Analyse database connections

Learn about the best practices to analyze database connections.

Configuration practices

Maximum open database connections to configure

The number of database connections that should be configured depends on the following:

• number of concurrent users (not every user opens a database connection)

• number of active threads ( the type of how and which censhare modules are configured)

• number of remote servers connected (only those that are tunneling the database connection through RMI)

• number of configured invocations in DataObjectService

In general, you can calculate a number of 50 maximum open database connections per application server.

Example: Everyone of 4 node application servers should have a value of 50 maximum open database connections configured.

Since Versions 2017.2.0 and higher, the default maximum open database connections per application server were increased from 50 to 150. This was necessary as more and more resources will be used by the censhare Server (e.g. censhare Web, CDB) instead of the resources of the backend database system (e.g. Oracle/PostgreSQL).

Now, if a remote server doesn't have a direct JDBC database connection and requests it indirectly over RMI from another (mostly the master) server, then the master server needs +50 open database connections, i.e. whose value increases to a maximum of 100 open database connections.

With this info, you can determine the values for your environment by a rule of thumb.

Don't forget that that increasing the value can require to increase the JVM heap size as well, because JDBC database connections cost heap memory (a few MB) as well.

Note that the value is only a soft limit. The real limit of database connections is set by the Oracle parameter processes, respectively its RAM (sga size) allocation.

Note that Oracle's default value of 150 processes is sufficient for two censhare servers, but we change it in our DBCA template to 300 processes. We never change the 'sessions' parameter on Oracle, because it's default value is OK = (1.1 * PROCESSES) + 5.

The value of the processes parameter in Oracle should be set to the result of the following formula:

"processes parameter" > ( "censhare max open JDBC connections" * "number of censhare servers") + "number of oracle background processes"

Example

4 censhare servers: 300 > ( 50 * 4 ) + 50

When to increase the value of maximum open database connections

Increase the value, if you notice the following messages in the ~/work/logs/server-0.0.log even if the DatabaseServiceWatchdog is appearing every 5 minutes within the server log file:

WARNING: S001: DatabaseService: EventServiceWatchdog: maximum number of connections in use, 
blocked further access

INFO : T013: HotfolderCheckinNew.processEvents: Service: uk-mal-lin-orac-01.20080915.140146.094[system]: waiting for service: 
DatabaseService[uk-mal-lin-orac-01:DatabaseService] database, since: 30000ms

ServiceException[srv-ex-unavailable]: Service 'DatabaseService[uk-mal-lin-orac-01:DatabaseService]' 
is not available

SEVERE : DatabaseServiceWatchdog: DatabaseService: DatabaseServiceWatchdog: corpus: 
limit of maxOpen connections [100] has been reached, could not open new connection

Where to configure the open connections and what to consider

Configure the max-open parameter in one of the following ways:

• in the censhare Admin Client under Configuration | Services | Database
• in the file ~/cscs/app/services/database/config.<server>.xml

Normally, a (remote) configuration refresh is sufficient. Sometimes (e.g. if all database connections are currently exhausted) this requires an application server (JVM) restart.

Performance optimization

Set the keep-open parameter to the average used database connections. Check the value via the Diagrams / Database Connections.

From a resource perspective, it's cheaper and faster (in the area of milliseconds) to keep a database connection open, compared to opening a new database connection each time.

Troubleshooting

DatabaseServiceWatchdog doesn't appear in the log file

Sometimes, the DatabaseServiceWatchdog is blocked by a thread or while trying to close a physical database connection. As a result, unused database connections are not released and the pool of maximum open database connections gets exhausted. With the following command you can check if the DatabaseServiceWatchog is appearing every 5 minutes:

grep "DatabaseServiceWatchdog:.*checking connections" ~/work/logs/server-0.0.log

If it doesn't appear, this means that it is blocked. For further analysis, you need to create a jstack output of the application server java process. Keep in mind that an older (lower than ~/css/app/common/lib/ojdbc6-v11.2.0.2.4.jar) version of the JDBC driver could lead to blocked physical connection, too.

The following example jstack output shows a BLOCKED DatabaseServiceWatchdog. It describes only one of many possible causes to show the analysis method.

"DatabaseServiceWatchdog" prio=3 tid=0x000000001a67d000 nid=0xa823 waiting for monitor entry [0xffff80ff7bd0d000] 
java.lang.Thread.State: BLOCKED (on object monitor) at 
oracle.jdbc.driver.PhysicalConnection.closeLogicalConnection(PhysicalConnection.java:4010) 
- waiting to lock <0x000000046623c4c8> (a oracle.jdbc.driver.T4CConnection) at oracle.jdbc.driver.LogicalConnection.closeInternal(LogicalConnection.java:258) at oracle.jdbc.pool.OraclePooledConnection.getConnection(OraclePooledConnection.java:306) 
- locked <0x000000046623c408> (a oracle.jdbc.pool.OraclePooledConnection) at com.censhare.manager.databasemanager.DatabaseServiceImpl$DBCachedConnection.closePhysical(DatabaseServiceImpl.java:294) ...

To find out which thread blocks the DatabaseServiceWatchdog, take the id 0x000000046623c4c8 offline - waiting to lock <0x000000046623c4c8> (an oracle.jdbc.driver.T4CConnection) and search for it within the stack output.

You find out that it's blocked by thread T004 . This thread corresponds to checkConstraints(WConnectionImpl.java:210).

"T004" prio=3 tid=0x00000000028e7000 nid=0x1c runnable [0xffff80ffb74dc000] java.lang.Thread.State: RUNNABLE ... at oracle.jdbc.driver.OracleStatement.executeQuery(OracleStatement.java:1477) 
- locked <0x000000046623c4c8> (an oracle.jdbc.driver.T4CConnection) at oracle.jdbc.driver.OracleStatementWrapper.executeQuery(OracleStatementWrapper.java:392) 
at com.censhare.server.support.wsql.WConnectionImpl.checkConstraints(WConnectionImpl.java:210) 
at com.censhare.server.support.wsql.WConnectionImpl.prepareStep(WConnectionImpl.java:428) 
at com.censhare.manager.dataobject.DataObjectServiceImpl.executeUpdate(DataObjectServiceImpl.java:114) ...

It means that within the DatabaseService configuration the parameter is set to constraint-check-enabled="true".

For a productive system that is inappropriate and shouldn't be enabled, because if a constraint error appears every database foreign key will be checked for violation. This could possibly take up hours which results in a blocked DatabaseServiceWatchdog.

Find out which thread uses a database connection

Since censhare Server version 5, there's an option within the XML of the database service called track-allocations="false". If you set it to true and refresh the database service, you can see the transaction ID of the thread which uses a database connection. With a grep for the transaction ID you can find out the censhare command/module which holds the database connection.