On different sites it is written that, datasource maintain the connection pool. Yes i agree it will maintain the pool
if we dont close the connection once we are done with transaction. But generally we close the connection once we are done
with transaction. So practically datasource will never have any connection to maintain the pool. Is this correct?
It will be the scenario only when programmer forget to close the connection.Right?
The connection pool maintains open connections for a certain period of time and usually closes them after some time of inactivity. Forgetting to close a connection prevents it from being released back to the pool and later reused, that's why you should always close them. What is the question anyways?
Well, if this is MS .NET, as long as the connection string is identical, the web server will pool the connections automatically. This will occur even if you 'close' them, so it is very efficient.
Related
This is my understanding after reading the Documents:
Pooling, like many other DBs, we have only a number of allowed connections, so you guys all line-up and wait for a free connection returned to the pool. (a connection is like a token in a sense)
at any given time, number of active and/or available connections is controlled in the range of 0-max.
idleTimeoutMillis said is "milliseconds a client must sit idle in the pool and not be checked out before it is disconnected from the backend and discarded." Not clear on this. Generally supposed when a client say a web app has done its CRUD but not return the connection voluntarily believed is idle. node-postgres will start the clock, once reaches the number of milliseconds will take the connection back to the pool for next client. So what is not be checked out before it is disconnected from the backend and discarded?
Say idleTimeoutMillis: 100, does it mean this connection will be literally disconnected (log-out) after idle for 100 millisecond? If yes then it's not returning to the pool and will result in frequent login connection as the doc said below:
Connecting a new client to the PostgreSQL server requires a handshake
which can take 20-30 milliseconds. During this time passwords are
negotiated, SSL may be established, and configuration information is
shared with the client & server. Incurring this cost every time we
want to execute a query would substantially slow down our application.
Thanks in advance for the stupid questions.
Sorry this question was not answered for so long but I recently came across a bug which questioned my understanding of this library too.
Essentially when you're pooling you're saying to the library you can have a maximum of X connections to the Database simultaneously open. So every request that comes into a CRUD API for example will open a new connection and you will have a total of X requests possible as each request opens a new connection. Now that means as soon as a request comes in it 'checks out' a connection from the pool. This also means another request cannot use this connection as it is currently blocked by another request.
So in order to let's say 'reuse' the same connection when one request is done with that connection you have to release it and say it's ready to use again 'checking out'. Now when another request comes in it is able to use this connection and do the aforementioned query.
idleTimeoutMillis this variable to me is very confusing to me and took a while to get my head around. When there is an open connection to a DB which has been released or 'checked out' it is in an IDLE state, which means that anyone wanting to make a request can make a request with this connection as it is not being used. This variable says that when a connection is in an IDLE state how long do we wait until we can close this connection. For various things this may be used. Obviously having open DB connections requires memory and so forth so closing them might be beneficial. Also when autoscaling - let's say you been at max requests/second and and you're using all DB conns then this is useful to keep IDLE connections open for a bit. However if this is too long and you scale down then you can run into prolonged memory as each IDLE connection will require some memory space.
The benefit of this is when you have an open connection and just send a query with it you don't need to re-authenticate with the DB it's authenticated and ready to go.
I'm coding in Java EE and I have some class that manage all the actions with my DB.
I was asking myself when should I open/close the connection with the DB.
Is it better to open and close it in each method ?
Or is it better to open it in the constructor and close it when I finished using my class ?
Thx
there is no generic solution. all decisions depends of concrete task. you should remember next things:
every connect to the database is the application time. if your method will be called too often, your application will waste lot of time to connect and disconnect tasks and much more with the slow network. in case of rarely calls, it will not so important;
if your method connects to database in the constructor for the long time without any operations the connection may be dropped. this is not neccesary, but may cause of the network issues or database connections policy. so before every query should be checked with fast and simple operation like select 'some random text' from dual;
database resources is not infinite and total number of connections is limited. this limit can be very large, but it still exist. so, if your application can be used in parallel several (hungred, thousands) times, it may reach the limits with permanent connections.
if you have no information of future usage of method, I advise use time limited permament connection. it should be opened with first query and closed with timer if method did no queries through this connection for some time like 3-5 seconds. sure, any querier should check connection status before query. open it, if it closed, with touching the closing timer. and don't forget explicitly close connection at destructor.
I would argue that you shouldn't be managing it in code. Every EE server I've worked on has connection pooling to remove the need for you to care about this. Basically you "open" the connection when you need and "close" it when you need. Those words are in quotes because it is up to the pool to manage when a connection is truly opened and closed.
From a design perspective then use the connection only when you need it. Object construction doesn't make sense - what if a method in the class doesn't get called for an hour? What is the purpose of having it open when you don't need it? So if a method needs a connection open and close it in the method.
In Go, when using a SQL database, does one need to close the DB (db.Close) before closing an application? Will the DB automatically detect that the connection has died?
DB will do its best to detect but with no luck, it may not be able to detect. Better to release what is acquired as soon as possible.
send() system call will wait for TCP connection to send data but client won't receive anything.
Power failure, network issue or bare exit happened without properly releasing resources. TCP keepalive mechanism will kick in and try to detect that connection is dead.
Client is paused and doesn't receive any data, in this case send() will block.
As a result, it may prevent
Graceful shutdown of cluster.
Advancing event horizon if it was holding exclusive locks as a part of transaction such as auto vacuum in postgresql.
Server keepalive config could be shortened to detect it earlier. (For example, ~2h 12m default in postgresql will be very long according to workload).
There may be a hard limit on max open connections, until detection, some connections will be zombie (there, unusable but decreases limit).
The database will notice the connection had died and take appropriate actions: for instance, all uncommitted transactions active on that connection will be rolled back and the user session will be terminated.
But notice that this is a "recovery" scenario from the point of view of the database engine: it cannot just throw up when a client disconnects; it rather have to take explicit actions to have consistent state.
On the other hand, to shut down property when the program goes down "the normal way" (that is, not because of a panic or log.Fatal()) is really not that hard. And since the sql.DB instance is usually a program-wide global variable, its even simpler: just try closing it in main() like Matt suggested.
If you're initialising a connection in any function, you're normally better off deferring the call to close immediately, i.e.
conn := sql.Connect() // for example
defer conn.Close()
Which will close the connection once the enclosing function exits.
This is handy when used in a main function since once the program exits, the call to Close() will happen.
I have NHibernate configured with Fluent NNibernate connecting to a PostgreSQL database.
I have a worker class that takes an ISessionFactory as a constructor parameter and consumes messages from a queue. For each message the worker process calls ISessionFactory.OpenSession() and it does some database processing.
When I add more worker processes the performance of the system remains the same which is odd.
After some more investigation I realized that all worker processes are using a single database connection. For example I would add 8 worker processes but on the database I can see only one database connection.
My understanding is that ISessionFactory.OpenSession() will open a new database connection unless the Connection Pool is full.
So is my understanding wrong or is this and issue with the Postgres NHibernate driver.
OpenSession does not open a database connection until needed, and it closes it (i.e. releases it back into the pool) as soon as possible.
By default the session will keep the connection open for the life time of a transaction and as Diego said, it only opens it when needed.
If you want to manage your own connections you can call
ISessionFactory.OpenSession(myConnection);
this question is related with this one.
The problem I'm having is that our application has a bug and it leaves an unclosed transaction at SQL. I know for sure that the connection has been released with SQLFreeHandle but looks like somehow ODBC maintains the connection open to reuse it latter. As the connection remains open the transaction is not rolled back and I'm having a lock.
Is there a way to force ODBC to shutdown the connection and to close the underlying socket?
Thanks in advance.
It sounds like connection pooling is turned on. This article describes how it can be turned off at the driver level.
As Mark said, connection pooling is turned on.
Typically the wrappers for the sql drivers make a rollback call prior to releasing the connection back to the pool.
If you guys have written your own ODBC driver (or wrapper for it), you might consider just modifying the connection dispose/close/release code to do that rollback if there was an open transaction.