The DGC abstraction is used for the server side of the distributed
garbage collection algorithm. This interface contains the two
methods: dirty and clean. A dirty call is made when a remote
reference is unmarshaled in a client (the client is indicated by
its VMID). A corresponding clean call is made when no more
references to the remote reference exist in the client. A failed
dirty call must schedule a strong clean call so that the call's
sequence number can be retained in order to detect future calls
received out of order by the distributed garbage collector.
A reference to a remote object is leased for a period of time by
the client holding the reference. The lease period starts when the
dirty call is received. It is the client's responsibility to renew
the leases, by making additional dirty calls, on the remote
references it holds before such leases expire. If the client does
not renew the lease before it expires, the distributed garbage
collector assumes that the remote object is no longer referenced by
that client.
The dirty call requests leases for the remote object references
associated with the object identifiers contained in the array
'ids'. The 'lease' contains a client's unique VM identifier (VMID)
and a requested lease period. For each remote object exported
in the local VM, the garbage collector maintains a reference
list-a list of clients that hold references to it. If the lease
is granted, the garbage collector adds the client's VMID to the
reference list for each remote object indicated in 'ids'. The
'sequenceNum' parameter is a sequence number that is used to
detect and discard late calls to the garbage collector. The
sequence number should always increase for each subsequent call
to the garbage collector.
Some clients are unable to generate a VMID, since a VMID is a
universally unique identifier that contains a host address
which some clients are unable to obtain due to security
restrictions. In this case, a client can use a VMID of null,
and the distributed garbage collector will assign a VMID for
the client.
The dirty call returns a Lease object that contains the VMID
used and the lease period granted for the remote references (a
server may decide to grant a smaller lease period than the
client requests). A client must use the VMID the garbage
collector uses in order to make corresponding clean calls when
the client drops remote object references.
A client VM need only make one initial dirty call for each
remote reference referenced in the VM (even if it has multiple
references to the same remote object). The client must also
make a dirty call to renew leases on remote references before
such leases expire. When the client no longer has any
references to a specific remote object, it must schedule a
clean call for the object ID associated with the reference.
The clean call removes the 'vmid' from the reference list of
each remote object indicated in 'id's. The sequence number is
used to detect late clean calls. If the argument 'strong' is
true, then the clean call is a result of a failed dirty call,
thus the sequence number for the client 'vmid' needs to be
remembered.
clean(ObjID[], long, VMID, boolean)
dirty