Brocade Communications Systems 12.4.00 manual Gslb hash-based persistence, Hashing scheme

Weighted distribution of sites with hash-based persistence 1

•“Disabling rehash on change in hash weight configuration” on page 79

GSLB hash-based persistence

GSLB provides two methods for persistence- Sticky method and Hash-based persistence. Sticky GSLB is suitable for single-box and HA (hot standby, symmetric, sym-active) topologies. However, if there are two GSLB controllers across a network providing GSLB for the same domain but are not in an HA configuration, and if persistence is desired when the same client is directed to either of these two GSLB controllers, then hash-based GSLB persistence should be used. hash-based Persistence provides GSLB controller persistence in multiple GSLB controller environments. When users perform a DNS query for a domain, the users will get the same IP address for that domain regardless of which GSLB controller is contacted. Currently hash-based persistence distributes hash buckets in a round robin fashion.

GSLB weighted hash-based persistence

In addition to providing hash-based persistence, we will now provide weighted hash-based persistence. Weighted hash-based persistence allocates the hash buckets in a weighted round robin fashion. This enables the user not only to maintain persistence, but also to determine what percentage of the traffic goes to a particular domain IP address.

Hashing scheme

Each domain maintains a separate hash table. For instance, if GSLB controller has the following two domains www.foo.com and www.test.com configured, then it will maintain one hash table for each domain. The number of hash buckets for each hash table is 256.

The client IP address is hashed to generate a value between 0 and 255.

After the Client IP address is hashed to an index in the hash table, the IP address associated with the hash index in the hash table is selected as the best IP address for the client. The GSLB controller reorders the IP address in the DNS server's response so that the best IP address is placed in the first position. It then forwards the modified response to the client.

IP address allocation

Firstly, IP addresses are ordered with the lowest IP having rank 1. IPs will be allocated to hash buckets in a weighted round robin fashion starting with lowest IP first. This is done so that no synchronization is required across Controllers.

Example

Consider the example where user has configured IPs 1.1.1.44, 1.1.1.43 and 1.1.1.42 for www.foo.com. The IP addresses are first sorted in ascending order.

1.1.1.42(rank 1)

1.1.1.43(rank 2)

1.1.1.44(rank 3)

User also configures hash weights for these IP addresses. Say the weights for the IP addresses are as follows.

1.1.1.42: weight 1

1.1.1.43: weight 1

1.1.1.44: weight 2

ServerIron ADX Global Server Load Balancing Guide	77
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