Add option for wsrep_sync_wait

When using Galera, the wsrep_sync_wait option [1]
can change the behavior of a variety of Galera DQL/DML statements
such that a particular operation will pause until outstanding
write-sets are fully persisted to the local node.  The setting
supersedes the previous boolean parameter wsrep_causal_reads
which only affected SELECT, with an updated approach that allows
for fine-grained control of so-called "causality checks"
on individual statement types.  The legacy-compatible setting
of '1' indicates that READ/SELECT/BEGIN operations should
proceed only after any pending writesets are fully available.

The use case for this setting is for an application that
is running operations on multiple Galera nodes simultaenously.
An application that commits data on one node, and then immediately
uses a different connection (on a potentially different node)
to SELECT that data, may fail to see those changes if
"causality checks" for SELECT are not enabled.  While
a COMMIT operation in Galera will block locally until all other
nodes approve of the writeset, the operation does not block
for the subsequent period of time when other nodes are actually
persisting that writeset.  Setting up "causal reads"
in this case indicates that a SELECT operation will wait until
any writesets in progress are available, thus maintaining
serialization between the COMMIT and subsequent SELECT.

As the name implies, wsrep_sync_wait adds...waiting!  to the
operation, and thus directly impacts performance by adding
latency to SELECT operations or to the operations that have
been selected for causality checks, to the degree that
concurrent writesets are expected to be present.

Since it's not expected that most if any Openstack applications
actually need this setting in order to be effective with
Galera multi-master operation, and as the setting is available
within client session scope and also impacts performance,
making it available on a per-application basis means that
specific applications which may see issues under load can
choose to enable this setting, much in the way any other
"transaction isolation" settings might be made, without having
to add a cluster-wide performance penalty by setting it at the
Galera server level.

[1] https://mariadb.com/docs/ent/ref/mdb/system-variables/wsrep_sync_wait/

Change-Id: Iee7afcac8ba952a2d67a9ad9dd0e4eae3f42518e

1 files changed, 8 insertions, 0 deletions

diff --git a/releasenotes/notes/add_wsrep_sync_wait-e3c5a9f4bc08b203.yaml b/releasenotes/notes/add_wsrep_sync_wait-e3c5a9f4bc08b203.yaml
new file mode 100644
index 0000000..b61a80f
--- /dev/null
+++ b/releasenotes/notes/add_wsrep_sync_wait-e3c5a9f4bc08b203.yaml
@@ -0,0 +1,8 @@
+---
+features:
+  - |
+    Added new option mysql_wsrep_sync_wait which sets the Galera
+    "wsrep_sync_wait" variable on server login. This session-level variable
+    allows Galera to ensure that writesets are fully up to date before running
+    new queries, and may be used to tune application behavior when multiple
+    Galera masters are targeted for SQL operations simultaneously.