Docker Architecture (v1.3)

Docker
Architecture
based
on
v
1.3
Compiled
by
Rajdeep
Dua
Twi?er
:
@rajdeepdua
Oct
2014
Tuesday, November 4, 14

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.

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.

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)

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

Docker
RunNme
Components

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

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

Docker
IniNalizaNon
5. Run
the
Job
job.run()
6. Start
AccepNng
ConnecNons
eng.Job(“AcceptConnections”).run()

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)

Daemon
• Main
Entry
point
for
all
the
requests
to
manage
containers
• Data
Structure
which
maintains
following
references
– ImageGraph
– Volume
Graph
– Engine
– ExecDriver
– Server
– ContainerStore

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

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

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

Lifecycle
of
a
Container
15

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

Container
store
• Persistent
backend
for
Container
data
• Implemented
using
SQLite
• Referenced
from
Daemon
containerGraph: graph
Used
to
load
container
informaNon
during
Daemon
restore

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

ExecDriver
• AbstracNon
for
the
underlying
Linux
Containment
• Called
from
the
daemon
• Supports
following
implementaNon
– LXC
– NaNve

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()
}

Driver
Interfaces
-‐
Networking
• Abstract
Interface
to
interact
with
the
underlying
implementaNon.
type Network struct {
Interface *NetworkInterface
Mtu int
ContainerID string
HostNetworking bool
}

libcontainer
• Underlying
naNve
implementaNon
of
the
Container
• Used
by
the
naNve
driver
• Container.config
–
representaNon
of
a
container
data
• Wrapper
over
cgroups
and
Namespaces

NaNve
Driver
ImplementaNon

Steps
in
Container
CreaNon
Container
Create

Container
Commit

Summary
• Linux
Containment
Principles
• Docker
Architectural
components
• NaNve
Driver
ImplementaNon
–
libcontainer
• ContainerCreaNon

Docker Architecture (v1.3)

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