Replication

Automatic Failover

Moped will automatically retry lost connections and attempt to detect dead connections before sending an operation. Note, that it will not retry individual operations! For example, these cases will work and not raise any exceptions:

session[:users].insert(name: "John")
# kill primary node and promote secondary
session[:users].insert(name: "John")
session[:users].find.count # => 2.0

# primary node drops our connection
session[:users].insert(name: "John")

However, you'll get an operation error in a case like:

# primary node goes down while reading the reply
session.with(safe: true)[:users].insert(name: "John")

And you'll get a connection error in a case like:

# primary node goes down, no new primary available yet
session[:users].insert(name: "John")

If your session is running with eventual consistency, read operations will never raise connection errors as long as any secondary or primary node is running. The only case where you'll see a connection failure is if a node goes down while attempting to retrieve more results from a cursor, because cursors are tied to individual nodes.

When two attempts to connect to a node fail, it will be marked as down. This removes it from the list of available nodes for :down_interval (default 30 seconds). Note that the :down_interval only applies to normal operations; that is, if you ask for a primary node and none is available, all nodes will be retried. Likewise, if you ask for a secondary node, and no secondary or primary node is available, all nodes will be retreied.

Node Discovery

The addresses you pass into your session are used as seeds for setting up replica set connections. After connection, each seed node will return a list of other known nodes which will be added to the set.

This information is cached according to the :refresh_interval option (default: 5 minutes). That means, e.g., that if you add a new node to your replica set, it should be represented in Moped within 5 minutes.