> For the complete documentation index, see [llms.txt](https://docs.aevo.xyz/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.aevo.xyz/api-reference/orderbook-checksum.md).

# Orderbook Checksum

Every orderbook message contains a 32-bit integer checksum (represented as a base-10 string). This checksum is used to compare a client's orderbook state versus the matching engine's orderbook state.

Should these checksums differ between client and server, the Websocket client is encouraged to re-subscribe to the channel to fetch the new orderbook snapshot in order to re-build the orderbook state.

* The preimage of the checksum is generated by concatenating each order's price and size, sorted by their price levels
* Only the **best 100 price levels** are included for purposes of checksumming.
* Format: `<best_bid_price>:<best_bid_size>:<best_ask_price>:<best_ask_size>:<second_best_bid_price>:<second_best_bid_size>:...`

**Example**:

* orderbook: `{ "bids": ["9", "2"]], "asks": [["10", "1"] }`
* resulting concatenation: `9:2:10:1`

```python
// Code Example

import zlib

def checksum(bids, asks):
    preimage = ""
    iterations = max(len(bids), len(asks))

    for index in range(min(iterations, 100)):
        if len(bids) > index:
            price, size = bids[index]
            preimage += price + ":" + size + ":"

        if len(asks) > index:
            price, size = asks[index]
            preimage += price + ":" + size + ":"
    
    preimage = preimage[:-1] # strip last colon
    crc = zlib.crc32(preimage.encode("utf8")) & 0xFFFFFFFF
    return crc

print(checksum([["9", "2"]], [["10", "1"]])) # 1226559413
```