mCaptcha-cache/docs/mechanism.md

mCaptcha Redis Module Mechanism

Data types

1. Bucket
2. Bucket safety
3. mCaptcha
4. mCaptcha safety

Bucket

• Timer queue, used for scheduling decrements.
• Timer used for scheduling can't be persisted so requires a safety

Bucket Safety

• Has expiry timer(Redis EXIPRE)
• Key name includes bucket name for which this safety was created for
• When Redis is started after crash, bucket safety expires and expire event is fired, the corresponding bucket is fetched and executed.

mCaptcha

• Contains mCaptcha defense details and current state

• When redis performs resharding, it's possible that mCaptcha gets separated from its bucket.

• We can't pin mCaptcha by using hashtags because the client figures out where a key should be placed/is available and will have no knowledge about node IDs that we use for pinning.

• So this too requires a safety to make sure that when recovering from a crash, it's counter doesn't have residues permanently.

mCaptcha safety

• Has expire timer(Redis EXIPRE)
• Key name includes mCaptcha name for which this safety was created for
• when expiry event is fired for this type, a bucket for corresponding mCaptcha is created to decrement it by x(where x is the count of mCaptcha when this event is fired). It's not perfect but at least we get eventual consistency.