The Docker way...

What is Docker Container?

Docker is an open-source engine that automates the deployment of any application as a lightweight, portable, self-sufficient container that will run virtually anywhere.

Docker containers can encapsulate any payload, and will run consistently on and between virtually any server. The same container that a developer buildsand tests on a laptop will run at scale, in production*, on VMs, bare-metal servers, OpenStack clusters, public instances, or combinations of the above.

[Source: As in package description, Vendor: Fedora Project]

Difference between Docker and VM [Pic source, Internet ]

Let's try out few basic stuff using Docker (Tested on a CentOS-6x based VM);

1. Install epel repo to get Docker-io

[root@docker-vm ~]# yum install epel-release -y

2. Install docker-io package and start the service;

[root@docker-vm ~]# yum install docker-io -y

[root@docker-vm ~]# service docker start
Starting cgconfig service:           [  OK  ]
Starting docker:                        [  OK  ]

3. List if we have any Docker image avaialble locally;
[root@docker-vm ~]# docker images
REPOSITORY          TAG                 IMAGE ID            CREATED             VIRTUAL SIZE

4. Since images are not available locally, I would pull a CentOS-6.9 image and try out few things;

Pulling a specific Centos docker image version;

[root@docker-vm ~]# docker pull centos:6.9
6.9: Pulling from centos

055b9989266a: Pull complete
f1070d829305: Pull complete
e071bce628ba: Pull complete
Digest: sha256:e7bdc458659b6e644ae85694f2baaf3727c06ad82186fca80f4e3a8e88907cc3
Status: Downloaded newer image for centos:6.9

5. List images once more;

[root@docker-vm ~]# docker images
REPOSITORY          TAG                 IMAGE ID            CREATED             VIRTUAL SIZE8.
centos              6.9                 e071bce628ba        5 weeks ago         194.7 MB

6. Now run our first container [docker run -it stands for docker run --interactive --tty, do a man docker-run for options] ;

[root@docker-vm ~]# docker run -it e071bce628ba /bin/bash
[root@bd93eb2b1ccf /]#

["e071bce628ba" is the IMAGE ID]

7. Now, within our container, let's install few packages [eg. iproute, httpd, nmap etc.];

[root@bd93eb2b1ccf /]# yum install iproute nmap httpd -y

8. Start HTTPD ;

[root@bd93eb2b1ccf /]# service httpd start

Starting httpd: httpd: Could not reliably determine the server's fully qualified domain name, using 172.17.0.1 for ServerName                                                [  OK  ]

9. Open a new terminal on the Host system and check what containers are running;

[centos@docker-vm ~]$ sudo docker ps -a

CONTAINER ID        IMAGE       COMMAND             CREATED             STATUS              PORTS               NAMES

bd93eb2b1ccf        e071bce628ba        "/bin/bash"         17 minutes ago      Up 17 minutes                           prickly_tesla

So, we have one running container from previous steps;

10. Detach and attach from/to an Docker container;

A. Detach from a container;
Press Ctrl-P + Ctrl-Q to detach from the container;

[root@docker-vm ~]# docker ps
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES
bd93eb2b1ccf        e071bce628ba        "/bin/bash"         24 minutes ago      Up 24 minutes                           prickly_tesla

B. Once detached, you can again connect to the running container. You need to provide the "CONTAINER ID" to attach to the reqd container;

[root@docker-vm ~]# docker attach bd93eb2b1ccf

[root@bd93eb2b1ccf /]#


11. Now, we will save our custom image with a different name [I named it as "centos6.9-httpd-bijit"]  with all our custom packages (remember we installed iproute, httpd, nmap etc)

Detach yourself [ctrl-p + ctrl-q] from the running container and save the container as new 
Docker image;

[root@docker-vm ~]# docker ps -a
CONTAINER ID        IMAGE          COMMAND         CREATED             STATUS              PORTS               NAMES
bd93eb2b1ccf        e071bce628ba        "/bin/bash"         36 minutes ago      Up 36 minutes prickly_tesla

[root@docker-vm ~]# docker commit bd93eb2b1ccf centos6.9-httpd-bijit

=================================
[root@docker-vm ~]# docker commit bd93eb2b1ccf centos6.9-httpd-bijit
dc988142a75d3f7d2308543223e987be89e31da7d6f579109f0f2bb4ee0c192a

[root@docker-vm ~]# docker images
REPOSITORY              TAG                 IMAGE ID            CREATED             VIRTUAL SIZE
centos6.9-httpd-bijit   latest              dc988142a75d        48 seconds ago      320.7 MB
centos                  6.9                 e071bce628ba        5 weeks ago         194.7 MB
=================================

12. Run a container using the newly built image;

[root@docker-vm ~]# docker run -it centos6.9-httpd-bijit /bin/bash
[root@29c8a4bd34d1 /]# service httpd start

So, we have successfully ran a container using our custom Docker image. 

13. Let's stop our container from previous step (ctrl+d from within container would do the job, [ docker stop also does that)

14. Now, we will Publish the container port (80 in our case) and map that to the Host OS, also provide a custom name for the container that we are going to start;

[root@docker-vm ~]# docker run -it -p 172.16.20.4:80:80 --name tux-container centos6.9-httpd-bijit /bin/bash
[root@a3923d777eaf /]#

[root@a3923d777eaf /]# service httpd start
Starting httpd: httpd: Could not reliably determine the server's fully qualified domain name, using 172.17.0.4 for ServerName


15. Now, try accessing Apache page using the Host IP address http://172.16.20.4/

[Remember our container IP is 172.17.0.4 while Host IP is 172.16.20.4 We are running HTTPD within the container and NOT on the Host systems]

============================================

List of basic commands;

============================================

Pushing an image to Docker Hub

A. Detach from an running Docker container and note down the container
id;

# docker ps
CONTAINER ID  IMAGE         COMMAND       CREATED       STATUS              PORTS               NAMES

3a8a3576695d  e071bce628ba  "/bin/bash"  11 minutes ago  Up 11 minutes                           tender_goldstine

B. Commit the changes using the following syntax;
docker commit /[:] to commit changes.

eg. in our case;

# docker commit 3a8a3576695d bijit/tuxb:centos6.9-httpd-bijit

4ff826f5c36ec9b6c452443ea1d759675456f53470fc30ea201b2be363e97627


C.    Now push this repository to the registry designated by its name or tag;

Syntax:  docker push /:

eg.  
====================
# docker push bijit/tuxb:centos6.9-httpd-bijit
The push refers to a repository [bijit/tuxb] (len: 1)
4ff826f5c36e: Image push failed

Please login prior to push:
Username: bijit
Password:
Email: xxxxxx@gmail.com
WARNING: login credentials saved in /root/.docker/config.json
Login Succeeded
The push refers to a repository [bijit/tuxb] (len: 1)
4ff826f5c36e: Image already exists
e071bce628ba: Image successfully pushed
f1070d829305: Image already exists
055b9989266a: Image successfully pushed

Digest: sha256:cce65ad4fc75b136f84b567ae3f281de2723115e13d21efa63660b36fbe53e42

====================

How to Pull the private Docker image?

1. Login to Docker Hub from command line.

docker login --username=yourhubusername --email=youremail@company.com

====================
[root@ins-2 ~]# docker login --username=bijit --email=xxxxx@gmail.com
Password:
WARNING: login credentials saved in /root/.docker/config.json
Login Succeeded
====================

2. Pull the image which was pushed to that private repository.

#docker pull bijit/tuxb:centos6.9-httpd-bijit

====================
[root@ins-2 ~]# docker pull bijit/tuxb:centos6.9-httpd-bijit
centos6.9-httpd-bijit: Pulling from bijit/tuxb
055b9989266a: Pull complete
f1070d829305: Pull complete
e071bce628ba: Pull complete
4ff826f5c36e: Already exists
Digest: sha256:cce65ad4fc75b136f84b567ae3f281de2723115e13d21efa63660b36fbe53e42
Status: Downloaded newer image for bijit/tuxb:centos6.9-httpd-bijit
====================

3. List the images available locally now;

# docker images
REPOSITORY          TAG                     IMAGE ID            CREATED             VIRTUAL SIZE
bijit/tuxb          centos6.9-httpd-bijit   4ff826f5c36e        10 hours ago        326.5 MB


4. Run a container using the image dowloaded from Private repository;

# docker run -it 4ff826f5c36e /bin/bash
[root@29c3296b8e48 /]#

["4ff826f5c36e" is the IMAGE ID ]



5. To stop the container either [ctrl+d] from within container or docker stop from Host

Replication Set in MongoDB – HowTo

Following write up describe how one can set up Replica Set in MongoDB (3x version). It is tested on CenOS-6x version. The same setup should work in any *nix based system. 

The IPs cited here are for example only.

A.  Participating Servers

1.   primary-ins   ========    172.16.20.3

2.   secondary-ins ========    172.16.20.4

3.   arbiter       ========     172.16.20.5

# cat /etc/centos-release

CentOS release 6.8 (Final)

    

     [On Openstack cloud platform]

B.   Host resolution (on all three servers)

     /etc/hosts

    

     172.16.20.3          primary-ins    

     172.16.20.4          secondary-ins  

     172.16.20.5          arbiter  

C.        Installed packages (On all three)

    

mongodb-org-shell-3.4.2-1.el6.x86_64

     mongodb-org-tools-3.4.2-1.el6.x86_64

     mongodb-org-server-3.4.2-1.el6.x86_64

     mongodb-org-mongos-3.4.2-1.el6.x86_64

     mongodb-org-3.4.2-1.el6.x86_64 

    

D.  Firewall considerations

* MongoDB listen on all interfaces instead of Loopback on all
  three hosts.

       [tcp 0  0 0.0.0.0:27017 0.0.0.0:*  LISTEN  22913/mongod]

    

     1. IPTABLES:

        Allowed entire subnet in Iptables INPUT chain for port 27017         (in all three hosts)

          

         # iptables -I INPUT -p tcp -s 172.16.20.0/24 --dport 27017 -j ACCEPT

         # service iptables save

         iptables: Saving firewall rules to /etc/sysconfig/iptables:[  OK  ]

         # service iptables restart

         iptables: Setting chains to policy ACCEPT: filter          [  OK  ]

         iptables: Flushing firewall rules:                         [  OK  ]

         iptables: Unloading modules:                               [  OK  ]

         iptables: Applying firewall rules:                         [  OK  ]

          

         # iptables -L | grep 27017

         ACCEPT     tcp  --  172.16.20.0/24   anywhere   tcp dpt:27017

              2.         SeLinux

          
           SeLinux changed from Enforcing to Permissive:

           # getenforce

            Enforcing

             

           #setenforce permissive

E.  Replication Setup (all three) /etc/mongod.conf

     replication:

       ## Replica Set name, consistent across all the systems

         replSetName: myRepSet

       ## Operation Log file size

         oplogSizeMB: 64

F.  Restart Mongo service (on all three, any order)

# service mongod restart 

G.  Initiate replica set

Connect to any Mongo Server and run rs.initiate() to initiate the new replica set (preferably connect to the one that would act as Primary):

    

# mongo

      

       > rs.initiate()

       {

"info2" : "no configuration specified. Using a default configuration for the set",

                     "me" : "primary-ins:27017",

                     "ok" : 1

       }

       myRepSet:OTHER>

    

    

     Note:

Once the initialization is done, the prompt would change to;

     myRepSet:PRIMARY>

    

H.  Add replica set members

Now, while on the Primary Server's prompt, add the other members (Secondary and the Arbiter) as follows;

     i.     Add seondary:

              myRepSet:PRIMARY> rs.add("secondary-ins:27017")

              { "ok" : 1 }

       ii.    Add arbiter

              myRepSet:PRIMARY> rs.addArb("arbiter:27017")

              { "ok" : 1 }

I.  Connect to other two Mongo servers and check the prompt(s):

     myRepSet:SECONDARY>

     myRepSet:ARBITER>

J.  Allow read from secondary members

Execute rs.slaveOk(). This is to allow you to read from secondary members (including the Arbiter);

    

myRepSet:SECONDARY> rs.slaveOk()

myRepSet:ARBITER> rs.slaveOk()

K.  Test

    Create a DB and insert a document to a collection on the primary

and check if it that has been replicated to the secondary;

    

     i.   Create DB and insert document on Primary Mongo server;

          

        myRepSet:PRIMARY> use test_replica

        switched to db test_replica

             

        myRepSet:PRIMARY> db.movie.insert({"name":"Jungle Book"})

        WriteResult({ "nInserted" : 1 })

     ii.  On the secondary Mongo server;

          

           myRepSet:SECONDARY> show dbs

           admin         0.000GB

           local         0.000GB

           test_replica  0.000GB

             

           myRepSet:SECONDARY> use test_replica

           switched to db test_replica

           myRepSet:SECONDARY> show collections

           movie

             

           myRepSet:SECONDARY> db.movie.find({})

           { "_id" : ObjectId("58963c37bede8f63243f5e52"), "name" : "Jungle Book" }

          

    

     iii. On Arbiter (Remember Arbiter does not take part in Data

replication, its only job is to elect member during a Failover)

          

myRepSet:ARBITER> show dbs

          admin  0.000GB

          local  0.000GB

               

L.   Check the configuration

myRepSet:PRIMARY> rs.conf()

{

        "_id" : "myRepSet",

        "version" : 3,

        "protocolVersion" : NumberLong(1),

        "members" : [

                {

                        "_id" : 0,

                        "host" : "primary-ins:27017",

                        "arbiterOnly" : false,

                        "buildIndexes" : true,

                        "hidden" : false,

                        "priority" : 1,

                        "tags" : {

                        },

                        "slaveDelay" : NumberLong(0),

                        "votes" : 1

                },

                {

                        "_id" : 1,

                        "host" : "secondary-ins:27017",

                        "arbiterOnly" : false,

                        "buildIndexes" : true,

                        "hidden" : false,

                        "priority" : 1,

                        "tags" : {

                        },

                        "slaveDelay" : NumberLong(0),

                        "votes" : 1

                },

                {

                        "_id" : 2,

                        "host" : "arbiter:27017",

                        "arbiterOnly" : true,

                        "buildIndexes" : true,

                        "hidden" : false,

                        "priority" : 1,

                        "tags" : {

                        },

                        "slaveDelay" : NumberLong(0),

                        "votes" : 1

                }

        ],

        "settings" : {

                "chainingAllowed" : true,

                "heartbeatIntervalMillis" : 2000,

                "heartbeatTimeoutSecs" : 10,

                "electionTimeoutMillis" : 10000,

                "catchUpTimeoutMillis" : 2000,

                "getLastErrorModes" : {

                },

                "getLastErrorDefaults" : {

                        "w" : 1,

                        "wtimeout" : 0

                },

                "replicaSetId" : ObjectId("589632c875b52533db9c7540")

        }

}