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From "James Baldassari (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (AVRO-1008) Allow IPC clients to perform the IPC handshake before the first RPC is invoked
Date Sat, 25 Feb 2012 23:59:48 GMT

    [ https://issues.apache.org/jira/browse/AVRO-1008?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13216586#comment-13216586
] 

James Baldassari commented on AVRO-1008:
----------------------------------------

bq. We should update the RPC specification to note that if the empty string is passed as the
message name during a handshake negotiation then it should be ignored. Or we might say that
an empty message name is always to be ignored, to permit ping messages in any protocol at
any time.

I think that both of these sound like good solutions.  Either way an RPC with an empty name
should be considered reserved for internal use by Avro.

bq. The change to Responder might also check !wasConnected, unless we decide that pings are
desired.

Makes sense.  I've been thinking about the question of whether pings should be a core part
of the protocol, and it doesn't seem like they would be useful in the general case.  Can you
think of a use case for pings that the majority of users would want/need?  I suppose that
one use case might be a type of pessimistic connection management scheme in which we ping
the server periodically to make sure the connection is alive before we attempt an RPC.  However,
there is nothing to prevent the connection from terminating between the ping and the RPC,
so even in this case the client would have to support an optimistic connection management
policy in which the connection is assumed to be alive until a failure occurs, at which time
the connection would have to be reestablished.  The only other use case I can think of right
now would be measuring client-server latency.  Now this might be useful, and we could consider
adding a method in Responder to give some kind of estimate of client-server latency by using
pings.

bq. What's the advantage of passing a boolean to the Requestor constructor to calling a method
like getRemote()? We might instead add a new method ensureHandshake() or somesuch that does
nothing but call getRemote().

I suppose there is no real advantage to having this flag in the constructor, other than possibly
making it more obvious to users who are more likely to look at the JavaDoc of the constructor
than all of the other methods in the class.  If we decide that we want to include pings as
a general feature of Avro protocols, I suppose we could simply have a method called ping()
or ensureHandshake() or measureLantency() that would establish this handshake if necessary.

bq. Why doesn't SpecificRequestor#getClient() just pass performHandshake to the requestor
constructor, so the only call to getRemote() is in the base Requestor class, avoiding the
duplication of that logic?

Good catch.  It should be changed as you suggested.

So I guess the next steps are:
* Decide whether to change the RPC spec to make an empty message name reserved for use by
Avro and to specify what should happen when a server receives a request with an empty message
name
* Decide whether pings should be supported
* Make the suggested changes to the patch
                
> Allow IPC clients to perform the IPC handshake before the first RPC is invoked
> ------------------------------------------------------------------------------
>
>                 Key: AVRO-1008
>                 URL: https://issues.apache.org/jira/browse/AVRO-1008
>             Project: Avro
>          Issue Type: Improvement
>    Affects Versions: 1.6.1
>            Reporter: James Baldassari
>            Assignee: James Baldassari
>              Labels: java
>         Attachments: AVRO-1008.patch
>
>
> Currently, the first RPC must complete before any subsequent RPCs may be invoked using
the same Transceiver instance.  The reason for this behavior is that the IPC client-server
handshake must be performed before any requests can be exchanged.  So while the first RPC
is being invoked, all other threads using the same Transceiver instance will block.  The goal
of this enhancement is to allow clients to perform a handshake with the server _before_ any
RPCs are invoked to avoid blocking any threads once the Transceiver is put into service.
> I have a patch for review that I think will enable clients to perform the handshake before
the first RPC is sent.  The changes consist of:
> * Modification to the Responder to handle a request that contains only a handshake (without
an RPC)
> * Addition of overridden SpecificRequestor.getClient(...) methods that take an boolean
indicating whether the handshake should be performed immediately upon initialization of the
Requestor
> * Unit test which is essentially the same as the test I wrote for AVRO-1001, but it uses
a pre-RPC handshake rather than invoking the add(...) RPC to perform the handshake

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