Differences between Strict Serializable and External Consistency

Question

I follow this great blog. In this blog, the author has drawn a complete picture of all types of isolation and consistency and the relationship between them.

But based on the Google's blog, there is another type of consistency named External Consistency which is provided by Google's Spanner database. As I understood:

External consistency = Strongly Consistency + Strict Serializable

After some research, the definition of external consistency might be:

For any two transactions, 𝑇1 and 𝑇2 (even if on opposite sides of the globe): if 𝑇2 starts to commit after 𝑇1 finishes committing, then the timestamp for 𝑇2 is greater than the timestamp for 𝑇1.

I still don't see the differences between External consistency and Strict Serializability. Please give me an example that it satisfies Strict Serializability but not External Consistency.

Thanks

Answer 1

Serializability requires that transactions appear to occur sequentially. Serializability does not require any particular ordering on the serial schedule to which the transaction executions be equivalent to.

Strict serializability requires serializability, but also imposes a condition on the order of the serial schedule to which the transaction execution is equivalent to: a transaction that commits before a different transaction starts must appear to happen first. Suppose A commits before B starts--A must appear to take effect before B. With a single node system this comes for free with serializability, and no one really discusses it in this context. In a distributed system it's very hard. See https://cs.brown.edu/~mph/HerlihyW90/p463-herlihy.pdf.

External consistency is a little different. External consistency requires that a transaction that commits before a different transaction commits must appear to happen first. Suppose A commits before B commits--A must appear to take effect first. See here for a definition of external consistency.

The subtle distinction here is that strict serializability imposes no order on concurrent transactions, whereas external consistency imposes a total order on all transactions. External consistency is therefore a stronger condition in the sense that any externally consistent system is also strictly serializable.