Or better titled ; A case for []byte over interface{}.

Somedays ago, I released the initial version of a Go library, onecache i have been working on. It is a caching library that supports caching data in the filesystem,memory (an hash table), Redis and Memcached.

The main goal of this library was to build a single caching library i could reuse easily as something I often use in PHP. A major part of achieving that has to be making the library expose a nice API for client code and it’s ease of use.

While designing the API, I ran into an issue with creating a cached data. The method name i chose was Set. I initially had a signature like Set(key string, data []byte, ttl time.Duration). While that is fine, I was concerned about client code and the added responsibility they would get by having to convert cacheable pieces of data into a byte array, so i decided to play a fast one. Save them the entire hard work. I converted the method to

package onecache

import (
	"time"
)

type Store interface {
	Set(key string, data interface{}, ttl time.Duration)
}

If you aren’t a Gopher, interface{} is different from an Interface. The former is a way of getting around Go’s type system (or a type that can accept any type) while the latter is OO Interface.

I decided to make the library should accept any thing whatsoever. A string, struct, any thing valid as a Go data type. This is obviously a great case for usability as client code can cache any thing and retrieve it since the underlying work (conversion to []byte) is done by the library.

I had success doing this for primitives and structs by making use of encoding/gob for encoding and decoding the cached data into/from a byte buffer. Something as simple as the code block below worked ;

package onecache

import (
	"bytes"
	"encoding/gob"
)

func MarshalBytes(i interface{}) ([]byte, error) {

	var buf bytes.Buffer

	enc := gob.NewEncoder(&buf)

	if err := enc.Encode(i); err != nil {
		return nil, err
	}

	return buf.Bytes(), nil
}

func UnMarshalBytes(data []byte, i interface{}) error {
	return gob.NewDecoder(bytes.NewBuffer(data)).Decode(i)
}

So what is the problem ?

While this worked, there were a few problems.

Here, the 80% is for primitives and datatypes known to Go but what about the 20% ? Well, you could work around it. I am going to illustrate the 20% with 2 examples ; the filesystem and the Redis cache store.

For the filesystem, i obviously was making use of io/ioutil to write the data into a file on disk. The io package requires data to be written is of []byte type. That isn’t a problem as conversion from primitives and even structs is pretty easy (the MarshalBytes function above), but there might be a problem. Custom types might need to be registered with encoding/gob else the library wouldn’t be able to help convert the type into a byte array.

Actually that piece of data would never get into the cache as you get an error all the time gob: type not registered for interface. And after debugging, you find out it was a certain caching library you installed days ago.

For the Redis store, you add some data into the cache. Everything is fine until you want to retrieve. If it is a primitive, you are good. If not, … ?

One thing i found out is this, clients for Redis implements the SET command with a byte array but might require an interface{} for client code. Internally, they would try converting the interface{} to a []byte but only with safe checks.

Now you see the problem ? If you have to implement encoding/BinaryMarshaler yourself, one that doesn’t cause an everlasting recursion and a stackoverflow (eventually). If you can register all types you create with encoding/gob. Who says you need some library helping you out with stuffs it wouldn’t even do better than you can OR stuffs it solely relies on you to do - the problems posed above - ? Well, not me.

The problem with onecache making use of interface{} is the fact that it delegates to other clients which are picky (for the right reasons) about the way they deal with the interface{}. They did the most elaborate (sensible ?) checks. Onecache wasn’t picky. It accepted anything while it was built on abstractions that happened to serve general purpose while still being picky to work for the most sensible usecases.

In wanting to write a single caching API for multiple backends, I ended up screwing up - a lot.

While building the library, I was blind to most of this, but right now it sure looks like something that would cause a lot of problems - low adoption ?.

The other problems are no much of a deal but they still count as problems

After all is said and done, I chose type safety and have re-written the library to make use of a byte array. It is a version 2.0.0 since it follows semver. This is kinda weird since v1 was released just 2 days before v2. But the main goal was in fixing my shit (tons of users ?) before it gets late.

Lesson to self ; Type safety always. Only make use of interface{} if you really know what you are doing and can safely contain it e.g the conversion to []byte described above.

Do check out Onecache if you need a Golang caching library that works across multiple stores. I hope to add a couple more storage backends in the coming weeks.

Footnotes

[0] This isn’t hitting out at Javascript. I also write PHP which is dynamically typed but can optionally be strictly typed but Javascript doesn’t have that.