# WTI

**[WTI](https://en.wikipedia.org/wiki/Write-through)** (Write-Through Invalidate) is the simplest snoopy cache coherence protocol: every write goes directly to main memory, and all other caches invalidate their copy upon seeing the write. Only two states per line are needed: Valid (matches memory) and Invalid (stale).

WTI fell out of favor because write-through generates excessive memory traffic—every write, not just the first one, travels to memory, wasting bandwidth compared to write-back protocols like MSI.

## Example

WTI protocol state transitions.

```
Valid   --[read hit]-->  Valid
        --[write hit]--> Invalid (at other caches)
        --[read miss]--> Valid (from memory)
Invalid --[read miss]--> Valid (from memory)
```