[Gluster-devel] libgfapi changes to add lk_owner and lease ID

[Niels de Vos]

On Thu, Dec 03, 2015 at 09:11:34AM -0500, Poornima Gurusiddaiah wrote:
> Hi, 
> 
> Brief Background: 
> ============ 
> For the below two features, we need ligfapi to take 2 other parameters from the applications for most number of fops. 
> http://review.gluster.org/#/c/12014/ 
> http://review.gluster.org/#/c/11980/ 
> 
> For leases to work as explained in the above doc, every file data read/write fop needs to be associated with a lease ID. This is specially required for Samba and NFS-Ganesha as they inturn serve other clients who request for leases. For Gluster to identify the end client (which is not Samba/NFS Ganesha) we need lease ID to be filled by Samba/NFS Ganesha. 
> 
> For mandatory locks feature to work as explained, every file data read/write fop needs to be associated with a lk_owner. In linux Kernel VFS takes care of filling the lk_ownere for the file system. In libgfapi case, the applications calling into libgfapi should be providing lk_owner with every fop. This is again required mainly for Samba and NFS Ganesha, as they serve multiple clients. 
> 
> Possible solutions: 
> ============= 
> 1. Modify all the required APIs to take 2 other parameter, lease ID and lk_owner. But that would mean backward compatibility issues and is a programming overhead for applications not interested in Leases and mandatory lock feature. 
> 2. Add an API called glfs_set_fop_attrs (lease ID, lk_owner) which works similar to glfs_set_uid(uid). The API sets a thread local storage (pthread_key) with the values provided, the further fops on that thread will pick the lease ID and lk_owner from the thread local storage (pthread_key). There are few minor details that needs to be worked out: 
> - In case of async API will end up using lease ID and lk_owner from wrong thread. 
> - unset lease ID and lk_owner after every fop to ensure there is no stale lease ID or lk_owner set? 
> - For fd based fops we can store the lease ID and lk_owner in glfd, so that the application writed need not set it for every fop. But for handle based fops lease ID and lk_owner needs to be set explicitly every-time. 
> 
> Solution 2 is more preferable except for that it adds overhead of calling another API, for the libgfapi users who intends to use these features. 
> A prototype of solution 2 can be found at http://review.gluster.org/#/c/12876/ 
> 
> Please let me know if you have any suggestions. 

We want to have some way to do compound operations in libgfapi. The
basis for that would be re-usable for many other use-cases, including
object orented APIs (PUT=CREATE+WRITE, GET=OPEN+READ, ..) and Kerberos
where lk_owner can not be set in the RPC-header.

A compound API in libgfapi would roughly look like a series of functions
that build a structure. Once the structure has been built, it gets
passed on to a call that actually starts the work. I am thinking of
something like this (naming things without putting much thought in it):

   glfs_compound_call_t compound;
   glfs_compound_reply_t result;

   ret = glfs_compound_new (&compound);

   glfs_compound_fop (compound, OPEN, ...);
   glfs_compound_fop (compound, READ, ...);

   ret = glfs_compound_call (compound, &reply);

The 'reply' would be a similar structure as the one built with
glfs_compound_fop(), but with the replies of the FOPs.

I think that is makes sense to create a SETLKOWNER and SETLEASEID (or
something like that) 'pseudo FOP'. These pseudo FOPs would be able to
carry some metadata that does not fit in the RPC-header. This makes it
pretty simple to have one function setting the lk_owner or lease_id.

Is there something that would not be solved by this?

Now, I heard that there are some ideas for compound operations already.
I have not seen any details about those yet, but am most interested! The
above only shows the very minimal way that I think we can use
immediately. But, it is possible to build much more complex compound
operations, maybe someone would like if/else structures in there or
other fancy statements. Please poke any developers that have ideas on
compound operations ;-)

Thanks,
Niels
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