mirror of
https://github.com/scylladb/scylladb.git
synced 2026-05-01 13:45:53 +00:00
f582bfd453
The AppendReg state machine stores a sequence of integers. It supports `append` inputs which append a single integer to the sequence and return the previous state (before appending). The implementation uses the `append_seq` data structure representing an immutable sequence that uses a vector underneath which may be shared by multiple instances of `append_seq`. Appending to the sequence appends to the underlying vector, but there is no observable effect on the other instances since they use only the prefix of the sequence that wasn't changed. If two instances sharing the same vector try to append, the later one must perform a copy. This allows efficient appends if only one instance is appending, which is useful in the following context: - a Raft server stores a copy in the underlying state machine replica and appends to it, - clients send append operations to the server; the server returns the state of the sequence before it was appended to, - thanks to the sharing, we don't need to copy all elements when returning the sequence to the client, and only one instance (the server) is appending to the shared vector, - summarizing, all operations have amortized O(1) complexity. We use AppendReg instead of ExReg in `basic_generator_test` with a generator which generates a sequence of append operations with unique integers. This implies that the result of every operation uniquely identifies the operation (since it contains the appended integer, and different operations use different integers) and all operations that must have happened before it (since it contains the previous state of the append register), which allows us to reconstruct the "current state" of the register according to the results of operations coming from Raft calls, giving us an on-line serializability checker with O(1) amortized complexity on each operation completion. We also enforce linearizability by checking that every completed operation was previously invoked. We also perform a simple liveness check at the end of the test by ensuring that a leader becomes eventually elected and that we can successfully execute a call. * kbr/linearizability-v2: test: raft: randomized_nemesis_test: check consistency and liveness in basic_generator_test test: raft: randomized_nemesis_test: introduce append register