*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](https://raft.github.io/) 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`|
-**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](https://golang.org/doc/code.html) environment
- [Protobuf 3.x or higher](https://developers.google.com/protocol-buffers/docs/downloads) to regenerate protocol buffer files (e.g. using `make generate`)
$ watch -n1 "swarmctl service inspect redis" # watch the update
```
This will update 2 tasks, wait for them to become *RUNNING*, then wait an additional 10 seconds before moving to other tasks.
Update options can be set at service creation and updated later on. If an update command doesn't specify update options, the last set of options will be used.
### Node Management
*SwarmKit* monitors node health. In the case of node failures, it re-schedules tasks to other nodes.
An operator can manually define the *Availability* of a node and can *Pause* and *Drain* nodes.
Let's put `node-1` into maintenance mode:
```
$ swarmctl node drain node-1
$ swarmctl node ls
ID Name Membership Status Availability Manager Status
