数据复制

Replication allows you to move data between streams in either a 1:1 mirror style or by multiplexing multiple source streams into a new stream. In future builds this will allow data to be replicated between accounts as well, ideal for sending data from a Leafnode into a central store.

Here we have 2 main streams - ORDERS and RETURNS - these streams are clustered across 3 nodes. These Streams have short retention periods and are memory based.

We create a ARCHIVE stream that has 2 sources set, the ARCHIVE will pull data from the sources into itself. This stream has a very long retention period and is file based and replicated across 3 nodes. Additional messages can be added to the ARCHIVE by sending to it directly.

Finally, we create a REPORT stream mirrored from ARCHIVE that is not clustered and retains data for a month. The REPORT Stream does not listen for any incoming messages, it can only consume data from ARCHIVE.

Mirrors

A mirror copies data from 1 other stream, as far as possible IDs and ordering will match exactly the source. A mirror does not listen on a subject for any data to be added. A mirror can filter by subject and the Start Sequence and Start Time can be set. A stream can only have 1 mirror and if it is a mirror it cannot also have any source.

Sources

A source is a stream where data is copied from, one stream can have multiple sources and will read data in from them all. The stream will also listen for messages on it's own subject. We can therefore not maintain absolute ordering, but data from 1 single source will be in the correct order but mixed in with other streams. You might also find the timestamps of streams can be older and newer mixed in together as a result.

A Stream with sources may also listen on subjects, but could have no listening subject. When using the nats CLI to create sourced streams use --subjects to supply subjects to listen on.

A source can have Start Time or Start Sequence and can filter by a subject.

Configuration

The ORDERS and RETURNS streams as normal, I will not show how to create them.

nats s report

Obtaining Stream stats

+---------+---------+-----------+----------+-------+------+---------+----------------------+
| Stream  | Storage | Consumers | Messages | Bytes | Lost | Deleted | Cluster              |
+---------+---------+-----------+----------+-------+------+---------+----------------------+
| ORDERS  | Memory  | 0         | 0        | 0 B   | 0    | 0       | n1-c2, n2-c2*, n3-c2 |
| RETURNS | Memory  | 0         | 0        | 0 B   | 0    | 0       | n1-c2*, n2-c2, n3-c2 |
+---------+---------+-----------+----------+-------+------+---------+----------------------+

We now add the ARCHIVE:

nats s add ARCHIVE --source ORDERS --source RETURNS

? Storage backend file
? Retention Policy Limits
? Discard Policy Old
? Stream Messages Limit -1
? Message size limit -1
? Maximum message age limit -1
? Maximum individual message size -1
? Duplicate tracking time window 2m0s
? Allow message Roll-ups No
? Allow message deletion Yes
? Allow purging subjects or the entire stream Yes
? Replicas 1
? Adjust source "ORDERS" start Yes
? ORDERS Source Start Sequence 0
? ORDERS Source UTC Time Stamp (YYYY:MM:DD HH:MM:SS)
? ORDERS Source Filter source by subject
? Import "ORDERS" from a different JetStream domain No
? Import "ORDERS" from a different account No
? Adjust source "RETURNS" start No
? Import "RETURNS" from a different JetStream domain No
? Import "RETURNS" from a different account No
Stream ARCHIVE was created

Information for Stream ARCHIVE created 2022-01-21T11:49:52-08:00

Configuration:

     Acknowledgements: true
            Retention: File - Limits
             Replicas: 1
       Discard Policy: Old
     Duplicate Window: 2m0s
    Allows Msg Delete: true
         Allows Purge: true
       Allows Rollups: false
     Maximum Messages: unlimited
        Maximum Bytes: unlimited
          Maximum Age: unlimited
 Maximum Message Size: unlimited
    Maximum Consumers: unlimited
              Sources: ORDERS
                       RETURNS


State:

             Messages: 0
                Bytes: 0 B
             FirstSeq: 0
              LastSeq: 0
     Active Consumers: 0

And we add the REPORT:

nats s add REPORT --mirror ARCHIVE

? Storage backend file
? Retention Policy Limits
? Discard Policy Old
? Stream Messages Limit -1
? Message size limit -1
? Maximum message age limit -1
? Maximum individual message size -1
? Allow message Roll-ups No
? Allow message deletion Yes
? Allow purging subjects or the entire stream Yes
? Replicas 1
? Adjust mirror start No
? Import mirror from a different JetStream domain No
? Import mirror from a different account No
Stream REPORT was created

Information for Stream REPORT created 2022-01-21T11:50:55-08:00

Configuration:

     Acknowledgements: true
            Retention: File - Limits
             Replicas: 1
       Discard Policy: Old
     Duplicate Window: 2m0s
    Allows Msg Delete: true
         Allows Purge: true
       Allows Rollups: false
     Maximum Messages: unlimited
        Maximum Bytes: unlimited
          Maximum Age: unlimited
 Maximum Message Size: unlimited
    Maximum Consumers: unlimited
               Mirror: ARCHIVE


State:

             Messages: 0
                Bytes: 0 B
             FirstSeq: 0
              LastSeq: 0
     Active Consumers: 0

When configured we'll see some additional information in a nats stream info output:

nats stream info ARCHIVE

Output extract

...
Source Information:

          Stream Name: ORDERS
                  Lag: 0
            Last Seen: 2m23s

          Stream Name: RETURNS
                  Lag: 0
            Last Seen: 2m15s
...

$ nats stream info REPORT
...
Mirror Information:

          Stream Name: ARCHIVE
                  Lag: 0
            Last Seen: 2m35s
...

Here the Lag is how far behind we were reported as being last time we saw a message.

We can confirm all our setup using a nats stream report:

nats s report

+--------------------------------------------------------------------------------------------------------+
|                                            Stream Report                                               |
+---------+---------+-------------+-----------+----------+-------+------+---------+----------------------+
| Stream  | Storage | Replication | Consumers | Messages | Bytes | Lost | Deleted | Cluster              |
+---------+---------+-------------+-----------+----------+-------+------+---------+----------------------+
| ARCHIVE | File    | Sourced     | 1         | 0        | 0 B   | 0    | 0       | n1-c2*, n2-c2, n3-c2 |
| ORDERS  | Memory  |             | 1         | 0        | 0 B   | 0    | 0       | n1-c2, n2-c2*, n3-c2 |
| REPORT  | File    | Mirror      | 0         | 0        | 0 B   | 0    | 0       | n1-c2*               |
| RETURNS | Memory  |             | 1         | 0        | 0 B   | 0    | 0       | n1-c2, n2-c2, n3-c2* |
+---------+---------+-------------+-----------+----------+-------+------+---------+----------------------+

+---------------------------------------------------------+
|                   Replication Report                    |
+---------+--------+---------------+--------+-----+-------+
| Stream  | Kind   | Source Stream | Active | Lag | Error |
+---------+--------+---------------+--------+-----+-------+
| ARCHIVE | Source | ORDERS        | never  | 0   |       |
| ARCHIVE | Source | RETURNS       | never  | 0   |       |
| REPORT  | Mirror | ARCHIVE       | never  | 0   |       |
+---------+--------+---------------+--------+-----+-------+

We then create some data in both ORDERS and RETURNS:

nats req ORDERS.new "ORDER {{Count}}" --count 100
nats req RETURNS.new "RETURN {{Count}}" --count 100

We can now see from a Stream Report that the data has been replicated:

nats s report --dot replication.dot

Obtaining Stream stats

+---------+---------+-----------+----------+---------+------+---------+----------------------+
| Stream  | Storage | Consumers | Messages | Bytes   | Lost | Deleted | Cluster              |
+---------+---------+-----------+----------+---------+------+---------+----------------------+
| ORDERS  | Memory  | 1         | 100      | 3.3 KiB | 0    | 0       | n1-c2, n2-c2*, n3-c2 |
| RETURNS | Memory  | 1         | 100      | 3.5 KiB | 0    | 0       | n1-c2*, n2-c2, n3-c2 |
| ARCHIVE | File    | 1         | 200      | 27 KiB  | 0    | 0       | n1-c2, n2-c2, n3-c2* |
| REPORT  | File    | 0         | 200      | 27 KiB  | 0    | 0       | n1-c2*               |
+---------+---------+-----------+----------+---------+------+---------+----------------------+

+---------------------------------------------------------+
|                   Replication Report                    |
+---------+--------+---------------+--------+-----+-------+
| Stream  | Kind   | Source Stream | Active | Lag | Error |
+---------+--------+---------------+--------+-----+-------+
| ARCHIVE | Source | ORDERS        | 14.48s | 0   |       |
| ARCHIVE | Source | RETURNS       | 9.83s  | 0   |       |
| REPORT  | Mirror | ARCHIVE       | 9.82s  | 0   |       |
+---------+--------+---------------+--------+-----+-------+

Here we also pass the --dot replication.dot argument that writes a GraphViz format map of the replication setup.