MemcachedStorage and IdP3

O'Dowd, Josh Josh.O'Dowd at
Wed Aug 23 14:48:17 EDT 2017

Thanks Marvin,

Reading the docs[1], the sentence:
“Any keys that hash to the unavailable host will be written or retrieved from a backup host.” kind of leads me to think that session keys are replicated to all the hosts, if a key that hashes to an unavailable host can be retrieved from a backup host.  But what you said makes me think that keys are not replicated across nodes.

Basically, I am trying to figure out if a session is lost when Idp tries to retrieve it from a now unavailable(timing out) node; or is that session available for retrieval from the backup node?


From: users [mailto:users-bounces at] On Behalf Of Marvin Addison
Sent: Wednesday, August 23, 2017 12:07 PM
To: Shib Users <users at>
Subject: Re: MemcachedStorage and IdP3

On Wed, Aug 23, 2017 at 1:18 PM O'Dowd, Josh <Josh.O'Dowd at<mailto:Josh.O%27Dowd at>> wrote:
My question is, in “Redistribute” mode, if the storage piece only writes to one of our 2 nodes, due to a failure on one of them, would a retrieval attempt check both nodes for that session, since our clients are configured to know about both nodes?

No. A node is selected based on a hash function applied to the key; if the selected node is not available, a backup node is selected. If backup node is not available, another backup is chosen and so on until either a functional node is located or there are no more nodes available. The key is written to or read from the first selected node. If an out-of-service node comes back up, it will be selected in preference to a backup and the read will be served from that node. In any case it's a first-one-wins approach.


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