SwarmKit

SwarmKit is a toolkit for orchestrating distributed systems at any scale. It includes primitives for node discovery, raft-based consensus, task scheduling and more.

Its main benefits are:

• Distributed: SwarmKit uses the Raft Consensus Algorithm in order to coordinate and does not rely on a single point of failure to perform decisions.
• Secure: Node communication and membership within a Swarm are secure out of the box. SwarmKit uses mutual TLS for node authentication, role authorization and transport encryption, automating both certificate issuance and rotation.
• Simple: SwarmKit is operationally simple and minimizes infrastructure dependencies. It does not need an external database to operate.

Overview

Machines running SwarmKit can be grouped together in order to form a Swarm, coordinating tasks with each other. Once a machine joins, it becomes a Swarm Node. Nodes can either be worker nodes or manager nodes.

• Worker Nodes are responsible for running Tasks using an Executor. SwarmKit comes with a default Docker Container Executor that can be easily swapped out.
• Manager Nodes on the other hand accept specifications from the user and are responsible for reconciling the desired state with the actual cluster state.

An operator can dynamically update a Node's role by promoting a Worker to Manager or demoting a Manager to Worker.

Tasks are organized in Services. A service is a higher level abstraction that allows the user to declare the desired state of a group of tasks. Services define what type of task should be created as well as how to execute them (e.g. run this many replicas at all times) and how to update them (e.g. rolling updates).

Features

Some of SwarmKit's main features are:

• Orchestration

  • Desired State Reconciliation: SwarmKit constantly compares the desired state against the current cluster state and reconciles the two if necessary. For instance, if a node fails, SwarmKit reschedules its tasks onto a different node.

  • Service Types: There are different types of services. The project currently ships with two of them out of the box

    • Replicated Services are scaled to the desired number of replicas.
    • Global Services run one task on every available node in the cluster.

  • Configurable Updates: At any time, you can change the value of one or more fields for a service. After you make the update, SwarmKit reconciles the desired state by ensuring all tasks are using the desired settings. By default, it performs a lockstep update - that is, update all tasks at the same time. This can be configured through different knobs:

    • Parallelism defines how many updates can be performed at the same time.
    • Delay sets the minimum delay between updates. SwarmKit will start by shutting down the previous task, bring up a new one, wait for it to transition to the RUNNING state then wait for the additional configured delay. Finally, it will move onto other tasks.

  • Restart Policies: The orchestration layer monitors tasks and reacts to failures based on the specified policy. The operator can define restart conditions, delays and limits (maximum number of attempts in a given time window). SwarmKit can decide to restart a task on a different machine. This means that faulty nodes will gradually be drained of their tasks.

• Scheduling

  • Resource Awareness: SwarmKit is aware of resources available on nodes and will place tasks accordingly.

  • Constraints: Operators can limit the set of nodes where a task can be scheduled by defining constraint expressions. Multiple constraints find nodes that satisfy every expression, i.e., an AND match. Constraints can match node attributes in the following table. Note that engine.labels are collected from Docker Engine with information like operating system, drivers, etc. node.labels are added by cluster administrators for operational purpose. For example, some nodes have security compliant labels to run tasks with compliant requirements.

    | node attribute | matches | example | |:------------- |:-------------| :-------------| | node.id | node's ID | node.id == 2ivku8v2gvtg4| | node.hostname | node's hostname | node.hostname != node-2| | node.ip | node's IP address | node.ip != 172.19.17.0/24| | node.role | node's manager or worker role | node.role == manager| | node.platform.os | node's operating system | node.platform.os == linux| | node.platform.arch | node's architecture | node.platform.arch == x86_64| | node.labels | node's labels added by cluster admins | node.labels.security == high| | engine.labels | Docker Engine's labels | engine.labels.operatingsystem == ubuntu 14.04|

  • Strategies: The project currently ships with a spread strategy which will attempt to schedule tasks on the least loaded nodes, provided they meet the constraints and resource requirements.

• Cluster Management

  • State Store: Manager nodes maintain a strongly consistent, replicated (Raft based) and extremely fast (in-memory reads) view of the cluster which allows them to make quick scheduling decisions while tolerating failures.
  • Topology Management: Node roles (Worker / Manager) can be dynamically changed through API/CLI calls.
  • Node Management: An operator can alter the desired availability of a node: Setting it to Paused will prevent any further tasks from being scheduled to it while Drained will have the same effect while also re-scheduling its tasks somewhere else (mostly for maintenance scenarios).

• Security

  • Mutual TLS: All nodes communicate with each other using mutual TLS. Swarm managers act as a Root Certificate Authority, issuing certificates to new nodes.
  • Token-based Join: All nodes require a cryptographic token to join the swarm, which defines that node's role. Tokens can be rotated as often as desired without affecting already-joined nodes.
  • Certificate Rotation: TLS Certificates are rotated and reloaded transparently on every node, allowing a user to set how frequently rotation should happen (the current default is 3 months, the minimum is 30 minutes).

Build

Requirements:

• Go 1.6 or higher
• A working golang environment
• Protobuf 3.x or higher to regenerate protocol buffer files (e.g. using make generate)

SwarmKit is built in Go and leverages a standard project structure to work well with Go tooling. If you are new to Go, please see BUILDING.md for a more detailed guide.

Once you have SwarmKit checked out in your $GOPATH, the Makefile can be used for common tasks.

From the project root directory, run the following to build swarmd and swarmctl:

$ make binaries

Test

Before running tests for the first time, setup the tooling:

$ make setup

Then run:

$ make all

Usage Examples

Setting up a Swarm

These instructions assume that swarmd and swarmctl are in your PATH.

(Before starting, make sure /tmp/node-N don't exist)

Initialize the first node:

$ swarmd -d /tmp/node-1 --listen-control-api /tmp/node-1/swarm.sock --hostname node-1

Before joining cluster, the token should be fetched:

$ export SWARM_SOCKET=/tmp/node-1/swarm.sock  
$ swarmctl cluster inspect default  
ID          : 87d2ecpg12dfonxp3g562fru1
Name        : default
Orchestration settings:
  Task history entries: 5
Dispatcher settings:
  Dispatcher heartbeat period: 5s
Certificate Authority settings:
  Certificate Validity Duration: 2160h0m0s
  Join Tokens:
    Worker: SWMTKN-1-3vi7ajem0jed8guusgvyl98nfg18ibg4pclify6wzac6ucrhg3-0117z3s2ytr6egmmnlr6gd37n
    Manager: SWMTKN-1-3vi7ajem0jed8guusgvyl98nfg18ibg4pclify6wzac6ucrhg3-d1ohk84br3ph0njyexw0wdagx

In two additional terminals, join two nodes. From the example below, replace 127.0.0.1:4242 with the address of the first node, and use the <Worker Token> acquired above. In this example, the <Worker Token> is SWMTKN-1-3vi7ajem0jed8guusgvyl98nfg18ibg4pclify6wzac6ucrhg3-0117z3s2ytr6egmmnlr6gd37n. If the joining nodes run on the same host as node-1, select a different remote listening port, e.g., --listen-remote-api 127.0.0.1:4343.

$ swarmd -d /tmp/node-2 --hostname node-2 --join-addr 127.0.0.1:4242 --join-token <Worker Token>
$ swarmd -d /tmp/node-3 --hostname node-3 --join-addr 127.0.0.1:4242 --join-token <Worker Token>

If joining as a manager, also specify the listen-control-api.

$ swarmd -d /tmp/node-4 --hostname node-4 --join-addr 127.0.0.1:4242 --join-token <Manager Token> --listen-control-api /tmp/node-4/swarm.sock --listen-remote-api 127.0.0.1:4245

In a fourth terminal, use swarmctl to explore and control the cluster. Before running swarmctl, set the SWARM_SOCKET environment variable to the path of the manager socket that was specified in --listen-control-api when starting the manager.

To list node