Re: Question about source IP persistence (balance source) when a server goes down:

From: Willy Tarreau <>
Date: Sat, 17 Jan 2009 22:36:24 +0100

Hi Malcolm,

On Fri, Jan 16, 2009 at 02:48:18PM +0000, Malcolm Turnbull wrote:
> The manual states that when using balance source:
> "The source IP address is hashed and divided by the total
> weight of the running servers to designate which server will
> receive the request. This ensures that the same client IP
> address will always reach the same server as long as no
> server goes down or up. If the hash result changes due to the
> number of running servers changing, many clients will be
> directed to a different server. This algorithm is generally
> used in TCP mode where no cookie may be inserted. It may also
> be used on the Internet to provide a best-effort stickyness
> to clients which refuse session cookies. This algorithm is
> static, which means that changing a server's weight on the
> fly will have no effect."
> Does this mean that if say 1 server out of a cluster of 5 servers
> fails, then it is likely
> that the hash result changes and lots of the clients could potentially
> loose their session? (i..e hit the wrong server because the hash has
> changed)

Absolutely. That's why hashing should only be used for cache optimization but not when strong persistency is required in case of server failure (cookies are far better then).

Willy Received on 2009/01/17 22:36

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