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Re: Question about MariaDB Non-blocking Client


Thanks, Kristian.  This cleared up a lot for me.  I'll implement the async
mysql_ping() and follow up if I have any serious issues.

On Wed, Aug 7, 2013 at 7:33 AM, Kristian Nielsen

> Mike Gibson <notmikegibson@xxxxxxxxx> writes:
> > Greetings,
> Hi, sorry for taking a few days to get back to you:
> > I'm using the MariaDB Non-blocking API to write a C++ client, but I've
> hit
> > a wall regarding connection checking. I've been referencing the binding
> > from node-mariasql (
> > https://github.com/mscdex/node-mariasql/blob/master/src/binding.cc).
> Ok, great that you can use the non-blocking API!
> > The problem I'm experiencing is if I have a connection and kill the
> server
> > (/etc/init.d/mysql stop) and then start it back (/etc/init.d/mysql
> start),
> > I can never get clean reconnects. It's usually a mixture of errcode 2058,
> > 2003, 2013.
> >
> > I'm really confused how to gracefully manage connections. Before I was
> > using the official MySQL C++ connector, and it provides a
> > connection->isConnected() method. I'm wondering how I can get something
> > similar with MariaDB's client, as I need the non-blocking interface.
> I am not familiar with the MySQL C++ connector. I tried downloading the
> source
> for mysql-connector-c++-1.1.3.tar.gz, but it does not seem to have any
> isConnected() method?
> Anyway, the usual way to do this is to issue a mysql_ping() to the server,
> to
> check if the connection is working. With the non-blocking API, you would do
> this with mysql_ping_start() and mysql_ping_cont(). This will issue a
> request
> to the server to check if the connection is ok. If you have autoconnect
> enabled (MYSQL_OPT_RECONNECT), then this will automatically reconnect if
> the
> old connection was broken for some reason.
> (Note that you will need to be aware of the usual issues with automatic
> reconnect. For example, even if ping is successful, the connection may
> break
> immediately afterwards. And an autoconnect will loose any existing state on
> the connection such as temporary tables, SET @user_var, BEGIN, etc.)
> > Given: MYSQL mysql, *mysqlHandle;
> >
> > Look at the mysql_real_connect_start() or cont() functions. I provide
> > struct and on connect I get a copy of the struct stored in *mysqlHandle.
> > It's not clear to me what the purpose of the copy is at this point as I
> > still use the initial struct to call query().
> Agree, it is not very useful, it is just a copy of the pointer to your own
> structure. You only need it to check for error (in which case the pointer
> will
> be NULL). After that you can just you the pointer to your own struct that
> you
> already have.
> > In the case that I can detect a disconnection, how do I properly clean up
> > the connection and attempt reconnect? Do I mysql_close(&mysql) and/or
> > mysql_close(mysqlHandle), shutdown/close the file descriptor,
> mysql_init()
> > a new handle and go through mysql_real_connect_start/cont()?
> I am not sure, but it seems to me from looking at the code that if you
> already
> got an error that the connection was closed, then you can just go through
> the
> mysql_real_connect_start/cont() sequence again to reconnect. But if that
> does
> not work, you can always mysql_close() and mysql_init() your struct again.
> There is no need to explicitly shutdown or close the file descriptor.
> > Does it even make sense for each object to have its own MYSQL struct
> that I
> > mysql_init(), or would it be better to have layer on top that
> mysql_init()s
> > a single MYSQL struct, connects, and passes the returned *mysqlHandle to
> > each query?
> This depends on your application. You can only have a single query in
> progress
> on one MYSQL struct at a time. So if you have multiple queries working at
> the
> same time on the server, you need one MYSQL struct for each. If you only
> have
> one query processing at a time, a single MYSQL struct will be sufficient.
> > Thanks for providing the async client, any help is appreciated.
> I hope this helps, though it is of a somewhat generic nature. If you need
> more
> details, please ask again, perhaps with some example code that shows your
> problems.
>  - Kristian.