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?
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