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From Devaraj Das <d...@hortonworks.com>
Subject Re: Handling protocol versions
Date Fri, 03 Aug 2012 18:40:24 GMT
Responses inline..

> On Wed, Aug 1, 2012 at 11:04 AM, Todd Lipcon <todd@cloudera.com> wrote:
>> One possibility:
>> During the IPC handshake, we could send the full version string /
>> source checksum. Then, have a client-wide map which caches which
>> methods have been found to be supported or not supported for an
>> individual version. So, we don't need to maintain the mapping
>> ourselves, but we also wouldn't need to do the full retry every time.

Yeah this is what I was thinking as the alternate to the current approach of using VersionedProtocol.

>> A different idea would be to introduce a call like
>> "getServerCapabilities()" which returns a bitmap, and define a bit per
>> time that we add a new feature.
>> The advantage of these approaches vs a single increasing version
>> number is that we sometimes want to backport a new IPC to an older
>> version, but not backport all of the intervening IPCs. Having a bitmap
>> allows us to "pick and choose" on backports without having to pull in
>> a bunch of things we didn't necessarily want.

Good point.

>> On Wed, Aug 1, 2012 at 1:41 AM, Stack <stack@duboce.net> wrote:
>>> On Tue, Jul 31, 2012 at 1:47 AM, Devaraj Das <ddas@hortonworks.com> wrote:
>>>> Wondering whether we should retain the VersionedProtocol now that we have
protobuf implementation for most (all?) of the protocols. I think we still need the version
checks and do them when we need to. Take this case:
>>>> 1. Protocol Foo has as one of the methods FooMethod(FooMethodRequest).
>>>> 2. Protocol Foo evolves over time, and the FooMethod(FooMethodRequest) now
has a better implementation called FooMethod_improved(FooMethodRequest).
>>>> 3. HBase installations have happened with both the protocol implementations.
>>>> 4. Clients should be able to talk to both old and new servers (and invoke
the newer implementation of FooMethod if the protocol implements it).
>>>> (4) is possible when the getProtocolVersion is implemented by the protocol
at the server. The client could check what the version of the protocol was (assuming VersionedProtocol
semantics where the protocol version number is upgraded for such significant changes) and
depending on that invoke the appropriate method...
>>>> Having to map version-numbers of protocols to the methods-supported is probably
arcane IMO but works..
>>>> The other approach (that wouldn't require the version#) is to do something
like - On the client side, get the protocol methods supported at the server (and cache it)
and then look this map up whenever needed to decide which method to invoke.
>>>> Any thoughts on whether we should invest time in the second approach yet?
>>> The VersionedProtocol w/ client being able to interrogate what methods
>>> a server supports strikes me as a facility that will be rarely used if
>>> at all and bringing it along, keeping up the directory of supported
>>> methods, will take a load of work on our part that we'll do less than
>>> perfectly so should it ever be needed, it won't work because we let it
>>> go stale.

Yeah, this won't be a common case. It'd (hopefully) be rare. The directory of methods would
be the methods in the protocol-interface at the server that could be figured by invoking reflection
(and hence staleness issue shouldn't happen). 

>>> What do you reckon?
>>> The above painted scenario too is a little on the exotic side.  We can
>>> do something like Jimmy suggests in those rare cases we need to add a
>>> new method because there is insufficient wiggle-room w/i the
>>> particular PB method call (If we get into the issue Ted raises where
>>> we'd have to go back to the server twice because there is a third new
>>> method call, we're doing our API wrong).

Agree that the exception handling hack can be played here.. In general, having some solution
around this might be really helpful *if* we get some API wrong (for e.g., indirect implication
on memory by the API semantics) and we need to fix it without breaking compatibility.. In
HDFS, listFile proved to be a memory killer for extremely large directories and people implemented
the iterator version of the same.

>>> The protocol needs a version though.  We'll be still sending that
>>> 'hrpc' long in the header preamble?  Should we add a version long
>>> after the 'hrpc' long?

The version in "hrpc" is the RPC version (as opposed to protocol version). I think that's
orthogonal to this discussion..

>>> As to a directory of supported methods, do we need this in the
>>> protocol at all?  Can't this be knowledge kept outside of the
>>> on-the-wire back and forth?
>>> St.Ack

As I answered above, and as Todd also says, it probably makes sense to have a client wide
cache for protocol<->supported-methods .. and look up the cache when and if the client
needs to decide between different versions of a method, or picking a new method, based on
the server it is talking to...
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