2017-09-28 14:55 GMT+02:00 Sijie Guo <guosi...@gmail.com>:
> On Sep 28, 2017 6:20 AM, "Enrico Olivelli" <eolive...@gmail.com> wrote:
>
> Hi,
> I am looking for an API to read ledger metadata without actually opening a
> ledger.
>
> Currently opening a ledger sets a watch on ZK and this is really expensive
> and in any case the desired action is not to "open a ledger" but to access
> meta information about the ledger.
>
> My real usecase is that I want to list ledgers and filter the results using
> custom metadata.
>
> If there is no available solution I would like to file and issue and a
> proposal to have a new method in the new API like
>
> org.apache.bookkeeper.client.api.LedgerMetadata {
> public long getId();
> public Map<String, byte[]> getCustomMetadata();
> public long getCtime();
> }
>
>
> I am fine with an interface for ledger metadata. It can help hide the
> implementation details.
>
>
> org.apache.bookkeeper.client.api.BookKeeper {
>
> LedgerMetadata getLedgerMetadata(long ledgerId);
> void listLedgers(String query, Consumer<LedgerMetadata> consumer)
>
> }
>
>
> I am not sure if we want this. Now, you can access the ledger manager for
> reading and listing metadata. And this is how it was used by bk shell. I
> would not suggest adding this to the public API until we really see a value
> there.
>
BookKeeper object does not expose LedgerManager, I don't know if we want to
expose it in the new API
we could add an API like:
interface org.apache.bookkeeper.client.api.LedgerMetadataFilter {
// marker interface
}
class org.apache.bookkeeper.client.api.CustomMetadataFilter implements
LedgerMetadataFilter {
// filter custom metadata field == value
CustomMetadataFilter(String key, byte[] value);
}
class org.apache.bookkeeper.client.api.CompositeMetadataFilter implements
LedgerMetadataFilter {
// apply a sequence of filters in AND
CompositeMetadataFilter(LedgerMetadataFilter ... filters);
}
void listLedgers(LedgerFilter filter, Consumer<LedgerMetadata> consumer)
Does this sound better to you ? it is more simpler but clear and the
implemenation will be really easy
>
>
> for the "query" we can define a simple "expression language"
>
> metadata.owner = 'xxx' and (metadata.type = 'yyyy' or metadata.type =
> 'zzzz')
>
>
> I am not a fan of having a query language for such purpose in bk. At least,
> I don't see a lot of use cases that need this kind of query capabilities.
> If you can share more use cases about this, that would be good for
> discussions.
>
>
> we can provide a basic implementation of that language which actually works
> on LedgerMetadata objects and internally we will loop over the ledgers and
> apply the expression to every ledger
> the basic implementation can leverage standard expression languages like EL
>
> smarter implementations of LedgerManager will be able to narrow the search
> and save resources
>
> Thoughts ?
>
> -- Enrico
>
