Service availability

Content Platform for Cloud Scale Administration Guide

Version
2.6.x
File Size
1945 KB
Audience
anonymous
Part Number
MK-HCPCS008-10

HCP for cloud scale services provide high availability as follows:

  • The Metadata Gateway service always has at least three service instances. When the system starts, the nodes "elect a leader" using the raft consensus algorithm. The other service instances follow the leader. The leader processes all GET and PUT requests. If the followers cannot identify the leader, they elect a new leader. The Metadata Gateway service tolerates the failure of one service instance without interruption. If more than one service instance is unavailable, some data can become unavailable until the instance recovers.
  • The Metadata Coordination service always has one service instance. If that instance fails, HCP for cloud scale automatically starts another instance. Until startup is complete, the Metadata Gateway service cannot scale.
  • The Metadata Cache service is deprecated but always has one service instance. If that instance fails, HCP for cloud scale automatically starts another instance.
  • To protect messaging consistency, the Message Queue service always has three service instances. To prevent being split into disconnected parts, the service shuts down if half of the service instances fail. In practice, messaging stops if two of the three instances fail. Do not let the service run with only two instances, because in that scenario if one of the remaining instances fails, the service shuts down. However, when one of the failed instances restarts, messaging services recover and resume.
  • To maintain access to the encryption key vault, the Key Management Server service uses an active-standby model. One service instance is the active instance and any other service instances are kept as standbys. If the active vault node becomes sealed or unavailable, one of the standbys takes over as active. You can scale up to the number of instances in the HCP for cloud scale system or your acceptable performance limits.

The rest of the HCP for cloud scale services remain available if HCP for cloud scale instances or service instances fail, as long as at least one service instance remains healthy. Even if a service that has only one service instance fails, HCP for cloud scale automatically starts a new service instance.