===xtm===Distributed transaction management tools for PostgreSQL.--------------------Communication scheme--------------------              ┏━━━━━━━━━┓     ┌────────┨ Backend ┠──────────┐     │        ┗━━━━━━━━━┛          │┏━━━━┷━━━━┓   ┏━━━━━━━━━┓   ┏━━━━━━┷━━━━━━┓┃ Arbiter ┠───┨ Backend ┠───┨ Coordinator ┃┗━━━━┯━━━━┛   ┗━━━━━━━━━┛   ┗━━━━━━┯━━━━━━┛     │        ┏━━━━━━━━━┓          │     └──┬─────┨ Backend ┠───────┬──┘        ┆     ┗━━━━━━━━━┛       ┆ libdtm + libsockhub      libpq + xtm procs-----------------------Coordinator-Backend API-----------------------This API includes a set of postgres procedures thatthe coordinator can call with "select" statement.FIXME: actualize the API------------------------Backend-Arbiter Protocol------------------------The underlying protocol (libsockhub) also transmits the message length, sothere is no need in 'argc'. Every command or reply is a series of int64numbers.The format of all commands:[cmd, argv[0], argv[1], ...]'cmd' is a command.'argv[i]' are the arguments.The commands:'r': reserve(minxid, minsize)Claims a sequence ≥ minsize of xids ≥ minxid for local usage. This willprevent the arbiter from using those values for global transactions.The arbiter replies with:[RES_OK, min, max] if reserved a range [min, max][RES_FAILED] on failure'b': begin(size)Starts a global transaction and assign a 'xid' to it. 'size' is usedfor vote results calculation. The arbiter also creates and returns thesnapshot.The arbiter replies with:[RES_OK, xid, *snapshot] if transaction started successfully[RES_FAILED] on failureSee the 'snapshot' command description for the snapshot format.'s': status(xid, wait)Asks the arbiter about the status of the global transaction identifiedby the given 'xid'.If 'wait' is 1, the arbiter will not reply until it considers thetransaction finished (all nodes voted, or one dead).The arbiter replies with:[RES_TRANSACTION_UNKNOWN] if not started[RES_TRANSACTION_COMMITTED] if committed[RES_TRANSACTION_ABORTED] if aborted[RES_TRANSACTION_INPROGRESS] if in progress[RES_FAILED] if failed'y': for(xid, wait)Tells the arbiter that this node votes for commit of the globaltransaction identified by the given 'xid'.The reply and 'wait' logic is the same as for the 'status' command.'n': against(xid, wait)Tells the arbiter that this node votes againts commit of the globaltransaction identified by the given 'xid'.The reply and 'wait' logic is the same as for the 'status' command.'h': snapshot(xid)Tells the arbiter to generate a snapshot for the global transactionidentified by the given 'xid'. The arbiter will create a snapshot forevery participant, so when each of them asks for the snapshot it willreply with the same snapshot. The arbiter generates a fresh version ifthe same client asks for a snapshot again for the same transaction.Joins the global transaction identified by the given 'xid', if notjoined already.The arbiter replies with [RES_OK, gxmin, xmin, xmax, xcnt, xip[0], xip[1]...],where 'gxmin' is the smallest xmin among all available snapshots.In case of a failure, the arbiter replies with [RES_FAILED].