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Birman Schiper Stephenson Protocol
The Birman-Schiper-Stephenson (BSS) Protocol is a distributed computing protocol that allows a group of computers to maintain a consistent shared state despite the possibility of failures. It was first described in a paper by Michael Birman, Roger Schiper, and Tom Stephenson in 1985.
In the BSS Protocol, each computer in the group is assigned a unique identifier and acts as a "replica" of the shared state. The replicas communicate with each other using a series of message passing operations to maintain consistency of the shared state. The protocol is based on a "token" that is passed between replicas, which serves as a marker of the current state of the shared state.
The BSS Protocol is considered a "partially synchronous" protocol, meaning that there is a bound on the time it takes for a message to be delivered, but the bound may be arbitrarily large. This means that the protocol can tolerate arbitrary delays in message delivery, but it cannot handle the case where messages are permanently lost.
BSS protocol is a practical and efficient solution for a number of use cases like replicated state machine, distributed file system, replicated database, and even blockchain protocols.
If you want to know more about the details of the protocol and its properties, I would recommend reading the original paper, or other resources that explain the protocol and its implementation.
Features
Here are some key features of the Birman-Schiper-Stephenson (BSS) Protocol −
Fault tolerance − The BSS Protocol is designed to tolerate the failure of one or more of the replicas in the group. When a replica fails, the remaining replicas can continue to maintain consistency of the shared state.
Replication − Each replica in the group maintains a copy of the shared state, allowing for multiple copies to exist in case of failure of one of them.
Token-based − The protocol uses a "token" that is passed between replicas, which serves as a marker of the current state of the shared state. The token is used to ensure consistency among replicas.
Partially synchronous − The BSS protocol is considered "partially synchronous" meaning that there is a bound on the time it takes for a message to be delivered, but the bound may be arbitrarily large.
Non-blocking − The BSS protocol provides a non-blocking message passing operations, this means it will not block the other replica's operation even when the state of some replicas are not consistent.
Consistency Guarantee − The protocol is designed to ensure that the shared state is always consistent across all replicas, meaning that all replicas will have the same view of the shared state at all times, as long as all replicas are working correctly.
Simple message-passing − The protocol is based on simple message-passing which makes it easy to implement.
Efficient − BSS protocol is relatively efficient in terms of communication and computation complexity.
It is worth noting that all distributed systems have trade-offs and BSS protocol has some weak points like partial synchrony, which might not suitable for some use cases.
Key Points
Here are some key points about the Birman-Schiper-Stephenson (BSS) Protocol −
The BSS Protocol is a distributed computing protocol that allows a group of computers to maintain a consistent shared state despite the possibility of failures.
It is based on a "token" that is passed between replicas, which serves as a marker of the current state of the shared state.
The protocol is designed to tolerate the failure of one or more of the replicas in the group.
Each replica in the group maintains a copy of the shared state, allowing for multiple copies to exist in case of failure of one of them.
The BSS protocol is considered "partially synchronous" meaning that there is a bound on the time it takes for a message to be delivered, but the bound may be arbitrarily large.
The protocol provides a non-blocking message passing operations, this means it will not block the other replica's operation even when the state of some replicas are not consistent.
The protocol is designed to ensure that the shared state is always consistent across all replicas, as long as all replicas are working correctly.
The BSS protocol is relatively efficient in terms of communication and computation complexity.
BSS protocol can be used in multiple distributed systems like replicated state machine, distributed file system, replicated database, and even blockchain protocols.
Examples
The Birman-Schiper-Stephenson (BSS) Protocol has been used in a variety of distributed systems, including −
Replicated State Machine − The BSS protocol can be used to implement a replicated state machine, where multiple copies of a state machine are maintained across a group of replicas. Each replica can process requests and update its local copy of the state machine, and then use the BSS protocol to ensure that all replicas have the same view of the state machine.
Distributed File System − The BSS protocol can be used to implement a distributed file system, where multiple copies of a file are maintained across a group of replicas. Each replica can process requests to read and write files, and then use the BSS protocol to ensure that all replicas have the same view of the files.
Replicated Database − The BSS protocol can be used to implement a replicated database, where multiple copies of a database are maintained across a group of replicas. Each replica can process requests to read and write data, and then use the BSS protocol to ensure that all replicas have the same view of the data.
Blockchain − BSS protocol is used to implement a consensus mechanism in blockchain technology where multiple nodes, which can act as replicas, need to agree on the state of the ledger.
Distributed consensus algorithm − BSS protocol is used as a consensus algorithm in distributed systems, as it ensures that the shared state among all replicas is consistent, and in case of failure, it can continue to function correctly by removing faulty nodes.
It's worth noting that this is not an exhaustive list, and the BSS protocol can be used in other distributed systems as well.
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