Practical Microservices

Writing write

Let’s start by examining the write_message^[31] function signature that the Eventide schema provides:

CREATE OR REPLACE FUNCTION write_message(
  _id varchar,
  _stream_name varchar,
  _type varchar,
  _data jsonb,
  _metadata jsonb DEFAULT NULL,
  _expected_version bigint DEFAULT NULL
)

This is a user-defined function and is the write interface that Message DB gives us. From this, we can tell that when writing a message, we supply the id, streamName, type, data, and metadata properties. The last parameter this function receives, _expected_version, is used when we’re implementing optimistic concurrency control later in this chapter under ​Adding Optimistic Concurrency Control to Our Writes​. Our job then ...