Leader/Follower Architecture

A common pattern in high availability (HA) systems is the leader/follower concept. This is also sometimes referred to as: master/agent, primary/replica, or some combination thereof. This architecture is used when you have one authoritative process, with N standby processes. In some systems, the standby processes might also be capable of serving requests or performing other operations. For example, when running a database like MySQL with a master and replica, the replica is able to serve read-only requests, but it cannot accept writes; only the master will accept writes. In DC/OS, a number of components follow the leader/follower pattern. We will discuss some of them and how they work.

Mesos

Mesos can be run in high availability mode, which requires running three or five masters. When run in HA mode, one master is elected as the leader, while the other masters are followers. Each master has a replicated log which contains some state about the cluster. ZooKeeper performs an election to select the leading master. For more detail on this, see the Mesos HA documentation.

Marathon

Marathon can be run in high availability mode, which allows you to run multiple Marathon instances (at least two for HA), with one elected leader. Marathon uses ZooKeeper for leader election. The followers do not accept writes or API requests; instead, the followers proxy all API requests to the leading Marathon instance.

ZooKeeper

Several services in DC/OS use ZooKeeper to provide consistency. ZooKeeper can be used as a distributed locking service, a state store, and a messaging system. ZooKeeper uses Paxos-like log replication and a leader/follower architecture to maintain consistency across multiple ZooKeeper instances. For a more detailed explanation of how ZooKeeper works, see the ZooKeeper internals document.

Fault Domain Isolation

Fault domain isolation is an important part of building HA systems. To correctly handle failure scenarios, systems must be distributed across fault domains to survive outages. There are different types of fault domains, a few examples of which are:

• Physical domains: this includes machine, rack, datacenter, region, and availability zone.
• Network domains: machines within the same network may be subject to network partitions. For example, a shared network switch may fail or have invalid configuration.

With DC/OS, you can distribute masters across racks for HA. Agents can be distributed across regions, and it is recommended that you tag agents with attributes to describe their location. Synchronous services like ZooKeeper should also remain within the same region to reduce network latency. For more information, see Configuring zones and regions for high-availability.

Applications which require HA should be distributed across fault domains. Marathon can accomplish this by using the UNIQUE and GROUP_BY constraints operator.

Separation of Concerns

High availability services should be decoupled, with responsibilities divided amongst services. For example, web services should be decoupled from databases and shared caches.

Eliminating Single Points of Failure

Single points of failure come in many forms. For example, a service like ZooKeeper can become a single point of failure when every service in your system shares one ZooKeeper cluster. You can reduce these risks by running multiple ZooKeeper clusters for separate services. There is an Exhibitor Catalog package that makes this easy.

Other common single points of failure include:

• Single database instances (like a MySQL)
• One-off services
• Non-HA load balancers

Fast Failure Detection

Fast failure detection comes in many forms. Services like ZooKeeper can be used to provide failure detection, such as detecting network partitions or host failures. Service health checks can also be used to detect certain types of failures. As a matter of best practice, services should expose health check endpoints, which can be used by services like Marathon.

Fast Failover

When failures do occur, failover should be as fast as possible. Fast failover can be achieved by:

• Using an HA load balancer like Marathon-LB, or the internal Layer 4 load balancer.
• Building apps in accordance with the 12-factor app manifesto.
• Following REST best practices when building services: in particular, avoiding storing client state on the server between requests.

A number of DC/OS services follow the fail-fast pattern in the event of errors. Specifically, both Mesos and Marathon will shut down in the case of unrecoverable conditions such as losing leadership.