The append-only journal
advanced16 min readLesson 159 of 180
JSONL framing, explicit charsets, and rotation by policy.
An append-only journal is the honest way to persist events: writes are cheap appends, corruption cannot destroy history (only tail it), and replay reconstructs state deterministically.
Files.writeString(file, line + "\n",
StandardCharsets.UTF_8, CREATE, APPEND);
for (String line : Files.readAllLines(file, StandardCharsets.UTF_8)) { ... }
Three disciplines: (1) explicit charset everywhere — UTF-8 in and out;
the platform-default overloads are deprecated precisely because "default"
differs between machines. (2) one record per line (JSONL): the newline is
the frame, so a truncated final line is detectable and skippable. (3) rotation
by policy — when the file exceeds a bound, Files.move it aside atomically
and keep appending to a fresh file. Kafka, databases, and every log shipper
in existence run on this shape.