In Cloud Pub/Sub you can format your message data however you want, as long as it can be encoded into a string.  However, its free form nature introduces some problems.  What happens if your subscribers are expecting a JSON object and they get a string instead?  It would be ideal if you could define and enforce a specific format.  Well, you can do just that with a message schema.

A message schema creates an enforced contract between the publisher and subscribers.  It makes it much easier for multiple people and teams to share the same topics.  Everyone knows and understands the format that the message data must follow.  If the schema defines a JSON object, then you cannot publish a message containing just a string.

Now you can not assign a schema to a topic that has already been created.  You must create your schema first, and then create the topic that will use it.  This also means you can reuse schemas, and enforce them across multiple topics.  So it is possible to create a single, company-wide standard for all messages.

Schemas can be defined either using ​​the Apache Avro format (which uses JSON) or the Protocol Buffer format (which is Google’s method of serializing structured data).  Attempting to publish a message that does not validate against the set schema will produce an error and the operation will fail.

So now I want to show you how to create and enforce a message schema.

The first step is to create the schema.  I’m going to call it “schema-1”.  Next, I have two options.  I can use the Avro format.  Or I can use the Protocol Buffer format.  I am going to pick Avro, since most viewers probably are already familiar with JSON.

The default provided schema works, but I am going to change it slightly to make it easier to understand.  So you can see it has two fields.  The first is a name field which will contain a string.  The second is a user ID field which will contain a number.  Any message published will need to have these two fields.

To make sure that you created a valid schema, you can click on the “Validate” button here.

Now that I have a schema, I can create a topic.  I’ll call it “topic-4”.  Now I need to be sure to check “Use a schema”.  If I forget to do this, I am going to have to delete the topic and create another.  You cannot go back and add a schema later.

Now I need to pick the schema.  And I’ll leave the format as JSON.

So this will create the topic and it’s going to enforce the specified message format on all messages.  Any message I try to publish must validate against the schema.

Before I can test this, I need to create a subscription.  So let me do that.

Now let’s see what happens when I try to publish two messages.  The first message will be in a valid format.  The second message is going to be invalid.

As you can see, in order to successfully publish a message to this topic, it must validate against the provided schema.  Any subscriber can now assume all messages must fit this format.

So now you know how to create and use schemas.