Add a proposal for multiple cluster managers

Signed-off-by: David Scott <[email protected]>

Author: djs55

xapi/design/multiple-cluster-managers.md

@@ -0,0 +1,63 @@
+---
+title: multiple cluster managers
+layout: default
+design_doc: true
+revision: 1
+status: proposed
+---
+
+Xapi currently uses a cluster manager called
+[xhad](../../features/HA/HA.html). Sometimes other software comes with its own
+built-in way of managing clusters, which would clash with xhad (example:
+xhad could choose to fence node 'a' while the other system could fence node
+'b' resulting in a total failure). To integrate xapi with this other software
+we have 2 choices:
+
+1. modify the other software to take membership information from xapi; or
+2. modify xapi to take membership information from this other software.
+
+This document proposes a way to modify xapi to take membership information from
+external software.
+
+API changes
+===========
+
+- New RO field `Pool.ha_cluster_stack` of type string which will be set to 'xhad'
+  on upgrade.
+- New parameter to `Pool.enable_ha` called `cluster_stack` of type string which
+  will have the default value of empty string (meaning: let the implementation
+	choose).
+
+Since other software will require specific `cluster_stack` settings we will
+define new exceptions:
+
+- `UNKNOWN_CLUSTER_STACK`: the operation cannot be performed because the
+  requested cluster stack does not exist. The user should check the
+	type was entered correctly and, failing that, check to see if the software
+	is installed. The exception will have a single parameter: the name of the
+	cluster stack which was not found.
+- `INCOMPATIBLE_CLUSTER_STACK_ACTIVE`: the operation cannot be performed because an
+  incompatible cluster stack is active. The single parameter will be the name
+	of the required cluster stack. This could happen (or example) if you tried to
+	create an OCFS2 SR with XenServer HA already enabled.
+- `CLUSTER_STACK_CONSTRAINT`: HA cannot be enabled with the provided cluster
+  stack because some third-party software is already active which requires
+	a different cluster stack setting. The two parameters are: a reference to
+	an object (such as an SR) which has created the restriction, and the name
+	of the cluster stack that this object requires.
+
+Implementation
+==============
+
+The `xapi.conf` file will have a new field: `cluster-stack-root` which will
+have the default value `/usr/libexec/xapi/cluster-stack`. The existing `xhad`
+scripts and tools will be moved to `/usr/libexec/xapi/cluster-stack/xhad/`.
+A hypothetical cluster stack called `foo` would be placed in
+`/usr/libexec/xapi/cluster-stack/foo/`.
+
+In `Pool.enable_ha` with `cluster_stack="foo"` we will verify that the
+subdirectory `<cluster-stack-root>/foo` exists. If it does not exist, then
+the call will fail with `UNKNOWN_CLUSTER_STACK`.
+
+Alternative cluster stacks will need to conform to the exact same interface
+as [xhad](../../features/HA/HA.md).