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in google's bigtable context, what does a commit log mean? and what is the use of a commit log?
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Commitlogs are an append only log of all mutations local to a Cassandra node. Any data written to Cassandra will first be written to a commit log before being written to a memtable. This provides durability in the case of unexpected shutdown. On startup, any mutations in the commit log will be applied.
In Postgres, commit logs are called write ahead logs. Each write to a Postgres database must first be recorded in the write ahead log before the data is changed in either a table or an index. The first benefit is that it speeds up database writes. Writing to a commit log is relatively fast, even on disk.
Database logging is an important part of your highly available database solution design because database logs make it possible to recover from a failure, and they make it possible to synchronize primary and secondary databases. All databases have logs associated with them. These logs keep records of database changes.
A log, in a computing context, is the automatically produced and time-stamped documentation of events relevant to a particular system. Virtually all software applications and systems produce log files.
In any database context, a "commit" is the application of a single transaction to the DB. A commit log is a record of transactions. It's used to keep track of what's happening, and help with e.g. disaster recovery - generally, all commits are written to the log before being applied, so transactions that were in flight when the server went down can be recovered and re-applied by checking the log.
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