Add `explain` flag support to the reroute API
By specifying the `explain` flag, an explanation for the reason a
command can or cannot be executed is returned. No allocation commands
are actually performed.
Returns a response similar to:
{
"state": {...cluster state...},
"acknowledged": true,
"explanations" : [ {
"command" : "cancel",
"parameters" : {
"index" : "decide",
"shard" : 0,
"node" : "IvpoKRdtRiGrQ_WKtt4_4w",
"allow_primary" : false
},
"decisions" : [ {
"decider" : "cancel_allocation_command",
"decision" : "YES",
"explanation" : "..."
} ]
}, {
"command" : "move",
"parameters" : {
"index" : "decide",
"shard" : 0,
"from_node" : "IvpoKRdtRiGrQ_WKtt4_4w",
"to_node" : "IvpoKRdtRiGrQ_WKtt4_4w"
},
"decisions" : [ {
"decider" : "same_shard",
"decision" : "NO",
"explanation" : "shard cannot be allocated on same node [IvpoKRdtRiGrQ_WKtt4_4w] it already exists on"
},
etc
]
}]
}
also removes AllocationExplanation from cluster state
Closes #2483
Closes #5169
e53a43800e