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...and stores small files. Typically there is one Chubby instance, or cell, per data center. Several Google systems " such as the Google Filesystem (GFS) and Bigtable " use Chubby for distributed coordination and to store a small amount of metadata.

Chubby achieves fault-tolerance through replication. A typical Chubby cell consists of five replicas, running the same code, each running on a dedicated machine. Every Chubby object (e.g., a Chubby lock, or file) is stored as an entry in a database. It is this database that is replicated. At any one time, one of these replicas is considered to be the master.

Chubby clients (such as GFS and Bigtable) contact a Chubby cell for service. The master replica serves all Chubby requests. If a Chubby client contacts a replica that is not the master, the replica replies with the masters network address. The Chubby client may then contact the master. If the master fails, a new master is automatically elected, which will then continue to serve traffic based on the contents of its local copy of the replicated database. Thus, the replicated database ensures continuity of Chubby state across master failover.

The first version of Chubby was based on a commercial, third-party, fault-tolerant database; we will refer to this database as 3DB" for the rest of this paper. This database had a history of bugs related to replication. In fact, as far as we know, the replication mechanism was not based on a proven replication algorithm and we do not know if it is correct. Given the history of problems associated with that product and the importance of Chubby, we eventually decided to replace 3DB with our own solution based on the Paxos algorithm.

If this made sense to you, you may be interested to read the full PDF from 2007 in which Tushar Chandra, Robert Griesemer and Joshua Redstone describe the engineering challenges building a Paxos algo based fault-tolerant database, which proved to be non-trivial, as the paper says. To find out more about Chubby itself, another PDF from 2006 is available. As Greg Linden once noted, it includes amusingly direct semi-rants by Googles Mike Burrows (co-creator of the Burrows-Wheeler transform used to optimize compression efficiency):

• Despite attempts at education, our developers regularly write loops that retry indefinitely when a file is not present, or poll a file by opening it and closing it repeatedly when one might expect they would open the file just once.
• We find that our developers rarely think about failure probabilities, and are inclined to treat a service like Chubby as though it were always available.
• Developers also fail to appreciate the difference between a service being up, and that service being available to their applications.
• Developers are often unable to predict how their services will be used in the future, and how use will grow
• A module written by one team may be reused a year later by another team with disastrous results
• Our developers are confused by non-intuitive caching semantics.
• Our developers sometimes do not plan for high availability in the way one would wish.

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