need to cache the data once any change in the database is done? How is it possible? Some one help me in clearing this issue?
I would like to know how to cache the database data once any updates is made in the database?
You can register a stored procedure as trigger that notifies your Java application about the update. Read about triggers here:
https://dev.mysql.com/doc/refman/5.7/en/trigger-syntax.html
Your question can be interpreted in different ways:
You want to minimize the latency between update in the database and the cache update. Eventual consistency is okay.
The cache is kept consistent with the database content
The latter is much more difficult to achieve. If consistency is required, then, at some point, the database transaction must wait until the application is notified and has acknowledged. You will run into lots of interesting questions in the area of distributed computing.
Application is hosted on multiple Virtual Machines and DB is on single server. All VMs are pointing to single Instance of DB.
In this architecture, I have a table having very few record. But this table is accessed and updated by threads running on VMs very heavily. This is causing a performance bottleneck and sometimes record level exception. Database level locking does not seem to be best option as it is introducing significant delays in request processing.
Please suggest if there is any other technique to solve this problem.
Few questions first!
Is your application using connection pooling? If not, please use it. Creating a JDBC connection is expensive!
Is your application read heavy/write heavy?
What kind of storage engine are you using in your MySQL tables? InnoDB or MyISAM. If your application is write heavy, please use InnoDB based tables as it uses row level locking and will serve concurrent requests better.
One special case - if you are implementing queues on top of database tables, find a database that has a built-in queue operation and use that, or use a reliable messaging service. Building queues on top of databases is typically not efficient. See e.g. http://mikehadlow.blogspot.co.uk/2012/04/database-as-queue-anti-pattern.html
In general, running transactions on databases is slow because at the end of each transaction the database needs to be sure that enough has been saved out to disk that if the system died right now the changes made by the transaction would be safely preserved. If you don't need this you might find it faster to write a single non-database application that does what the database does but doesn't write anything out to disk, or still does database IO but does the minimum possible. Then instead of all of the VMs talking to the database directly they would all talk to this application.
We have a situation where we have to perform a lengthy query to the database based on human input. As the input changes, the query has to be done over and over again, and the input may change once per second.
The problem is, we know that this will cause a spike in server activity for several seconds, and since it is not critical to have an answer immediately or on every input change, it means we can afford executing or not executing the query.
The criteria we would like to use is the current state of the database server, and only allow the query to be done if it is in a low or medium load state, skipping the query when the database server is under stress.
We use Oracle database for this, and so far we have not found any way, from Java, to do this except by actually loading into the server a known query and benchmark it, but that is essentially adding some load to the server. So my question: is there any other way, specifically in Oracle database, where we can discover from the Java side of the application the load of the database?
Depending on how you define "low or medium load state", I'd guess that hitting v$osstat would give you the information you're after Of course, hitting v$osstat constantly will also add to the load on the server. You may want to write a job that copies the v$osstat data to a table you control periodically (and can thus index appropriately) so that your application can hit that table rather than hitting the dynamic performance view constantly. Depending on the goal (i.e. are you trying to ensure that other users have enough resources or are you trying to ensure that your app remains responsive), you may want to use Resource Manager to control resource utilization among users, you may want to run the query asynchronously from the application, and/or you may want to use some sort of cache at the middle tier to avoid hitting the database every time.
The underlying problem I want to solve is running a task that generates several temporary tables in MySQL, which need to stay around long enough to fetch results from Java after they are created. Because of the size of the data involved, the task must be completed in batches. Each batch is a call to a stored procedure called through JDBC. The entire process can take half an hour or more for a large data set.
To ensure access to the temporary tables, I run the entire task, start to finish, in a single Spring transaction with a TransactionCallbackWithoutResult. Otherwise, I could get a different connection that does not have access to the temporary tables (this would happen occasionally before I wrapped everything in a transaction).
This worked fine in my development environment. However, in production I got the following exception:
java.sql.SQLException: Lock wait timeout exceeded; try restarting transaction
This happened when a different task tried to access some of the same tables during the execution of my long running transaction. What confuses me is that the long running transaction only inserts or updates into temporary tables. All access to non-temporary tables are selects only. From what documentation I can find, the default Spring transaction isolation level should not cause MySQL to block in this case.
So my first question, is this the right approach? Can I ensure that I repeatedly get the same connection through a Hibernate template without a long running transaction?
If the long running transaction approach is the correct one, what should I check in terms of isolation levels? Is my understanding correct that the default isolation level in Spring/MySQL transactions should not lock tables that are only accessed through selects? What can I do to debug which tables are causing the conflict, and prevent those tables from being locked by the transaction?
I consider keeping transaction open for an extended time evil. During my career the definition of "extended" has descended from seconds to milli-seconds.
It is an unending source of non-repeatable problems and headscratching problems.
I would bite the bullet in this case and keep a 'work log' in software which you can replay in reverse to clean up if the batch fails.
When you say your table is temporary, is it transaction scoped? That might lead to other transactions (perhaps on a different transaction) not being able to see/access it. Perhaps a join involving a real table and a temporary table somehow locks the real table.
Root cause: Have you tried to use the MySQL tools to determine what is locking the connection? It might be something like next row locking. I don't know the MySQL tools that well, but on oracle you can see what connections are blocking other connections.
Transaction timeout: You should create a second connection pool/data source with a much longer timeout. Use that connection pool for your long running task. I think your production environment is 'trying' to help you out by detecting stuck connections.
As mentioned by Justin regarding Transaction timeout, I recently faced the problem in which the connection pool ( in my case tomcat dbcp in Tomcat 7), had setting which was supposed to mark the long running connections mark abandon and then close them. After tweaking those parameters I could avoid that issue.
I'm doing a school software project with my class mates in Java.
We store the info on a remote db.
When we start the application we pull all the information from the database and transform it into objects to use in our application (using java sql statemens).
In the application we edit some of these objects and then when we exit the application
we save or update information in the database using Hibernate.
As you see we dont use Hibernate for pulling in information, we use it just for saving and updating.
We have 2, but very similar problems.
The loading of object(when we start the app) and the saving of objects(with Hibernate) in the db(when closing the app) is taking too much time.
And our project its not a huge enterprise application, its a quite small app, we just manage some students, teachers, homeworks and tests. So our db is also very very small.
How could we increase performance ?
later edit: if we use a local database it runs very quick, it just runs slow on remote databases
Are you saying you are loading the entire database into memory and then manipulating it? If that is the case, why don't you instead simply use the database as a storage device, and do lookups and manipulation as necessary (using Hibernate if you like, or something else if you don't)? The key there is to make sure that you are using connection pooling, as that will reduce the connection time.
If this is what you are doing, then you could be running into memory issues as well - first, by not caching the entire database in memory, you will reduce memory and will spread out the network load from the beginning/end to the times when it needs to happen.
These 2 sentences are red flags for me :
When we start the application we pull
all the information from the database
and transform it into objects to use
in our application (using java sql
statemens). In the application we edit
some of these objects and then when we
exit the application we save or update
information in the database using
Hibernate.
Is there a requirements reason that you are loading all the information from the database into memory at startup, or why you're waiting until shutdown to save changes back in the database?
If not, I'd suggest a design change. If you've already got Hibernate mappings for the tables in the DB, I'd use Hibernate for both all of your CRUD (create, read, update, delete) operations. And, I'd only load the data that each page in your app needs, as it needs it.
If you can't make that kind of design change at this point, I think you've got to look closely at how you're managing the database connections. Are you using connection pools? Are you opening up multiple connections? Forgetting to release them?
Something else to look at. How are you using Hibernate to save the entities to the db? Are you doing a getHibernateTemplate().get on each one and then doing an entity.save or entity.update on each one? If so, that means you are also causing Hibernate to run a select query for each database object before it does a save or update. So, essentially, you'd be loading each database object twice (once at the beginning of the program, once before saving). To see if that's what's happening, you can turn on the show_sql property or use P6Spy to see exactly what queries Hibernate is running.
For what you are doing, you may very well be better off serializing your objects and writing them out to a flat file.
But, much more likely, you should just read / update objects directly from your database as needed instead of all at once, for all the reasons aperkins gives.
Also, consider what happens if your application crashes? If all of your updates are saved only in memory until the application is closed, everything would be lost if the app closes unexpectedly.
The difference in loading everything from a remote DB server versus loading everything from a local DB server is the network latency / pipe size. The network is a much smaller pipe than anything else. Two questions: first, how much data are we really talking about? Second, what is your network speed? 10/100/1000? Figure between 10 and 20% of your pipe size is going to be overhead due to everything from networking protocols to the actual queries themselves.
As others have stated, the way you've architected is usually high on the list of "don't do". When starting, pull only enough data to initialize the app. As the user works through it, pull what you need for that task.
The ONLY time you pull everything is when they are working in a disconnected state. In that case, you still don't load everything as objects in the application, you just work from a local data store which gets sync'ed with the remote server every so often.
The project its pretty much complete. we cant do large refactoring on it now.
I tried to use a second level cache for Hibernate when saving. EhCacheProvider.
in hibernate.xml:
net.sf.ehcache.hibernate.EhCacheProvider
i have done a config for the cache, ehcache.xml:
i have put the cache.jar in the project build path
and i have set the hibernate property for every class and set in the mapping.
But this cache doesn't seem to have an effect. I dont know if it works(if it is used).
Try minimising number of SQL queries, since every query has its own overhead.
You can enable database compression, which should speed things up when there is a lot of data.
Maybe you are connecting to the database many times?
Check the ping time of remote database server - it might be the problem.
As your application is just slow when running on a remote database server, I'd assume that the performance loss is due to:
Connecting to the server: try to reuse connections (pass the instance around) or use connection pooling
Query round-trip time: use as few queries as possible, see here in case of a hand-written DAL:
Preferred way of retrieving row with multiple relating rows
For hibernate you may use its batch functionality and adjust hibernate.batch_size.
In all cases, especially when you can't refactor larger parts of the codebase, use a profiler (method time or sql queries) to find the bottleneck. I bet you'll find thousands of queries, each taking 10ms RTT) which could be merged into one.
Some other things you can look into:
You can allocate more memory to the JVM
Use the jconsole tool to investigate what the bottlenecks are.
Why dont you have two separate threads?
Thread 1 will load your objects one by one.
Thread 2 will process objects as they are loaded.
Your app will seem more interactive at startup.
It never hurts to review the basics:
Improving speed means reducing time (obviously), and to do that, you find activities that take significant time but can be eliminated or replaced with something that uses less time. What I mean by activity is almost always a function call, method call, or property call, performed on a specific line of code for a specific purpose. If may invoke I/O or it may invoke computation, or both. If its purpose is not essential, then it can be optimized.
Many people use profilers to try to find these time-wasting lines of code, but most profilers miss the target because they look at functions, not lines, they go to sleep during I/O, and they worry about "self time".
Many more people try to guess what could be the problem, or they ask others to guess, such as by asking on SO. Such guesses, in the nature of guesses, are sometimes right - more often not, but people still invest time and resources in them.
There's a very simple way to find out for sure, without guessing, what could fruitfully be optimized, and here is one way to do it in Java.
Thanks for your answers. Their were more than helpful.
We completely solved this problem like so:
Refactored the LOAD code. Now it uses Hibernate with Lazy Fetching.
Refactored the SAVE code. Now it saves, just the data that was modified and right after the time it was modified. This way we dont have a HUGE save an the end.
Im amazed of how good it all went. The amount of new code we had to write was very very small.