[infinispan-dev] ISPN-384 - Implementing hash distribution aware headers in Hot Rod

[galder at redhat.com]

Hi,

Re: https://jira.jboss.org/jira/browse/ISPN-384

The topology headers are in, so the next step is to get the hash distribution headers in. The first step here is for Hot Rod servers to be able to query an Address' position in the wheel so that this is sent back to clients (This is what I refer to by hashcode in http://community.jboss.org/wiki/HotRodProtocol#HashDistributionAware_Client_Topology_Change_Header - it really is the hash wheel position). However, there's no such API at the  moment that allows clients to query it. Having had a look, the most reasonable thing would be to add something like this to the ConsistentHash interface:

int getPosition(Address a)

However, this might be somehow limiting if we end up implementing virtual nodes. ExperimentalDefaultConsistentHash hints at the possibility of that happening and so something like this might more future proof:

List[Integer] getPositions(Address a)

This also highlights the limitation of the Hot Rod spec where it's assumed that a server has a single position. 

Moreover, these brings up another interesting topic which is the order in which Hot Rod orders the servers in the headers. For topology headers, although not written down, I'm following the same kind of pattern used at the JGroups level where serves started first appear in first in the list returned. I should probably add this to the protocol wiki. 

In the case of hash distribution headers, I think it'd make sense for the order to be based on the hash wheel position in ascendant order. That way it would make life easier for clients to find the target node for the operation, since it'd avoid them having to do the sorting and finding out the next node. If we take this into account with the fact  that a node might map to multiple positions, I think the hash distribution header might be look better this way:

[Response header][Topology Id][Num Key Owners][Hash Function Version][Hash space size][Num servers in topology]
-> New: 
[*m1: Server Id*][m1: Host/IP length][m1: Host/IP address]
[*m2: Server Id*][m2: Host/IP length][m2: Host/IP address]...
[*m3: Server Id*][m3: Host/IP length][m3: Host/IP address]...
[Num total positions]
[*m2: Server Id*][m2: hash wheel position 1]
[*m3: Server Id*][m3: hash wheel position 2]
[*m2: Server Id*][m2: hash wheel position 2]
[*m3: Server Id*][m3: hash wheel position 1]
[*m3: Server Id*][m3: hash wheel position 3]
[*m1: Server Id*][m1: hash wheel position 1]

So, above I've splitted the server host/port definitions in one side and then the hash wheel positions. I did this to avoid repeating the host/port definition which each hash wheel definition. I've also added a number of total positions and the list following it is ordered in ascendant order of position. The server id would be a vInt.

WDYT?

--
Galder Zamarreño
Sr. Software Engineer
Infinispan, JBoss Cache