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The CAP theorem expands to the PACELC theorem. PACELC stands for Partition, Availability, Consistency, Else, Latency, and Consistency. The theorem states that i

by jph 3y ago

The CAP theorem expands to the PACELC theorem. PACELC stands for Partition, Availability, Consistency, Else, Latency, and Consistency. The theorem states that in the event of a network partition, a distributed system must choose between availability and consistency; otherwise, it must choose between latency and consistency. https://en.wikipedia.org/wiki/PACELC_theorem https://en.wikipedia.org/wiki/PACELC_theorem

peterbell_nyc 3y ago
It seems like people make this really complicated. There will occasionally be network partitions. When there are, a given node can either respond (potentially inconsistently) or not. So you pick some balance between consistency and availability. Latency is really just a proxy for availability - as latency tends towards infinite, availability tends towards zero. Of course you can wait until the network partition is resolved and the nodes are caught up - but I think it's simpler to consider that as not being available for a period of time rather than as having a high latency at that time. Consistency or Availability - you don't have to pick one, but the more consistent you want to be (in the case of network partitions) the less available you'll be.
- pachico 3y ago
  If your decision between Consistency and Availability has business implications, you definitely aren't making this "really complicated" but rather a business logic to adhere to. Failing to understand the implications of one over the other is often sign of immature architecture.
- justin_hancock 3y ago
  Disagree on latency, for certain classes of applications it really matters. Latency can be caused by a myriad of things, but if that thing is your means for cut over, you haven't failed to respond, you've failed to respond in a timely way. The telco space has strict latency requirements. If you have a failure which results in erratic behaviour on the network due to the database taking time to handle a node failure you aren't in a good place, it has a financial impact. There are databases that can provide high consistency and availability.
  - harkinian 3y ago
    Put an SLA on the latency, and it becomes availability.
    - DandyDev 3y ago
      That’s a great way of putting it!
- harkinian 3y ago
  Was going to say, latency doesn't need to be in there.

by jph 3y ago

peterbell_nyc 3y ago

It seems like people make this really complicated. There will occasionally be network partitions. When there are, a given node can either respond (potentially inconsistently) or not. So you pick some balance between consistency and availability. Latency is really just a proxy for availability - as latency tends towards infinite, availability tends towards zero. Of course you can wait until the network partition is resolved and the nodes are caught up - but I think it's simpler to consider that as not being available for a period of time rather than as having a high latency at that time. Consistency or Availability - you don't have to pick one, but the more consistent you want to be (in the case of network partitions) the less available you'll be.

pachico 3y ago
If your decision between Consistency and Availability has business implications, you definitely aren't making this "really complicated" but rather a business logic to adhere to. Failing to understand the implications of one over the other is often sign of immature architecture.
justin_hancock 3y ago
Disagree on latency, for certain classes of applications it really matters. Latency can be caused by a myriad of things, but if that thing is your means for cut over, you haven't failed to respond, you've failed to respond in a timely way. The telco space has strict latency requirements. If you have a failure which results in erratic behaviour on the network due to the database taking time to handle a node failure you aren't in a good place, it has a financial impact. There are databases that can provide high consistency and availability.
- harkinian 3y ago
  Put an SLA on the latency, and it becomes availability.
  - DandyDev 3y ago
    That’s a great way of putting it!
harkinian 3y ago
Was going to say, latency doesn't need to be in there.