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