# Does total ordering imply causal ordering?

## Does total ordering imply causal ordering?

Causal ordering implies FIFO ordering • Total ordering does not imply causal ordering. Causal ordering does not imply total ordering. Hybrid mode: causal-total ordering, FIFO-total ordering.

What is causal ordering?

Causal order is concerned with the problem that the occurrence of some events may affect other events in the future, while other events may not influence each other.

### What is meant by causal order and total order in distributed system?

For total ordering, there are two variants: total-causal order and total order. The total-causal order is the strictest ordering in distributed systems; it establishes only one linearization, consistent with the causal ordering, among all the events that occur in the system, even those that occur concurrently.

What is casual ordering in distributed system?

Causal message ordering is a partial ordering of messages in a distributed com- puting environment. Causal message ordering is either needed or desirable in a variety of distributed algorithms such as fault-tolerant object replication, distributed resource allocation and teleconferencing.

#### Why do we need causal ordering of the message?

The purpose of causal ordering of messages is to insure that the same causal relationship for the “message send” events correspond with “message receive” events. (i.e. All the messages are processed in order that they were created.)

Which algorithm is used for causal ordering messages?

This algorithm is used to maintain the causal ordering of the messages i.e. the message which is sent first should be received first. If send (M1)–>send(M2) then for all processes which receive the messages M1 and M2 should receive M1 before M2.

## What is casual order?

Causal Order refers to the practice of organizing information to illustrate cause-and-effect relationships.

What are the limitations of Lamport logical clock?

The problem with Lamport Timestamps is that they can’t tell if events are concurrent or not. This problem is solved by Vector Clocks.

### What is the advantage of vector clock over Lamport clock?

Vector Clocks represent an extension of Lamport Timestamps in that they guarantee the strong clock consistency condition which (additionally to the clock consistency condition) dictates that if one event’s clock comes before another’s, then that event comes before the other, i.e., it is a two-way condition.

Why do we need vector clock?

A vector clock is a data structure used for determining the partial ordering of events in a distributed system and detecting causality violations. Just as in Lamport timestamps, inter-process messages contain the state of the sending process’s logical clock.

#### What’s the difference between causal ordering and time order?

We are used to thinking of ordering by time which is a total order – every pair of events can be placed in some order. In contrast, causal ordering is only a partial order – sometimes events happen with no possible causal relationship i.e. not (A -> B or B -> A). This image shows a nice way to picture these relationships.

How is causal ordering conveyed in a distributed system?

Between machines causal ordering is conveyed by messages. Since sending messages is the only way for machines to affect each other this gives rise to a nice property: Since we don’t have a single global time this is the only thing that allows us to reason about causality in a distributed system.

## When does a distributed system impose a global total order?

If the final state is dependent on the order of updates then the system must choose a single serialisation of the events, imposing a global total order. A distributed system is consistent exactly when the outside world can never observe two different serialisations. The CAP (Consistency-Availability-Partition) theorem also boils down to causality.

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