return value size limit

[yinanUCSD]

For specpaxos protocol, is there a limit to the size of return string? We want to have a relatively long string, like thousands of bytes.

[drkp]

Not explicitly, but the underlying UDP messaging library does a pretty terrible job of handling packets that exceed its maximum packet size -- it has rudimentary support for fragmentation, but it doesn't work well at all. So you don't want the return value to exceed the maximum message size minus a few bytes for the rest of the reply message.

That's defined by MAX_UDP_MESSAGE_SIZE in lib/udptransport.c, and set to 9000 by default. In theory this could be set as high as 65507 bytes, but it probably shouldn't exceed the MTU of your network unless you are willing to deal with fragmentation (which could cause problems with multicast delivery).

I don't think it would be too hard to remove this limitation, especially for the reply message, where there's no underlying reason it would need to fit in one message, but we've never done that.