This video featuring Benjamin Hindman, formerly of Twitter, now of Mesosphere, is a few years old but still a great watch for understanding Mesos. What follows is basically my notes from watching this video.

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A Scheduler is a piece of custom software written by a developer which interacts with a Mesos master node. Recall that a Mesos Framework consists of a Scheduler and its Tasks.

The interaction between a Scheduler and the Mesos Master is not a request/response cycle of HTTP, but more of an Actor model, with unidirectional message passing.

Here’s a quick overview of an Scheduler/Master interaction:

  1. Scheduler - I want to Register
  2. Mesos - You have been Registered
  3. Scheduler - I want to Request these specific resources (requests are optional)
  4. Mesos - Here is an offer of some available resources
  5. Scheduler - I don’t need those - I decline your offer
  6. Mesos - Here is an offer of some more available resources
  7. Scheduler - Nice. I want to Launch a task using these specific resources
  8. Mesos - here is an Update on your task status after launch

The messages generally fall into 3 categories:

1. Lifecycle Management Messages

Scheduler ===> Mesos Master

  • Register Unregister Reregister (failover)

Mesos Master ===> Scheduler

  • Registered Reregistered

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2. Resource Allocation Messages

Scheduler ===> Mesos Master

  • Request Decline Revive

Mesos Master ===> Scheduler

  • Offer Rescind

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3. Task Management Messages

Scheduler ===> Mesos Master

  • Launch Kill Acknowledge(status) Reconcile(status)

Mesos Master ===> Scheduler

  • Update

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Launching a task

The scheduler creates a TaskInfo struct and sends a Launch message to the Mesos Master

  • TaskInfo includes SlaveID and the resources, and a Launch Command or an optional Executor

The Master sends the task to the Mesos slave which has the resources

  1. Launch Command - literally run a shell command to launch the task
  2. Executor - Mesos slave hands the task to an Executor which is running on that slave, which launches the task

When a task is launched it is put into a container inside the Mesos slave (the executor or the task are inside the container)