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+# Migrating from 0.3 to 0.4
+
+**Migrating from 0.3 to 0.4 is unsupported. This document is only intended to
+document the process internally for the Deuxfleurs cluster where we have to do
+it. Do not try it yourself, you will lose your data and we will not help you.**
+
+**Migrating from 0.2 to 0.4 will break everything for sure. Never try it.**
+
+The internal data format of Garage hasn't changed much between 0.3 and 0.4.
+The Sled database is still the same, and the data directory as well.
+
+The following has changed, all in the meta directory:
+
+- `node_id` in 0.3 contains the identifier of the current node. In 0.4, this
+  file does nothing and should be deleted. It is replaced by `node_key` (the
+  secret key) and `node_key.pub` (the associated public key). A node's
+  identifier on the ring is its public key.
+
+- `peer_info` in 0.3 contains the list of peers saved automatically by Garage.
+  The format has changed and it is now stored in `peer_list` (`peer_info`
+  should be deleted).
+
+When migrating, all node identifiers will change. This also means that the
+affectation of data partitions on the ring will change, and lots of data will
+have to be rebalanced.
+
+- If your cluster has only 3 nodes, all nodes store everything, therefore nothing has to be rebalanced.
+
+- If your cluster has only 4 nodes, for any partition there will always be at
+  least 2 nodes that stored data before that still store it after. Therefore
+  the migration should in theory be transparent and Garage should continue to
+  work during the rebalance.
+
+- If your cluster has 5 or more nodes, data will disappear during the
+  migration. Do not migrate (fortunately we don't have this scenario at
+  Deuxfleurs), or if you do, make Garage unavailable until things stabilize
+  (disable web and api access).
+
+
+The migration steps are as follows:
+
+1. Prepare a new configuration file for 0.4. For each node, point to the same
+   meta and data directories as Garage 0.3. Basically, the things that change
+   are the following:
+
+  - No more `rpc_tls` section
+  - You have to generate a shared `rpc_secret` and put it in all config files
+  - `bootstrap_peers` has a different syntax as it has to contain node keys.
+    Leave it empty and use `garage node-id` and `garage node connect` instead (new features of 0.4)
+  - put the publicly accessible RPC address of your node in `rpc_public_addr` if possible (its optional but recommended)
+  - If you are using Consul, change the `consul_service_name` to NOT be the name advertised by Nomad.
+    Now Garage is responsible for advertising its own service itself.
+
+2. Disable api and web access for some time (Garage does not support disabling
+   these endpoints but you can change the port number or stop your reverse
+   proxy for instance).
+
+3. Do `garage repair -a --yes tables` and `garage repair -a --yes blocks`,
+   check the logs and check that all data seems to be synced correctly between
+   nodes.
+
+4. Save somewhere the output of `garage status`. We will need this to remember
+   how to reconfigure nodes in 0.4.
+
+5. Turn off Garage 0.3
+
+6. Backup metadata folders if you can (i.e. if you have space to do it
+   somewhere). Backuping data folders could also be usefull but that's much
+   harder to do. If your filesystem supports snapshots, this could be a good
+   time to use them.
+
+7. Turn on Garage 0.4
+
+8. At this point, running `garage status` should indicate that all nodes of the
+   previous cluster are "unavailable". The nodes have new identifiers that
+   should appear in healthy nodes once they can talk to one another (use
+   `garage node connect` if necessary`). They should have NO ROLE ASSIGNED at
+   the moment.
+
+9. Prepare a script with several `garage node configure` commands that replace
+   each of the v0.3 node ID with the corresponding v0.4 node ID, with the same
+   zone/tag/capacity. For example if your node `drosera` had identifier `c24e`
+   before and now has identifier `789a`, and it was configured with capacity
+   `2` in zone `dc1`, put the following command in your script:
+
+```bash
+garage node configure 789a -z dc1 -c 2 -t drosera --replace c24e
+```
+
+10. Run your reconfiguration script. Check that the new output of `garage
+	status` contains the correct node IDs with the correct values for capacity
+	and zone. Old nodes should no longer be mentioned.
+
+11. If your cluster has 4 nodes or less, and you are feeling adventurous, you
+	can reenable Web and API access now. Things will probably work.
+
+12. Garage might already be resyncing stuff. Issue a `garage repair -a --yes
+	tables` and `garage repair -a --yes blocks` to force it to do so.
+
+13. Wait for resyncing activity to stop in the logs. Do steps 12 and 13 two or
+	three times, until you see that when you issue the repair commands, nothing
+	gets resynced any longer.
+
+14. Your upgraded cluster should be in a working state. Re-enable API and Web
+	access and check that everything went well.