Understanding Difference between Docker Swarm(Classic), Swarm Mode & SwarmKit

Docker Swarm is an older (2014) Docker native orchestration tool. It is standalone from the Docker engine and serves to connect Docker engines together to form a cluster. It's then possible to connect to the Swarm and run containers on the cluster. Swarm has a few features:

Allows us to specify a discovery service
Some control over where containers are placed (using filters / constraints / distribution strategies, etc...)
Exposes the same API as the Docker engine itself, allowing 3rd-party tools to interact seamlessly

Swarmkit is a new (2016) tool developed by the Docker team which provides functionality for running a cluster and distributing tasks to the machines in the cluster. Here are the main features:

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.

Docker Swarm Mode (Version 1.12 >) uses Swarmkit libraries & functionality in order to make container orchestration over multiple hosts (a cluster) very simple & secure to operate. There is a new set of features (the main one being docker swarm) which are now built into Docker itself to allow us to initiate a new Swarm and deploy tasks to that cluster.

Docker Swarm is not being deprecated, and is still a viable method for Docker multi-host orchestration, but Docker Swarm Mode (which uses the Swarmkit libraries under the hood) is the recommended way to begin a new Docker project where orchestration over multiple hosts is required.

One of the big features in Docker 1.12 release is Swarm mode. Docker had Swarm available for Container orchestration from 1.6 release. Docker released Swarmkit as an opensource project for orchestrating distributed systems few weeks before Docker 1.12(RC) release.

"Swarm" refers to traditional Swarm functionality, "Swarm Mode" refers to new Swarm mode added in 1.12, "Swarmkit" refers to the plumbing open source orchestration project.

Swarm, Swarm Mode and Swarmkit

Following table compares Swarm and Swarm Mode :

Swarm	Swarm Mode
Separate from Docker Engine and can run as Container	Integrated inside Docker engine
Needs external KV store like Consul	No need of separate external KV store
Service model not available	Service model is available. This provides features like scaling, rolling update, service discovery, load balancing and routing mesh
Communication not secure	Both control and data plane is secure
Integrated with machine and compose	Not yet integrated with machine and compose as of release 1.12. Will be integrated in the upcoming releases

Following table compares Swarmkit and Swarm Mode:

Swarmkit	Swarm Mode
Plumbing opensource project	Swarmkit used within Swarm Mode and tightly integrated with Docker Engine
Swarmkit needs to built and run separately	Docker 1.12 comes integrated with Swarm Mode
No service discovery, load balancing and routing mesh	Service discovery, load balancing and routing mesh available
Use swarmctl CLI	Use regular Docker CLI

Swarmkit has primitives to handle orchestration features like node management, discovery, security and scheduling.

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