Stage 2 · Lesson 4 of 10
ACID transactions and the write-ahead log
A groups several changes so they all succeed or all fail together. ACID names four guarantees. : all or nothing.
Consistency: constraints, rules such as "a balance cannot go negative", hold before and after. Isolation: transactions running at the same time do not see each other's half-done work. Durability: once committed, the change survives a crash.
Durability usually comes from a . Before confirming a commit, the database adds the change to the end of a log file and forces it onto disk. It updates the main data files later.
After a crash, it replays the log to recover. The same log also feeds replication (copying data to other machines) and (streaming changes to other systems).
Common trapassuming every store gives you this: many systems only make a single record atomic, so a transfer between two accounts needs a different design.
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Resources for this stage
Start with· follow one of these
- CoursePaidGrokking Database Fundamentals for Tech Interviews
Design Gurus · Beginner · Subscription
Storage engines, indexes, transactions, isolation, replication, sharding and NoSQL models
- GitHub repoFreeSystem Design Interview Roadmap by karanpratapsingh
Karan Pratap Singh · Intermediate
Free: the databases, indexes, normalisation, ACID, sharding and replication chapters