The main purpose of Swarm was already defined, but how does it accomplish its goals? Here are its key features:
- Swarm v1 supports Docker Engine of version 1.6.0 or more recent. Swarm v2 has been in built for Docker Engine since version 1.12.
- APIs of each release of Swarm will be compatible with Docker APIs on the same release train. API compatibility is maintained for one version backward.
- In Swarm v1, the leader-election mechanism is implemented for multiple Swarm masters using the leadership library (only supported when deploying Swarm with a discovery service, such as Etcd, Consul, or Zookeeper).
- In Swarm v2, leader election has been built using the decentralized mechanism. Swarm v2 does not need a dedicated set of discovery services anymore because it integrates Etcd, an implementation of the Raft consensus algorithm (see Chapter 2, Discover the Discovery Services).
- In the Swarm v1 terminology, the leader Swarm master is called primary, where others are called replica. In Swarm v2, there is a concept of Master and Worker nodes. While the leader nodes are managed automatically by the cluster using Raft.
- Basic and advanced scheduling options. The scheduler is an algorithm that decides the hosts on which the containers must be physically placed. Swarm comes with a set of built-in schedulers.
- Constraints and affinities to let the operator take decisions on scheduling; for example, one wants to keep the database containers geographically near and suggest the scheduler to do that. Constraints and affinities use Docker Swarm labels.
- In Swarm v2, in-cluster load balancing is implemented with the built-in DNS Round-Robin, while it supports external load balancing via the routing mesh mechanism, which is implemented over IPVS.
- High-availability and failover mechanism means that you can create a Swarm with more than one master; so if they go down, there will be other master/s ready to take control. Swarm v2 is available, by default, when we form a cluster of at least 3 nodes. All nodes can be the master nodes. Also, Swarm v2 includes health indicator information.
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