HSet() object, if hash not exists

[numberinho]

Goal: Set a Hash with multiple fields, only if the hash does not already exist.

Why: I want to store a "flattened" object (containing only primitives as fields). Each field in the hash maps to a object property.

As far as I can see I cannot use HSetNX, because it only checks for the field to not already exist. Also, I cannot chain multiple HSetNX, because in concurrent access another routine may call HGetAll and not the whole object is stored in Redis yet. So HGetAll will miss some fields and return an incomplete object. Therefore I need some blocking mechanism.

Like a HMSetNX (this does not exist in Redis!), that would only set the fields if the hash does not exist.
This is a bit counter-intuitive, because one may interpret it to only set the fields if the fields do not exist.

For my understanding the function would look like:

lock
if hash not exists {
    HSet(object) -> HSetNX(key, value, key, value, ..)
}
lock

Can you give me any suggestions on how to do that using redis/rueidis? Maybe I need to chain the check for exist into a transaction with the HSet commands, to make sure nothing changed between exist and HSet?

Thank you!

[rueian]

Hi @numberinho,

The most performant way is using a Lua script that first checks if HLEN(key) returns 0 and then does HSET, but you won't be able to use the HSet function of rueidiscompat.

[numberinho]

Hi @rueian

That's a nice suggestion, thank you!

Not being able to use rediscompat is unfortunate. Rueidiscompat's argsToSlice in adapter.go (line 3327) would be is super useful in this use case, but is not exported. On the other hand I think it would not make to much sense to export this function to keep consistency throughout the library.

[rueian]

Though making argToSlice and argsToSlice be public may let people confuse about how to use them, I think we can still make them public and give some usage examples.

[numberinho]

Hi @rueian I think I’m trying to reinvent the wheel…
Just got into the rueidis/om which provides what I want to achieve. Downside: It does not support all primitives and respective pointers , which I would love to add. What do you think?

Also, I can add some examples to argToSlice in the next days, if you want.

[numberinho]

Doing this, im pretty stuck at catching time.Time, which is a reflect.struct.

[rueian]

Hi @numberinho,

You could submit a draft PR first so that we can know how to help you.