CachedCalculations

Coordinated and Cached Calculations (Golang)

CachedCalculation

CachedCalculation is a small and simple implementation to coordinate and cache calculations.

Requirements

For backend, it is a frequent situation when some resource is requested many times a minute and takes a few seconds to compose and return the result. Most of the time the result will be the same as the underlying data is not changed that often. For example it could be the list of countries, the list of commodities and their prices at warehouse, etc. Still, all of this calculation's tasks, if not properly coordinated, will be making the same queries to the database, combine the results, and return the same JSON data. The situation could be improved by :

1. Avoiding concurrent calculation of the same resource which decrease the load of the database, etc.
2. Cache the response result based on the request and return subsequent responses from the cache rather than calculating them again.

Meet the CachedCalculations, which I created to implement these specific requirements as a backend developer.

Implementation

1. Provided, that some calculation is "cached", it guarantees that when it does not hit the cache, it automatically performs necessary calculation, caches response, and returns it to the client. All the requests for the same resource will be waiting for the result if calculation for the resource has been started, so the same calculation is never completed concurrently, either on the same machine or distributed.
2. It provides the facility for cache entry expiration:

• If the data isn't too old (MinTTL), let's say 1-10 seconds, for MinTTL time just cached response is returned for the subsequent requests of the same resource.
• When MinTTL for the data expired, it still returns cached response to the client but immediately performs the calculation of the resource in background (refresh). When refresh finished, it stores the response to the cache, and will return the refreshed response for subsequent calls.
• When the data has expired totally (MaxTTL), it's forcefully removed and will be recalculated on demand.

3. CachedCalculation provides a simple method GetCachedCalc of refactoring existing backend methods to use its infrastructure. The idea is to wrap all the existing methods that handle some request to a function of type

CalculateValue func(context.Context) (any, error)

where any in the type of the result, in most cases will be the string with JSON. To do this, wrap your implementation into a closure called e.g. slowGet, which sees all the parameters of your method and use it like this:

result, err := cachecalc.GetCachedCalc(ctx, key, maxTTL, minTTL, true, slowGet)

It's a straightforward refactoring process which will drastically improves the performance of your backend!

4. CachedCalculation takes lock before calculation using external cache if provided. Those who were unable to take lock patiently wait when the result appears in the external cache. This allows to avoid concurrency issues mentioned in point #1. When the calculated value is ready, all clients are provided with it from the cache. If you know that you have only a single instance of the server, you don't need an external cache. In this case it will work significantly faster due the absence of networking delays. Although it works with an external cache asynchronously, it still takes time for coordination between processes. My rule of thumb in this case: if you have a simple REST API handler without heavy load, give it more RAM and CPU, and single Golang server will handle your request much faster than in Kubernetes or other distributed systems. Don't use a sledgehammer to crack a nut!

TODO

Make TTL returned by calculation. It might know better when returned value expires. For example ttl for token returned by calculation.