Pub/sub messages

Messages are pieces of information, such as business events taking place in an integrated environment, that endpoints exchange in the publish/subscribe architecture.

Each message may represent a single occurrence of a specific change of state - for instance, an update to a user's phone number which is information that may need to be delivered to multiple systems and services that keep track of the user's current state. On the other hand, an IoT system may want to publish a steady stream of events, e.g. continually broadcasting its current state to interested subscribers. Yet another example is a request-response scenario where the response is expected to arrive after a relatively longer time, e.g. after minutes, hours or days, which implies that the requesting application should not wait for the response in a blocking manner.

All messages exchanged through Zato publish/subscribe are always wrapped in JSON or Python dicts - this conveniently allows any kind of an application to send or receive them, regardless of whether they are implemented in Python, Java, JavaScript or any other technology.

Because all REST pub/sub message use JSON, it is possible to use standard and common tools, such as curl or Postman, during the development or testing.

Data of any type can be sent embedded in JSON, e.g. it is possible to send CSV other types of information. There are no message size limits although, typically, each message will not be bigger than a few dozen kilobytes.

Each message is composed of the actual business data as well as metadata - a set of attributes that clarify, refine and define additional routing or delivery options. No matter if it is REST, Python or other endpoint sending or receiving messages, all message attributes are always consistently named the same, e.g. a subscription key is always "sub_key" and the actual data is always in an attribute called "data".

The following is a discussion of each of the attributes that describe each pub/sub message. Some of the attributes are writable - they can be provided on input, during publication, whereas some are populated by Zato and they become only available to recipients. If an attribute is optional, it means that either such a key may not exist or it may contain an empty value.

All the attributes are discussed using the following sample message that has been already received by a subscriber.

{
 "data":"This is a sample message",
 "topic_name":"/zato/demo/sample",
 "msg_id":"zpsm3a91d3e76a325b60700c1c66",
 "correl_id":"abcdef123456",
 "in_reply_to":"",
 "priority":5,
 "mime_type":"application/json",
 "ext_client_id":"MyApp.1",
 "pub_time_iso":"2023-04-03T15:59:04.760628",
 "ext_pub_time_iso":"",
 "recv_time_iso":"2023-04-03T15:59:04.941140",
 "expiration":86400,
 "expiration_time_iso":"2023-04-04T15:59:04.760628",
 "size":24,
 "delivery_count":7,
 "sub_key":"zpsk.rest.4fed61",
}