[ovs-dev] [PATCH 1/7] ovsdb: Add support for transaction forwarding to the replication mode.

Ilya Maximets i.maximets at ovn.org
Sat Jun 12 02:04:46 UTC 2021


On 5/10/21 1:38 PM, Dumitru Ceara wrote:
> On 5/1/21 2:55 AM, Ilya Maximets wrote:
>> Current version of ovsdb replication allows to scale out read-only
>> access to the primary database.  However, many clients are not
>> read-only but read-mostly.  For example, ovn-controller.
>>
>> In order to scale out database access for this case ovsdb-server
>> need to process transactions that are not read-only.  Replica is
>> not allowed to do that, i.e. not allowed to modify the database,
>> but it can act like a proxy and forward transactions that includes
>> database modifications to the primary server and forward replies
>> back to client. At the same time it may serve read-only transactions
>> and monitor requests by itself greatly reducing the load on primary
>> server.
>>
>> To support that new command line option '--enable-txn-forward' added
>> to ovsdb-server.
>>
>> This configuration will slightly increase transaction latency, but
>> it's not very important for read-mostly use cases.
>>
>> With this change instead of creating a trigger to commit the
>> transaction, ovsdb-server will create a trigger for transaction
>> forwarding.  Later, replication_run() will send all new transactions
>> to the replication source.  Once transaction reply received from the
>> replication source, replication module will update the state of the
>> transaction forwarding with the reply.  After that, trigger_run()
>> will complete the trigger and jsonrpc_server_run() will send the
>> reply back to the client.  Since transaction reply from the replication
>> source will be received after all the updates, client will receive
>> all the updates before receiving the transaction reply as it is in
>> a normal scenario without transaction forwarding.
>>
>> Signed-off-by: Ilya Maximets <i.maximets at ovn.org>
>> ---
> 
> Hi Ilya,
> 
> I have a tiny nit below, except for that the code looks OK to me.
> 
> What I'm not very sure about is the fact that a replication server will
> service read-only transactions locally while other transactions are
> forwarded.
> 
> A client might generate two transactions, in this order:
> 
> T1: insert in table X
> T2: select from table X (read-only)
> 
> If the client doesn't wait for the result of T1, with txn forwarding
> enabled, it will potentially get a stale value from X at T2.
> 
> Arguably, this is a problem in the client because it's not waiting for
> T1 to complete, but should we worry about it?  Or should we at least
> document this change in behavior somewhere?

This is the same behavior as if if you're performing these transactions
over the the follower of a clustered database.  And this is documented
in 'Understanding Cluster Consistency' section of ovsdb(7).  I also
pointed to that in the relay service model documentation in v2.

> 
> Regards,
> Dumitru


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