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

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-external-master:

juju deploy memcached
juju deploy local:trusty/nrpe-external-master
juju add-relation memcached nrpe-external-master

For more details see the nrpe-external-master charm. Until there is a version of nrpe-external-master in trusty, you will need to use a local copy of the precise charm branched into an appropriate trusty directory.

Known Limitations and Issues


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

Contact Information



                            limit of requests a single client can make at one time
(memcached option -R) -1 takes memcached default (20)

                            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.

                            Enable memcached replication

                            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.

                            number of threads to use. default is 4. (memcached option -t)
                            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:
If you're running multiple environments with the same services in them
this allows you to differentiate between them.

                            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)
                            disable use of CAS (and reduce the per-item size by 8 bytes)
                            TCP port number for replication (default: 11212)

                            maximum simultaneous connections (memcached option -c)
                            UDP port to listen on (memcached option -U)
                            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)

                            Memcached + Repcached package location

                            do not remove things automatically from the cache on OOM
(memcached option -M)

                            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.

                            TCP port to listen on. (memcached option -p)
                            memcached has many other options documented in its man page. You may pass them here as a string which
                            Size of memcache pool in MiB (memcached option -m)