Once a JetStream cluster is operating interactions with the CLI and with nats CLI is the same as before. For these examples, lets assume we have a 5 server cluster, n1-n5 in a cluster named C1.
Within an account there are operations and reports that show where users data is placed and which allow them some basic interactions with the RAFT system.
When adding a stream using the nats CLI the number of replicas will be asked, when you choose a number more than 1, (we suggest 1, 3 or 5), the data will be stored o multiple nodes in your cluster using the RAFT protocol as above. The replica count must be less than the maximum number of servers.
$ nats str add ORDERS --replicas 3
....
Information for Stream ORDERS created 2021-02-05T12:07:34+01:00
....
Configuration:
....
Replicas: 3
Cluster Information:
Name: C1
Leader: n1-c1
Replica: n4-c1, current, seen 0.07s ago
Replica: n3-c1, current, seen 0.07s ago
Above you can see that the cluster information will be reported in all cases where Stream info is shown such as after add or using nats stream info.
Here we have a stream in the NATS cluster C1, its current leader is a node n1-c1 and it has 2 followers - n4-c1 and n3-c1.
The current indicates that followers are up to date and have all the messages, here both cluster peers were seen very recently.
The replica count cannot be edited once configured.
Users can get overall statistics about their streams and also where these streams are placed:
$ nats stream report
Obtaining Stream stats
+----------+-----------+----------+--------+---------+------+---------+----------------------+----------+
| Stream | Consumers | Messages | Bytes | Storage | Lost | Deleted | Cluster | Template |
+----------+-----------+----------+--------+---------+------+---------+----------------------+----------+
| ORDERS | 4 | 0 | 0 B | File | 0 | 0 | n1-c1*, n2-c1, n3-c1 | |
| ORDERS_3 | 4 | 0 | 0 B | File | 0 | 0 | n1-c1*, n2-c1, n3-c1 | |
| ORDERS_4 | 4 | 0 | 0 B | File | 0 | 0 | n1-c1*, n2-c1, n3-c1 | |
| ORDERS_5 | 4 | 0 | 0 B | File | 0 | 0 | n1-c1, n2-c1, n3-c1* | |
| ORDERS_2 | 4 | 1,385 | 13 MiB | File | 0 | 1 | n1-c1, n2-c1, n3-c1* | |
| ORDERS_0 | 4 | 1,561 | 14 MiB | File | 0 | 0 | n1-c1, n2-c1*, n3-c1 | |
+----------+-----------+----------+--------+---------+------+---------+----------------------+----------+
Every RAFT group has a leader that's elected by the group when needed. Generally there is no reason to interfere with this process, but you might want to trigger a leader change at a convenient time. Leader elections will represent short interruptions to the stream so if you know you will work on a node later it might be worth moving leadership away from it ahead of time.
Moving leadership away from a node does not remove it from the cluster and does not prevent it from becoming a leader again, this is merely a triggered leader election.
$ nats stream cluster step-down ORDERS
14:32:17 Requesting leader step down of "n1-c1" in a 3 peer RAFT group
14:32:18 New leader elected "n4-c1"
Information for Stream ORDERS created 2021-02-05T12:07:34+01:00
...
Cluster Information:
Name: c1
Leader: n4-c1
Replica: n1-c1, current, seen 0.12s ago
Replica: n3-c1, current, seen 0.12s ago
The same is true for consumers, nats consumer cluster step-down ORDERS NEW.
Systems users can view state of the Meta Group - but not individual Stream or Consumers.
We have a high level report of cluster state:
$ nats server report jetstream --user system
+--------------------------------------------------------------------------------------------------+
| JetStream Summary |
+--------+---------+---------+-----------+----------+--------+--------+--------+---------+---------+
| Server | Cluster | Streams | Consumers | Messages | Bytes | Memory | File | API Req | API Err |
+--------+---------+---------+-----------+----------+--------+--------+--------+---------+---------+
| n3-c2 | c2 | 0 | 0 | 0 | 0 B | 0 B | 0 B | 1 | 0 |
| n3-c1 | c1 | 6 | 24 | 2,946 | 27 MiB | 0 B | 27 MiB | 3 | 0 |
| n2-c2 | c2 | 0 | 0 | 0 | 0 B | 0 B | 0 B | 3 | 0 |
| n1-c2 | c2 | 0 | 0 | 0 | 0 B | 0 B | 0 B | 14 | 2 |
| n2-c1 | c1 | 6 | 24 | 2,946 | 27 MiB | 0 B | 27 MiB | 15 | 0 |
| n1-c1* | c1 | 6 | 24 | 2,946 | 27 MiB | 0 B | 27 MiB | 31 | 0 |
+--------+---------+---------+-----------+----------+--------+--------+--------+---------+---------+
| | | 18 | 72 | 8,838 | 80 MiB | 0 B | 80 MiB | 67 | 2 |
+--------+---------+---------+-----------+----------+--------+--------+--------+---------+---------+
+---------------------------------------------------+
| RAFT Meta Group Information |
+-------+--------+---------+---------+--------+-----+
| Name | Leader | Current | Offline | Active | Lag |
+-------+--------+---------+---------+--------+-----+
| n1-c1 | yes | true | false | 0.00s | 0 |
| n1-c2 | | true | false | 0.05s | 0 |
| n2-c1 | | true | false | 0.05s | 0 |
| n2-c2 | | true | false | 0.05s | 0 |
| n3-c1 | | true | false | 0.05s | 0 |
| n3-c2 | | true | false | 0.05s | 0 |
+-------+--------+---------+---------+--------+-----+
This is a full cluster wide report, the report can be limited to a specific account using --account.
Here we see the distribution of streams, messages, api calls etc by across 2 super clusters and an overview of the RAFT meta group.
In the Meta Group report the server n2-c1 is not current and has not been seen for 9 seconds, it's also behind by 2 raft operations.
This report is built using raw data that can be obtained from the monitor port on the /jsz url, or over nats using:
$ nats server req jetstream --help
...
--name=NAME Limit to servers matching a server name
--host=HOST Limit to servers matching a server host name
--cluster=CLUSTER Limit to servers matching a cluster name
--tags=TAGS ... Limit to servers with these configured tags
--account=ACCOUNT Show statistics scoped to a specific account
--accounts Include details about accounts
--streams Include details about Streams
--consumer Include details about Consumers
--config Include details about configuration
--leader Request a response from the Meta-group leader only
--all Include accounts, streams, consumers and configuration
$ nats server req jetstream --leader
This will produce a wealth of raw information about the current state of your cluster - here requesting it from the leader only.
Similar to Streams and Consumers above the Meta Group allows leader stand down. The Meta Group is cluster wide and spans all accounts, therefore to manage the meta group you have to use a SYSTEM user.
$ nats server raft step-down --user system
17:44:24 Current leader: n2-c2
17:44:24 New leader: n1-c2
Generally when shutting down NATS, including using Lame Duck Mode, the cluster will notice this and continue to function. A 5 node cluster can withstand 2 nodes being down.
There might be a case though where you know a machine will never return, and you want to signal to JetStream that the machine will not return. This will remove it from the Stream in question and all it's Consumers.
After the node is removed the cluster will notice that the replica count is not honored anymore and will immediately pick a new node and start replicating data to it. The new node will be selected using the same placement rules as the existing stream.
$ nats s cluster peer-remove ORDERS
? Select a Peer n4-c1
14:38:50 Removing peer "n4-c1"
14:38:50 Requested removal of peer "n4-c1"
At this point the stream and all consumers will have removed n4-c1 from the group, they will all start new peer selection and data replication.
$ nats stream info ORDERS
....
Cluster Information:
Name: c1
Leader: n3-c1
Replica: n1-c1, current, seen 0.02s ago
Replica: n2-c1, outdated, seen 0.42s ago
We can see a new replica was picked, the stream is back to replication level of 3 and n4-c1 is not active any more in this Stream or any of its Consumers.