I am building a Java application which queries my SQL server once a minute. Right now, the application is using a connection pool with a single connection (min pool size 1, max pool size 1).
I figured that a pool size of 1 would be enough because of the infrequent queries (once a minute, as mentioned before).
Do I need a connection pool at all, and if the answer is yes, is 1 connection enough? Or should I just not use a pool and open a new connection every minute?
using a connection pool with a single connection (min pool size 1, max
pool size 1).
In such case don't see any need or benefit of using connection pool since at any point in time there will be only one connection object and if it's in use then other request has to wait (or) create a non-pooled connection object.
Connection pooling is generally used to save time/resource from creation/tearing-up the connection object.
In your specific case, you can probably create a connection instance and dispose it off once done with your work.
I would like to give some reason to use the connection pool, potentially with multiple connections. Not sure if you are considering this negative case.
In real world, a query might run more than 1 minute with various reason.
Do you want the application to wait for the hung connection? Or what is your expected behavior for this?
Also if you use connection pool, DB connection initialize process(time and resource consuming) is done while the pool is generated. When you actually use the DB connection pool, some of initialization step should be already done so it reduce repetitive overhead when the application is running query.
If you're accessing the database that rarely, then in practice having a connection pool will not provide any significant benefit. However, I'd keep it if it's already written. Maybe one day your application will grow and you'll find the pool useful. It's not a bad thing to have.
Related
According to lettuce, we don't need connection pool and it uses single thread safe shared connection
https://github.com/lettuce-io/lettuce-core/wiki/Connection-Pooling
But, according to hikari-cp we need to have pool of connections of preferably size connections = ((core_count * 2) + effective_spindle_count)
https://github.com/brettwooldridge/HikariCP/wiki/About-Pool-Sizing
I am confused why we don't need pooling in one case but required in other?
Hikari is a JDBC connection pool. Connection pooling is required because database has long things to do and while that connection open you can not run another query over it, so it is better to have multiple "open" connection ready. Read more here
On the other hand Redis is not a database. Redis is a key value based in memory store/cache used for fast access. There you do not need connection pool because it is simple and fast, you ask to redis is "key1" exist give me the data and you get it or not. On the other hand with database you run long running SQLs sometime stored procedures depends to complexity but it is not one step work.
I have a scenario in production for a web app, where when a form is submitted the data gets stored in 3 tables in Oracle DB through JDBC. Sometimes I am seeing connection time out errors in logs while the app is trying to connect to Oracle DB through Java code. This is intermittent.
Below is the exception:
SQL exception while storing data in table
java.sql.SQLRecoverableException: IO Error: Connection timed out
Most of the times the web app is able to connect to data base and insert values in it but some times and I am getting this time out error and unable to insert data in it. I am not sure why am I getting this intermittent issue. When I checked the connections pool config in my application, I noticed the following things:
Pool Size (Maximum number of Connections that this pool can open) : 10
Pool wait (Maximum wait time, in milliseconds, before throwing an Exception if all pooled Connections are in use) : 1000
Since the pool size is just 10 and if there are multiple users trying to connect to data base will this connection time out issue occur ?
Also since there are 3 tables where the data insertion occurs we are doing the whole insertion in just one connection itself. We are not opneing each DB connection for each individual table.
NOTE: This application is deployed on AEM (Content Management system) server and connections pool config is provided by them.
Update: I tried setting the validation query in the connections pool but still I am getting the connection time out error. I am not sure whether the connections pool has checked the validation query or not. I have attached the connections pool above for reference.
I would try two things:
Try setting a validation query so each time the pool leases a connection, you're sure it's actually available. select 1 from dual should work. On recent JDBC drivers that should not be required but you might give it a go.
Estimate the concurrency of your form. A 10 connections pool is not too small depending on the complexity of your work on DB. It seems you're saving a form so it should not be THAT complex. How many users per day do you expect? Then, on peak time, how many users do you expect to be using the form at the same time? A 10 connections pool often leases and retrieves connections quite fast so it can handle several transactions per second. If you expect more, increase the size slightly (more than 25-30 actually degrades DB performance as more queries compete for resources there).
If nothing seems to work, it would be good to check what's happening on your DB. If possible, use Enterprise Manager to see if there are latches while doing stuff on those three tables.
I give this answer from programming point of view. There are multiple possibilities for this problem. These are following and i have added appropriate solution for it. As connection timeout occurs, means your new thread do not getting database access within mentioned time and it is due to:
Possibility I: Not closing connection, there should be connection leakage somewhere in your application Solution
You need to ensure this thing and need to check for this leakage and close the connection after use.
Possibility II: Big Transaction Solution
i. Is these insertion synchronized, if it is so then use it very carefully. Use it at block level not method level. And your synchronized block size should be minimum as much as possible.
What happen is if we have big synchronized block, we give connection, but it will be in waiting state as this synchronized block needs too much time for execution. so other thread waiting time increases. Suppose we have 100 users, each have 100 threads for that operation. 1st is executing and it takes too long time. and others are waiting. So there may be a case where 80th 90th,etc thread throw timeout. And For some thread this issue occurs.
So you must need to reduce size of the synchronized block.
ii. And also for this case also check If the transaction is big, then try to cut the transaction into smaller ones if possible:-
For an example here, for one insertion one small transaction. for second other small transaction, like this. And these three small transaction completes operation.
Possibility III: Pool size is not enough if usability of application is too high Solution
Need to increase the pool size. (It is applicable if you properly closes all the connection after use)
You can use Java Executor service in this case .One thread One connection , all asynchronous .Once transaction completed , release the connection back to pool.That way , you can get rid of this timeout issue.
If one connection is inserting the data in 3 tables and other threads trying to make connection are waiting, timeout is bound to happen.
Why should I prefer using a connection pool instead of static variable from a static class in Tomcat to hold the database connection?
This seems to me equivalent of using a connection pool having the capacity to store just one connection. So, a related question is: why the capacity of a connection pool needs to be bigger than one connection?
Thank's in advance.
With a pool, you can have multiple threads using different connections. Do you really want to limit your web application to handling one db-related request at a time? :) (And adding the complication of synchronizing to make sure that one thread doesn't try to use that single connection while another request is doing so...)
It would be generally a very bad idea to have a connection pool with a capacity of 1 - but at least if you did so, you could later increase the capacity without changing any other code, of course.
(EDIT: And as noted in other answers, connections in a pool can be closed and reopened if they become stale or broken in some way.)
The reason is to increase scalability, robustness and speed.
If you're creating a web application, there can be many concurrent HTTP requests coming in, each served by a different thread.
If you have only one static connection to the database, you need synchronization around that connection. You can't share a connection between several threads, That means each HTTP request have to wait for someone else using the database. And you need to fix up/reconnect that connection if something goes wrong with it at one point or another.
You could open a connection at the start of each HTTP request - however opening a new database connection can be expensive, and you don't get much control over how many database connections you have. Databases can be overwhelmed by having too many connections.
A connection pool solves this, as you have a pool of already opened connections that can be handed out to serve an HTTP request, or to different parts of the applications that needs to do database operations, and is returned to the pool when the database operation is finished, ready to be used again by something else.
A connection pool of just 1 connection rarely makes sense though - however connection pools take care of many other things as well, such a closing the connection and opening a new one if a connection goes stale or is otherwise in a bad state, as well as it takes care of the synchronization when there is are no more connections to hand out at a particular time.
If you're using a connection pool with only one connection it is equivalent to have one static connection - like you mentioned and there's no advantage for connection pool in regards.
The strength of a connection pool is when you're using multiple connections (multiple threads) - it saves you the effort of managing the connections (open/close connections, boilerplate-code, smart resource handling etc).
Using a connection pool for one connection only is kind of like paving a 10-lane road that will be used by one car only - lot of overhead with (almost) no gain.
Using a connection pool is not just about sharing connections: it is about leveraging years of experience with broken JDBC drivers and all the weird ways in which a Connection can become unusable. Using a single static connection is not only a bottleneck, but a very fragile solution. It will cause your application to break on a regular basis, and even restarting the application will not clean up the problems: you may get a connection leak.
I'm trying to write a multithreading program that connects to a MySQL database and processes the returned set for a query (which has thousands of rows). The problem is that I have implemented the connection pool and I get every thread to open the connection to the database and get the resulting set. But I don't understand which is the advantage of using connection-pooling if retrieving that big set takes such a lot of time. It wouldn't be better if I get the whole set with only one connection (without using pooling) and then I use thread pooling to process it? Or is there a way that every thread takes the next row of the resulting set?
If you have a limited number of threads, I would have a connection per thread.
A connection pool is more efficient if the number of threads which could use a connection is too high and those thread use the connections a relatively low percentage of the time.
I am using a connection pool in my project. I want to know how many connections are opened?
I have given 20 connections as max no of connections. What will happen if it exceeds 20? Will the pool manage this or it will through error?
Its more of a configuration related problem. But in general the pool will throw exception when the new request for a connection results in the number of connection exceeding the max connection setting.
Must depend on the particular connection pool implementation.
Conceptually it would be possible to wait, fail or victimise.
Waiting indefinitely is potentially a very bad thing, in a badly configured system with antisocial clients keeping connections for too long the number of waiters could grow very large.
Arbitrarily victimising some existing client and taking their connection is usually not reasonable (or indeed possible) for DB connections. So that's not likely to be seen.
Which leaves a failure - you'll get an exception on the lines of "No Connections Available". Some connection pools actually wait a while (for a configurable period of time) before throwing that exception - this deals better with cases of occasional peaks in demand.
To know the number of connections opened in Oracle you can write a query like this.
SELECT s.program, s.server, p.spid, s.username FROM v$session s, v$process p
WHERE s.type = 'USER'
and s.username != 'sys';
The query assumes that your connected to Oracle in a dedicated server mode also the query counts connections made other than the user sys.
Then you can use the result in your application. And the second question, "What will happen if it exceeds 20?" depends on your implementation of connection pool and I won't say more since it has been answered.
Hope this is helpful.