Under the Hood with Docker Swarm Mode - Drew Erny and Nishant Totla, Docker

With Docker Swarm Mode
Under the Hood

Nishant Totla
Software Engineer
Docker
Drew Erny
Software Engineer
Docker
Under the Hood with Docker Swarm Mode

1. Overview of Swarm
2. Dive into New Features
3. Questions
Contents

Background: What Is Swarm?
You start with this
Database
Web Server 2
Web Server 1
Server

Then you need more servers
Database
Replica
Web Server 2
Web Server 2
Server 2
Database
Replica
Web Server 2
Web Server 1
Server 1
Database
Replica
Web Server 1
Web Server 1
Server 3

Database
Replica
Web Server 2
Web Server 2
Server 2
Database
Replica
Web Server 2
Web Server 1
Server 1
Database
Replica
Web Server 1
Web Server 1
Server 3
You have to figure out where to put new things
New
Service
New
Service
New
Service
?
?
?

Database
Replica
Web Server 2
Web Server 2
Server 2
Database
Replica
Web Server 2
Web Server 1
Dead Server
Database
Replica
Web Dead 1
Web Server 1
Server 3
You have to manually compensate for failures

Swarm is Cluster Orchestration
And it’s simple!
Many Discrete Computers One Cluster
Hand Architecting Algorithmic Scheduling
Manual Recovery Automatic Rescheduling

Swarm is Cluster
OrchestrationBuilt on services
Service Spec Service
Image Name
# of replicas
Network
Attachments
Exposed ports
...
Orchestrated

Swarm is Cluster
OrchestrationUsing Desired State Reconciliation
Cluster StateDesired State
Make Changes
Compare Differences

Swarm Topology
Manager ManagerManager Raft
Worker Worker Worker Worker Worker

What’s New in Swarm Mode
Improvements New Features
High-Availability Scheduling Topology-Aware Scheduling
Encrypted Raft Log Secrets
Health-Aware Orchestration Service Rollbacks
Service Logs

Prioritize spreading out containers in a service instead
of equalizing the number of containers per node
HA Scheduling

Service 2
Service 2
Worker 1
Service 1
Service 2
Worker 2
Service 1
What does this look
like?
Consider you have 2
nodes.
HA Scheduling

Service 2
Service 2
Worker 1
Service 1
Service 2
Worker 2
Service 1
What does this look
like?
And then you add a
third node.
HA Scheduling Worker 3

Service 2
Service 2
Worker 1
Service 1
Service 2
Worker 2
Service 1
What does this look
like?
And then you add a
new service with 3 new
replicas.
Service 3
Service 3
Service 3
?
?
?

Service 2
Service 2
Worker 1
Service 1
Service 2
Worker 2
Service 1
What does this look
like?
Under the old
algorithm, something
like this would happen.
Service 3
Service 3
Service 3

Service 2
Service 2
Worker 1
Service 1
Service 2
Worker 2
Service 1
What does this look
like?
With HA scheduling, the
service gets spread
across the nodes.
Service 3
Service 3
Service 3

Service 2
Service 2
Worker 1
Service 1
Service 2
Worker 2
Service 1
What does this look
like?
And if a service is
already evenly spread?
Service 3
Service 3
Service 3
Service 3
?

Service 2
Service 2
Worker 1
Service 1
Service 2
Worker 2
Service 1
What does this look
like?
Then absolute number
of containers is the
tiebreaker.
Service 3
Service 3
Service 3
Service 3

$ docker service create --replicas 3 dockercon
HA Scheduling: How to Use

$ docker service create --replicas 3 dockercon
HA Scheduling: How to Use
That’s it! You’re already using
it!

Spread in order across arbitrary labeled nodes
Topology-Aware Scheduling

Spreading across zones
Node 1 Node 2 Node 3 Node 4 Node 6Node 5
?
?
?

Availability Zone 1 Availability Zone 2

$ docker node update
--label-add az=1
q7uf8b
# and so on for each node
Placement Prefs: How to Use

$ docker service create
--placement-pref 'spread=node.labels.az'
dockercon
Placement Prefs: How to Use

Spreading Across Zones
Availability Zone 1 Availability Zone 2

• All cluster-wide communication is encrypted
• Security should be easy to use
Raft Log Encryption

Raft Log Encryption
/var/lib/docker/swarm
The Raft log is backed up on disk
It contains sensitive cluster info,
including state and membership

Raft Log Encryption
For security, it’s important to encrypt
your Raft log
Swarm can do it for you

Raft Log: How to Encrypt
$ docker swarm init
# basic cluster create command

Raft Log: How to Encrypt
$ docker swarm init
# basic cluster create command
That’s it! Your Raft log is
encrypted and secure!

• The Raft log and TLS encryption keys are still on disk
• Who protects the protector?
What’s the Catch

Protecting Encryption Keys
Swarm makes it easy to protect
encryption keys

$ docker swarm init --autolock
Creates a key to encrypt the
encryption keys
Key required to start manager from
existing log

$ docker swarm init --autolock
Swarm initialized: current node (u3hujejsk5plrmfn3uq10kmu5) is now
a manager.
[...]
To unlock a swarm manager after it restarts, run the `docker swarm unlock`
command and provide the following key:
SWMKEY-1-yDrZW4AyTzPiqpJvYGL5sKqkX5XFvQJBm1ztGwFDgiI
Please remember to store this key in a password manager, since without
it you will not be able to restart the manager.

• Services often require sensitive information (like
passwords)
• Need a way to securely distribute such information across
the cluster
Securely Distributing Passwords

$ docker service create -e password=TOTALLYSECURE dockercon
The Old Way
$ docker service create -v some/host/dir:/password dockercon

Passing a secret in an
environment variable
Environment
Variables
-e password=TOTALLYSECURE
dockercon
Service
Node

Node
Service
ENV:
password:
TOTALLYSECURE
-e password=TOTALLYSECURE
dockercon
Service is created with
password in the container
environment
Environment
Variables

Node
Service
ENV:
password:
TOTALLYSECURE
A user tries to debug the
service
Environment
Variables

Node
debug-log.txt
Service
ENV:
password:
TOTALLYSECURE
ENV:
password:
TOTALLYSECURE
service
Environment
Variables
The environment is dumped
into a debug log

debug-log.txt
Service
ENV:
password:
TOTALLYSECURE
ENV:
password:
TOTALLYSECURE
service
Environment
Variables
The environment is dumped
into a debug log
The log is often shared

Node
Service
ENV:
password:
TOTALLYSECURE
Environment
Variables

Node
Service
ENV:
password:
TOTALLYSECURE
crash-log.txt
ENV: 04/17/17 13:01:15
password:
TOTALLYSECURE
Service crashed 04/17/17 12:58:33
Service down 04/17/17 13:00:00
Service down 04/17/17 13:01:00
Service Config 04/17/17 13:01:30
Replicas: 3
…
Network Config 04/17/17 13:02:00
Aliases: net
…
The service crashes and
dumps out a crash log file
Environment
Variables

Node
crash-log.txt
Service crashed 04/17/17 12:58:33
Service down 04/17/17 13:00:00
Service down 04/17/17 13:01:00
Service Config 04/17/17 13:01:30
Replicas: 3
…
Network Config 04/17/17 13:02:00
Aliases: net
…
ENV: 04/17/17 13:01:15
password:
TOTALLYSECURE
The service crashes and
dumps out a crash log file
Environment
Variables
The log file contains a
plaintext password and is
saved to disk

Node 1
/password
-v some/host/dir:/password
dockercon
Node 2
/password
Volume must exist on every
node that service needs to
run on
Volumes

Node 1
Service
/password
-v some/host/dir:/password
dockercon
Node 2
/password
Service
Volume must exist on every
node that service needs to
run on
Volumes

Node 1
Service
/password
Node 2
/password
Service
Service instance goes down
Volumes

Node 1
/password
Node 2
/password
Service
ServiceService instance goes down
Volumes
Service instance is
rescheduled
But secret stays on the old
node

• Easy to use
• Mitigate the risk from workarounds
• Seamlessly work with Swarm Services
Docker Secrets

A basic Swarm cluster
The Raft log is encrypted and
secure
Secrets
Manager ManagerManager Raft Store
Client

Let’s encrypt the encryption
keys for added security
It takes just one command!
Secrets
$ docker swarm update --autolock=true
Client

Let’s start a service
Secrets
Client
Service Service Service
--replicas 3
dockercon

Ready to create a
secret (password)
Secrets
Client
$ docker secret create
my-password password.file
password

That was easy!
Secrets
Client

Secret shared across
managers via the Raft store
Your secret is safe with Swarm
Secrets
Client

Update service to use
the secret
Secrets
Client
$ docker service update
--secret-add=my-password
Dockercon

Secret only sent to nodes
running the service
Stored in tmpfs mounted into
the container
Secrets
Client

Node failure
Service instance needs
to be rescheduled
Secrets
Client

Secret moves with the service
Dead worker node does
not have secret
Secrets
Client

Secrets are now
first-class objects
The right way is also
the easy way
$ docker secret create my-password
password.file
x1r790346t2t3sofmpchee5pm
--secret-add=my-password
Dockercon
Secrets

Health-Aware Orchestration
Defining Image Healthchecks in the Dockerfile
FROM dockercon
. . .
HEALTHCHECK --interval 10s
--timeout 3s
--retries 5
CMD curl http://localhost/health
. . .
CMD [“start”]

Healthchecks can be defined on the image or via command line

Healthchecks can be defined on the image or via command line
Swarm now monitors container health

Manager
Worker Worker
Service

Manager
Worker Worker
Service
Container still running, but unhealthy

Roll back a service to the previous spec
Two Ways:
1. Manually through service update --rollback
2. Automatically as --update-failure-action=rollback
Service Rollbacks

--name Dockercon
--replicas 1845
--publish 80:80
texas
Manual Rollbacks

Manual Rollbacks
Service Object
Name:
Dockercon
Service State
Task States
Network Status
Service Spec
Image:
texas
# of replicas:
1845
Ports:
[80]

--replicas 2017
--publish-add 443:443
--image dockercon
Dockercon
Manual Rollbacks

Manual Rollbacks
Service Object
Name:
Dockercon
Service State
Task States
Network Status
Service Spec
Image: dockercon
# of replicas:
2017
Ports:
[80,443]
Previous Spec
Image:
texas
# of replicas:
1845
Ports:
[80]

$ docker service update --rollback Dockercon
Manual Rollbacks

Manual Rollbacks
Service Object
Name:
Dockercon
Service State
Task States
Network Status
Service Spec
Image: dockercon
# of replicas:
2017
Ports:
[80,443]
Service Spec
Image:
texas
# of replicas:
1845
Ports:
[80]

Manual Rollbacks
Service Object
Name:
Dockercon
Service State
Task States
Network Status
Previous Spec
Image:
dockercon
# of replicas:
2017
Ports:
[80,443]
Service Spec
Image:
texas
# of replicas:
1845
Ports:
[80]

Continue
Pause
Rollback
Rolling Update
Modes
t + 0 t + 1 t + 2 t + 3 t + 4

• Continue
Pause
Rollback
Rolling Update
Modes
t + 0 t + 1 t + 2 t + 3 t + 4

Continue
• Pause
Rollback
Rolling Update
Modes
t + 0 t + 1 t + 2 t + 3 t + 4

Continue
Pause
• Rollback
Rolling Update
Modes
t + 0 t + 1 t + 2 t + 3 t + 4

Rollback Failure Actions
Pause Continue

$ docker service create --name dockercon
--replicas 4
--rollback-failure-action=pause
OR
...
--rollback-failure-action=continue
Rolling Updates

$ docker service create --name Dockercon
--replicas 100
--update-failure-action rollback
--update-max-failure-ratio 0.1
--update-monitor-period 10s
. . .
Update Tuning

Update Tuning
Updating 10
instances at a time

Update Tuning
Updating 10
instances at a time
4 failures within
monitor period

Update Tuning
Updating 10
instances at a time
4 more failures in
monitor period

Update Tuning
Updating 10
instances at a time
Too many failures!

Update Tuning
Updating 10
instances at a time
Over 10% failures
Update failed

Update Tuning
Automatic rollback
Updating 10
instances at a time

SSH into each node?
Set up a logging system?
Getting Logs from a Service

Fetch logs from containers of a service
Includes logs from stopped containers
Use same API options as container logs
Sends log context (service, node, and task ids) as details
Service Logs

$ docker service logs --tail 10 dockercon | sort -k3 -k4
dockercon.3.vo3l16eyy4cl@moby | 2017/04/03 23:12:26 Got a healthcheck!
dockercon.1.wy9wq4m4rvtf@moby | 2017/04/03 23:12:28 Got a healthcheck!
dockercon.2.co0nmnczoz62@moby | 2017/04/03 23:12:28 Got a healthcheck!
Service Logs

Log Request Is Made
Log Model
Swarm
Worker
Swarm
Worker
Swarm
Worker
Client
Swarm
Manager

Swarm Manager creates a
Subscription and dispatches
to the workers
Log Model
Swarm
Worker
Swarm
Worker
Swarm
Worker
Client
Swarm
Manager

Swarm Workers start logs
for every container that
matches the selector
Logs come back as a single
aggregated stream
Log Model
Swarm
Worker
Stream to
Manager
Service Container
Service Container
Different Service

Swarm Workers start
streaming back logs to the
manager
Log Model
Swarm
Worker
Swarm
Worker
Swarm
Worker
Client
Swarm
Manager

Manager aggregates all of the
logs and returns them as one
stream to the client
Log Model
Swarm
Worker
Swarm
Worker
Swarm
Worker
Client
Swarm
Manager

Logs can be followed.
Streams from new replicas as
they come up.
Ends the stream when the
user cancels
Log Model
Swarm
Worker
Swarm
Worker
Swarm
Worker
Client
Swarm
Manager

Summary
Scheduling
- HA scheduling default
- Topologically aware scheduling available

Summary
Security
- Raft log encrypted by default
- Docker Secrets used to pass sensitive
information to services

Summary
Orchestration
- Healthchecks incorporated
- Rolling-updates improvements

Summary
Service Logs
- Easiest way to get aggregate logs

Thank You!
@dperny
@nishanttotla

Under the Hood with Docker Swarm Mode - Drew Erny and Nishant Totla, Docker

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