Subject Mapping and Traffic Shaping
Subject mapping is a very powerful feature of the NATS server, useful for canary deployments, A/B testing, chaos testing, and migrating to a new subject namespace.

Simple Mapping

The example of foo:bar is straightforward. All messages the server receives on subject foo are remapped and can be received by clients subscribed to bar.

Subject Token Reordering

Wildcard tokens may be referenced via lt;position>. For example, the first wildcard token is $1, the second is $2, etc. Referencing these tokens can allow for reordering.
For example with this mapping bar.*.*: baz.$2.$1, messages that were originally published to bar.a.b are remapped in the server to baz.b.a. Messages arriving at the server on would be mapped to, and so forth.

Weighted Mappings for A/B Testing or Canary Releases

Traffic can be split by percentage from one subject to multiple subjects. Here's an example for canary deployments, starting with version 1 of your service.
Applications would make requests of a service at myservice.requests. The responders doing the work of the server would subscribe to myservice.requests.v1. Your configuration would look like this:
myservice.requests: [
{ destination: myservice.requests.v1, weight: 100% }
All requests to myservice.requests will go to version 1 of your service.
When version 2 comes along, you'll want to test it with a canary deployment. Version 2 would subscribe to myservice.requests.v2. Launch instances of your service.
Update the configuration file to redirect some portion of the requests made to myservice.requests to version 2 of your service.
For example the configuration below means 98% of the requests will be sent to version 1 and 2% to version 2.
myservice.requests: [
{ destination: myservice.requests.v1, weight: 98% },
{ destination: myservice.requests.v2, weight: 2% }
Once you've determined Version 2 is stable you can switch 100% of the traffic over to it and you can then shutdown the version 1 instance of your service.

Traffic Shaping in Testing

Traffic shaping is also useful in testing. You might have a service that runs in QA that simulates failure scenarios which could receive 20% of the traffic to test the service requestor.
myservice.requests.*: [{ destination: myservice.requests.$1, weight: 80% }, { destination:$1, weight: 20% }

Artificial Loss

Alternatively, introduce loss into your system for chaos testing by mapping a percentage of traffic to the same subject. In this drastic example, 50% of the traffic published to foo.loss.a would be artificially dropped by the server.
foo.loss.>: [ { destination: foo.loss.>, weight: 50% } ]
You can both split and introduce loss for testing. Here, 90% of requests would go to your service, 8% would go to a service simulating failure conditions, and the unaccounted for 2% would simulate message loss.
myservice.requests: [{ destination: myservice.requests.v3, weight: 90% }, { destination:, weight: 8% }] the remaining 2% is "lost"
Last modified 10d ago