Database Session Manager

["Campbell, Lance" <la...@illinois.edu>]

Tomcat 8, 8.5, or 9
Is there a session manager for Tomcat that does not keep sessions in memory at all but stores the session attributes in a database?

Example:  If I create a session for the first time then the session manager would create a session ID and write it to a database table.  Now if you do setAttribute on the session it would then insert a row in the database based on the session ID and name of the attribute.  It would not delay.  It would write it at that moment.  If you do a getAttribute it would then select the value from the database based on the session ID and name.  At no time is a session object kept in memory by the session manager.

The other element is that if I have N number of tomcat instances that run behind a load balancer any one of them can receive an incoming request.  There is no communication that has to occur between tomcat instances.

I looked over the Tomcat JDBCStore.  But it looks like it still maintains sessions in memory for a time before writing them.  I would like an implementation that never keeps them in memory.  Direct writing and direct reading when a session does a setAttribute or getAttribute.

Is this even possible to do?  My feeling is that the Manager might have some fields that never do anything.  I don't want to create something that someone else has already done.

If this is able to be done and it has not been developed before I would be happy to do it if someone wants to give me some basic starting information on how to do this.  I would be happy to share it with the community.

Thanks,

Lance

Re: Database Session Manager

[Christopher Schultz <ch...@christopherschultz.net>]

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Lance,

On 6/21/17 11:09 AM, Campbell, Lance wrote:
> Tomcat 8, 8.5, or 9 Is there a session manager for Tomcat that does
> not keep sessions in memory at all but stores the session
> attributes in a database?

http://tomcat.apache.org/tomcat-8.0-doc/cluster-howto.html#Overview

http://tomcat.apache.org/tomcat-8.0-doc/config/manager.html
(search for "database")

> Example:  If I create a session for the first time then the
> session manager would create a session ID and write it to a
> database table. Now if you do setAttribute on the session it would
> then insert a row in the database based on the session ID and name
> of the attribute. It would not delay.  It would write it at that
> moment.  If you do a getAttribute it would then select the value
> from the database based on the session ID and name.  At no time is
> a session object kept in memory by the session manager.> I looked
> over the Tomcat JDBCStore. But it looks like it still maintains
> sessions in memory for a time before writing them. I would like an
> implementation that never keeps them in memory. Direct writing and
> direct reading when a session does a setAttribute or getAttribute.
I believe the JDBCStore doesn't prevent data from existing in memory.

> The other element is that if I have N number of tomcat instances 
> that run behind a load balancer any one of them can receive an 
> incoming request. There is no communication that has to occur
> between tomcat instances.
Using JDBCStore basically requires the use of Clustering, which does
in fact notify the other nodes in the cluster about new sessions being
created, etc.

It can't work any other way, because a node needs to know if a session
exists or not. If there is no data in the session, what is in the db?
If it's only session attributes, then there is no difference between
an empty session and an invalid session.

Clustering also generally batches the updates to the other cluster
members when the request is completed, so I expect JDBCStore does the
same thing.

> Is this even possible to do? My feeling is that the Manager might 
> have some fields that never do anything. I don't want to create 
> something that someone else has already done.

I don't believe there is a way to do what you are requesting with an
out-of-the-box Tomcat.

> If this is able to be done and it has not been developed before I 
> would be happy to do it if someone wants to give me some basic
> starting information on how to do this. I would be happy to share
> it with the community.

What would be great. I think you might want to look at the memcached
implementation(s) of a Tomcat Manager floating around on the
Internet... those will probably give you a good starting point. If you
want to share the code back to the Tomcat project, please take care to
obtain the proper permission and ensure that the original license of
anything you "borrow" is compatible with AL2.

I would be +1 on incorporating such a Manager into Tomcat, as long as
it was configurable enough to be useful to others (e.g. batch updates
at request-completion as opposed to immediate-write, customizable
serialization mechanism e.g. XML/JSON/Java Serialization).

- -chris
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