4 ms·

Why does session affinity not solve the problem of session caching? The author says it's "a whole other world of pain and suffering" but doesn't explain why.

jadc 15y ago
Seconded. It would be great if the author could expand on this point.
- jpetazzo 15y ago
  I guess that it's not easy to implement session affinity on the SSL level. You cannot access information like cookies or other HTTP headers (since they are inside the SSL payload that you're trying to handle). So you have to use the little information you have: source IP address and port, and session data. The idea would be to make sure that a given client is always sent to the same SSL handler. We could imagine having two layers of load balancers: - first layer would use source IP address and/or session data to determine to which server of the second layer the connection should be forwarded; - second layer would receive the connection and to the proper SSL handling. I believe that this would work, but it seems that it would require a custom "half-implementation" of SSL on the first layer of load balancers. I don't know if there is any provision for that in OpenSSL or GNUTLS. Also, since there are already hooks to do session caching in most SSL-enabled servers, using those hooks to plug in a memcached backend seems to be less "disruptive" (read "easier to understand, implement and debug").
  - ominous_prime 15y ago
    You can balance ssl traffic at a TCP level using the client's IP to set affinity. The only real catch is if you have a large base of clients behind a single NAT, but in most cases traffic will balance out pretty well. Sometimes though, you don't want affinity at all. If you don't care what backend server takes the request, you can balance the load more efficiently, and more easily rotate servers in and out of service.
ominous_prime 15y ago
I think that was a bit hyperbolic, but affinity does have drawbacks. Balancing the requests is much easier without affinity, and you don't run the risk of the load ever getting severely skewed. Maintenance is also easier, because you don't have to "drain" the load from a server, you just drop it out of service, and the requests can go on to something else. The latter point isn't that big a deal if the only reason for affinity is SSL session caching, because you could yank the server even if it has active sessions, and the clients would simply re-establish with the next backend. I often load-balance ssl using session affinity, and would also like to know if this author has encountered other issues, or just hasn't looked at the capabilities of haproxy in a while.

jadc 15y ago

Seconded. It would be great if the author could expand on this point.

jpetazzo 15y ago
I guess that it's not easy to implement session affinity on the SSL level. You cannot access information like cookies or other HTTP headers (since they are inside the SSL payload that you're trying to handle). So you have to use the little information you have: source IP address and port, and session data. The idea would be to make sure that a given client is always sent to the same SSL handler. We could imagine having two layers of load balancers: - first layer would use source IP address and/or session data to determine to which server of the second layer the connection should be forwarded; - second layer would receive the connection and to the proper SSL handling. I believe that this would work, but it seems that it would require a custom "half-implementation" of SSL on the first layer of load balancers. I don't know if there is any provision for that in OpenSSL or GNUTLS. Also, since there are already hooks to do session caching in most SSL-enabled servers, using those hooks to plug in a memcached backend seems to be less "disruptive" (read "easier to understand, implement and debug").
- ominous_prime 15y ago
  You can balance ssl traffic at a TCP level using the client's IP to set affinity. The only real catch is if you have a large base of clients behind a single NAT, but in most cases traffic will balance out pretty well. Sometimes though, you don't want affinity at all. If you don't care what backend server takes the request, you can balance the load more efficiently, and more easily rotate servers in and out of service.

ominous_prime 15y ago

I think that was a bit hyperbolic, but affinity does have drawbacks. Balancing the requests is much easier without affinity, and you don't run the risk of the load ever getting severely skewed. Maintenance is also easier, because you don't have to "drain" the load from a server, you just drop it out of service, and the requests can go on to something else. The latter point isn't that big a deal if the only reason for affinity is SSL session caching, because you could yank the server even if it has active sessions, and the clients would simply re-establish with the next backend. I often load-balance ssl using session affinity, and would also like to know if this author has encountered other issues, or just hasn't looked at the capabilities of haproxy in a while.