## TL;DR
After a restart, shards stay unassigned while the cluster waits (delayed allocation) or because allocation is genuinely blocked. Check `_cluster/allocation/explain` for the reason: wait it out, fix the blocker, or reroute. Shards assign once the decider is satisfied.

## Error
```text
opensearch: unassigned shards after node restart
```

## Steps
1. Check cluster health and the pending tasks: confirm the restart finished and the cluster is otherwise green or yellow. Expected: the scope of the problem is just the unassigned shards.
2. Ask the decider: call the allocation explain API for an unassigned shard. Expected: the exact reason (delayed allocation timer, disk decider, awareness, no valid node).
3. If it is delayed allocation, wait: the timer exists to let the node rejoin without shuffling data. Expected: shards reassign when the timer expires.
4. If a decider blocks (disk, awareness, allocation disabled), fix the underlying condition. Expected: the decider permits allocation.
5. As a last resort, use the reroute API to manually assign, understanding the data-safety tradeoff. Expected: shards assigned, cluster green.

## When to use
- Yellow or red cluster with unassigned shards after restarts
- Planned node maintenance
- Understanding allocation decisions

## When not to use
- Read-only blocks from disk watermarks (different fix)
- Brand-new indices with no replicas configured (expected state)

## Tool compatibility
- OpenSearch `_cluster/allocation/explain`, reroute API; allocation deciders

## Variant phrasings
### shards unassigned delayed allocation
Wait for the timer; it avoids pointless data movement when the node is coming back.

### allocation explain no valid node
Fix the decider's complaint (disk, zone awareness) rather than forcing assignment.

## Why it happens
OpenSearch is cautious about moving shard copies: delayed allocation avoids re-replicating data for a node that is briefly gone, and deciders veto placements that would violate disk, awareness, or filtering rules. Unassigned is the safe default until a placement is genuinely valid.

## Provenance

Resolved from the public thread: https://vectle.com/posts/pst_oHWnp_1x7XlMDDezZdfwZA