LE.2.1 Characteristics of a Lease

I'm trying to understand some of the nuances of Landlord implementation, and the 
behavior of the LRM related to my previous post, asking about when 
LeaseListener.notify() gets called.  I've included the text of LE.2.1 below for 
reference.

One of the interesting issues, for me, is that lease renewal, indicates a 
continued interest in a resource.  The lease grantor, who created the lease, 
specified an upper bound on the length of time that the lease was valid.  That 
entity, also could have specified a LeaseStateListener, to watch "renew" come in.

I'm trying to decide at which place, it makes the most since, for renewal 
failure, to be handled.  My basic concern, is that I am providing leases, which 
are "forever" leases when a client adds a listener for particular kinds of 
events that the service generates.  These leases involve UDP traffic, and are 
important to allow the server to stop sending traffic when a client goes away.

I'd like for the service to be notified when any particular renewal interval 
expires, and I'd like the client to be in charge of what interval it might 
request as a max value, while I can configure the maximum that I'm willing to 
grant.  All of the values are provided by what is in place today.

My issue is that if the client misses a renewal, I want the service to stop 
sending events.  I can make my Landlord implementation track the grantor's 
specified duration initially, but then set the expiration time to the value that 
is specified by the renewal so that a sliding window occurs.

My issue is that this seems to invalidate my implementation, because of the 
text, "However, when renewing a lease the grantor cannot, unless explicitly 
requested to do so, shorten the duration of the lease so that it expires before 
it would have if it had not been renewed."

So, should I make the LeaseStateListener deal with missing renews?

It seems like renew is worthless, if it doesn't represent an opportunity for 
expiry to occur, when renews are missing.

Gregg Wonderly


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LE.2.1 Characteristics of a Lease

There are a number of characteristics that are important for understanding what 
a lease is and when it is appropriate to use one. Among these characteristics are:

     * A lease is a time period during which the grantor of the lease ensures 
(to the best of the grantor's abilities) that the holder of the lease will have 
access to some resource. The time period of the lease can be determined solely 
by the lease grantor, or can be a period of time that is negotiated between the 
holder of the lease and the grantor of the lease. Duration negotiation need not 
be multi-round; it often suffices for the requestor to indicate the time desired 
and the grantor to return the actual time of grant.

     * During the period of a lease, a lease can be cancelled by the entity 
holding the lease. Such a cancellation allows the grantor of the lease to clean 
up any resources associated with the lease and obliges the grantor of the lease 
to not take any action involving the lease holder that was part of the agreement 
that was the subject of the lease.

     * A lease holder can request that a lease be renewed. The renewal period 
can be for a different time than the original lease, and is also subject to 
negotiation with the grantor of the lease. The grantor may renew the lease for 
the requested period or a shorter period or may refuse to renew the lease at 
all. However, when renewing a lease the grantor cannot, unless explicitly 
requested to do so, shorten the duration of the lease so that it expires before 
it would have if it had not been renewed. A renewed lease is just like any other 
lease and is itself subject to renewal.

     * A lease can expire. If a lease period has elapsed with no renewals, the 
lease expires, and any resources associated with the lease may be freed by the 
lease grantor. Both the grantor and the holder are obliged to act as though the 
leased agreement is no longer in force. The expiration of a lease is similar to 
the cancellation of a lease, except that no communication is necessary between 
the lease holder and the lease grantor.

Leasing is part of a programming model for building reliable distributed 
applications. In particular, leasing is a way of ensuring that a uniform 
response to failure, forgetting, or disinterest is guaranteed, allowing 
agreements to be made that can then be forgotten without the possibility of 
unbounded resource consumption, and providing a flexible mechanism for 
duration-based agreement.

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