https://bugzilla.wikimedia.org/show_bug.cgi?id=62528
Bug ID: 62528
Summary: Flow: don't perform reads on DB_MASTER
Product: MediaWiki extensions
Version: unspecified
Hardware: All
OS: All
Status: NEW
Severity: normal
Priority: Unprioritized
Component: Flow
Assignee: wikibugs-l@lists.wikimedia.org
Reporter: mmul...@wikimedia.org
CC: ebernhard...@wikimedia.org, mpinc...@wikimedia.org,
pandiculat...@gmail.com, sp...@wikimedia.org
Web browser: ---
Mobile Platform: ---
There's a couple of places where we perform read queries against DB_MASTER. We
shouldn't be doing that, it kind of defeats the purpose of having slave DBs
anyway, if we keep directing reads to the same master server, and it may
eventually get overloaded.
Breaking down the problem:
* we have to make sure no (potentially) lagged slave data is cached.
We only cache DB-hits, not DB-misses. That's good, this means that if a lagged
DB says some data is not there, that won't be saved to cache where it could
persist for 3 more days. Instead, as soon as slave DBs have caught up, the most
recent changes will appear.
However, there may be cases where we've *updated* existing DB data. In that
case, it may still be possible for a lagged DB server to respond with
(outdated) data that will be stored in cache until it expires.
* we have to make sure chronology is respected.
Core has ChronologyProtector in place to make sure that every consecutive
request is always served by a DB at least as recent as the previous request.
This already works on external DBs too
(https://gerrit.wikimedia.org/r/#/c/58691/)
* we have to make sure same-request queries don't query lagged slaves after
data was written to master.
ChronologyProtector only kicks in at the beginning of a request, not for every
query. So when we're performing writes to DB_MASTER, we should make sure that
any follow-up read query to DB_SLAVE is to a slave that is at least in the same
state DB_MASTER is in.
In AFTv5, I've fixed this by checking in the own getDB() wrapper. If $db is
DB_MASTER, I
mark some variable as true, and make sure that any follow-up DB_SLAVE call is
converted to DB_MASTER if that variable was set to true. This auto-direct all
(and only those) calls to DB_SLAVE if we've just written new data to master.
That code is at:
https://git.wikimedia.org/blob/mediawiki%2Fextensions%2FArticleFeedbackv5/60ea34d65207b5b71a6a617ff0a2129326ac9442/data%2FDataModelBackend.LBFactory.php#L36
We should address #3, and make sure that #1 can never happen.
Anything else I missed?
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