Birman Schiper Stephenson Protocol

The Birman Schiper Stephenson (BSS) protocol is a fundamental protocol used in distributed systems to achieve reliable ordering of messages in a distributed environment. It ensures that messages are delivered to processes in the same order they were sent, despite the possibility of message delays or failures.

Basics

In distributed systems, processes communicate with each other by sending messages. However, due to network delays and failures, messages may be delivered out of order, leading to inconsistencies and incorrect results. The BSS protocol provides a solution to this problem by defining a total ordering of messages.

Total Order Broadcast

The key concept in the BSS protocol is total order broadcast. It is a communication primitive that ensures that all correct processes deliver the same set of messages, in the same order. The protocol works as follows:

1. Each process assigns a unique timestamp to every outgoing message, using Lamport timestamps or similar techniques.
2. When a process receives a message, it checks the timestamp of the message and compares it with its own timestamp. Based on that, the process delivers the message or delays it until the required order is achieved.
3. Processes also multicast messages containing their locally delivered messages to inform other processes.
4. Each process maintains a buffer to store messages until they can be delivered in the correct order, based on the timestamps.

Properties

The BSS protocol guarantees several important properties:

1. Agreement: If a correct process delivers a message m, all other correct processes will also deliver m.
2. Integrity: No process delivers a message that was never sent by any process.
3. Total Order: If a correct process delivers messages m1 and m2 in that order, no other correct process will deliver m2 before delivering m1.

Implementation

Implementing the BSS protocol requires knowledge of the underlying distributed system and the ability to handle message timestamps. Various algorithms and techniques can be used to achieve total order broadcast, such as Lamport timestamps, vector clocks, or causal ordering.

Here's an example of using Lamport timestamps:

// Pseudocode for message sending
localTimestamp++; // Increment the local timestamp
Message message = new Message(localTimestamp, "Hello");
send(message, destination);

// Pseudocode for message receiving
receive(message);
if (message.timestamp > localTimestamp) {
    localTimestamp = message.timestamp + 1;
}
buffer.store(message); // Store the message in the buffer until it can be delivered in order

Conclusion

The Birman Schiper Stephenson protocol is an essential protocol in distributed systems that ensures reliable ordering of messages. It provides a total order broadcast primitive, which guarantees agreement, integrity, and total order properties. Implementing the protocol requires handling message timestamps and buffering messages until they can be delivered correctly.

For more detailed information and alternative algorithms, refer to the original Birman Schiper Stephenson paper: link.