This assumes you have created a RabbitMQ user with the administrator tag.

Refer to the RabbitMQ REST API documentation.

Let's say there is a policy named foo.policy in virtual host vhost001. Here is how you would return the JSON of the policy using the REST API.

Or to list a policy in a virtual host matching a policy name.

curl
--request GET
--user john.doe:itsasecret
--url http://hostname:15671/api/policies/vhost001/foo.policy

If no matches are found, something like this should be returned.

{
 'reason' => 'Not Found',
 'error' => 'Object Not Found'
}

On the other hand, if the policy exists, something like this should be returned. Let's say you want to update the max-lenght-bytes value from 10000000 to 50000000.

{
  'priority' => 0,
  'pattern' => '^foo.queue',
  'vhost' => 'foo',
  'name' => 'deadletter.foo.queue.policy',
  'apply-to' => 'queues',
  'definition' => {
    'dead-letter-exchange' => 'deadletter.foo.queue.exchange',
    'max-length-bytes' => 10000000,
    'dead-letter-routing-key' => '*',
    'overflow' => 'reject-publish-dlx'
  }
}

Updating the policy is exactly the same as creating the policy. Here is how you would update the policy max-lenght-bytes to 50000000.

curl
--request PUT
--user john.doe:itsasecret
--url http://hostname:15671/api/policies/vhost001/deadletter.policy
--header 'content-type: application/json'
--data '{ "pattern": "foo.queue", "apply-to": "queues", "priority": 0, "definition": {"dead-letter-exchange": "deadletter.exchange", "max-length-bytes": 50000000, "overflow": "reject-publish-dlx" } }'
--write-out "%{http_code}"

No output will be returned, regardless if the policy is or is not update. For this reason, the -w or --write-out option is included to get the return code of the curl command.