We Keep Coding

I want to know about mongodb

I'm currently creating an API server that reads and writes. Using MongoDB
The library uses Mongoose.
I wonder if db.close() must be used when reading and writing.
datamodel.js:
var db = mongoose.connect('mongodb://localhost/testdb', {useNewUrlParser: true,useUnifiedTopology:true});
mongoose.Promise = global.Promise;
.....
Boards = mongoose.model("boards", BoardSchema);
exports.Boards = Boards;
routes/getList.js:
let result = await Boards.find().sort({"date": -1});
Should I close the DB declared above with db.close() when reading or writing?

(Very generic answer, but should help you get started with what to research)
Closing MongoDB connection depends on how is the connection established in the first place.
Are you initialising the connection on server startup: If yes, you should not close the connection. (But initialising the connection on server startup is bad idea because, if connection is lost to server (like database server restart), then you would also have to restart the application or set reconnectTries)
Are you using connection pool: If you are using connection pool, then closing and opening of connections is taken care by Mongoose itself. All you have to do is, release the connection after use, so that, it's available for other requests.
Are you creating connection per request: If yes, then you should close the connection before returning the response or you would quickly run out of available connections at database server.

you can call mongoose.disconnect() to close the connection

How to enable JDBC connection pooling on Exasol?

We use Hibernate with Exasol in a web app. When we set Hibernate's hibernate.connection.pool_size property to a value greater than 1 (or the corresponding property of any JDBC connection pool, e.g, HikariCP, c3p0, to a value greater than 1), we get:
java.sql.SQLException: syntax error, unexpected FOR_READ_ONLY_, expecting end_of_input or ';' [line 1, column 93] (Session: 1622929410050974585)
at com.exasol.jdbc.ExceptionFactory.createSQLException(ExceptionFactory.java:164)
at com.exasol.jdbc.ExceptionFactory.createSQLException(ExceptionFactory.java:21)
at com.exasol.jdbc.AbstractEXAPreparedStatement.<init>(AbstractEXAPreparedStatement.java:62)
at com.exasol.jdbc.AbstractEXAPreparedStatement_14.<init>(AbstractEXAPreparedStatement_14.java:14)
at com.exasol.jdbc.EXAPreparedStatement.<init>(EXAPreparedStatement.java:12)
at com.exasol.jdbc.DialectGeneric.createPreparedStatement(DialectGeneric.java:10)
at com.exasol.jdbc.AbstractEXAConnection.prepareStatement(AbstractEXAConnection.java:608)
at org.hibernate.id.enhanced.TableGenerator.prepareStatement(TableGenerator.java:618)
at org.hibernate.id.enhanced.TableGenerator.access$200(TableGenerator.java:127)
From the Exasol's user manual,
The Connection Pooling of the driver manager is deactivated by
default. You can explicitly activate it in the configuration tool
"ODBC Data Source Administrator". But please note that in that case
reused connections keep their session settings which were set via SQL
commands (see ALTER SESSION).
Here, ODBC is mentioned, whereas we use JDBC. So my question is, how to enable JDBC connection pooling on Exasol?
Should we add/update a row in EXA_PARAMETERS view/table?

for JDBC, EXASOL itself doesn't have an integrated connection pooling.
But we find the following library very useful:
http://commons.apache.org/proper/commons-dbcp/download_dbcp.cgi
For instance, this is also used in JMeter and works fine there.
However, we do not have any experience combining this library with hibernate.
By the way, did you use https://github.com/exasol/hibernate-exasol?
Please let us know about the progress, we are happy to help! (Contact us: www.exasol.com/contact)
Best regards,
Team Exasol

How to differ between MSSQL JDBC error codes?

My server calls sometimes the MSSQL JDBC CallableStatement.execute() that works with a previously connected connection. Everything is ok while the connection is alive. But if the connection is disconnected somehow (e.g. somebody turn SQL server down) the execute() call throws SQLException. I need to differ between 'connection was dropped' error and any other JDBC error (like table I'm trying to use doesn't exist). Since if I hit the connection error - I need to reconnect. And in any other case I need just to give error message to the user.
SQLException.getSQLState() always returns null and getErrorCode() always returns 0.
Thanks

I am no Java expert but this seems to be mainly based on Java.
You'd need to catch the exception from the .execute() command and query the string to see what the error is, then make a decision on which method to execute next based on that information.

SQL execution time much slower in a Tomcat Servlet than in a normal Java program

For inexplicable reasons however, this morning the performance increased for two of my Queries that used to be slow. I have no idea why.
I have no authority over the server, maybe someone changed something.
The problem is no more.
In a nutshell:
s.executeQuery(sql) runs extremely slowly within a tomcat servlet on server
Same query runs fine without servlet (simple java program) on the same machine
Not all queries are slow within the servlet. Only a few bigger ones do
Same servlet runs fast on another machine
UPDATES
Please read the updates below the text !
I have a servlet that executes SQL requests and sends back the results via JSON. For some reason, some requests take a huge amount of time to execute, but when I run them in any Oracle SQL Client, they are executed in no time.
I am talking about a difference of 1 second vs 5 minutes for the same SQL (that is not that complex).
How can this be explained ?
Is there a way to improve the performance of a java based SQL request ?
I am using the traditional way of executing queries:
java.sql.Connection conn = null;
java.sql.Statement s = null;
ResultSet rs = null;
String dbDriver = "oracle.jdbc.driver.OracleDriver";
String dbConnectionString = "jdbc:oracle:thin:#" + dbHost + ":" + dbPort + ":" + dbSid;
Class.forName(dbDriver).newInstance();
conn = DriverManager.getConnection(dbConnectionString, dbUser, dbPass);
s = conn.createStatement();
s.setQueryTimeout(9999);
rs = s.executeQuery(newStatement);
ResultSetMetaData rsmd = rs.getMetaData();
// Get the results
while (rs.next()) {
// collect the results
}
// close connections
I tried with ojdbc14 and ojdbc6 but there was no difference.
UPDATE 1:
I tried the same SQL in a local Java project (not a servlet) on my client machine, and I get the results immediately. So I assume the problem is coming from my servlet or the tomcat configuration ?
UPDATE 2:
The culprit is indeed rs = s.executeQuery(mySql); I tried to use preparedStatement instead, but there is no difference.
UPDATE 3:
I created a new Servlet running on a local Tomcat and the Query comes back fast. The problem is therefore coming from my production server or Tomcat config. Any ideas what config items could affect this ?
UPDATE 4:
I tried the same code in a normal java program instead of a servlet (still on the same server) and the results are coming fast. Ergo the problem comes from the Servlet itself (or Tomcat ?). Still don't know what to do, but I narrowed it down :)
UPDATE 5:
Jstack shows the following (It starts where my servlet is, I cut the rest)
"http-8080-3" daemon prio=3 tid=0x00eabc00 nid=0x2e runnable [0xaa9ee000]
java.lang.Thread.State: RUNNABLE
at java.net.SocketInputStream.socketRead0(Native Method)
at java.net.SocketInputStream.read(SocketInputStream.java:129)
at oracle.net.ns.Packet.receive(Packet.java:311)
at oracle.net.ns.DataPacket.receive(DataPacket.java:105)
at oracle.net.ns.NetInputStream.getNextPacket(NetInputStream.java:305)
at oracle.net.ns.NetInputStream.read(NetInputStream.java:249)
at oracle.net.ns.NetInputStream.read(NetInputStream.java:171)
at oracle.net.ns.NetInputStream.read(NetInputStream.java:89)
at oracle.jdbc.driver.T4CSocketInputStreamWrapper.readNextPacket(T4CSocketInputStreamWrapper.java:123)
at oracle.jdbc.driver.T4CSocketInputStreamWrapper.read(T4CSocketInputStreamWrapper.java:79)
at oracle.jdbc.driver.T4CMAREngineStream.unmarshalUB1(T4CMAREngineStream.java:429)
at oracle.jdbc.driver.T4CTTIfun.receive(T4CTTIfun.java:397)
at oracle.jdbc.driver.T4CTTIfun.doRPC(T4CTTIfun.java:257)
at oracle.jdbc.driver.T4C8Oall.doOALL(T4C8Oall.java:587)
at oracle.jdbc.driver.T4CStatement.doOall8(T4CStatement.java:210)
at oracle.jdbc.driver.T4CStatement.doOall8(T4CStatement.java:30)
at oracle.jdbc.driver.T4CStatement.executeForDescribe(T4CStatement.java:762)
at oracle.jdbc.driver.OracleStatement.executeMaybeDescribe(OracleStatement.java:925)
at oracle.jdbc.driver.OracleStatement.doExecuteWithTimeout(OracleStatement.java:1104)
at oracle.jdbc.driver.OracleStatement.executeQuery(OracleStatement.java:1309)
- locked <0xe7198808> (a oracle.jdbc.driver.T4CConnection)
at oracle.jdbc.driver.OracleStatementWrapper.executeQuery(OracleStatementWrapper.java:422)
So i am Stuck at java.net.SocketInputStream.socketRead0(Native Method) ?

In some cases (not sure if this applies to yours) setting fetchSize on the Statement object yields great performance improvements. It depends on the size of the resultSet that is being fetched.
Try playing with it by setting it to something bigger than default 10 for Oracle (see this link).
See Statement.setFetchSize.

Given your symptoms, I believe that your issue is not with your SQL client code and you are in fact looking at issues with your server. The stack shows that your client is waiting for a response. This tallies with the fact that you can run the client without any problem in a separate process.
So what you probably need to look at is systemic reasons why the SQL server is running slowly and how that may be tied to Tomcat. My experience in cases like this is its usually the disk, so I'd be inclined to check whether you are paging due to a lack of RAM when Tomcat is loaded, or suffering from much higher disk ops due to a reduced disk cache. Assuming you are running on a UNIX variant, I'd have a look at vmstat and iostat for a working and broken case to eliminate such issues.

For inexplicable reasons however, this morning the performance increased and my problem is no more. I have no idea why. I have no authority over the server, maybe someone changed something.

Since your thread is waiting on socket read, which means is waiting for a response from the database server I would :
Check database performance, make sure not the instance nor the query is getting impacted at some point in time during the day?
Check your network latencies between Java and DB Servers. Same as above. Probably traceroute?

Since you have not put the query, I can give you a scenario where it is possible. If you use a function in your query like to_char etc. then your table indexes wouldn't be used while executing query via JDBC but will work fine you run it in console. I don't exactly know why but there's something with JDBC driver. I had the exact same issue in db2 and I resolved it removing the use of functions.
Other scenario could be that a huge no of records is being fetched and proper batching is not implemented.

Set timeout for Statement.executeBatch

I would like to find out how I can set a timeout for a Statement.executeBatch() in Java. The code works well when the database is up, but when I block the db IP in iptables, the JVM hangs at executeBatch(). I tried using .setQueryTimeout(10) but an exception is thrown:
SQLException cannot be cast to SybSQLException
On database side I use Sybase and jdbc2 for connection
Thank you.
PS: I can't post the code because it is company copyrighted.