On Wed, Jul 09, 2014 at 05:00:55PM -0700, Chris Hydon wrote: > On 2014-07-09 3:41 PM, Ben Pfaff wrote: > >On Wed, Jul 09, 2014 at 02:04:00PM -0700, Chris Hydon wrote: > >>I am part of a team developing a program that connects to a running > >>OVSDB server via the jsonrpc interface and performs transactions. I > >>noticed during testing that after sending a database REQUEST, if a > >>table has a monitor active on the same socket it will receive the > >>monitor NOTIFY before the REPLY corresponding to the REQUEST. That > >>means if we send a transaction and wait until we receive a REPLY > >>with matching ID, processing any monitor notifications that come in > >>the interim, our local cache is going to be consistent with OVSDB > >>when we're done. > >> > >>Can we rely on this behaviour, or is it incidental and subject to > >>change (i.e. is it conceivable that at some point in the future the > >>NOTIFY will arrive after the REPLY)? > >This is more or less an accidental property of the current > >implementation. It has always been true, and I don't have any plan to > >change it, but I don't think that there's a commitment to this > >behavior. > > > >One property that I can solidly commit to is that any given NOTIFY > >represents a complete transactional state of the database. That is, > >the database never breaks a transaction into two pieces and sends them > >in separate notifications. (Newer versions of ovsdb-server *will* > >combine multiple transactions into a single notification, if the > >database connection gets backlogged, but never splits them.) > > Thanks. Follow-up question: if I issue a transaction and it > succeeds, are all clients guaranteed to receive the same > notification (as far as what was specified in that client's monitor > request)? I'm considering whether the following edge case could be > possible in the future, based on your comment about combining > multiple transactions: > > - Client A sends REQUEST 1, making update X. > - Client A receives REPLY 1 and waits for NOTIFY of X. > - Client B receives NOTIFY of X. > - Client B sends REQUEST 2, making update Y. > - Client B receives REPLY 2 and NOTIFY of Y. > - Client A receives combined NOTIFY of X and Y. > > Could that happen, or would the NOTIFY of X to A be queued at the > same time as to B, and thus never be combined with Y?
Notifications sent to different clients can be combined differently. Combining of notification sent to a particular client depends on whether and when the socket buffer gets filled. This means that it's more likely that notifications will be combined if they are sent to a remote client (to which there is limited bandwidth) over TCP or SSL than if they sent to an on-box client. In the extreme, a client that resides on a network whose connectivity goes down for a few seconds might see all essentially all of its transactions combined until connectivity comes back up, whereas a client connected locally over a Unix domain socket would see each one individually. _______________________________________________ discuss mailing list discuss@openvswitch.org http://openvswitch.org/mailman/listinfo/discuss
