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https://issues.apache.org/jira/browse/ZOOKEEPER-2353?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15097762#comment-15097762
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Flavio Junqueira commented on ZOOKEEPER-2353:
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That's an independent codebase [~suda], I'm not sure it'd help much here.
> QuorumCnxManager protocol needs to be upgradable with-in a specific Version
> ---------------------------------------------------------------------------
>
> Key: ZOOKEEPER-2353
> URL: https://issues.apache.org/jira/browse/ZOOKEEPER-2353
> Project: ZooKeeper
> Issue Type: Improvement
> Affects Versions: 3.4.7, 3.5.1
> Reporter: Powell Molleti
>
> Currently 3.5.X sends its hdr as follows:
> {code:title=QuorumCnxManager.java|borderStyle=solid}
> dout.writeLong(PROTOCOL_VERSION);
> dout.writeLong(self.getId());
> String addr = self.getElectionAddress().getHostString() + ":" +
> self.getElectionAddress().getPort();
> byte[] addr_bytes = addr.getBytes();
> dout.writeInt(addr_bytes.length);
> dout.write(addr_bytes);
> dout.flush();
> {code}
> Since it writes length of host and port byte string there is no simple way to
> append new fields to this hdr anymore. I.e the rx side has to consider all
> bytes after sid for host and port parsing, which is what it does here:
> [QuorumCnxManager.InitialMessage.parse(): http://bit.ly/1Q0znpW]
> {code:title=QuorumCnxManager.java|borderStyle=solid}
> sid = din.readLong();
> int remaining = din.readInt();
> if (remaining <= 0 || remaining > maxBuffer) {
> throw new InitialMessageException(
> "Unreasonable buffer length: %s", remaining);
> }
> byte[] b = new byte[remaining];
> int num_read = din.read(b);
> if (num_read != remaining) {
> throw new InitialMessageException(
> "Read only %s bytes out of %s sent by server %s",
> num_read, remaining, sid);
> }
> // FIXME: IPv6 is not supported. Using something like Guava's
> HostAndPort
> // parser would be good.
> String addr = new String(b);
> String[] host_port = addr.split(":");
> {code}
> This has been captured in the discussion here: ZOOKEEPER-2186.
> Though it is possible to circumvent this problem by various means the request
> here is to design messages with hdr such that there is no need to bump
> version number or hack certain fields (i.e figure out if its length of
> host/port or length of different message etc, in the above case).
> This is the idea here as captured in ZOOKEEPER-2186.
> {code:java}
> dout.writeLong(PROTOCOL_VERSION);
> String addr = self.getElectionAddress().getHostString() + ":" +
> self.getElectionAddress().getPort();
> byte[] addr_bytes = addr.getBytes();
> // After version write the total length of msg sent by sender.
> dout.writeInt(Long.BYTES + addr_bytes.length);
> // Write sid afterwards
> dout.writeLong(self.getId());
> // Write length of host/port string
> dout.writeInt(addr_bytes.length);
> // Write host/port string
> dout.write(addr_bytes);
> {code}
> Since total length of the message and length of each variable field is also
> present it is quite easy to provide backward compatibility, w.r.t to parsing
> of the message.
> Older code will read the length of message it knows and ignore the rest.
> Newer revision(s), that wants to keep things compatible, will only append to
> hdr and not change the meaning of current fields.
> I am guessing this was the original intent w.r.t the introduction of protocol
> version here: ZOOKEEPER-1633
> Since 3.4.x code does not parse this and 3.5.x is still in alpha mode perhaps
> it is possible to consider this change now?.
> Also I would like to propose to carefully consider the option of using
> protobufs for the next protocol version bump. This will prevent issues like
> this in the future.
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