Eventual Consistency
Here’s my attempt at describing eventual consistency. If I send an update to one system, then a minute later send that update to another system, my system will be inconsistent for that minute. Not quite. In my system there’s only one application that creates new data. Think of that application as the author of a novel. Updates to the novel are sent to other applications in the system. Applications receive updates at different times. Any application reading page 42 of the novel has read the same 42 pages that any other application has or will read. When at page 42, that application’s understanding of the novel is consistent with all other applications’ understanding when they’re 42 pages into the novel.