0g-storage-node/docs/k-v-store.md

# K-V Store

0G Storage provides a Key-Value runtime upon the log layer. Each key-value node can access the key-value store state through the runtime interface. The key-value runtime provides the standard interface like `Put()` and `Get()`, and accepts serialized key-value pair from any application-specific structure. During the normal execution of the key-value store node, it maintains the latest key-value state locally. It updates the value of a key through `Put()` API which composes a log entry containing the updated key-value pair and appends to the log. The runtime constantly monitors the new log entries in the log and fetches them back to the key-value node and updates the local key-value state according to the log entry contents. In this sense, multiple key-value store nodes essentially synchronize with each other through the shared decentralized log.

A user-defined function will be used to deserialize the raw content in the log entry to the application-specific key-value structure. Application can use `Get()` API to access the latest value of a given key. To improve the efficiency of the updates for small key-value pairs, the `Put()` allows batched updates with multiple key-value pairs at once. Figure 1 illustrates the architecture of the decentralized key- value store. To manage the access control, the ownership information of each key can also be maintained in the log entries. All the honest key-value nodes follow the same update rule for the keys based on the ownership to achieve the state consistency.

When a new key-value node just joins the network, it connects to the log layer and plays the log entries from head to tail to construct the latest state of the key-value store. During the log entry playing, an application-specific key-value node can skip irrelevant log entries which do not contain stream IDs that it cares.

<figure><img src="../../.gitbook/assets/zg-storage-log.png" alt=""><figcaption><p>Figure 1. Decentralized K-V Store</p></figcaption></figure>