Re: Support for N synchronous standby servers - take 2

From: Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com>
To: Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>
Cc: Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>, Fujii Masao <masao(dot)fujii(at)gmail(dot)com>, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Thom Brown <thom(at)linux(dot)com>, Beena Emerson <memissemerson(at)gmail(dot)com>, Josh Berkus <josh(at)agliodbs(dot)com>, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Support for N synchronous standby servers - take 2
Date: 2016-03-03 14:30:49
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Thank you so much for reviewing this patch!

All review comments regarding document and comment are fixed.
Attached latest v14 patch.

> This accepts 'abc^Id' as a name, which is wrong behavior (but
> such appliction names are not allowed anyway. If you assume so,
> I'd like to see a comment for that.).

'abc^Id' is accepted as application_name, no?
postgres(1)=# set application_name to 'abc^Id';
postgres(1)=# show application_name ;
(1 row)

> addlit_xd_string(char *ytext) and addlitchar_xd_string(unsigned
> char ychar) requires differnt character types. Is there any reason
> for that?

Because addlit_xd_string() is for adding string(char *) to xd_string,
OTOH addlit_xd_char() is for adding just one character to xd_string.

> I personally don't like addlit*string() things for such simple
> syntax but itself is acceptble enough for me. However it uses
> StringInfo to hold double-quoted names, which pallocs 1024 bytes
> of memory chunk for every double-quoted name. The chunks are
> finally stacked up left uncollected until the current
> memorycontext is deleted or reset (It is deleted just after
> finishing config file processing). Addition to that, setting
> s_s_names runs the parser twice. It seems to me too greedy and
> seems that static char [NAMEDATALEN] is enough using the v12 way
> without palloc/repalloc.

I though that length of group name could be more than NAMEDATALEN, so
I use StringInfo.
Is it not necessary?

> I found that the name SyncGroupName.wait_num is not
> instinctive. How about sync_num, sync_member_num or
> sync_standby_num? If the last is preferable, .members also should
> be .standbys .

Thanks, sync_num is preferable to me.

> I am quite uncomfortable with the existence of
> WanSnd.sync_standby_priority. It represented the pirority in the
> old linear s_s_names format but nested groups or even
> single-level quarum list obviously doesn't fit it. Can we get rid
> of sync_standby_priority, even though we realize atmost
> n-priority for now?

We could get rid of sync_standby_priority.
But if so, we will not be able to see the next sync standby in
pg_stat_replication system view.
Regarding each node priority, I was thinking that standbys in quorum
list have same priority, and in nested group each standbys are given
the priority starting from 1.

> The function SyncRepGetSyncedLsnsUsingPriority doesn't seem to
> have specific code for every prioritizing method (which are
> priority, quorum, nested and so). Is there any reson to use it as
> a callback of SyncGroupNode?

The reason why the current code is so is that current code is for only
priority method supporting.
At first version of this feature, I'd like to implement it more simple.

Aside from this, of course I'm planning to have specific code for nested design.
- The group can have some name nodes or group nodes.
- The group can use either 2 types of method: priority or quorum.
- The group has SyncRepGetSyncedLsnFn() and SyncRepGetStandbysFn()
- SyncRepGetSyncedLsnsFn() function recursively determine synced LSN
at that moment using group's method.
- SyncRepGetStandbysFn() function returns standbys of its group,
which are considered as sync using group's method.

For example, s_s_name = '3(a, b, 2[c,d]::group1)', SyncRepStandbys
memory structure will be,

"main(quorum)" --- "a"
-- "b"
-- "group1(priority)" --- "c"
-- "d"

When determine synced LSNs, we need to consider group1's LSN using by
priority method at first, and then we can determine main's LSN using
by quorum method with "a" LSNs, "b" LSNs and "group1" LSNs.
So SyncRepGetSyncedLsnsUsingPriority() function would be,

SyncRepGetSyncedLsnsUsingPriority(*group, *write_lsn, *flush_lsn)
sync_num = group->SynRepGetSyncstandbysFn(group, sync_list);

if (sync_num < group->sync_num)
return false;

for (each member of sync_list)
if (member->type == group node)
call SyncRepGetSyncedLsnsFn(member, w, f) and store w and
f into lsn_list.
Store name node LSNs into lsn_list.

Determine synced LSNs of this group using lsn_list and priority method.
Store synced LSNs into write_lsn and flush_lsn.
return true;

> SyncRepClearStandbyGroupList is defined in syncrep.c but the
> other related functions are defined in syncgroup_gram.y. It would
> be better to place them together.

SyncRepClearStandbyGroupList() is used by
check_synchronous_standby_names(), so I put this function syncrep.c.

> SyncRepStandbys are to be in multilevel and the struct is
> naturally allowed to be so but SyncRepClearStandbyGroupList
> assumes it in single level.

Because I think that we don't need to implement to fully support
nested style at first version.
We have to carefully design this feature while considering
expandability, but overkill implementation could be cause of crash.
Consider remaining time for 9.6, I feel we could implement quorum
method at best.

> This is a comment from the aspect of abstractness of objects.
> The callers of SyncRepGetSyncStandbysUsingPriority() need to care
> the inside of SyncGroupNode but what the function should just
> return seems to be the list of wansnds element. Element number is
> useless when the SyncGroupNode nests.
> > int
> > SyncRepGetSyncStandbysUsingPriority(SyncGroupNode *group, volatile WalSnd **sync_list)
> This might need to expose 'volatile WalSnd*' (only pointer type)
> outside of walsender.
> Or it should return the list of index number of
> *WalSndCtl->walsnds*.

SyncRepGetSyncStandbysUsingPriority() already returns the list of
index number of "WalSndCtl->walsnd" as sync_list, no?
As I mentioned above, SyncRepGetSyncStandbysFn() doesn't need care the
inside of SyncGroupNode in my design.
Selecting sync nodes from its group doesn't depend on the type of node.
What SyncRepGetSyncStandbyFn() should do is to select sync node from
*its* group.


Masahiko Sawada

Attachment Content-Type Size
000_multi_sync_replication_v14.patch binary/octet-stream 44.2 KB

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