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Key-Value Storage#


Key-Value storage is just a high-level interface on top of NatsJS.

It is a regular JetStream, where the KV key is a subject.

Put/Update an object to KV by key - it's like publishing a new message to the corresponding subject in the stream.

Thus, the Get command returns not only the current key value but the latest one with an offset of it. Additionally, you can ask for a specific value based on its offset in the KV stream.

This interface provides you with rich abilities to use it like a regular KV storage (ignoring offset) + subscribe to KV key changes + ask for an old KV value revision. So you can use this feature in your application in a really different way. You can find some examples on the NATS developers' official YouTube channel

FastStream Details#

FastStream has some useful methods to help you with Key-Value NATS feature interacting.

First of all, you need to create a Key-Value storage object and put some value to it:

from faststream import FastStream
from faststream.nats import NatsBroker

broker = NatsBroker()
app = FastStream(broker)

async def setup_broker():
    key_value = await broker.key_value(bucket="bucket")
    await key_value.put("key", b"Hello!")


broker.key_value(bucket="bucket") is an idempotent method. It means that it stores all already created storages in memory and do not make new request to NATS if your are trying to call it for the same bucket.

Then we are able to use returned key_value object as a regular NATS one. But, if you want to watch by any changes by some key in the bucket, FastStream allows you to make it via regular @broker.subscriber interface:

@broker.subscriber("key", kv_watch="bucket")
async def handler(msg: str):
    assert msg == "Hello!"

Also, if you want more detail settings for you Key Value Storage, we have KvWatch object for it:

from faststream.nats import NatsBroker, KvWatch

    kv_watch=KvWatch("bucket", declare=False),
async def handler(msg: str):