Conflict Management - Essay

Conflict Management
Suppose you are sitting in a coffee shop working on your book. J. Chris comes over and tells you about his new phone. The new phone came with a new number, and you have J. Chris dictate it while you change it using your laptop’s address book application.

Luckily, your address book is built on CouchDB, so when you come home, all you need to do to get your home computer up-to-date with J. Chris’s number is replicate your address book from your laptop. Neat, eh? What’s more, CouchDB has a mechanism to maintain continuous replication, so you can keep a whole set of computers in sync with the same data, whenever a network connection is available.

Let’s change the scenario a little bit. Since J. Chris didn’t anticipate meeting you at the coffee shop, he also sent you an email with the new number. At the time you weren’t using WiFi because you wanted concentrate on your work, so you didn’t read his email until you got home. But it was a long day and by then you had forgotten that you changed the number in the address book on your laptop. When you read the email at home, you simply copy-and-pasted the number into the address book on your home computer. Now—and here’s the twist—it turns out you entered the wrong number in your laptop’s address book.

You now have a document in each of the databases that has different information. This situation is called a conflict. Conflicts occur in distributed systems. They are a natural state of your data. How does CouchDB’s replication system deal with conflicts?

When you replicate two databases in CouchDB and you have conflicting changes, CouchDB will detect this and will flag the affected document with the special attribute "_conflicts":true. Next, CouchDB determines which of the changes will be stored as the latest revision (remember, documents in CouchDB are versioned). The version that gets picked to be the latest revision is the winning revision. The losing revision gets stored as the previous...