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Consistency is a synonym for "guaranteed", and means "written to 2 remote, reliable, append-only storage endpoints" (for any reasonable definition of reliabili

by cduzz 3y ago

Consistency is a synonym for "guaranteed", and means "written to 2 remote, reliable, append-only storage endpoints" (for any reasonable definition of reliability) So -- a single system collecting a log event -- it is not reliable (guaranteed) if written just to some device on that system. Instances can be de-provisioned (and logs lost), filesystems or databases can be scrambled, badguys can encrypt your data, etc. In this context, a "network partition" prevents consistency (data not written to reliable media) or prevents availability (won't accept new requests until their activity can be logged reliably). If you define "reliably" differently, you may have a different interpretation of log consistency.

fnordpiglet 3y ago
I’m not sure I understand the way you’re using the vocabulary. Consistency is a read operation concept not write. There is no online reads for logs. Availability is achieved if at least one writer acknowledges a write. In a partition, it means when you have multiple parts of the system disagreeing about the write contents due to a partition in the network. But because logs are immutable and write only, this doesn’t happen in any situation. The only situation this might occur is if you’re maintaining a distributed ratchet with in delivery order semantics rather than eventually consistent temporal semantics- in which case you will never have CAP. But that’s an insanely rare edge case. Note CAP doesn’t ensure perfect durability. I feel like you’re confusing consistency with durability. Consistency means after I’ve durably written something all nodes agree on read it’s been written. Since logs don’t support read on the online data plane this is trivially not an issue. Any write acknowledgment is sufficient.
lmm 3y ago
> Consistency is a synonym for "guaranteed", and means "written to 2 remote, reliable, append-only storage endpoints" (for any reasonable definition of reliability) No it doesn't. Read your own wiki link. > In this context, a "network partition" prevents consistency (data not written to reliable media) or prevents availability (won't accept new requests until their activity can be logged reliably). A network partition doesn't matter for a log system because there is no way to have consistency issues with logs. Even a single partitioned-off instance can accept writes without causing any problem. Of course if you cannot connect to any instance of your log service then you cannot write logs. But that's got nothing to do with the CAP theorem.

by cduzz 3y ago

fnordpiglet 3y ago

I’m not sure I understand the way you’re using the vocabulary. Consistency is a read operation concept not write. There is no online reads for logs. Availability is achieved if at least one writer acknowledges a write. In a partition, it means when you have multiple parts of the system disagreeing about the write contents due to a partition in the network. But because logs are immutable and write only, this doesn’t happen in any situation. The only situation this might occur is if you’re maintaining a distributed ratchet with in delivery order semantics rather than eventually consistent temporal semantics- in which case you will never have CAP. But that’s an insanely rare edge case. Note CAP doesn’t ensure perfect durability. I feel like you’re confusing consistency with durability. Consistency means after I’ve durably written something all nodes agree on read it’s been written. Since logs don’t support read on the online data plane this is trivially not an issue. Any write acknowledgment is sufficient.

lmm 3y ago

> Consistency is a synonym for "guaranteed", and means "written to 2 remote, reliable, append-only storage endpoints" (for any reasonable definition of reliability) No it doesn't. Read your own wiki link. > In this context, a "network partition" prevents consistency (data not written to reliable media) or prevents availability (won't accept new requests until their activity can be logged reliably). A network partition doesn't matter for a log system because there is no way to have consistency issues with logs. Even a single partitioned-off instance can accept writes without causing any problem. Of course if you cannot connect to any instance of your log service then you cannot write logs. But that's got nothing to do with the CAP theorem.