DGC.java

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package java.rmi.dgc;

import java.rmi.*;
import java.rmi.server.ObjID;

/**
 * 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.
 *
 * @author Ann Wollrath
 */
public interface DGC extends Remote {

    /**
     * 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.
     *
     * @param ids IDs of objects to mark as referenced by calling client
     * @param sequenceNum sequence number
     * @param lease requested lease
     * @return granted lease
     * @throws RemoteException if dirty call fails
     */
    Lease dirty(ObjID[] ids, long sequenceNum, Lease lease)
        throws RemoteException;

    /**
     * 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.
     *
     * @param ids IDs of objects to mark as unreferenced by calling client
     * @param sequenceNum sequence number
     * @param vmid client VMID
     * @param strong make 'strong' clean call
     * @throws RemoteException if clean call fails
     */
    void clean(ObjID[] ids, long sequenceNum, VMID vmid, boolean strong)
        throws RemoteException;
}