Docker Explained: How To Create Docker Containers Running Memcached

Status: Deprecated #

This article is deprecated and no longer maintained.

Reason #

The techniques in this article are outdated and may no longer reflect Docker best-practices.

See Instead #

The Docker Ecosystem: An Introduction to Common Components
The Official memcached Container on Docker Hub

Introduction #

For the majority of web applications, it is extremely rare to find the CPU being the culprit for the dropped HTTP requests or choking the web server hosting them. It usually is an under-engineered setup with no caching layer involved, eating up all the resources of the backend data store (i.e. your database of choice).
Memcached – which by now should need no introduction – can increase the performance of your application deployment stack greatly without making any amendments to your available resources (enabling you to squeeze every last bit of its juices).
In this DigitalOcean article, especially keeping in mind those who host multiple web applications (e.g. multiple WordPress instances, Python Applications, etc.), we are going to create docker images to quickly start running (on-demand) Memcached containers which can be operated individually. These containers, kept and secured within their own environments, will work with the application being hosted to help them get better and faster.

Glossary #

1. Docker in Brief #

2. Memcached in Brief #

3. Installing Docker on Ubuntu #

4. Basic Docker Commands #

Running the docker daemon and CLI Usage
docker Commands

5. Getting Started with Creating Memcached Images #

Quick Recap: What Are Dockerfiles?
Dockerfile Commands Overview
Creating a Dockerfile
Final Dockerfile
Creating the Docker Image for Memcached Containers

6. Running dockerised Memcached Containers #

Creating a Memcached Installed Container
Limiting the Memory for a Memcached Container
Testing the Memcached Container

Docker in Brief #

The docker project offers higher-level tools, working together, which are built on top of some Linux kernel features. The goal is to help developers and system administrators port applications – with all of their dependencies conjointly – and get them running across systems and machines – headache free.
Docker achieves this by creating safe, LXC (i.e. Linux Containers) based environments for applications called docker containers. These containers are created using docker images, which can be built either by executing commands manually or automatically through Dockerfiles.
Note: To learn more about docker and its parts (e.g. docker daemon, CLI, images etc.), check out our introductory article to the project: docker Explained: Getting Started.

Memcached in Brief #

Memcached is a distributed, open-source data storage engine. It was designed to store certain types of data in RAM (instead of slower rate traditional disks) for very fast retrievals by applications, cutting the amount of time it takes to process requests by reducing the number of queries performed against heavier datasets or APIs such as traditional databases (e.g. MySQL).
By introducing a smart, well-planned, and optimized caching mechanism, it becomes possible to handle a seemingly larger amount of requests and perform more procedures by applications. This is the most important use case of Memcached, as it is with any other caching application or component.
Heavily relied upon and used in production for web sites and various other applications, Memcached has become one of the go-to tools for increasing performance without -necessarily – needing to utilize further hardware (e.g. more servers or server resources).
It works by storing keys and their matching values (up to 1 MB in size) onto an associative array (i.e. hash table) which can be scaled and distributed across a large number of virtual servers.

Installing Docker on Ubuntu (Latest) #

To start using the Docker project on your VPS, you can either use DigitalOcean’s docker image for Ubuntu 13.04 or install it yourself. In this section, we will quickly go over the basic installation instructions for Docker 0.7.1.

Installation Instructions for Ubuntu #

Update your droplet:

sudo aptitude    update
sudo aptitude -y upgrade

Make sure aufs support is available:

sudo aptitude install linux-image-extra-`uname -r`

Add docker repository key to apt-key for package verification:

sudo sh -c "wget -qO- https://get.docker.io/gpg | apt-key add -"

Add the docker repository to aptitude sources:

sudo sh -c "echo deb http://get.docker.io/ubuntu docker main
> /etc/apt/sources.list.d/docker.list"

Update the repository with the new addition:

sudo aptitude    update

Finally, download and install docker:

sudo aptitude install lxc-docker

Ubuntu’s default firewall (UFW: Uncomplicated Firewall) denies all forwarding traffic by default, which is needed by docker.
Enable forwarding with UFW:
Edit UFW configuration using the nano text editor.

sudo nano /etc/default/ufw

Scroll down and find the line beginning with DEFAULT_FORWARD_POLICY.
Replace:

DEFAULT_FORWARD_POLICY="DROP"

With:

DEFAULT_FORWARD_POLICY="ACCEPT"

Press CTRL+X and approve with Y to save and close.
Finally, reload the UFW:

sudo ufw reload

Basic Docker Commands #

Before we begin working with docker, let’s quickly go over its available commands to refresh our memory from our first Getting Started article.

Running the docker daemon and CLI Usage #

Upon installation, the docker daemon should be running in the background, ready to accept commands sent by the docker CLI. For certain situations where it might be necessary to manually run docker, use the following.
Running the docker daemon:

sudo docker -d &

docker CLI Usage:

sudo docker [option] [command] [arguments]

Note: docker needs sudo privileges in order to work.

Commands List #

Here is a summary of currently available (version 0.7.1) docker commands:

attach #

Attach to a running container

build #

Build a container from a Dockerfile

commit #

Create a new image from a container’s changes

cp #

Copy files/folders from the containers filesystem to the host path

diff #

Inspect changes on a container’s filesystem

events #

Get real time events from the server

export #

Stream the contents of a container as a tar archive

history #

Show the history of an image

images #

List images

import #

Create a new filesystem image from the contents of a tarball

info #

Display system-wide information

insert #

Insert a file in an image

inspect #

Return low-level information on a container

kill #

Kill a running container

load #

Load an image from a tar archive

login #

Register or Login to the docker registry server

logs #

Fetch the logs of a container

port #

Lookup the public-facing port which is NAT-ed to PRIVATE_PORT

ps #

List containers

pull #

Pull an image or a repository from the docker registry server

push #

Push an image or a repository to the docker registry server

restart #

Restart a running container

rm #

Remove one or more containers

rmi #

Remove one or more images

run #

Run a command in a new container

save #

Save an image to a tar archive

search #

Search for an image in the docker index

start #

Start a stopped container

stop #

Stop a running container

tag #

Tag an image into a repository

top #

Lookup the running processes of a container

version #

Show the docker version information

Getting Started with Creating Memcached Images #

Building on our knowledge gained from the previous articles in the docker series, let’s dive straight into building a Dockerfile to have docker automatically build Memcached installed images (which will be used to run sandboxed Memcached instances).

Quick Recap: What Are Dockerfiles? #

Dockerfiles are scripts containing commands declared successively which are to be executed, in the order given, by docker to automatically create a new docker image. They help greatly with deployments.
These files always begin with the definition of a base image by using the FROM command. From there on, the build process starts and each following action taken forms the final with commits (saving the image state) on the host.
Usage:

# Build an image using the Dockerfile at current location
# Tag the final image with [name] (e.g. *nginx*)
# Example: sudo docker build -t [name] .
sudo docker build -t memcached_img . 

Note: To learn more about Dockerfiles, check out our article: Docker Explained: Using Dockerfiles to Automate Building of Images.

Dockerfile Commands Overview #

Add #

Copy a file from the host into the container

CMD #

Set default commands to be executed, or passed to the ENTRYPOINT

ENTRYPOINT #

Set the default entrypoint application inside the container

ENV #

Set environment variable (e.g. “key = value”)

EXPOSE #

Expose a port to outside

FROM #

Set the base image to use

MAINTAINER #

Set the author / owner data of the Dockerfile

RUN #

Run a command and commit the ending result (container) image

USER #

Set the user to run the containers from the image

VOLUME #

Mount a directory from the host to the container

WORKDIR #

Set the directory for the directives of CMD to be executed

Creating a Dockerfile #

Since Dockerfiles constitute of plain-text documents, creating one translates to launching your favourite text editor and writing the commands you want docker to execute in order to build an image. After you start working on the file, continue with adding all the content below (one after the other) before saving the final result.
Note: You can find what the final Dockerfile will look like at the end of this section.
Let’s create an empty Dockerfile using nano text editor:

nano Dockerfile

We need to have all instructions (commands) and directives listed successively. However, everything starts with building on a base image (set with the FROM command).
Let’s define the purpose of our Dockerfile and declare the base image to use:

############################################################
# Dockerfile to run Memcached Containers
# Based on Ubuntu Image
############################################################

# Set the base image to use to Ubuntu
FROM ubuntu

# Set the file maintainer (your name - the file's author)
MAINTAINER Maintaner Name

After this initial block of commands and declarations, we can begin with listing the instructions for Memcached installation.

# Update the default application repository sources list
RUN apt-get update

# Install Memcached
RUN apt-get install -y memcached

Set the default port to be exposed to outside the container:

# Port to expose (default: 11211)
EXPOSE 11211

Set the default execution command and entrpoint (i.e. Memcached daemon):

# Default Memcached run command arguments
CMD ["-u", "root", "-m", "128"]

# Set the user to run Memcached daemon
USER daemon

# Set the entrypoint to memcached binary
ENTRYPOINT memcached

Final Dockerfile #

############################################################
# Dockerfile to run Memcached Containers
# Based on Ubuntu Image
############################################################

# Set the base image to use to Ubuntu
FROM ubuntu

# Set the file maintainer (your name - the file's author)
MAINTAINER Maintaner Name
  
# Update the default application repository sources list
RUN apt-get update

# Install Memcached
RUN apt-get install -y memcached

# Port to expose (default: 11211)
EXPOSE 11211

# Default Memcached run command arguments
CMD ["-m", "128"]

# Set the user to run Memcached daemon
USER daemon

# Set the entrypoint to memcached binary
ENTRYPOINT memcached

After having everything written inside the Dockerfile, save it and exit by pressing CTRL+X followed by Y.
Using this Dockerfile, we are ready to get started with dockerised Memcached containers!

Creating the Docker Image for Memcached Containers #

We can now create our first Memcached image by following the usage instructions explained in the Dockerfile Basics section.
Run the following command to create an image, tagged as “memcached_img”:

sudo docker build -t memcached_img .

Note: Do not forget the trailing . for docker to find the Dockerfile.

Running dockerised Memcached Containers #

It is very simple to create any number of perfectly isolated and self-contained memcached instances – now – thanks to the image we have obtained in the previous section. All we have to do is to create a new container with docker run.

Creating a Memcached Installed Container #

To create a new container, use the following command, modifying it to suit your requirements following this example:

# Example: sudo docker run -name [container name] -p [port to access:port exposed] -i -t [memcached image name]
sudo docker run -name memcached_ins -d -p 45001:11211 memcached_img

Now we will have a docker container named “memcached_ins”, accessible from port 45001, run using our image tagged “memcached_img”, which we built previously.

Limiting the Memory for a Memcached Container #

In order to limit the amount of memory a docker container process can use, simply set the -m [memory amount] flag with the limit.
To run a container with memory limited to 256 MBs:

# Example: sudo docker run -name [name] -m [Memory (int)][memory unit (b, k, m or g)] -d (to run not to attach) -p (to set access and expose ports) [image ID]
sudo docker run -name memcached_ins -m 256m -d -p 45001:11211 memcached_img

To confirm the memory limit, you can inspect the container:

# Example: docker inspect [container ID] | grep Memory
sudo docker inspect memcached_ins | grep Memory

Note: The command above will grab the memory related information from the inspection output. To see all the relevant information regarding your container, opt for sudo docker inspect [container ID].

Testing the Memcached Container #

There are various ways to try your newly created Memcached running container(s). We will use a simple Python CLI application for this. However, you can just get to production with your application using caching add-ons, frameworks, or libraries.
Make sure that your host has the necessary libraries for Python / Memcached:

sudo apt-get update && sudo apt-get -y upgrade 
sudo apt-get install -y python-pip
pip install python-memcached

Let’s create a simple Python script called “mc.py” using nano:

nano cache.py

Copy-and-paste the below (self-explanatory) content inside:

# Import python-memcache and sys for arguments
import memcache
import sys

# Set address to access the Memcached instance
addr="localhost"

# Get number of arguments
# Expected format: python cache.py [memcached port] [key] [value]
len_argv = len(sys.argv)

# At least the port number and a key must be supplied
if len_argv < 3:
    sys.exit("Not enough arguments.")

# Port is supplied and a key is supplied - let's connect!
port  = sys.argv[1]
cache = memcache.Client(["{0}:{1}".format(addr, port)])

# Get the key
key   = str(sys.argv[2])

# If a value is also supplied, set the key-value pair
if len_argv == 4:
    
    value = str(sys.argv[3])    
    cache.set(key, value)

    print "Value for {0} set!".format(key)

# If a value is not supplied, return the value for the key
else:

    value = cache.get(key)

    print "Value for {0} is {1}.".format(key, value) 

Press CTRL+X and approve with Y to save and close.
Testing a docker memcached instance using the script above from your host:

# Example: python cache.py [port] [key] [value]
python cache.py 45001 my_test_key test_value

# Return: Value for my_test_key set

# See if the key is set:
python cache.py 45001 my_test_key

# Return: Value for my_test_key is test_value.

For the full set of instructions to install and use docker, check out the docker documentation at docker.io.