Statemachine:
State: last_transaction_id - incrementing number to generate transaction IDs transactions - a map from pid to transaction ID number (read_/write_)locks - maps of lock item to transaction ID. Read lock can have multiple transaction IDs transaction_locks - simple directed graph, where verticies are transactions and edges are locks committed_transactions - map from transaction ID to pid to ignore commit messages
Start: create a mnesia table to store committed transactions
Commands:
start_transaction - generates a new transaction ID and monitors the process
cleans up all the old transactions for the process
effects:
monitor pid
rollback - cleans up transaction, removing locks and maybe unlocking locked transactions
pre_conditions:
if there is no such transaction - return error
effects:
demonitor pid
maybe send message to waiting locked transactions to unlock them
commit - writes commit data and transaction ID to the mnesia DB,
cleans up transaction, unlocking locked transactions
records committed transaction ID to committed_transactions
pre_conditions:
if transaction is already committed - noop
if there is no such transaction - return error
effects:
release_cursor if committed successfully
maybe send message to waiting locked transactions to unlock them
finish - cleans up committed transaction flag to make commit idempotent
effects:
demonitor pid
lock - check locks and maybe aquire a new lock. If locked - record locking
transactions to the state and return the locked error, if no locking
transactions (only higher transaction IDs can be locking) - return
the locked_instant error.
pre_conditions:
if there is no such transaction - return error
read/index_read/match_object/etc. - try to aquire a lock and read data from
the mnesia DB. Can return the locked error
the read data or aborted error.
pre_conditions:
if there is no such transaction - return error
prev/next - same as read, but can return key_not_found error.
down - a monitored process is stopped, remove all the trasaction data
from the state, same as rollback, but does not have preconditions
create_table/delete-table - perform a mnesia operation. TODO!
Leader effects: remonitor transaction processes.
Locks: Locks are similar to mnesia. Read locks can be aquired by multiple transactions, write locks - only one. Write locks are locked by both write and read, read locks are locked by write. If transaction is locked by other transactions, the higher transaction IDs are recorded as locking in the transaction locks graph. Locking relation is a directed edge in the graph from locking to locked. When transaction finishes, it removes itself from the graph, leaving some transactions without locking. Such transactions should receive an unlock message