Overview
Open Horizon generally treats nodes as entities with an independent lifecycle, apart from all other nodes. But there are use cases, such as using sensors for monitoring critical systems, where it is important to have redundant monitoring in place so that there is always at least one monitoring agent operating. This is of course similar to the principles of high availability and continuous availability that are commonly found within IT systems. Open Horizon already contains a little known feature, called HA Groups, that enables nodes to be associated as a group such that at least 1 copy of a service deployed to the group is always running. Further, Open Horizon also ensures that when services are upgraded, the upgrade will be rolled across all the members of a node group such that at least 1 copy of the service is always running. One of the problems with the existing HA Group support is that it is not dynamic. For example, nodes must be added to a group as part of registering them with the management hub. Nodes cannot be removed from a group. Nodes cannot be added to an existing group without unregistering the entire group and registering each node again with the new group members. Node registration is something that happens once for the lifetime of the node. A node should never need to be unregistered unless it is being decommissioned.
The design proposes to enhance the current concept of HA node groups by enabling organization administrators and node owners to create HA node groups at any time in the lifecycle of a node. Further, the design proposes to loosen the current restriction that all services deployed to a node in an HA group are deployed on all nodes in the group, enabling the use of heterogeneous node equipment within a group. Note that for the purposes of this design, the HA node group concept is intended to provide both HA and CA for the services running on those nodes. Following are
Agbot
The Agbot is responsible for ensuring that all deployment policies have been checked for compatibility against all nodes in the system, and making adjustments to the deployed state of services as node policy, deployment policy and service policy change over time. The Agbot already has support for ensuring that service upgrades are performed in a rolling fashion across an HA Group. The existing support will have to be augmented in the following ways:
- HA Group membership is obtained from the new /hagroup resource in the Exchange. The existing HA support obtains this info from an internal representation of the node (in the code it's called the producer policy, and the info is also saved in the Agbot's agreement object in the DB). The HA Group membership should be removed from this internal representation and obtained from the /hagroup resource. The /hagroup resources will also need to be added to the resource cache in the Agbot.
- The existing HA support assumes that ALL services running on a node in an HA Group are supposed to be running on ALL nodes in the group. This assumption is no longer true with this design. The Agbot needs to perform some additional checking (while managing a rolling upgrade) to understand which nodes in the group the service should be running on, and ensuring that it is always running on at least one of them. The Agbot MUST NOT assume that the service being upgraded is intended to be running on all nodes in the group. A service is intended to be running on a node if the node policy is compatible with the service's policy and all deployment policies that reference the service.
Exchange Changes API
When new resources are added to the system, the scope of change notification of those resources needs to be defined. Both the Agbot and the agent need to be aware of /hagroup resource creation/update/deletion.
Agent Upgrades
In addition to rolling upgrade support for services, agent upgrades also need to be performed in a rolling fashion across all the nodes in an HA node group. Agents are responsible for autonomously upgrading based on node management policy (NMP) as defined by the administrator, therefore there is no central entity that is able to coordinate across agents within a group. The only entity in the system capable of assisting with the coordination is the Agbot.
To create, modify and delete HA Groups, use the following commands:
hzn exchange hagroup create <name> --nodeId node1 --nodeId node2 [ --nodeId node3 ]
hzn exchange hagroup update <name> --nodeId node1 --nodeId node2 [ --nodeId node3 ]
hzn exchange hagroup delete <name>
hzn exchange hagroup list [ <name> ]
To check if a service will be deployed to all nodes in an HA Group, we need a new flag on the command. This flag will cause the command to retrieve the list of nodes in the HA group and compare them all against the policy inputs:
hzn deploycheck --checkHA
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Agent - Use of the Agbot API to direct agent upgrades.
Agbot - Awareness of a node's HA Group membership for making agreements, and an API for tracking rolling agent upgrades.
CLI - To list, add, remove nodes from an HA Group.
Exchange - To hold the new HA Group membership API.
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Exchange APIs
The following new APIs are introduced in this design. Any user in an org can use these APIs (or corresponding CLI). Org users can only create/modify/delete HA groups containing nodes that the user has permission to modify.
The HA Group object schema:
{
"name": "hagroup name",
"members": [ "node1234", ... ],
"updated": <update time stamp>
}Create a new node group. The caller must have permission to modify all the nodes listed in the body (shown above). The Exchange will set a reference to this object onto all the node resources listed in the body. The Exchange will return an error (409) if one of the nodes is already in an hagroup. To remove a node from a group, use the PUT API to provide the list of nodes that should be in the group.
POST /org/<org>/hagroups
Modify the group membership of an existing group. All the desired members of the group MUST be listed in the body. This API behaves like a full replace.
PUT /org/<org>/hagroups
List all the hagroups.
GET /org/<org>/hagroups
List all members of an hagroup.
GET /org/<org>/hagroups/<name>
Delete an hagroup.
DELETE /org/<org>/hagroups/<name>
Node Resource:
In addition to the new hagroup resource, the node resource is also extended with a new field called "ha_group" that contains a reference to the HA group in which this node is a member. This field is updated for all nodes in an HA group when a new HA group is created, updated or deleted. Updates to this field are atomic with updates to the hagroup resource.
Agbot APIs
To tell the Agbot that the node is ready to upgrade the agent.
POST /org/<org>/node/<node-id>/upgrade
200 - yes, go ahead with the upgrade
409 - no, another node is upgrading
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- Need an Overview of how HA Groups work on the OH doc site. Hopefully the material from this doc can be used for that content.
- Need a new article describing how to use HA Groups, this would be focused toward the administrator and node owners. It could be the same article for both roles (but we might change our mind on this AFTER we have tried to write it). This article would show the use of the CLI and probably need to include or refer to a CLI reference section.
- Update/remove HA doc in the anax repo for the HA /attribute API. Support for this capability is being removed.
- Edge Clusters are out of scope for this support. Edge clusters already natively support HA and CA, and therefore don't need any special assistance from OpenHorizon.
- Test service lifecycles with services deployed to HA groups that are homogeneous (all members have the same services) and heterogeneous (there is at least 1 service common to all members, but some services are only running on a subset of members).
- Performance test of service upgrades with and without HA groups in the system.