[openstack-dev] [oslo.db][nova] Deprecating use_slave in Nova

Matt Riedemann mriedem at linux.vnet.ibm.com
Sat Jan 31 00:32:45 UTC 2015

On 1/30/2015 11:05 AM, Matthew Booth wrote:
> At some point in the near future, hopefully early in L, we're intending
> to update Nova to use the new database transaction management in
> oslo.db's enginefacade.
> Spec:
> http://git.openstack.org/cgit/openstack/oslo-specs/plain/specs/kilo/make-enginefacade-a-facade.rst
> Implementation:
> https://review.openstack.org/#/c/138215/
> One of the effects of this is that we will always know when we are in a
> read-only transaction, or a transaction which includes writes. We intend
> to use this new contextual information to make greater use of read-only
> slave databases. We are currently proposing that if an admin has
> configured a slave database, we will use the slave for *all* read-only
> transactions. This would make the use_slave parameter passed to some
> Nova apis redundant, as we would always use the slave where the context
> allows.
> However, using a slave database has a potential pitfall when mixed with
> separate write transactions. A caller might currently:
> 1. start a write transaction
> 2. update the database
> 3. commit the transaction
> 4. start a read transaction
> 5. read from the database
> The client might expect data written in step 2 to be reflected in data
> read in step 5. I can think of 3 cases here:
> 1. A short-lived RPC call is using multiple transactions
> This is a bug which the new enginefacade will help us eliminate. We
> should not be using multiple transactions in this case. If the reads are
> in the same transaction as the write: they will be on the master, they
> will be consistent, and there is no problem. As a bonus, lots of these
> will be race conditions, and we'll fix at least some.
> 2. A long-lived task is using multiple transactions between long-running
> sub-tasks
> In this case, for example creating a new instance, we genuinely want
> multiple transactions: we don't want to hold a database transaction open
> while we copy images around. However, I can't immediately think of a
> situation where we'd write data, then subsequently want to read it back
> from the db in a read-only transaction. I think we will typically be
> updating state, meaning it's going to be a succession of write transactions.
> 3. Separate RPC calls from a remote client
> This seems potentially problematic to me. A client makes an RPC call to
> create a new object. The client subsequently tries to retrieve the
> created object, and gets a 404.
> Summary: 1 is a class of bugs which we should be able to find fairly
> mechanically through unit testing. 2 probably isn't a problem in
> practise? 3 seems like a problem, unless consumers of cloud services are
> supposed to expect that sort of thing.
> I understand that slave databases can occasionally get very behind. How
> behind is this in practise?
> How do we use use_slave currently? Why do we need a use_slave parameter
> passed in via rpc, when it should be apparent to the developer whether a
> particular task is safe for out-of-date data.
> Any chance they have some kind of barrier mechanism? e.g. block until
> the current state contains transaction X.
> General comments on the usefulness of slave databases, and the
> desirability of making maximum use of them?
> Thanks,
> Matt

I'd recommend talking to Mike Wilson (geekinutah) about this.



Matt Riedemann

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