Berkeley DB Reference Guide: Master Leases

Berkeley DB Reference Guide:
Berkeley DB Replication

Master Leases

Some applications have strict requirements about the consistency of data read on a master site. Berkeley DB provides a mechanism called master leases to provide such consistency. Without master leases, it is sometimes possible for Berkeley DB to return old data to an application when newer data is available due to unfortunate scheduling as illustrated below:

Application on master site: Read data item foo via Berkeley DB DB->get or DBcursor->get call.
Application on master site: sleep, get descheduled, etc.
System: Master changes role, becomes a client.
System: New site is elected master.
System: New master modifies data item foo.
Application: Berkeley DB returns old data for foo to application.

By using master leases, Berkeley DB can provide guarantees about the consistency of data read on a master site. The master site can be considered a recognized authority for the data and consequently can provide authoritative reads. Clients grant master leases to a master site. By doing so, clients acknowledge the right of that site to retain the role of master for a period of time. During that period of time, clients cannot elect a new master, become master, nor grant their lease to another site.

By holding a collection of granted leases, a master site can guarantee to the application that the data returned is the current, authoritative value. As a master performs operations, it continually requests updated grants from the clients. When a read operation is required, the master guarantees that it holds a valid collection of lease grants from clients before returning data to the application. By holding leases, Berkeley DB provides several guarantees to the application:

Authoritative reads: A guarantee that the data being read by the application is the current value.
Durability from rollback: A guarantee that the data being written or read by the application is permanent across a majority of client sites and will never be rolled back.
The rollback guarantee also depends on the DB_TXN_NOSYNC flag. The guarantee is effective as long as there isn't total replication group failure while clients have granted leases but are holding the updates in their cache. The application must weigh the performance impact of synchronous transactions against the risk of total replication group failure. If clients grant a lease while holding updated data in cache, and total failure occurs, then the data is no longer present on the clients and rollback can occur if the master also crashes.

The guarantee that data will not be rolled back applies only to data successfully committed on a master. Data read on a client, or read while ignoring leases can be rolled back.
Freshness: A guarantee that the data being read by the application on the master is up-to-date and has not been modified or removed during the read.
The read authority is only on the master. Read operations on a client always ignore leases and consequently, that operation can return stale data.
Master viability: A guarantee that a current master with valid leases cannot encounter a duplicate master situation.
Leases remove the possibility of a duplicate master situation that forces the current master to downgrade to a client. However, it is still possible that old masters with expired leases can discover a later master and return DB_REP_DUPMASTER to the application.

There are several requirements of the application using leases:

Replication Manager applications must configure a majority (or larger) acknowledgement policy via the DB_ENV->repmgr_set_ack_policy method. Base API users must implement and enforce such a policy on their own.
Base API users must return an error from the send callback function when the majority acknowledgement policy is not met for permanent records marked with DB_REP_PERMANENT. Note that the Replication Manager automatically fulfills this requirement.
Applications must set the number of sites in the group using the DB_ENV->rep_set_nsites method before starting replication and cannot change it during operation.
Using leases in a replication group is all or none. Behavior is undefined when some sites configure leases and others do not. Use the DB_ENV->rep_set_config method to turn on leases.
The configured lease timeout value must be the same on all sites in a replication group, set via the DB_ENV->rep_set_timeout method.
The configured clock_scale_factor value must be the same on all sites in a replication group. This value defaults to no skew, but can be set via the DB_ENV->rep_set_clockskew method.
Applications that care about read guarantees must perform all read operations on the master. Reading on a client does not guarantee freshness.
The application must use elections to choose a master site. It must never simply declare a master without having won an election (as is allowed without Master Leases).

Master leases are based on timeouts. Berkeley DB assumes that time always runs forward. Users who change the system clock on either client or master sites when leases are in use void all guarantees and can get undefined behavior. See the DB_ENV->rep_set_timeout method for more information.

Read operations on a master that should not be subject to leases can use the DB_IGNORE_LEASE flag to the DB->get method or the DBcursor->get method. Read operations on a client always imply leases are ignored.

Clients are forbidden from participating in elections while they have an outstanding lease granted to a master. Therefore, if the DB_ENV->rep_elect method is called, then Berkeley DB will block, waiting until its lease grant expires before participating in any election. While it waits, the client attempts to contact the current master. If the client finds a current master, then it returns from the DB_ENV->rep_elect method. When leases are configured and the lease has never yet been granted (on start-up), clients must wait a full lease timeout before participating in an election.