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Update upgrading_cluster.md

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nats_admin/upgrading_cluster.md
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@ -27,7 +27,7 @@ Let's fix that, by starting the second server:
```bash ```bash
nats-server -D -p 4333 -cluster nats://localhost:6333 -routes nats://localhost:6222,nats://localhost:6333 nats-server -D -p 4333 -cluster nats://localhost:6333 -routes nats://localhost:6222,nats://localhost:6333
``` ```
The second server was started on port 4333 with its cluster port on 6333. Otherwise the same as 'A.' The second server was started on port 4333 with its cluster port on 6333. Otherwise the same as 'A'.
Let's get one client, so we can observe it moving between servers as servers get removed: Let's get one client, so we can observe it moving between servers as servers get removed:
```bash ```bash
@ -36,8 +36,8 @@ nats-sub -s nats://localhost:4222 ">"
`nats-sub` is a subscriber sample included with all NATS clients. `nats-sub` subscribes to a subject and prints out any messages received. You can find the source code to the go version of `nats-sub` [here)(https://github.com/nats-io/nats.go/tree/master/examples). After starting the subscriber you should see a message on 'A' that a new client connected. `nats-sub` is a subscriber sample included with all NATS clients. `nats-sub` subscribes to a subject and prints out any messages received. You can find the source code to the go version of `nats-sub` [here)(https://github.com/nats-io/nats.go/tree/master/examples). After starting the subscriber you should see a message on 'A' that a new client connected.
We have two servers and a client. Time to simulate our rolling upgrade. But wait, before we upgrade 'A,' let's introduce a new server 'T.' Server 'T' will join the existing cluster while we perform the upgrade. Its sole purpose is to provide an additional place where clients can go other than 'A.' and ensure we don't end up with a single server serving all the clients after the upgrade procedure. Clients will randomly select a server when connecting unless a special option is provided that disables that functionality (usually called 'DontRandomize' or 'noRandomize'). You can read more about ["Avoiding the Thundering Herd"](/developer/reconnect/random.md/). We have two servers and a client. Time to simulate our rolling upgrade. But wait, before we upgrade 'A', let's introduce a new server 'C'. Server 'C' will join the existing cluster while we perform the upgrade. Its sole purpose is to provide an additional place where clients can go other than 'A' and ensure we don't end up with a single server serving all the clients after the upgrade procedure. Clients will randomly select a server when connecting unless a special option is provided that disables that functionality (usually called 'DontRandomize' or 'noRandomize'). You can read more about ["Avoiding the Thundering Herd"](/developer/reconnect/random.md/).
Suffice it to say that clients redistribute themselves about evenly between all servers in the cluster. In our case 1/2 of the clients on 'A' will jump over to 'B' and the remaining half to 'T.' Suffice it to say that clients redistribute themselves about evenly between all servers in the cluster. In our case 1/2 of the clients on 'A' will jump over to 'B' and the remaining half to 'C'.
Let's start our temporary server: Let's start our temporary server:
@ -45,9 +45,9 @@ Let's start our temporary server:
nats-server -D -p 4444 -cluster nats://localhost:6444 -routes nats://localhost:6222,nats://localhost:6333 nats-server -D -p 4444 -cluster nats://localhost:6444 -routes nats://localhost:6222,nats://localhost:6333
``` ```
After an instant or so, clients on 'A' learn of the new cluster member that joined. On our hands-on tutorial, `nats-sub` is now aware of 3 possible servers, 'A' (specified when we started the tool) and 'B' and 'T' learned from the cluster gossip. After an instant or so, clients on 'A' learn of the new cluster member that joined. On our hands-on tutorial, `nats-sub` is now aware of 3 possible servers, 'A' (specified when we started the tool) and 'B' and 'C' learned from the cluster gossip.
We invoke our admin powers and turn off 'A' by issuing a `CTRL+C` to the terminal on 'A,' and observe that either 'B' or 'T' reports that a new client connected. That is our `nats-sub` client. We invoke our admin powers and turn off 'A' by issuing a `CTRL+C` to the terminal on 'A' and observe that either 'B' or 'C' reports that a new client connected. That is our `nats-sub` client.
We perform the upgrade process, update the binary for 'A', and restart 'A': We perform the upgrade process, update the binary for 'A', and restart 'A':
@ -55,13 +55,13 @@ We perform the upgrade process, update the binary for 'A', and restart 'A':
nats-server -D -p 4222 -cluster nats://localhost:6222 -routes nats://localhost:6222,nats://localhost:6333 nats-server -D -p 4222 -cluster nats://localhost:6222 -routes nats://localhost:6222,nats://localhost:6333
``` ```
We move on to upgrade 'B'. Notice that clients from 'B' reconnect to 'A' and 'T'. We upgrade and restart 'B': We move on to upgrade 'B'. Notice that clients from 'B' reconnect to 'A' and 'C'. We upgrade and restart 'B':
```bash ```bash
nats-server -D -p 4333 -cluster nats://localhost:6333 -routes nats://localhost:6222,nats://localhost:6333 nats-server -D -p 4333 -cluster nats://localhost:6333 -routes nats://localhost:6222,nats://localhost:6333
``` ```
If we had more servers, we would continue the stop, update, restart rotation as we did for 'A' and 'B.' After restarting the last server, we can go ahead and turn off 'T.' Any clients on 'T' will redistribute to our permanent cluster members. If we had more servers, we would continue the stop, update, restart rotation as we did for 'A' and 'B'. After restarting the last server, we can go ahead and turn off 'C.' Any clients on 'C' will redistribute to our permanent cluster members.
### Seed Servers ### Seed Servers