Happy to answer any questions. And thanks to Dominik Tornow of Resonate for writing this up as a guest post! It was a little rule we had coined, to help people remember how to preserve consistency across different DBMS's, and I think Dominik gave (beautiful) voice to it.
I concur with https://news.ycombinator.com/item?id=45886850, the article doesn’t actually spell out what the “Write Last, Read First” rule is supposed to say. The name of the rule also isn’t great, because it leaves open what to write to/read from, and doesn’t even suggest that there might be several systems to write to/read from, which however is essential to the rule.
The what here is the “system of record”. There might also be no other systems to write to/read from—and that's fine. The important thing is the order. Hopefully "Write Last, Read First" is unforgettable in that respect!
The article mixes two things: (1) That you should (depending on requirements) have one system of records while the other systems are merely systems of reference, and (2) that in that architecture, when (*) you write to both systems or read from both systems for the same logical record, then in the writing case you have to write to the system of records last, and in the reading case you have to read from the system of records first. The article however doesn’t make it clear that the titular rule is specifically (2), and that it is conditioned on (1) and on (*).
I have no suggestion for a better name off the top of my head. The issue I see is that you already have to know well its context and when it applies, also in order to not misremember it as “Write First, Read Last”, and to not mistake it as LIFO, or to relate it to a read-modify-write scenario in which you naturally would read first and write last anyway, though in a different sense. You see how the name is confusing?
IMO the example about checkpointing is not that demonstrative because creating entities is easy to make idempotent. What about transactions? Do they need idempotency and if so, how does it work?
I was hoping this was going to be advice for being a good citizen of the ‘net, but this week seems to be Brought to You by Tiger Beetle, so here we are.
I found the article interesting, but I don't think I understand what is meant by 'Write Last, Read first' rule - even after reading it a few times. It seems to be too ambiguous a statement to be helpful.
Under the section 'Order of Operations':
> "Since the system of reference doesn’t determine existence, we can safely write to it first without committing anything. [...]"
Then the next paragraph
> "This principle—Write Last, Read First—ensures that we maintain application level consistency."
What I think it means is, 'writing-last to the system-of-record' and 'a read-first from the system of record' yields authoritative results, but I don't get that just from the title. Is my understanding correct?
Yes, write last to the system of record, read first from the system of record. Or in other words, commit to the system of record, and then read from the system of record to see what's committed.
(This is similar also to how chain replication preserves consistency.)
No, in two-phase commit all target systems perform two phases (staging the commit, which may fail, and then actually committing it in a failsafe way), which isn’t the case here.
Joran from TigerBeetle!
Happy to answer any questions. And thanks to Dominik Tornow of Resonate for writing this up as a guest post! It was a little rule we had coined, to help people remember how to preserve consistency across different DBMS's, and I think Dominik gave (beautiful) voice to it.
I concur with https://news.ycombinator.com/item?id=45886850, the article doesn’t actually spell out what the “Write Last, Read First” rule is supposed to say. The name of the rule also isn’t great, because it leaves open what to write to/read from, and doesn’t even suggest that there might be several systems to write to/read from, which however is essential to the rule.
The what here is the “system of record”. There might also be no other systems to write to/read from—and that's fine. The important thing is the order. Hopefully "Write Last, Read First" is unforgettable in that respect!
Or, can you suggest something pithier?
The article mixes two things: (1) That you should (depending on requirements) have one system of records while the other systems are merely systems of reference, and (2) that in that architecture, when (*) you write to both systems or read from both systems for the same logical record, then in the writing case you have to write to the system of records last, and in the reading case you have to read from the system of records first. The article however doesn’t make it clear that the titular rule is specifically (2), and that it is conditioned on (1) and on (*).
I have no suggestion for a better name off the top of my head. The issue I see is that you already have to know well its context and when it applies, also in order to not misremember it as “Write First, Read Last”, and to not mistake it as LIFO, or to relate it to a read-modify-write scenario in which you naturally would read first and write last anyway, though in a different sense. You see how the name is confusing?
IMO the example about checkpointing is not that demonstrative because creating entities is easy to make idempotent. What about transactions? Do they need idempotency and if so, how does it work?
I was hoping this was going to be advice for being a good citizen of the ‘net, but this week seems to be Brought to You by Tiger Beetle, so here we are.
I found the article interesting, but I don't think I understand what is meant by 'Write Last, Read first' rule - even after reading it a few times. It seems to be too ambiguous a statement to be helpful.
Under the section 'Order of Operations':
> "Since the system of reference doesn’t determine existence, we can safely write to it first without committing anything. [...]"
Then the next paragraph
> "This principle—Write Last, Read First—ensures that we maintain application level consistency."
What I think it means is, 'writing-last to the system-of-record' and 'a read-first from the system of record' yields authoritative results, but I don't get that just from the title. Is my understanding correct?
Yes, write last to the system of record, read first from the system of record. Or in other words, commit to the system of record, and then read from the system of record to see what's committed.
(This is similar also to how chain replication preserves consistency.)
I think they just reinvented 2-phase commit? I'm not sure either tbh.
No, in two-phase commit all target systems perform two phases (staging the commit, which may fail, and then actually committing it in a failsafe way), which isn’t the case here.
Not sure why this was downvoted, the comment is completely right.