I am using MySQL database and hibernate and JSP.using hibernate select database store value and prepared view and display using Ajax.i am polling database every 1 seconds using Java script timer that called a ajax function and return the new responds,it result me an error
JDBCExceptionReporter:78 - Data source rejected establishment of connection, message from server: "Too many connections"".
Help me to sort-out the above define problem.
Make sure you close the session (and connection) after using it
Make sure the maximum connection configured for mysql is sufficient
Use of some caching layer. It is insane to hit the database every 1 second from each user.
If you are making some chat application, consider comet and server-side pub-sub solutions (jmx for example).
Related
I'm trying to understand whether it is possible to achieve the following:
I have multiple instances of an application server running behind a round-robin load balancer. The client expects GET after POST/PUT semantics, in particular the client will make a POST request, wait for the response and immediately make a GET request expecting the response to reflect the change made by the POST request, e.g:
> Request: POST /some/endpoint
< Response: 201 CREATED
< Location: /some/endpoint/123
> Request: GET /some/endpoint/123
< Response must not be 404 Not Found
It is not guaranteed that both requests are handled by the same application server. Each application server has a pool of connections to the DB. Each request will commit a transaction before responding to the client.
Thus the database will on one connection see an INSERT statement, followed by a COMMIT. One another connection, it will see a SELECT statement. Temporally, the SELECT will be strictly after the commit, however there may only be a tiny delay in the order of milliseconds.
The application server I have in mind uses Java, Spring, and Hibernate. The database is MySQL 5.7.11 managed by Amazon RDS in a multiple availability zone setup.
I'm trying to understand whether this behavior can be achieved and how so. There is a similar question, but the answer suggesting to lock the table does not seem right for an application that must handle concurrent requests.
Under ordinary circumstances, you will not have any issue with this sequence of requests, since your MySQL will have committed the changes to the database by the time the 201 response has been sent back. Therefore, any subsequent statements will see the created / updated record.
What could be the extraordinary circumstances under which the subsequent select will not find the updated / inserted record?
Another process commits an update or delete statement that changes or removes the given record. There is not too much you can do about this, since it is part of the normal operation. If you do not want such thing to happen, then you have to implement application level locking of data.
The subsequent GET request is routed not only to a different application server, but that one uses (or is forced to use) a different database instance, which does not have the most updated state of that record. I would envisage this to happen if either application or database server level there is a severe failure, or routing of the request goes really bad (routed to a data center at a different geographical location). These should not happen too frequently.
If you're using MyISAM tables, you might be seeing the effects of 'concurrent inserts' (see 8.11.3 in the mysql manual). You can avoid them by either setting the concurrent_insert system variable to 0, or by using the HIGH_PRIORITY keyword on the INSERT.
Say I have app.js as the main app on the server, and it imports(requires) 4 other scripts that route login, join, write article, delete article requests. I currently have connection instances through
var connection = mysql.createConnection(mysqlConfig) in EACH of the login.js, join.js, write.js, delete.js.
I have a feeling that this is bad practice. What should I do? Should I create a mysqlconnection.js that exports a connection instance, and require that in each of the scripts that require a connection?
Deciding the number of connections depends on your application. If you have a sporadically used application with very few calls to mysql, then a single connection might work. However, looking at your use case, it seems that multiple users will be trying to login, write, etc. In such a case I recommend using Connection Pooling. https://github.com/coopernurse/node-pool
Well I have an issue with a Codeigniter application and the database connections. As far as I've read on the Internet, CodeIgniter uses singleton to handle instances of the objects, such as the database object. So, with this, I understand that always that there is an instance of the database live there would be only 1 connection in mysql (I am using mysql).
So, I am using an Ajax cross domain request to save "visits" of several sites in different servers in a centralized database. The problem now is that as the sites that I am saving the requests from are worldwide I am getting a huuuuge amount of requests, I have no problem saving the data in the database, the server is very powerful, the thing is that some how, every time a request happen the I have a bran new connection to the database, so I am getting more than 1000 live connections in mysql per minute... and then mysql crashes because it only supports up to 9000 live connections.
Isn't is supposed that using singleton (natively by codeigniter) I would only have 1 live connection and not one per request?
Thank you all for reading, hope you can help with this.
Regards
Hi am confused with sql servers session. What does it actually mean? Does it keep track of the client like httpSession? I have read some documents on query life cycle. None talks about the sesion. Most of the documents say that after the query is recived by the server it gets parsed and then maintains a syntax tree and then execution plan and then executes the query and then a dispatch palan and then dispatches the resultset to the client who issued the query on the server. In the whole story where does the session on sql server like mysql server fits in and what actually it does? or There is no session concept on Mysql server(any sql server)? am i in wrong imagination?
A session in this context usually just refers to a single client connection.
The client connects to the DB server and authenticates; this is the start of the session.
When the client disconnects (gracefully or not) the session ends.
This is relevant for things like temporary tables or transactions: Un-committed transactions will be rolled back by the DBMS and all temporary tables created through this connection (=session) are discarded when the client disconnects, i.e. when the session ends.
Note that a client does not necessarily actively end a session or connection. The client may crash, or the network connection may break, or the server may shut down &c. Any of this implicitly terminates the session.
Problems may arise when a (client) application uses a connection pool keeping connections (and sessions) open and handing them out transparently to different application components. When not handled correctly, errors may occur because a given session may already be 'spoiled' by a previous operation. If, for example, one routine on the client creates a temporary table named 'X' and fails to explicitly drop it afterwards, the next routine that 'inherits' this session may encounter an error when trying to create another temporary table of that name, because it already exists in this specific session; which couldn't be the case if the connection/session was freshly created.
"Session" is mainly a generic term. You connect to a server (MySQL, Oracle, FTP, IRC... whatever), you do your stuff and finally disconnect when you're done. That has been a session.
HTTP is a particular case. It's a stateless protocol: if you spend an hour reading a web site, you don't remain connected for a whole hour. You make a quick connection, fetch an item at a time (an HTML document, a style sheet, a picture...) and close the connection. (Internals are actually more complex but that's the general idea.) When you ask for a second page, the server doesn't know who you are: that makes it impossible to keep track of your whole browsing session at protocol level. Thus HTTP sessions were invented: they're a way to emulate physical sessions.
The MySQL session starts when you open a connection to the server. A connection ID is assigned which can be read via the SELECT CONNECTION_ID() statement. The session is terminated when the connection is closed or, in case of persistent connections, after a certain timeout or when the server shuts down.
I have an application which connects to a MySql database using Delphi's TAdoConnection object. This is a very query intensive application. So I create the connection and keep it open to avoid the high resource expense of open/closing database connections. But obviously problems can arise (database restart, network connection failure, etc). So I have built in code to free my database object, recreate it, and reconnect when queries fail.
I have a common function to connect to the database. The relevant code is this:
try
AdoConnection.open;
result := Adoconnection.Connected
except
result := False;
......
end;
I ran some test by turning on and off the MySql database. Everything works fine if the database is off on application startup (i.e. it properly throws an exception). However, if I turn off the database after the application has already successfully connected, subsequent re-connections do not throw exceptions, and additionally falsley report true for AdoConnection.Connected. I am sure the connection object had been freed/recreated first.
It seems there is some sort of caching mechanism going on here (most likely at the hardware/driver level - not application level). Anyone have any ideas?
I observed this also.
Ideas for dealing with it:
If you get an error on running a query then check if it's a connection issue and if so try to reconnect and then run the query again.
If your program uses a timer and waits a while between running batches of queries then perform a simple query (maybe "select now()") before each batch and if this fails then try to reconnect.