Il giorno lun 30 lug 2018 alle ore 09:17 Ivan Kelly <iv...@apache.org> ha scritto:
> >> Thank you for putting this together. It is also good to put this as a > BP, > >> since it is about the metadata layout. > > I'll put a BP up this week once I have initial feedback. > > >> > - When writing a metadata, check what is in /ledgers/LAYOUT. If it is > >> > as above, write using the current text protobuf. If is bumped, use the > >> > new binary format. > > > > isn't it too costly? Adding a zk read for each write. We could add a > watch > > but is has an important cost > > This cost can be amortized. In simplest case, we only read when client > starts (as already happens). Client are able both read and write the > old format, so clients that read the pre-upgrade version will still > write in text format, and this is fine. > For me it is fine to read it when client starts. A rolling restart of the client application will be enough to load the new configuration. > > However, we would want them to eventually see the new format without a > restart. At one extreme, we could put a watch on the znode, but this > could be quite heavy (I think the only per-client watch we have now is > on /ledgers/available). Alternatively, we could just read > /ledgers/LAYOUT once per day. This way clients who saw the old version > will eventually get updated. > I prefer NOT to use the watch, this cost will be payed forever even when there will be no configuration changes. I would go with the 'load at start' option, it is clear and works fine > > > What about having a client side config writeMetadataVersion ? > > We start a new metadata version, the new one will be encoded as binary. > > By default 4.8 clients will use previous version, as we already do for > > journal and fileinfo on bookies. > > > > Each ledger is independent from the others, there is no need of a global > > flag written on zk. > > There is a global property here though,which only the administrator > can control. It is "Can all clients in the cluster/application read > the new ledger metadata version?". This is why I'd like to keep it as > a centralized flag rather than allowing each client to decide for > themselves. > ok, let's keep it Enrico > > > Don't we already have some tool? (Maybe I only have seen such tool in my > > company applications) > > It may already exist, I haven't checked. > > -Ivan >
