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PSYNC2: reorganize comments related to recent fixes.

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Related to PR #4412 and issue #4407.
This commit is contained in:
antirez 2017-11-24 11:08:22 +01:00
parent 9d86ae4597
commit 4d063bb6ba
2 changed files with 24 additions and 26 deletions
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27
src/rdb.c
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@ -2039,19 +2039,16 @@ rdbSaveInfo *rdbPopulateSaveInfo(rdbSaveInfo *rsi) {
* only when repl_backlog is not NULL. If the repl_backlog is NULL,
* it means that the instance isn't in any replication chains. In this
* scenario the replication info is useless, because when a slave
* connect to us, the NULL repl_backlog will trigger a full synchronization,
* at the same time we will use a new replid and clear replid2.
* And remember that after free backlog if we reach repl_backlog_time_limit,
* we will use a new replid and clear replid2 too. So there is only one
* scenario which can make repl_stream_db be -1, that is the instance is
* a master, and it have repl_backlog, but server.slaveseldb is -1. */
* connects to us, the NULL repl_backlog will trigger a full
* synchronization, at the same time we will use a new replid and clear
* replid2. */
if (!server.masterhost && server.repl_backlog) {
rsi->repl_stream_db = server.slaveseldb == -1 ? 0 : server.slaveseldb;
/* Note that when server.slaveseldb is -1, it means that this master
* didn't apply any write commands after a full synchronization.
* So we can let repl_stream_db be 0, this allows a restarted slave
* to reload replication ID/offset, it's safe because the next write
* command must generate a SELECT statement. */
rsi->repl_stream_db = server.slaveseldb == -1 ? 0 : server.slaveseldb;
return rsi;
}
@ -2061,16 +2058,12 @@ rdbSaveInfo *rdbPopulateSaveInfo(rdbSaveInfo *rsi) {
rsi->repl_stream_db = server.master->db->id;
return rsi;
}
/* It is useful to persist cached_master's db id inside RDB file.
* When a slave lost master's connection, server.master will be
* cached as server.cached_master, after that a slave can not
* increment the master_repl_offset because slave only apply data
* from connected master, so the cached_master can hold right
* replication info. But please note that this action is safe
* only after we fix the free backlog problem, because when a master
* turn to be a slave, it will use itself as the server.cached_master,
* that is dangerous if we didn't use a new replication ID after
* free backlog. */
/* If we have a cached master we can use it in order to populate the
* replication selected DB info inside the RDB file: the slave can
* increment the master_repl_offset only from data arriving from the
* master, so if we are disconnected the offset in the cached master
* is valid. */
if (server.cached_master) {
rsi->repl_stream_db = server.cached_master->db->id;
return rsi;

23
src/replication.c
View File

@ -2614,15 +2614,20 @@ void replicationCron(void) {
if (idle > server.repl_backlog_time_limit) {
/* When we free the backlog, we always use a new
* replication ID and clear the ID2. Since without
* backlog we can not increment master_repl_offset
* even do write commands, that may lead to inconsistency
* when we try to connect a "slave-before" master
* (if this master is our slave before, our replid
* equals the master's replid2). As the master have our
* history, so we can match the master's replid2 and
* second_replid_offset, that make partial sync work,
* but the data is inconsistent. */
* replication ID and clear the ID2. This is needed
* because when there is no backlog, the master_repl_offset
* is not updated, but we would still retain our replication
* ID, leading to the following problem:
*
* 1. We are a master instance.
* 2. Our slave is promoted to master. It's repl-id-2 will
* be the same as our repl-id.
* 3. We, yet as master, receive some updates, that will not
* increment the master_repl_offset.
* 4. Later we are turned into a slave, connecto to the new
* master that will accept our PSYNC request by second
* replication ID, but there will be data inconsistency
* because we received writes. */
changeReplicationId();
clearReplicationId2();
freeReplicationBacklog();