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https://issues.apache.org/jira/browse/HADOOP-8990?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13559262#comment-13559262
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Binglin Chang commented on HADOOP-8990:
---------------------------------------
bq. Note that since rpc has pluggable rpc engine, the rpc request itself is
serialized based on the pluggable rpc engine. For protobuf-rpc-engine the
length is serialized as varint.
I don't understand why protobuf-rpc-engine has to be varint prefixd? I mean
originally it is 4 byte int prefixed, and HADOOP-8084 changed it to varint
prefixed, and then at some time later RpcResponseWritable changed to using
varint, and RpcRequestWritable keep using 4 byte int.
I just think 4 byte int is a better choice, because it easy to implement in
non-blocking code.
Just for example, see Todd's ipc implementation:
https://github.com/toddlipcon/libhrpc/blob/master/rpc_client.cpp
{code}
uint32_t length;
CodedInputStream cis(&rbuf_[rbuf_consumed_], rbuf_available());
bool success = cis.ReadVarint32(&length);
if (!success) {
if (rbuf_available() >= kMaxVarint32Size) {
// TODO: error handling
LOG(FATAL) << "bad varint";
}
// Not enough bytes in buffer to read varint, ask for at least another byte
EnsureAvailableLength(rbuf_available() + 1,
boost::bind(&RpcClient::ReadResponseHeaderLengthCallback, this,
asio::placeholders::error));
return;
}
{code}
There are lot of retry logic to read varint in non-blocking code, it is much
easier if it's fixed 4 byte, and hence unified.
> Some minor issus in protobuf based ipc
> --------------------------------------
>
> Key: HADOOP-8990
> URL: https://issues.apache.org/jira/browse/HADOOP-8990
> Project: Hadoop Common
> Issue Type: Improvement
> Reporter: Binglin Chang
> Priority: Minor
>
> 1. proto file naming
> RpcPayloadHeader.proto include not only RpcPayLoadHeaderProto, but also
> RpcResponseHeaderProto, which is irrelevant to the file name.
> hadoop_rpc.proto only include HadoopRpcRequestProto, and the filename
> "hadoop_rpc" is strange comparing to other .proto file names.
> How about merge those two file into HadoopRpc.proto?
> 2. proto class naming
> In rpc request RpcPayloadHeaderProto includes callId, but in rpc response
> callId is included in RpcResponseHeaderProto, and there is also
> HadoopRpcRequestProto, this is just too confusing.
> 3. The rpc system is not fully protobuf based, there are still some Writables:
> RpcRequestWritable and RpcResponseWritable.
> rpc response exception name and stack trace string.
> And RpcRequestWritable uses protobuf style varint32 prefix, but
> RpcResponseWritable uses int32 prefix, why this inconsistency?
> Currently rpc request is splitted into length, PayLoadHeader and PayLoad, and
> response into RpcResponseHeader, response and error message.
> I think wrap request and response into single RequstProto and ResponseProto
> is better, cause this gives a formal complete wire format definition,
> or developer need to look into the source code and hard coding the
> communication format.
> These issues above make it very confusing and hard for developers to use
> these rpc interfaces.
> Some of these issues can be solved without breaking compatibility, but some
> can not, but at least we need to know what will be changed and what will stay
> stable?
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