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Docker 
Architecture 
based 
on 
v 
1.3 
Compiled 
by 
Rajdeep 
Dua 
Twi?er 
: 
@rajdeepdua 
Oct 
2014 
Tuesday, November ...
Before 
we 
get 
started 
• What 
is 
a 
Container? 
– Group 
of 
processes 
contained 
in 
a 
Isolated 
Environment 
– Is...
CGroup 
• Limit, 
account, 
and 
isolate 
resource 
usage 
(CPU, 
memory, 
disk 
I/O, 
etc.) 
of 
process 
groups. 
• Reso...
Namespace 
• ParNNon 
essenNal 
kernel 
structures 
to 
create 
virtual 
environments 
• Different 
Namespaces 
– pid 
(pr...
Docker 
• Manages 
Images 
and 
Container 
runNmes 
• Supports 
mulNple 
file 
system 
back-­‐ends 
• MulNple 
Execdrivers...
Docker 
RunNme 
Components 
Tuesday, November 4, 14
Docker 
Engine 
• Core 
of 
Docker 
: 
Store 
for 
Containers 
• Manages 
containers 
using 
Jobs 
(similar 
to 
Unix 
job...
Docker 
IniNalizaNon 
1. Main 
funcNon 
of 
Docker 
: 
docker.main() 
2. Calls 
: 
mainDaemon() 
3. InstanNate 
Engine 
en...
Docker 
IniNalizaNon 
5. Run 
the 
Job 
job.run() 
6. Start 
AccepNng 
ConnecNons 
eng.Job(“AcceptConnections”).run() 
Tue...
Docker 
IniNalizaNon 
: 
4 
4. 
Register 
built-­‐ins 
Instantiate daemon(eng) 
//see later slides for details 
eng.Regist...
Daemon 
• Main 
Entry 
point 
for 
all 
the 
requests 
to 
manage 
containers 
• Data 
Structure 
which 
maintains 
follow...
Daemon 
-­‐ 
Graph 
• Graph 
is 
a 
(structure) 
store 
of 
versioned 
file 
system 
and 
rela@onship 
between 
images 
• ...
Concept 
of 
Images 
and 
Containers 
in 
Docker 
• Docker 
image 
is 
a 
Layer 
in 
the 
file 
System 
• Containers 
are ...
Container 
in 
Docker 
• DataStructure 
which 
resides 
in-­‐memory 
and 
is 
persisted 
in 
SQLite 
store 
• References 
...
Lifecycle 
of 
a 
Container 
15 
Tuesday, November 4, 14
Graph 
Driver 
• Referenced 
by 
the 
Daemon 
• Used 
to 
abstract 
mulNple 
storage 
backends 
• Loads 
one 
of 
the 
fol...
Container 
store 
• Persistent 
backend 
for 
Container 
data 
• Implemented 
using 
SQLite 
• Referenced 
from 
Daemon 
c...
Volume 
Graph 
• Simple 
vfs 
based 
Graph 
to 
keep 
track 
of 
container 
volumes 
• Volumes 
used 
volume 
driver 
in 
...
ExecDriver 
• AbstracNon 
for 
the 
underlying 
Linux 
Containment 
• Called 
from 
the 
daemon 
• Supports 
following 
im...
Driver 
Interfaces 
• Abstract 
Interface 
to 
interact 
with 
the 
underlying 
implementaNon. 
type driver Interface{ 
Ru...
Driver 
Interfaces 
-­‐ 
Networking 
• Abstract 
Interface 
to 
interact 
with 
the 
underlying 
implementaNon. 
type Netw...
libcontainer 
• Underlying 
naNve 
implementaNon 
of 
the 
Container 
• Used 
by 
the 
naNve 
driver 
• Container.config 
...
NaNve 
Driver 
ImplementaNon 
Tuesday, November 4, 14
Steps 
in 
Container 
CreaNon 
Container 
Create 
Tuesday, November 4, 14
Container 
Commit 
Tuesday, November 4, 14
Summary 
• Linux 
Containment 
Principles 
• Docker 
Architectural 
components 
• NaNve 
Driver 
ImplementaNon 
– 
libcont...
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Docker Architecture (v1.3)

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Docker Architecture (based on v 1.3)

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Docker Architecture (v1.3)

  1. 1. Docker Architecture based on v 1.3 Compiled by Rajdeep Dua Twi?er : @rajdeepdua Oct 2014 Tuesday, November 4, 14
  2. 2. Before we get started • What is a Container? – Group of processes contained in a Isolated Environment – IsolaNon provided by Concepts like cgroups and namespaces • What is Docker? – ImplementaNon of a container which is portable using a concept of image. Tuesday, November 4, 14
  3. 3. CGroup • Limit, account, and isolate resource usage (CPU, memory, disk I/O, etc.) of process groups. • Resource limi@ng: groups can be set to not exceed a set memory limit — this also includes file system cache. • Priori@za@on: some groups may get a larger share of CPU[8] or disk I/O throughput. • Accoun@ng: to measure how much resources certain systems use • Control: freezing groups or checkpoin@ng and restar@ng. Tuesday, November 4, 14
  4. 4. Namespace • ParNNon essenNal kernel structures to create virtual environments • Different Namespaces – pid (processes) – net (network interfaces, rouNng...) – ipc (System V IPC) – mnt (mount points, filesystems) – uts (hostname) – user (UIDs) Tuesday, November 4, 14
  5. 5. Docker • Manages Images and Container runNmes • Supports mulNple file system back-­‐ends • MulNple Execdrivers for container implementaNon • Client and server components – interacNon using HTTP using unix sockets Tuesday, November 4, 14
  6. 6. Docker RunNme Components Tuesday, November 4, 14
  7. 7. Docker Engine • Core of Docker : Store for Containers • Manages containers using Jobs (similar to Unix jobs) • Contains Handlers a funcNon which wraps Jobs • All the acNons performed using Jobs Engine n 1 1 1 Handler Job Tuesday, November 4, 14
  8. 8. Docker IniNalizaNon 1. Main funcNon of Docker : docker.main() 2. Calls : mainDaemon() 3. InstanNate Engine eng := engine.New() 4. Register built-­‐ins builtsin.Register(eng) 5. InstanNate job job := eng.Job(“initserver”) 6. Set Env variables for the Job Tuesday, November 4, 14
  9. 9. Docker IniNalizaNon 5. Run the Job job.run() 6. Start AccepNng ConnecNons eng.Job(“AcceptConnections”).run() Tuesday, November 4, 14
  10. 10. Docker IniNalizaNon : 4 4. Register built-­‐ins Instantiate daemon(eng) //see later slides for details eng.Register("initserver", server.InitServer) //see later slides for details eng.Register(“init_networkdriver”, bridge.InitDriver) Tuesday, November 4, 14
  11. 11. Daemon • Main Entry point for all the requests to manage containers • Data Structure which maintains following references – ImageGraph – Volume Graph – Engine – ExecDriver – Server – ContainerStore Tuesday, November 4, 14
  12. 12. Daemon -­‐ Graph • Graph is a (structure) store of versioned file system and rela@onship between images • For each container a Graph is instan@ated • References a graphdriver.Driver • Ac@ons on a Graph – Create a New Graph – Get image from a Graph – Restores a Graph – Creates an Image and Register in the Graph – Registers a pre-­‐exis@ng image on the Graph Tuesday, November 4, 14
  13. 13. Concept of Images and Containers in Docker • Docker image is a Layer in the file System • Containers are two Layers – Layer one is init layer based on Image – Layer two is the actual container content Container Content Image Content Layer Init Layer Docker Container Tuesday, November 4, 14
  14. 14. Container in Docker • DataStructure which resides in-­‐memory and is persisted in SQLite store • References other components like – Daemon – Volumes – Has a lifecycle which is controlled by Daemon – Daemon has in-­‐memory dicNonary of containerIDs and containers 14 Tuesday, November 4, 14
  15. 15. Lifecycle of a Container 15 Tuesday, November 4, 14
  16. 16. Graph Driver • Referenced by the Daemon • Used to abstract mulNple storage backends • Loads one of the following File System Backends – aufs – Device mapper (devmapper) – vfs – btrfs Tuesday, November 4, 14
  17. 17. Container store • Persistent backend for Container data • Implemented using SQLite • Referenced from Daemon containerGraph: graph Used to load container informaNon during Daemon restore Tuesday, November 4, 14
  18. 18. Volume Graph • Simple vfs based Graph to keep track of container volumes • Volumes used volume driver in Daemon to create and a?ach volumes to the container • Each container is associated with one of more volumes Tuesday, November 4, 14
  19. 19. ExecDriver • AbstracNon for the underlying Linux Containment • Called from the daemon • Supports following implementaNon – LXC – NaNve Tuesday, November 4, 14
  20. 20. Driver Interfaces • Abstract Interface to interact with the underlying implementaNon. type driver Interface{ Run(c *Command,..) Kill(c *Command) Pause(c *Command) Name() GetProcessIdsForContainer(id string) Terminate() } Tuesday, November 4, 14
  21. 21. Driver Interfaces -­‐ Networking • Abstract Interface to interact with the underlying implementaNon. type Network struct { Interface *NetworkInterface Mtu int ContainerID string HostNetworking bool } Tuesday, November 4, 14
  22. 22. libcontainer • Underlying naNve implementaNon of the Container • Used by the naNve driver • Container.config – representaNon of a container data • Wrapper over cgroups and Namespaces Tuesday, November 4, 14
  23. 23. NaNve Driver ImplementaNon Tuesday, November 4, 14
  24. 24. Steps in Container CreaNon Container Create Tuesday, November 4, 14
  25. 25. Container Commit Tuesday, November 4, 14
  26. 26. Summary • Linux Containment Principles • Docker Architectural components • NaNve Driver ImplementaNon – libcontainer • ContainerCreaNon Tuesday, November 4, 14

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