This question illustrates the theoretical differences between docker run and docker service.
What I don't understand is when would one need to use the exact same container replicated multiple times (as per the Docker documentation example)?
There, they run the same web app replicated 5 times.
Is deployment on Kubernetes (for example) a potential use case, where the developer does not want to centralize the app on one host, in order to make it more resilient, hence why 5 replicas are created?
To understand, can someone please please with an example use case, where the docker service is useful?
swarm is an orchestrator just like kubernetes. docker service deploys services to swarm just as you deploy your services to kubernetes using kubectl.
swarm is essentially built-in primitive orchestrator. One possible case for replicas is running a proxy that directs requests to proper containers. You could expose multiple machines and have one take place of another in case another fails. Or any other high availability case you could think of.
Your question could be rephrased as "What's the difference between running a single container and running containers in a cluster?", which would be another question altogether, but that rephrasing might help illustrate what docker service does.
If you want to scale your application, you can run multiple instances of it (horizontal scaling) or you beef up the machine(s) that it runs on (vertical scaling). For the first, you would have to put a load balancer in front of your application so that the traffic is evenly distributed between the different instances. The idea is that those instances run on different hosts, so if one goes down, your application is still up. Some controlling instance (a Kubernetes service, for example) will notice that one of your instances has gone south and won't direct any more traffic to it. Nowadays, with all the cloud stuff going on, this is typically the way to go.
You don't need Kubernetes for such a setup, but you're right, this would be a typical use case for it. At least if you run your application in a Docker container.
Once use case is running on Docker swarm which consists of n number of nodes in your swarm cluster. You can run replicas of your application on the swarm cluster with a load balancer/reverse proxy to load balance your setup. If any one of the nodes goes down the application can still run.
But the exact use case for running multiple instances is scalabilty. Suppose you know that one instance of your app can serve 10000 users (Assume Bank authentication) at a time.
If you want your application to serve 50K users just run 5 replicas(using docker service create) .
I am familiarizing with the architecture and practices to package, build and deploy software or unless, small pieces of software.
If suddenly I am mixing concepts with specific tools (sometimes is unavoidable), let me know if I am wrong, please.
On the road, I have been reading and learning about the images and containers terms and their respective relationships in order to start to build the workflow software systems of a better possible way.
And I have a question about the services orchestration in the context of the docker :
The containers are lightweight and portable encapsulations of an environment in which we have all the binary and dependencies we need to run our application. OK
I can set up communication between containers using container links --link flag.
I can replace the use of container links, with docker-compose in order to automate my services workflow and running multi-containers using .yaml file configurations.
And I am reading about of the Container orchestration term, which defines the relationship between containers when we have distinct "software pieces" separate from each other, and how these containers interact as a system.
Well, I suppose that I've read good the documentation :P
My question is:
A docker level, are container links and docker-compose a way of container orchestration?
Or with docker, if I want to do container orchestration ... should I use docker-swarm?
You should forget you ever read about container links. They've been obsolete in pure Docker for years. They're also not especially relevant to the orchestration question.
Docker Compose is a simplistic orchestration tool, but I would in fact class it as an orchestration tool. It can start up multiple containers together; of the stack it can restart individual containers if their configurations change. It is fairly oriented towards Docker's native capabilities.
Docker Swarm is mostly just a way to connect multiple physical hosts together in a way that docker commands can target them as a connected cluster. I probably wouldn't call that capability on its own "orchestration", but it does have some amount of "scheduling" or "placement" ability (Swarm, not you, decides which containers run on which hosts).
Of the other things I might call "orchestration" tools, I'd probably divide them into two camps:
General-purpose system automation tools that happen to have some Docker capabilities. You can use both Ansible and Salt Stack to start Docker containers, for instance, but you can also use these tools for a great many other things. They have the ability to say "run container A on system X and container B on system Y", but if you need inter-host communication or other niceties then you need to set them up as well (probably using the same tool).
Purpose-built Docker automation tools like Docker Compose, Kubernetes, and Nomad. These tend to have a more complete story around how you'd build up a complete stack with a bunch of containers, service replication, rolling updates, and service discovery, but you mostly can't use them to manage tasks that aren't already in Docker.
Some other functions you might consider:
Orchestration: How can you start multiple connected containers all together?
Networking: How can one container communicate with another, within the cluster? How do outside callers connect to the system?
Scheduling: Which containers run on which system in a multi-host setup?
Service discovery: When one container wants to call another, how does it know who to call?
Management plane: As an operator, how do you do things like change the number of replicas of some specific service, or cause an update to a newer image for a service?
Being a novice to docker/elastic search worlds, I am trying to build a deployment model of using elastic search via containers in one of my project.
I have few application servers, each of which have some logs. I would like to have all these logs at one place. Below is what I have in my mind.
All application servers install filebeat to push data to a Logstash server (in a docker image). This LogStash server forward these logs to elasticsearch docker image that also have kibana.
Does this make sense? Is it OK to have logstash in one image and ElasticSearch/Kibana on a different one? Are there any pros/cons of this approach? What could be alternative approaches to architect this?
The policy of Docker is that 1 container does 1 thing and 1 thing good. So I would go for a docker image for ElasticSearch, 1 for Kibana and one for LogStash. Add them together with docker compose.
https://docs.docker.com/v17.09/engine/userguide/eng-image/dockerfile_best-practices/#use-multi-stage-builds
Each container should have only one concern
Decoupling applications into multiple containers makes it much easier to scale horizontally and reuse containers. For instance, a web application stack might consist of three separate containers, each with its own unique image, to manage the web application, database, and an in-memory cache in a decoupled manner.
You may have heard that there should be “one process per container”. While this mantra has good intentions, it is not necessarily true that there should be only one operating system process per container. In addition to the fact that containers can now be spawned with an init process, some programs might spawn additional processes of their own accord. For instance, Celery can spawn multiple worker processes, or Apache might create a process per request. While “one process per container” is frequently a good rule of thumb, it is not a hard and fast rule. Use your best judgment to keep containers as clean and modular as possible.
If containers depend on each other, you can use Docker container networks to ensure that these containers can communicate.
I watched this YouTube video on Docker and at 22:00 the speaker (a Docker product manager) says:
"You're probably thinking 'Docker does not support multi-tenancy'...and you are right!"
But never is any explanation of why actually given. So I'm wondering: what did he mean by that? Why Docker doesn't support multi-tenancy?! If you Google "Docker multi-tenancy" you surprisingly get nothing!
One of the key features most assume with a multi-tenancy tool is isolation between each of the tenants. They should not be able to see or administer each others containers and/or data.
The docker-ce engine is a sysadmin level tool out of the box. Anyone that can start containers with arbitrary options has root access on the host. There are 3rd party tools like twistlock that connect with an authz plugin interface, but they only provide coarse access controls, each person is either allowed or disallowed from an entire class of activities, like starting containers, or viewing logs. Giving users access to either the TLS port or docker socket results in the users being lumped into a single category, there's no concept of groups or namespaces for the users connecting to a docker engine.
For multi-tenancy, docker would need to add a way to define users, and place them in a namespace that is only allowed to act on specific containers and volumes, and restrict options that allow breaking out of the container like changing capabilities or mounting arbitrary filesystems from the host. Docker's enterprise offering, UCP, does begin to add these features by using labels on objects, but I haven't had the time to evaluate whether this would provide a full multi-tenancy solution.
Tough question that others might know how to answer better than me. But here it goes.
Let's take this definition of multi tenancy (source):
Multi-tenancy is an architecture in which a single instance of a software application serves multiple customers.
It's really hard to place Docker in this definition. It can be argued that it's both the instance and the application. And that's where the confusion comes from.
Let's break Docker up into three different parts: the daemon, the container and the application.
The daemon is installed on a host and runs Docker containers. The daemon does actually support multi tenancy, as it can be used my many users on the same system, each of which has their own configuration in ~/.docker.
Docker containers run a single process, which we'll refer to as the application.
The application can be anything. For this example, let's assume the Docker container runs a web application like a forum or something. The forum allows users to sign in and post under their name. It's a single instance that serves multiple customers. Thus it supports multi tenancy.
What we skipped over is the container and the question whether or not it supports multi tenancy. And this is where I think the answer to your question lies.
It is important to remember that Docker containers are not virtual machines. When using docker run [IMAGE], you are creating a new container instance. These instances are ephemeral and immutable. They run a single process, and exit as soon as the process exists. But they are not designed to have multiple users connect to them and run commands simultaneously. This is what multi tenancy would be. Instead, Docker containers are just isolated execution environments for processes.
Conceptually, echo Hello and docker run echo Hello are the same thing in this example. They both execute a command in a new execution environment (process vs. container), neither of which supports multi tenancy.
I hope this answers is readable and answers your question. Let me know if there is any part that I should clarify.
I've heard about Docker some days ago and wanted to go across.
But in fact, I don't know what is the purpose of this "container"?
What is a container?
Can it replace a virtual machine dedicated to development?
What is the purpose, in simple words, of using Docker in companies? The main advantage?
VM: Using virtual machine (VM) software, for example, Ubuntu can be installed inside a Windows. And they would both run at the same time. It is like building a PC, with its core components like CPU, RAM, Disks, Network Cards etc, within an operating system and assemble them to work as if it was a real PC. This way, the virtual PC becomes a "guest" inside an actual PC which with its operating system, which is called a host.
Container: It's same as above but instead of using an entire operating system, it cut down the "unnecessary" components of the virtual OS to create a minimal version of it. This lead to the creation of LXC (Linux Containers). It therefore should be faster and more efficient than VMs.
Docker: A docker container, unlike a virtual machine and container, does not require or include a separate operating system. Instead, it relies on the Linux kernel's functionality and uses resource isolation.
Purpose of Docker: Its primary focus is to automate the deployment of applications inside software containers and the automation of operating system level virtualization on Linux. It's more lightweight than standard Containers and boots up in seconds.
(Notice that there's no Guest OS required in case of Docker)
[ Note, this answer focuses on Linux containers and may not fully apply to other operating systems. ]
What is a container ?
It's an App: A container is a way to run applications that are isolated from each other. Rather than virtualizing the hardware to run multiple operating systems, containers rely on virtualizing the operating system to run multiple applications. This means you can run more containers on the same hardware than VMs because you only have one copy of the OS running, and you do not need to preallocate the memory and CPU cores for each instance of your app. Just like any other app, when a container needs the CPU or Memory, it allocates them, and then frees them up when done, allowing other apps to use those same limited resources later.
They leverage kernel namespaces: Each container by default will receive an environment where the following are namespaced:
Mount: filesystems, / in the container will be different from / on the host.
PID: process id's, pid 1 in the container is your launched application, this pid will be different when viewed from the host.
Network: containers run with their own loopback interface (127.0.0.1) and a private IP by default. Docker uses technologies like Linux bridge networks to connect multiple containers together in their own private lan.
IPC: interprocess communication
UTS: this includes the hostname
User: you can optionally shift all the user id's to be offset from that of the host
Each of these namespaces also prevent a container from seeing things like the filesystem or processes on the host, or in other containers, unless you explicitly remove that isolation.
And other linux security tools: Containers also utilize other security features like SELinux, AppArmor, Capabilities, and Seccomp to limit users inside the container, including the root user, from being able to escape the container or negatively impact the host.
Package your apps with their dependencies for portability: Packaging an application into a container involves assembling not only the application itself, but all dependencies needed to run that application, into a portable image. This image is the base filesystem used to create a container. Because we are only isolating the application, this filesystem does not include the kernel and other OS utilities needed to virtualize an entire operating system. Therefore, an image for a container should be significantly smaller than an image for an equivalent virtual machine, making it faster to deploy to nodes across the network. As a result, containers have become a popular option for deploying applications into the cloud and remote data centers.
Can it replace a virtual machine dedicated to development ?
It depends: If your development environment is running Linux, and you either do not need access to hardware devices, or it is acceptable to have direct access to the physical hardware, then you'll find a migration to a Linux container fairly straight forward. The ideal target for a docker container are applications like web based API's (e.g. a REST app), which you access via the network.
What is the purpose, in simple words, of using Docker in companies ? The main advantage ?
Dev or Ops: Docker is typically brought into an environment in one of two paths. Developers looking for a way to more rapidly develop and locally test their application, and operations looking to run more workload on less hardware than would be possible with virtual machines.
Or Devops: One of the ideal targets is to leverage Docker immediately from the CI/CD deployment tool, compiling the application and immediately building an image that is deployed to development, CI, prod, etc. Containers often reduce the time to move the application from the code check-in until it's available for testing, making developers more efficient. And when designed properly, the same image that was tested and approved by the developers and CI tools can be deployed in production. Since that image includes all the application dependencies, the risk of something breaking in production that worked in development are significantly reduced.
Scalability: One last key benefit of containers that I'll mention is that they are designed for horizontal scalability in mind. When you have stateless apps under heavy load, containers are much easier and faster to scale out due to their smaller image size and reduced overhead. For this reason you see containers being used by many of the larger web based companies, like Google and Netflix.
Same questions were hitting my head some days ago and what i found after getting into it, let's understand in very simple words.
Why one would think about docker and containers when everything seems fine with current process of application architecture and development !!
Let's take an example that we are developing an application using nodeJs , MongoDB, Redis, RabbitMQ etc services [you can think of any other services].
Now we face these following things as problems in application development and shipping process if we forget about existence of docker or other alternatives of containerizing applications.
Compatibility of services(nodeJs, mongoDB, Redis, RabbitMQ etc.) with OS(even after finding compatible versions with OS, if something unexpected happens related to versions then we need to relook the compatibility again and fix that).
If two system components requires a library/dependency with different versions in application in OS(That need a relook every time in case of an unexpected behaviour of application due to library and dependency version issue).
Most importantly , If new person joins the team, we find it very difficult to setup the new environment, person has to follow large set of instructions and run hundreds of commands to finally setup the environment And it takes time and effort.
People have to make sure that they are using right version of OS and check compatibilities of services with OS.And each developer has to follow this each time while setting up.
We also have different environment like dev, test and production.If One developer is comfortable using one OS and other is comfortable with other OS And in this case, we can't guarantee that our application will behave in same way in these two different situations.
All of these make our life difficult in process of developing , testing and shipping the applications.
So we need something which handles compatibility issue and allows us to make changes and modifications in any system component without affecting other components.
Now we think about docker because it's purpose is to
containerise the applications and automate the deployment of applications and ship them very easily.
How docker solves above issues-
We can run each service component(nodeJs, MongoDB, Redis, RabbitMQ) in different containers with its own dependencies and libraries in the same OS but with different environments.
We have to just run docker configuration once then all our team developers can get started with simple docker run command, we have saved lot of time and efforts here:).
So containers are isolated environments with all dependencies and
libraries bundled together with their own process and networking
interfaces and mounts.
All containers use the same OS resources
therefore they take less time to boot up and utilise the CPU
efficiently with less hardware costs.
I hope this would be helpful.
Why use docker:
Docker makes it really easy to install and running software without worrying about setup or dependencies. Docker is really made it easy and really straight forward for you to install and run software on any given computer not just your computer but on web servers as well or any cloud based computing platform. For example when I went to install redis in my computer by using bellow command
wget http://download.redis.io/redis-stable.tar.gz
I got error,
Now I could definitely go and troubleshoot this install that program and then try installing redis again, and I kind of get into endless cycle of trying to do all bellow troubleshooting as you I am installing and running software.
Now let me show you how easy it is to run read as if you are making use of Docker instead. just run the command docker run -it redis, this command will install docker without any error.
What docker is:
To understand what is docker you have to know about docker Ecosystem.
Docker client, server, Machine, Images, Hub, Composes are all projects tools pieces of software that come together to form a platform where ecosystem around creating and running something called containers, now if you run the command docker run redis something called docker CLI reached out to something called the Docker Hub and it downloaded a single file called an image.
An image is a single file containing all the dependencies and all the configuration required to run a very specific program, for example redis this which is what the image that you just downloaded was supposed to run.
This is a single file that gets stored on your hard drive and at some point time you can use this image to create something called a container.
A container is an instance of an image and you can kind of think it as being like a running program with it's own isolated set of hardware resources so it kind of has its own little set or its own little space of memory has its own little space of networking technology and its own little space of hard drive space as well.
Now lets examine when you give bellow command:
sudo docker run hello-world
Above command will starts up the docker client or docker CLI, Docker CLI is in charge of taking commands from you kind of doing a little bit of processing on them and then communicating the commands over to something called the docker server, and docker server is in charge of the heavy lifting when we ran the command Docker run hello-world,
That meant that we wanted to start up a new container using the image with the name of hello world, the hello world image has a tiny tittle program inside of it whose sole purpose or sole job is to print out the message that you see in the terminal.
Now when we ran that command and it was issued over to the docker server a series of actions very quickly occurred in background. The Docker server saw that we were trying to start up a new container using an image called hello world.
The first thing that the docker server did was check to see if it already had a local copy like a copy on your personal machine of the hello world image or that hello world file.So the docker server looked into something called the image cache.
Now because you and I just installed Docker on our personal computers that image cache is currently empty, We have no images that have already been downloaded before.
So because the image cache was empty the docker server decided to reach out to a free service called Docker hub. The Docker Hub is a repository of free public images that you can freely download and run on your personal computer. So Docker server reached out to Docker Hub and and downloaded the hello world file and stored it on your computer in the image-cache, where it can now be re-run at some point the future very quickly without having to re-downloading it from the docker hub.
After that the docker server will use it to create an instance of a container, and we know that a container is an instance of an image, its sole purpose is to run one very specific program. So the docker server then essentially took that image file from image cache and loaded it up into memory to created a container out of it and then ran a single program inside of it. And that single programs purpose was to print out the message that you see.
What a container is:
A container is a process or a set of processes that have a grouping of resource specifically assigned to it, in the bellow is a diagram that anytime that we think about a container we've got some running process that sends a system call to a kernel, the kernel is going to look at that incoming system call and direct it to a very specific portion of the hard drive, the RAM, CPU or what ever else it might need and a portion of each of these resources is made available to that singular process.
Let me try to provide as simple answers as possible:
But in fact, I don't know what is the purpose of this "container"?
What is a container?
Simply put: a package containing software. More specifically, an application and all its dependencies bundled together. A regular, non-dockerised application environment is hooked directly to the OS, whereas a Docker container is an OS abstraction layer.
And a container differs from an image in that a container is a runtime instance of an image - similar to how objects are runtime instances of classes in case you're familiar with OOP.
Can it replace a virtual machine dedicated to development?
Both VMs and Docker containers are virtualisation techniques, in that they provide abstraction on top of system infrastructure.
A VM runs a full “guest” operating system with virtual access to host resources through a hypervisor. This means that the VM often provides the environment with more resources than it actually needs In general, VMs provide an environment with more resources than most applications need. Therefore, containers are a lighter-weight technique. The two solve different problems.
What is the purpose, in simple words, of using Docker in companies?
The main advantage?
Containerisation goes hand-in-hand with microservices. The smaller services that make up the larger application are often tested and run in Docker containers. This makes continuous testing easier.
Also, because Docker containers are read-only they enforce a key DevOps principle: production services should remain unaltered
Some general benefits of using them:
Great isolation of services
Great manageability as containers contain everything the app needs
Encapsulation of implementation technology (in the containers)
Efficient resource utilisation (due to light-weight os virtualisation) in comparison to VMs
Fast deployment
If you don't have any prior experience with Docker this answer will cover the basics needed as a developer.
Docker has become a standard tool for DevOps as it is an effective application to improve operational efficiencies. When you look at why Docker was created and why it is very popular, it is mostly for its ability to reduce the amount of time it takes to set up the environments where applications run and are developed.
Just look at how long it takes to set up an environment where you have React as the frontend, a node and express API for backend, which also needs Mongo. And that's just to start. Then when your team grows and you have multiple developers working on the same front and backend and therefore they need to set up the same resources in their local environment for testing purposes, how can you guarantee every developer will run the same environment resources, let alone the same versions? All of these scenarios play well into Docker's strengths where it's value comes from setting containers with specific settings, environments and even versions of resources. Simply type a few commands to have Docker set up, install, and run your resources automatically.
Let's briefly go over the main components. A container is basically where your application or specific resource is located. For example, you could have the Mongo database in one container, then the frontend React application, and finally your node express server in the third container.
Then you have an image, which is from what the container is built. The images contains all the information that a container needs to build a container exactly the same way across any systems. It's like a recipe.
Then you have volumes, which holds the data of your containers. So if your applications are on containers, which are static and unchanging, the data that change is on the volumes.
And finally, the pieces that allow all these items to speak is networking. Yes, that sounds simple, but understand that each container in Docker have no idea of the existence of each container. They're fully isolated. So unless we set up networking in Docker, they won't have any idea how to connect to one and another.
There are really good answers above which I found really helpful.
Below I had drafted a simpler answer:
Reasons to dockerize my web application?
a. One OS for multiple applications ( Resources are shared )
b. Resource manangement ( CPU / RAM) is efficient.
c. Serverless Implementation made easier -Yes, AWS ECS with Fargate, But serverless can be achieved with Lamdba
d. Infra As Code - Agree, but IaC can be achieved via Terraforms
e. "It works in my machine" Issue
Still, below questions are open when choosing dockerization
A simple spring boot application
a. Jar file with size ~50MB
b. creates a Docker Image ~500MB
c. Cant I simply choose a small ec2 instance for my microservices.
Financial Benefits (reducing the individual instance cost) ?
a. No need to pay for individual OS subscription
b. Is there any monetary benefit like the below implementation?
c. let say select t3.2xlarge ( 8 core / 32 GB) and start 4-5 docker images ?