memcached #0

Supports: xenial trusty bionic zesty artful cosmic disco


memcached optimizes specific high-load serving applications that are designed to take advantage of its versatile no-locking memory access system. Clients are available in several different programming languages, to suit the needs of the specific application. Traditionally this has been used in mod_perl apps to avoid storing large chunks of data in Apache memory, and to share this burden across several machines.


Memcached is a Free & open source, high-performance, distributed memory object caching system, generic in nature, but intended for use in speeding up dynamic web applications by alleviating database load.

Memcached is an in-memory key-value store for small chunks of arbitrary data (strings, objects) from results of database calls, API calls, or page rendering.


You can deploy a memcached instance with

juju deploy memcached

Memcached listens to port 11211 on the internal network, it is not recommended to expose memcached directly to the open internet. If you want to check that it has deployed correctly you can ssh into the unit and then check the port by hand:

juju status memcached
juju ssh memcached/3
echo "stats settings" | nc localhost 11211

The "3" in this case is an example, the juju status memcached will show you which machine number the service is running on so you can juju ssh to it.


The charm uses the repcached patch ( ), this patch has some limitations, as the ability to just replicate between 2 nodes.

For enabling replication create a config.yaml file with the following content: yaml memcached: repcached: True

Then deploy a maximum of 2 units of memcached:

juju deploy -n 2 config.yaml memcached

Caution : As per design limitations, If you try to add another unit of memcached, all the units will be set as standalone you decide to disable repcached by using juju set memcached repcached=false and re-deploy the unit.

Or you can deploy 2 units and then enable replication by running juju deploy -n 2 memcached juju set memcached repcached=true

Removing a unit

Removing one of the cluster units, means remove replication, please disable replication first and then remove the unit safely.

juju set memcached repcached=false
juju remove-unit memcached/0

Removing replication

For turning the replication support off on memcached, you need to run the following command:

juju set memcached repcached=false

Example Usage

This charm can be used with other charms, in particular make note of these possible relations

To use memcached with the Rails charm follow these instructions

Scale out Usage

You can

juju add-unit memcached

To add more units. Memcached doesn't share load, it's very simple and the clients have the intelligence to know which server to pick.

Nagios Monitoring

To use this charm with nrpe (assuming you want to use the nrpe-external-master relation):

juju deploy memcached
juju deploy nrpe
juju add-relation memcached:nrpe-external-master nrpe:nrpe-external-master

For more details see the nrpe charm.

Known Limitations and Issues

Juju Network Space support

The charm supports Juju network space bindings on its interfaces. This is activated in Juju 2.0+ by either using a --bind during a deploy, or using --constraints. If an interface is not bound to a space then the charm falls back to the default behaviour of using the unit's private address.

The following network space strings are used in the charm to support binding to specific interfaces:

  • 'cluster' for the interface: memcached-replication
  • 'cache' for the interface: memcache
  • 'munin' for the interface: munin-node
  • 'monitors' for the interface: monitors
  • 'nrpe-external-master' for the interface: nrpe-external-master

Thus, for a Juju network space called 'memcache-space', the binding would be

juju ... --bind='cache:memcache-space' ...

Juju 1.25.x continues to use the private address of the unit.


Standard configuration options are provided, we recommend scanning the Memcached documentation before tweaking the default configuration.

Contact Information



(boolean) When this option is set to True the charm will disable the IPv6 support in ufw in case ip6tables couldn't be activated, situations where this could happen is in a LXC container running on top of a host that doesn't have loaded the ip6_tables. If this option is False (the default) and ip6_tables module couldn't be loaded, the charm will fail to install.
(int) maximum simultaneous connections (memcached option -c)
(boolean) do not remove things automatically from the cache on OOM (memcached option -M)
(boolean) disable use of CAS (and reduce the per-item size by 8 bytes)
(boolean) The charm will will try to use large pages if given more than 2GB of RAM. You may want to disable this behavior. (memcached option -L)
(string) memcached has many other options documented in its man page. You may pass them here as a string which
(float) Use <factor> as the multiplier for computing the sizes of memory chunks that items are stored in. A lower value may result in less wasted memory depending on the total amount of memory available and the distribution of item sizes. The default is 1.25.
(int) Allocate a minimum of <size> bytes for the item key, value, and flags. The default is 48. If you have a lot of small keys and values, you can get a significant memory efficiency gain with a lower value. If you use a high chunk growth factor (-f option), on the other hand, you may want to increase the size to allow a bigger percentage of your items to fit in the most densely packed (smallest) chunks. (memcached option -n)
(string) Used by the nrpe-external-master subordinate charm. A string that will be prepended to instance name to set the host name in nagios. So for instance the hostname would be something like: juju-memcached-0 If you're running multiple environments with the same services in them this allows you to differentiate between them.
(boolean) Enable memcached replication
(string) Memcached + Repcached package location
(string) TCP port number for replication (default: 11212)
(int) limit of requests a single client can make at one time (memcached option -R) -1 takes memcached default (20)
(int) Size of memcache pool in MiB (memcached option -m)
(int) Override the size of each slab page in bytes. In mundane words, it adjusts the maximum item size that memcached will accept. You can use the suffixes K and M to specify the size as well, so use 2000000 or 2000K or 2M if you want a maximum size of 2 MB per object. It is not recommended to raise this limit above 1 MB due just to performance reasons. The default value is 1 MB. (memcached option -I) -1 takes the memcached default.
(int) TCP port to listen on. (memcached option -p)
(int) number of threads to use. default is 4. (memcached option -t)
(int) UDP port to listen on (memcached option -U)