Brewer’s CAP Theorem

Brewer’s CAP theorem is an important concept in scalability discussions. The theorem states that only two of the three items, Consistency, Availability, and Partition tolerance are achievable. Below are three illustrations of how this works. For the purposes of these examples, we will imagine a cluster of three storage nodes used to store user profiles.

Scenario A: Sacrificing Partition Tolerance

On each of the three nodes, we will only store a subset of the user profiles. This is called sharding. Node one will have users A-H, node two I-S, and node three T-Z. As long as each node is up and running, we have achieved a three times higher throughput than with a single node as each node only server a third of the traffic (assuming of course that user profile querying and updating is uniformly distributed through the alphabet). Consistency is achieved because immediately after data is written, it is accessible. Availability is achieved because each server is accessible in real time. However, we have lost the concept of partition tolerance as the disabling of one server has rendered a certain section of users unreachable. This carries the notion that upon hardware failure, data could have permanently been lost. All in all, not a good sacrifice under most circumstances.

Scenario B: Sacrificing Availability

On each of the three nodes, we will store all the user profiles. And furthermore, to guarantee data consistency and data loss prevention, we will ensure that every write into the system happens on all three nodes before it is completed. So, if were to update a profile for Bob McBob, any subsequent queries or writes on Bob McBob’s profile would be blocked until the update has completed. Even worse is when one of the nodes is lost but the requirement of three writes is still required, our entire system is unavailable until it is restored. This means that while our data is consistent and protected, we have sacrificed the availability of the data. This is a reasonable sacrifice in some systems. However, our goal is scalability and this does not fit that requirement.

Scenario C: Sacrificing Consistency

On each of the three nodes, we will store all the user profiles. However (and different than scenario B), we will acknowledge a completed write immediately and not wait for the other two nodes. This means that if a read comes in on node two for data written on node one, it may or may not be up-to-date depending on the latency of replication. We are still highly available and still partition tolerant (with respect to the latency it takes to replicate to another second node).

A majority of the time, scenario C is the chosen path for a couple of reasons. First, most business use cases do not require up-to-the-second information. Take, for instance, a generated report on the sales in a given region. While the business user may request “live” data, monitoring the usage of such a report will likely look as follows: 1) User prints a report and waits for x time (perhaps some coffee is obtained). 2) User imports report into Excel and slices and dices the data for y time. 3) User acts upon information. Overall, the decision is delayed from “live” data by x + y, which is most likely in the order of hours and is definitely not based on “live” data.

Second, a business benefit can be garnered in some cases. Let’s take an ATM for instance. Upon a withdrawal of funds, the ATM looks at the data available for a decision on whether or not to allow the transaction to proceed. It is not aware of any pending transfers to or from the account and is definitely not aware of what occurred in the last x minutes. If you were to use a mobile phone, move some money from your ATM account, and then inquire the ATM for a balance, the account would look no different and allow an overdraft. Ultimately, the bank, by choosing “eventual” consistency, has appropriated a fee.

In my next post, I’d like to discuss how this eventually consistent model applies to Event Sourcing and how we can structure our applications to take advantage of this in the context of enforcing transactional consistency.

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