CSSE4004 - Lecture 6

good idea to ensure that all conflicting operations done in same order everywhere

guaranteeing global ordering on conflicting operations may be costly, downgrading scalability

replication can greatly increase performance of non-conflicting operations

Weakening consistency requirements to increases global synchronization requirements

Read Your Writes: Effect of write operation by process on data item x, always seen by successive read operation on x by same process

Writes Follow Reads: Write operation by process on data item x following previous read operation on x by same process is guaranteed to take place on same or more recent value of x that was read.

Read write read: Write operation by process on data item x following previous read operation on x by same process is guaranteed to take place on same or more recent value of x that was read.

Read Your Writes: Write operation by process on data item x following previous read operation on x by same process is guaranteed to take place on same or more recent value of x that was read.

Read-your-writes Consistency

Sequential Consistency

Causal Consistency

Eventual consistency

More difficult for developers

Allows for application specific solutions

Middleware handles communication

Middleware is responsible for ensuring objects are consistent across nodes

If process reads value of data item x, any successive read operation on x by that process will always return that same or more recent value

Write operation by process on data item x completed before any successive write on x by same process

If process reads value of data item x, any successive read operation on x by that process will always return a value no older than the latency between the client and the server

Write operation by process on data item x completed before any successive write by other users in the system

Increase availability

Enhance reliability

Improve performance

Improved data consistency

Database must tolerate a high degree of inconsistency

Updates guaranteed to propagate to all replicas

if no updates for long time, all replicas gradually become consistent

Eventual consistency increases the time an insert operation takes

To prevent read-write conflicts: Nr + Nw > N

To prevent write-write conflicts: Nw > N/2

To prevent read-write conflicts: Nw > N/2

To prevent write-write conflicts: Nr + Nw > N + 1

Push: Server must have list of replicas

Push: No polling required

Pull: Updates are immediate

Pull: Polling required