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From Rainer Jung <>
Subject Distributable contexts and non-serializable session attributes
Date Sun, 10 Jul 2011 19:38:47 GMT
I implemented an addition to DeltaManager which allows to filter which
session attributes actually get replicated (not yet committed).

This is useful, because many applications

- use non-serializable session attributes, so
  it is not possible to simply replicate everything

- do not actually need everything replicated, because they
  can bootstrap all they need from a login context if a filter
  detects, that a failover has happened.

In these cases it can also help to reduce replication traffic a lot.

Unfortunately I stu,bled into some code in StandardSession:

public void setAttribute(String name, Object value, boolean notify) {
if ((manager != null) && manager.getDistributable() &&
  !(value instanceof Serializable))
    throw new IllegalArgumentException
        (sm.getString("standardSession.setAttribute.iae", name));

This means if you enable "distributable", then you can no longer set any
non-serializable session attribute. The code is not recent, it goes back
to at least TC 3.3.

I understand, that it is helpful to devs being informed about
serialization/replication problems. But I think it is too strict to
enforce this in StandardSession whch is the base class of all our
cluster sessions and we always delegate to
StandardSession.setAttribute() after some cluster specific handling.

The SPEC says:

The container must accept objects that implement the Serializable interface.
- The container may choose to support storage of other designated
objects in the
HttpSession, such as references to Enterprise JavaBeans components and
- Migration of sessions will be handled by container-specific facilities.
The distributed servlet container must throw an IllegalArgumentException for
objects where the container cannot support the mechanism necessary for
of the session storing them.

(and some more in 7.7.2).

I think checking for serializability in StandardSession isn't good,
because AFAIK StandardSession isn't used by any mechanism Tomcat
supports to implement "distributable".

And in DeltaSession I would like to have the check optional, i.e. only
check if the configuration wants the attribute to be replicated.

By default all attributes will be replicated as is today, but by
configuration one will be able to choose attributes to replicate using a
regexp against the attribute name.

Any remarks?


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