zqr10159 commented on issue #3744:
URL: https://github.com/apache/hertzbeat/issues/3744#issuecomment-3254456681
Hi,
Thanks for reaching out and providing the detailed logs.
The error you're encountering, `java.lang.NullPointerException: Cannot
invoke "com.mysql.cj.NativeSession.getCancelTimer()" because "this.session" is
null`, is a common issue related to database connection management.
### Root Cause Analysis
This error occurs deep inside the MySQL JDBC driver. It essentially means
that HertzBeat tried to execute a query using a database connection that was no
longer valid or had already been closed. The underlying `session` object was
`null`, leading to the `NullPointerException`.
This is typically not a bug in the application code but rather an issue of
stale connections in the connection pool. The most common reasons for this are:
1. **MySQL Server `wait_timeout`**: This is the most likely cause. MySQL
database servers have a `wait_timeout` setting (often defaulting to 8 hours).
If a connection in the pool remains idle for longer than this period, the MySQL
server will automatically close it to free up resources. However, the HertzBeat
collector still thinks the connection is valid. When it tries to use this
"stale" connection, the driver fails.
2. **Network Issues**: Any network interruption (like a firewall closing
idle TCP sessions, a brief outage, or a proxy restart) between the HertzBeat
collector and the MySQL server can invalidate the connection.
3. **Database Restart**: If the MySQL server was restarted, all existing
connections would be terminated.
The second error, `Jdbc close statement error: No operations allowed after
statement closed`, is a direct consequence of the first one. It happens when
the code enters its cleanup phase and tries to close a statement that is
already in a closed state due to the initial connection failure.
### Recommended Solution
The most effective way to solve this is to configure the JDBC connection to
validate itself before being used. This ensures that the connection pool
doesn't hand out stale connections.
You can do this by modifying the **JDBC URL** in your MySQL monitor
configuration within HertzBeat.
Please try updating your JDBC URL to include connection validation and
auto-reconnect parameters.
**Example:**
```
jdbc:mysql://YOUR_HOST:3306/YOUR_DATABASE?autoReconnect=true&failOverReadOnly=false&validationQuery=SELECT
1
```
**Key Parameters:**
* `autoReconnect=true`: Instructs the driver to try and re-establish a
connection if it has been lost.
* `validationQuery=SELECT 1`: Provides a simple, lightweight query that
can be used to test the connection's validity before executing the actual
monitoring query. When a connection is borrowed from the pool, this query is
run to ensure it's still alive.
By adding these parameters, you should be able to prevent this error from
occurring. Please update your monitor's configuration and let us know if this
resolves the issue.
Thanks\!
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