A transaction log, called a commit log in ScyllaDB, is an append-only file that preserves writes before they reach durable SSTables. It protects data when a node or process fails: Scylla replays the log into the memtable during startup. section 6.2.2 section 8.3.1
A write follows this path:
- Scylla writes the data to the in-memory memtable and appends it to the commit log. section 6.2.2
- Appending avoids inserting into a particular disk location, reducing disk I/O while preserving durability. section 8.3.1
- When the memtable and log reach configured limits, Scylla flushes data into a new immutable SSTable. section 8.3.1
- After a process death, Scylla replays the commit log to reconstruct the memtable and prevent data loss. section 8.3.1
A restaurant review write
When a new restaurant review arrives, Scylla appends the write to the node’s commit-log file and places it in the memtable. If the process stops before an SSTable is created, startup replay restores the write. section 8.3.1
Figure 8.10 When data is written to the node, it first goes to the memtable and the commit log before eventually being flushed to disk into an SSTable.
Trace data from the memtable and commit log to its eventual SSTable. section 8.3.1
