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HealthManager 2.0

Health Manager monitors the state of the applications and ensures that started applications are indeed running, their versions and number of instances correct.

Conceptually, this is done by maintaining a Actual State of applications and comparing it against the Desired State. When discrepancies are found, actions are initiated to bring the applications to the Desired State, e.g., start/stop commands are issued for missing/extra instances, respectively.

Additionally, Health Manager collects and exposes statistics and health status for individual applications, as well as aggregates for frameworks, runtimes, etc.

AppState

The state of each application is represented by an instance of an aptly named class AppState. AppState gets forwarded important state-changing messages (i.e. hearbeats and exit signals), updates its internal state accordingly and then invokes registered event handlers. It is the job of these handlers (housed in the Harmonizer, see below) to enforce complex policies, e.g., whether to restart application, if so, with which priority, etc.

Components

HM is comprised of the following components:

  • Manager
  • Harmonizer
  • Scheduler
  • DesiredState
  • ActualState
  • Nudger
  • Reporter

Manager

Provides an entry point, configures, initializes and registers other components.

Harmonizer

Expresses the policy of bringing the applications to the Desired State by observing the Actual State.

Harmonizer sets up the interactions between other components, and aims to achieve clarity of the intent through delegation:

Actual State and Desired State are compared periodically with the use of the Scheduler and Nudger actions are Scheduled to bring the States into harmony.

Scheduler

Encapsulates EventMachine-related functionality such as timer setup and cancellation, quantization of long-running tasks to prevent EM Reactor loop blocking.

Desired State

Provides the desired state of the application, e.g., whether the application was Started or Stopped, how many instances should be running, etc. This information comes from the Cloud Controller by way of http-based Bulk API.

The Bulk API contains the state of the world as the Cloud Controller says it should be. This is a dump of the CCs database. It might differ from what the world actually looks like, and the Harmonizer will attempt to make the current state match this desired state.

Actual State

The ActualState listens to heartbeat and other messages on the NATS bus from the DEA.

The State of each application is represented by an instant of object AppState. That object receives updates of the application state, stores them and notifies registered listeners about events, such as instances_missing, etc.

Nudger

Nudger is the interface for health manager to affect the change on the world, by dispatching cloudcontrollers.hm.requests messages that instruct CCs to start or stop instances. Nudger maintains a priority queue of these requests, and deques the messages by a batchful.

Reporter

Reporter responds to healthmanager.status and healthmanager.health requests.

Harmonization Policy in Detail

Conceptually, harmonization happens in two ways:

  • by reacting to messages (such as droplet.exited);
  • by periodically scanning the world and enumerating applications, looking for anomalies.

droplet.exited signal

There are three distinct scenarios possible when droplet.exited signal arrives:

  • application is stopped; means the application was stopped explicitly, no action required;

  • DEA evacuation; the DEA is being evacuated and all instances from that DEA need to be restarted somewhere else. HM-2 initiates that restarting;

  • application instance crashed; That instance needs to be restarted unless it crashed multiple times in short period of time, in which case it is declared flapping. See more on this below.

flapping instances

An instance of application is declared flapping if it crashed more than flapping_death times within flapping_timeout seconds. There are several possible reasons for flapping:

  • app is completely broken and simply does not start;
  • app has a bug that results in a crash every once in a while;
  • app has a dependency on the external world or a CF-provisioned service, and that dependency is unavailable, perhaps temporarily, resulting in app repeatedly crashing.

Handling flapping apps is hard. We'd like to:

  • make the best effort to restart an app, when it makes sense;
  • provide the crashlogs for crashing instances;
  • cut down on the overhead associated with restarting an app, particularly relating to moving application bits to DEA and storing it there.
  • avoid IO spikes due to massive simultaneous restarts

In order to accomodate these conflicting requirements, the following policy for flapping instances (FI) adopted:

  • initially the FI is restarted with a delay defined by min_restart_delay config value;
  • for each subsequent crash, the delay is doubled, but not to exceed max_restart_delay config value;
  • a random noise is added to the value of delay, its maximum absolute value defined by delay_time_noise config value;
  • if the number of crashes for a given FI exceeds giveup_crash_number, give up restarting attempts. This behavior can be turned off.

Heartbeat processing

DEAs peridically send out heartbeat messages on NATS bus. These heartbeats contain DEA identifying information, as well as information on application instances running on respective DEAs.

The heartbeats are used to establish "missing" and "extra" indices. Missing indices are then commanded to start, extra indices are commanded to stop.

AppState object tracks heartbeats for each instance of each version.

An instance is "missing" if a live version of this instance has not received a heartbeat in the last droplet_lost seconds.

However, an instance_missing event is only triggered if the AppState was not reset recently, and if check_for_missing_instances method has been invoked.

Configuration

HealthManager reads its configuration from a YAML file. Look at the example config file for an explanation of all the configurable variables.

Logs

HealthManager uses Steno to manage its logs. The logging key in the config file provides information for Steno configuration.

Here are the log levels, with examples of what they're being used for:

  • error - HM received an error response from the Cloud Controller bulk API
  • warn - a droplet analysis was initiated while the previous droplet analysis was still going
  • info - HM registered a new VCAP component, HM is shutting down
  • debug2 - HM received a heartbeat from a DEA, HM compares an app's desired and known states
  • debug - HM starts/stops an instance

Contributing

Please read the contributors' guide