Summary: Fixes T11159. We get two different values here (`NULL` and `0`) with different meanings.
Test Plan:
- Ran `STOP SLAVE;`.
- Saw this:
{F1710181}
- Ran `START SLAVE;`.
- Back to normal.
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T11159
Differential Revision: https://secure.phabricator.com/D16225
Summary:
Ref T4571. Write more of the missing documentation sections and clarify a few things.
Since the "replicating master" check needs a special permission, imposes a performance penalty, is probably very difficult to misconfigure, and likely not a big deal anyway, just drop the idea of trying to automatically detect + prevent it. We still show if it's an issue on the status page, provided we have permission to check.
When you don't have any cluster databases configured, never stop trying to connect to the default master database. We might want to do this eventually as load reduction, but just don't muddy the waters too much for now while things stabilize.
Test Plan:
- Tested functionality in cluster, non-cluster, and degraded-cluster modes.
- Used status console to monitor a health check cycle.
- Read docs.
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T4571
Differential Revision: https://secure.phabricator.com/D15679
Summary:
Ref T4571. When a database goes down briefly, we fall back to replicas.
However, this fallback is slow (not good for users) and keeps sending a lot of traffic to the master (might be bad if the root cause is load-related).
Keep track of recent connections and fully degrade into "severed" mode if we see a sequence of failures over a reasonable period of time. In this mode, we send much less traffic to the master (faster for users; less load for the database).
We do send a little bit of traffic still, and if the master recovers we'll recover back into normal mode seeing several connections in a row succeed.
This is similar to what most load balancers do when pulling web servers in and out of pools.
For now, the specific numbers are:
- We do at most one health check every 3 seconds.
- If 5 checks in a row fail or succeed, we sever or un-sever the database (so it takes about 15 seconds to switch modes).
- If the database is currently marked unhealthy, we reduce timeouts and retries when connecting to it.
Test Plan:
- Configured a bad `master`.
- Browsed around for a bit, initially saw "unrechable master" errors.
- After about 15 seconds, saw "major interruption" errors instead.
- Fixed the config for `master`.
- Browsed around for a while longer.
- After about 15 seconds, things recovered.
- Used "Cluster Databases" console to keep an eye on health checks: it now shows how many recent health checks were good:
{F1213397}
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T4571
Differential Revision: https://secure.phabricator.com/D15677
Summary:
Ref T4571. If we fail to connect to the master, automatically try to degrade into a temporary read-only mode ("UNREACHABLE") for the remainder of the request, if possible.
If the request was something like "load the homepage", that'll work fine. If it was something like "submit a comment", there's nothing we can do and we just have to fail.
Detecting this condition imposes a performance penalty: every request checks the connection and gives the database a long time to respond, since we don't want to drop writes unless we have to. So the degraded mode works, but it's really slow, and may perpetuate the problem if the root issue is load-related.
This lays the groundwork for improving this case by degrading futher into a "SEVERED" mode which will persist across requests. In the future, if several requests in a short period of time fail, we'll sever the database host and refuse to try to connect to it for a little while, connecting directly to replicas instead (basically, we're "health checking" the master, like a load balancer would health check a web application server). This will give us a better (much faster) degraded mode in a major service disruption, and reduce load on the master if the root cause is load-related, giving it a better chance of recovering on its own.
Test Plan:
- Disabled master in config by changing the host/username, got degraded automatically to UNREACAHBLE mode immediately.
- Faked full SEVERED mode, requests hit replicas and put me in the mode properly.
- Made stuff work, hit some good pages.
- Hit some non-cluster pages.
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T4571
Differential Revision: https://secure.phabricator.com/D15674
Summary: Ref T4571. If `cluster.databases` is configured but only has replicas, implicitly drop to read-only mode and send writes to a replica.
Test Plan:
- Disabled the `master`, saw Phabricator automatically degrade into read-only mode against replicas.
- (Also tested: explicit read-only mode, non-cluster mode, properly configured cluster mode).
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T4571
Differential Revision: https://secure.phabricator.com/D15672
Summary: Fixes T6710. After D15669, we support a proper timeout parameter, so we don't need this hack anymore.
Test Plan: See D15669: forced a MySQL connector, set a low timeout, set a bad database, saw fast failures.
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T6710
Differential Revision: https://secure.phabricator.com/D15670
Summary:
Ref T4571. Ref T10759. Ref T10758. This isn't complete, but gets most of the job done:
- When `cluster.databases` is set up, most things ignore `mysql.host` now.
- You can `bin/storage upgrade` and stuff works.
- You can browse around in the web UI and stuff works.
There's still a lot of weird tricky stuff to navigate, and this has real no advantages over configuring a single server yet (no automatic failover, etc).
Test Plan:
- Configured `cluster.databases` to point at my `t1.micro` hosts in EC2 (master + replica).
- Ran `bin/storage upgrade`, got a new install setup on them properly.
- Survived setup warnings, browsed around.
- Switched back to local config, ran `bin/storage upgrade`, browsed around, went through setup checks.
- Intentionally broke config (bad hosts, no masters) and things seemed to react reasonably well.
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T4571, T10758, T10759
Differential Revision: https://secure.phabricator.com/D15668
Summary: Ref T4571. The configuration option still doesn't do anything, but add a status panel for basic setup monitoring.
Test Plan:
Here's what a good version looks like:
{F1212291}
Also faked most of the errors it can detect and got helpful diagnostic messages like this:
{F1212292}
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T4571
Differential Revision: https://secure.phabricator.com/D15667
