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You can use nats-top to monitor in realtime NATS server connections and message statistics.
Prerequisites
​Set up your Go environment​
​Install the NATS server​
1. Install nats-top
go install github.com/nats-io/nats-top
You may need to run the following instead:
sudo -E go install github.com/nats-io/nats-top
2. Start the NATS server with monitoring enabled
nats-server -m 8222
3. Start nats-top
nats-top
Result:
nats-server version 0.6.6 (uptime: 2m2s)
Server:
  Load: CPU:  0.0%  Memory: 6.3M  Slow Consumers: 0
  In:   Msgs: 0  Bytes: 0  Msgs/Sec: 0.0  Bytes/Sec: 0
  Out:  Msgs: 0  Bytes: 0  Msgs/Sec: 0.0  Bytes/Sec: 0
​
Connections: 0
  HOST                 CID      SUBS    PENDING     MSGS_TO     MSGS_FROM   BYTES_TO    BYTES_FROM  LANG     VERSION
4. Run NATS client programs
Run some NATS client programs and exchange messages.
For the best experience, you will want to run multiple subscribers, at least 2 or 3. Refer to the example pub-sub clients.
5. Check nats-top for statistics
nats-server version 0.6.6 (uptime: 30m51s)
Server:
  Load: CPU:  0.0%  Memory: 10.3M  Slow Consumers: 0
  In:   Msgs: 56  Bytes: 302  Msgs/Sec: 0.0  Bytes/Sec: 0
  Out:  Msgs: 98  Bytes: 512  Msgs/Sec: 0.0  Bytes/Sec: 0
​
Connections: 3
  HOST                 CID      SUBS    PENDING     MSGS_TO     MSGS_FROM   BYTES_TO    BYTES_FROM  LANG     VERSION
  ::1:58651            6        1       0           52          0           260         0           go       1.1.0
  ::1:58922            38       1       0           21          0           105         0           go       1.1.0
  ::1:58953            39       1       0           21          0           105         0           go       1.1.0
6. Sort nats-top statistics
In nats-top, enter the command o followed by the option, such as bytes_to. You see that nats-top sorts the BYTES_TO column in ascending order.
nats-server version 0.6.6 (uptime: 45m40s)
Server:
  Load: CPU:  0.0%  Memory: 10.4M  Slow Consumers: 0
  In:   Msgs: 81  Bytes: 427  Msgs/Sec: 0.0  Bytes/Sec: 0
  Out:  Msgs: 154  Bytes: 792  Msgs/Sec: 0.0  Bytes/Sec: 0
sort by [bytes_to]:
Connections: 3
  HOST                 CID      SUBS    PENDING     MSGS_TO     MSGS_FROM   BYTES_TO    BYTES_FROM  LANG     VERSION
  ::1:59259            83       1       0           4           0           20          0           go       1.1.0
  ::1:59349            91       1       0           2           0           10          0           go       1.1.0
  ::1:59342            90       1       0           0           0           0           0           go       1.1.0
7. Use different sort options
Use some different sort options to explore nats-top, such as:
cid, subs, pending, msgs_to, msgs_from, bytes_to, bytes_from, lang, version
You can also set the sort option on the command line using the -sort flag. For example: nats-top -sort bytes_to.
8. Display the registered subscriptions.
In nats-top, enter the command s to toggle displaying connection subscriptions. When enabled, you see the subscription subject in nats-top table:
nats-server version 0.6.6 (uptime: 1h2m23s)
Server:
  Load: CPU:  0.0%  Memory: 10.4M  Slow Consumers: 0
  In:   Msgs: 108  Bytes: 643  Msgs/Sec: 0.0  Bytes/Sec: 0
  Out:  Msgs: 185  Bytes: 1.0K  Msgs/Sec: 0.0  Bytes/Sec: 0
​
Connections: 3
  HOST                 CID      SUBS    PENDING     MSGS_TO     MSGS_FROM   BYTES_TO    BYTES_FROM  LANG     VERSION SUBSCRIPTIONS
  ::1:59708            115      1       0           6           0           48          0           go       1.1.0   foo.bar
  ::1:59758            122      1       0           1           0           8           0           go       1.1.0   foo
  ::1:59817            124      1       0           0           0           0           0           go       1.1.0   foo
9. Quit nats-top
Use the q command to quit nats-top.
10. Restart nats-top with a specified query
For example, to query for the connection with largest number of subscriptions:
nats-top -n 1 -sort subs
Result: nats-top displays only the client connection with the largest number of subscriptions:
nats-server version 0.6.6 (uptime: 1h7m0s)
Server:
  Load: CPU:  0.0%  Memory: 10.4M  Slow Consumers: 0
  In:   Msgs: 109  Bytes: 651  Msgs/Sec: 0.0  Bytes/Sec: 0
  Out:  Msgs: 187  Bytes: 1.0K  Msgs/Sec: 0.0  Bytes/Sec: 0
​
Connections: 3
  HOST                 CID      SUBS    PENDING     MSGS_TO     MSGS_FROM   BYTES_TO    BYTES_FROM  LANG     VERSION
  ::1:59708            115      1       0           6           0           48          0           go       1.1.0
nats-top
Next - Using NATS
Developing With NATS
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Outline
Prerequisites
1. Install nats-top
2. Start the NATS server with monitoring enabled
3. Start nats-top
4. Run NATS client programs
5. Check nats-top for statistics
6. Sort nats-top statistics
7. Use different sort options
8. Display the registered subscriptions.
9. Quit nats-top
10. Restart nats-top with a specified query