Add context cache as a request level cache

[lunny]

To avoid duplicated load of the same data in an HTTP request, we can set a context cache to do that. i.e. Some pages may load a user from a database with the same id in different areas on the same page. But the code is hidden in two different deep logic. How should we share the user? As a result of this PR, now if both entry functions accept context.Context as the first parameter and we just need to refactor GetUserByID to reuse the user from the context cache. Then it will not be loaded twice on an HTTP request.

But of course, sometimes we would like to reload an object from the database, that's why RemoveContextData is also exposed.

The core context cache is here. It defines a new context

type cacheContext struct {
	ctx  context.Context
	data map[any]map[any]any
        lock sync.RWMutex
}

var cacheContextKey = struct{}{}

func WithCacheContext(ctx context.Context) context.Context {
	return context.WithValue(ctx, cacheContextKey, &cacheContext{
		ctx:  ctx,
		data: make(map[any]map[any]any),
	})
}

Then you can use the below 4 methods to read/write/del the data within the same context.

func GetContextData(ctx context.Context, tp, key any) any
func SetContextData(ctx context.Context, tp, key, value any)
func RemoveContextData(ctx context.Context, tp, key any)
func GetWithContextCache[T any](ctx context.Context, cacheGroupKey string, cacheTargetID any, f func() (T, error)) (T, error)

Then let's take a look at how system.GetString implement it.

func GetSetting(ctx context.Context, key string) (string, error) {
	return cache.GetWithContextCache(ctx, contextCacheKey, key, func() (string, error) {
		return cache.GetString(genSettingCacheKey(key), func() (string, error) {
			res, err := GetSettingNoCache(ctx, key)
			if err != nil {
				return "", err
			}
			return res.SettingValue, nil
		})
	})
}

First, it will check if context data include the setting object with the key. If not, it will query from the global cache which may be memory or a Redis cache. If not, it will get the object from the database. In the end, if the object gets from the global cache or database, it will be set into the context cache.

An object stored in the context cache will only be destroyed after the context disappeared.

[zeripath]

I have a feeling that making this an unlimited map instead of something limited in size will bite us as more and more things use to use the cache.

Whilst LRUs have an overhead it might be a better idea to make this a generous LRU.

I guess it might be sensible to provide a few implementations of a context cache (LRU/hard limit map/no limit map) here, AND/OR to add some trace logging/pprof metrics that counts the number of entities that gets stored in these maps.

---

As an aside the complicated changes within models/settings do show the failure of the current design of our db services - these probably need to be migrated to service interfaces which would allow for simpler cache wrapping interfaces.

[lunny]

For a request, I cannot imagine it's an unlimited map. And in fact, all the values are pointers, only keys are in this map. When a request is end, the map and related objects will be GC.

[wxiaoguang]

For my understanding, this PR does 2 things:

1. Introduce ctx argument for many functions
2. Introduce the context cache package, and only use it in system.GetSetting

For 2, I think using generic function in the future would help a lot, like this:

func GetWithCache[T any](ctx context.Context, cacheGroupKey string, cacheTargetID any, f func() (T, error)) (T, error) {
	// call function f and cache the result if there is no cache before
	return t, err
}

.....

func GetSettingNoCache(ctx context.Context, key string) (string, error) {
	// some slow operation
	// ...
	return "", nil
}

func GetUserByID(ctx context.Context, id int64) (*User, error) {
	// some slow operation
	// ...
	return nil, nil
}

func doSomething(ctx context.Context) {
	val, err := ctxcache.GetWithCache(ctx, "GetSetting", "my_setting_key", func() (string, error) {
		return GetSettingNoCache(ctx, "my_setting_key")
	})
	println(val, err)

	user, err := ctxcache.GetWithCache(ctx, "GetUserByID", 12345, func() (*User, error) {
		return GetUserByID(ctx, 12345)
	})
	println(user, err)
}

[lunny]

LG-TM bot

[KN4CK3R]

Isn't it harmful to use db.DefaultContext now because this uses a context cache without request lifetime? db.DefaultContext is valid for the whole application lifetime. I try to replace #22491 but there db.DefaultContext would be used sometimes.

[lunny]

Here we use db.DefaultContext then there is no request level cache will be triggered. The context cache will be ignored here.