How do they choose which shards are included in the first 10% canary group?
This is a tricky problem. It's tempting to include only small (less valuable) accounts in the first group. But some bugs only occur with large accounts, so you need some of those in the first 10%.
Many bugs affect only a small portion of customers. There are many categories. A canary becomes more effective when it includes members from each category. Example: account type, number of users, client type (web/ios/android/macos/windows/linux), client version, web browser type and version, ipv4/ipv6, vpn, TLS MITM proxy, language, timezone, payment currency, country, tax region, mobile service provider, etc.
This is a tricky problem. It's tempting to include only small (less valuable) accounts in the first group. But some bugs only occur with large accounts, so you need some of those in the first 10%.
Many bugs affect only a small portion of customers. There are many categories. A canary becomes more effective when it includes members from each category. Example: account type, number of users, client type (web/ios/android/macos/windows/linux), client version, web browser type and version, ipv4/ipv6, vpn, TLS MITM proxy, language, timezone, payment currency, country, tax region, mobile service provider, etc.