Lack of docs for default memcached connection loss behaviour

[m6w6]

• Reported as: lp:1132725
• Reported by: lp:~tim-bunce
• Reported at: 2013-02-25T10:29:44Z

Imported from Launchpad using lp2gh.

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I can't seem to find documentation that describes exactly what happens when a server goes down, and how best to handle its graceful recovery.

The documentation for the behaviours seems to assume a certain level of knowledge that I don't have, or at least I'm not sure about. It's a reference manual. I'm looking for a guide.

My question is similar to http://stackoverflow.com/questions/10029432/libmemcached-fail-over-of-a-clusters-node
except that a) I'd like more background information, and b) there's no discussion of handling the recovery of a node.

---

As an example, the venerable and original Cache::Memcached module (https://metacpan.org/source/DORMANDO/Cache-Memcached-1.30/lib/Cache/Memcached.pm) seems to recalculate the 'buck2sock' mapping (by $socket_cache_generation++;) whenever a socket is marked dead by _dead_sock() being called.

But some of calls to _dead_sock, specifically those from connection setup, also provide a duration ($dead_for) during which the host should be regarded as dead.

So a single communications error on an established socket doesn't remove a server, but does cause a connection drop and reconnect. If that reconnect fails then the server is removed from the buck2sock calculation for a period.

This seems reasonable and is self-repairing. Sadly the libmemcached docs don't seem to discuss this behaviour at all.

---

I'm rather surprised I can't find this kind of discussion in the libmemcached docs. Am I missing something?

The use-case is simply what I imagine a typical installation would be: a list of memcached servers and consistent hashing enabled.

I was asked that question recently and realised that I wasn't sure and was then doubly surprised by not being able to find it documented.

I'm looking for a simple description of exactly what happens by default in libmemcached when a memcached server goes down (both for a clean tcp close and for a network outage leading to tcp timeouts) and when it comes back again later.

That would provide the background detail against which the various optional behaviours could be explained and understood.

[m6w6]

• Comment by: lp:~brianaker:
• Created at: 2013-02-25T19:29:25Z

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Thanks, tracking this via bug reports is the best way to make this happen.

Sent from my Ti85

On Feb 25, 2013, at 2:35, TimBunce wrote:

Public bug reported:

I can't seem to find documentation that describes exactly what happens
when a server goes down, and how best to handle its graceful recovery.

The documentation for the behaviours seems to assume a certain level of
knowledge that I don't have, or at least I'm not sure about. It's a
reference manual. I'm looking for a guide.

My question is similar to http://stackoverflow.com/questions/10029432/libmemcached-fail-over-of-a-clusters-node
except that a) I'd like more background information, and b) there's no discussion of handling the recovery of a node.

---

As an example, the venerable and original Cache::Memcached module
(https://metacpan.org/source/DORMANDO/Cache-
Memcached-1.30/lib/Cache/Memcached.pm) seems to recalculate the
'buck2sock' mapping (by $socket_cache_generation++;) whenever a socket
is marked dead by _dead_sock() being called.

But some of calls to _dead_sock, specifically those from connection
setup, also provide a duration ($dead_for) during which the host should
be regarded as dead.

So a single communications error on an established socket doesn't remove
a server, but does cause a connection drop and reconnect. If that
reconnect fails then the server is removed from the buck2sock
calculation for a period.

This seems reasonable and is self-repairing. Sadly the libmemcached docs
don't seem to discuss this behaviour at all.

---

I'm rather surprised I can't find this kind of discussion in the
libmemcached docs. Am I missing something?

The use-case is simply what I imagine a typical installation would be: a
list of memcached servers and consistent hashing enabled.

I was asked that question recently and realised that I wasn't sure and
was then doubly surprised by not being able to find it documented.

I'm looking for a simple description of exactly what happens by default
in libmemcached when a memcached server goes down (both for a clean tcp
close and for a network outage leading to tcp timeouts) and when it
comes back again later.

That would provide the background detail against which the various
optional behaviours could be explained and understood.

** Affects: libmemcached
Importance: Undecided
Status: New

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https://bugs.launchpad.net/bugs/1132725

Title:
Lack of docs for default memcached connection loss behaviour

Status in libmemcached - A C and C++ client library for memcached:
New

Bug description:
I can't seem to find documentation that describes exactly what happens
when a server goes down, and how best to handle its graceful recovery.

The documentation for the behaviours seems to assume a certain level
of knowledge that I don't have, or at least I'm not sure about. It's a
reference manual. I'm looking for a guide.

My question is similar to http://stackoverflow.com/questions/10029432/libmemcached-fail-over-of-a-clusters-node
except that a) I'd like more background information, and b) there's no discussion of handling the recovery of a node.

---

As an example, the venerable and original Cache::Memcached module
(https://metacpan.org/source/DORMANDO/Cache-
Memcached-1.30/lib/Cache/Memcached.pm) seems to recalculate the
'buck2sock' mapping (by $socket_cache_generation++;) whenever a socket
is marked dead by _dead_sock() being called.

But some of calls to _dead_sock, specifically those from connection
setup, also provide a duration ($dead_for) during which the host
should be regarded as dead.

So a single communications error on an established socket doesn't
remove a server, but does cause a connection drop and reconnect. If
that reconnect fails then the server is removed from the buck2sock
calculation for a period.

This seems reasonable and is self-repairing. Sadly the libmemcached
docs don't seem to discuss this behaviour at all.

---

I'm rather surprised I can't find this kind of discussion in the
libmemcached docs. Am I missing something?

The use-case is simply what I imagine a typical installation would be:
a list of memcached servers and consistent hashing enabled.

I was asked that question recently and realised that I wasn't sure and
was then doubly surprised by not being able to find it documented.

I'm looking for a simple description of exactly what happens by
default in libmemcached when a memcached server goes down (both for a
clean tcp close and for a network outage leading to tcp timeouts) and
when it comes back again later.

That would provide the background detail against which the various
optional behaviours could be explained and understood.

To manage notifications about this bug go to:
https://bugs.launchpad.net/libmemcached/+bug/1132725/+subscriptions

[m6w6]

• Comment by: lp:~tim-bunce:
• Created at: 2014-04-30T19:30:56Z

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Any news on this? It's been over a year now. It seems like a simple, and important, request.