The priority field in an SRV RR indicates a preferential order at which
the underlying targets should be utilised. We need to prefer healthy
services in order of priority, where 0 is highest.
Prior to this commit, we relied on whatever order the
dnsclient.getresources method returned. As it turns out, this assumption
is incorrect. The order returned is likely whatever order the system
resolver received DNS responses in, which may not be ordered according
to the spec.
This introduces a ResolvedAddress type which holds the priority value
for SRV targets, or a stand-in priority of zero for A/AAAA RRs. This
type is used as a return value from the underlying name resolution
routines in Name and SRVName.
In this manner, all ordering by priority and resolved time can be
performed directly within the ResolverCache class and calling code can
continue to be none-the-wiser.
Before sorting, we still ensure that we only consider targets with a
priority within the given threshold as previously implemented.
See t/115911.
Fixes an assumption that the PostgreSQL port will always be reachable at
the discovered host on the default port when performing the healthcheck.
Instead we should be sourcing this from the same environment variable
that the application will be using to connect to.
Defaults to the standard PG port, 5432.
There are two failure modes that can be expected - no target SRV DNS RRs
found or no healthy service available at target addresses. Prior to this
patch, there was no way to differentiate from log messages between the
two cases.
Introduce an EmptyCache exception which may be raised by either the
ResolverCache or HealthyCache. The exception message contains enough
information about where the exception occurred to troubleshoot issues.
An existing bug was fixed in this commit. Previously if a target address
changed during runtime, an old cached (healthy) address would be
returned.. The behaviour has been corrected to return the newly cached
address.
We'd like to lean on the DNS caching service for more than the standard
DB and Redis hosts, but without having to add additional code each time.
Define a new environment variable
DISCOURSE_DNS_CACHE_ADDITIONAL_SERVICE_NAMES (admittedly a mouthful)
which is a list of service names to be added to the static list at
process execution time.
For example, plugin foo may reference two services that you want to
cache the address of. By specifying the following two variables in the
process environment, cache_critical_dns will perform the lookup
alongside the DB and Redis host variables.
```
DISCOURSE_DNS_CACHE_ADDITIONAL_SERVICE_NAMES='FOO_SERVICE1,FOO_SERVICE2'
FOO_SERVICE1='foo.service1.example.com'
FOO_SERVICE1_SRV='foo._tcp.example.com'
FOO_SERVICE2='foo.service2.example.com'
```
The behaviour when it comes to SRV record lookup is the same as
previously implemented for the `DISCOURSE_DB_..` and
`DISCOURSE_REDIS_..` variables.
For the purposes of the health checks, services defined in the list _are
always considered healthy_. This is a compromise for conveniences sake.
Defining a dynamic method for health checks at runtime is not practical.
See t/88457/32.
We are already caching any DB_HOST and REDIS_HOST (and their
accompanying replicas), we should also cache the resolved addresses for
the MessageBus specific Redis. This is a noop if no MB redis is defined
in config. A side effect is that the MB will also support SRV lookup and
priorities, following the same convention as the other cached services.
The port argument was added to redis_healthcheck so that the script
supports a setup where Redis is running on a non-default port.
Did some minor refactoring to improve readability when filtering out the
CRITICAL_HOST_ENV_VARS. The `select` block was a bit confusing, so the
sequence was made easier to follow.
We were coercing an environment variable to an int in a few places, so
the `env_as_int` method was introduced to do that coercion in one place and
for convenience purposes default to a value if provided.
See /t/68301/30.
There are situations where a container running Discourse may want to
cache the critical DNS services without running the cache_critical_dns
service, for example running migrations prior to running a full bore
application container.
Add a `--once` argument for the cache_critical_dns script that will
only execute the main loop once, and return the status code for the
script to use when exiting. 0 indicates no errors occured during SRV
resolution, and 1 indicates a failure during the SRV lookup.
Nothing is reported to prometheus in run_once mode. Generally this
mode of operation would be a part of a unix pipeline, in which the exit
status is a more meaningful and immediate signal than a prometheus metric.
The reporting has been moved into it's own method that can be called
only when the script is running as a service.
See /t/69597.
Describes the behaviour and configuration of the cache_critical_dns
script, mainly cribbed from commit messages. Tries to make this program
a bit less of an enigma.
The `PG::Connection#ping` method is only reliable for checking if the
given host is accepting connections, and not if the authentication
details are valid.
This extends the healthcheck to confirm that the auth details are
able to both create a connection and execute queries against the
database.
We expect the empty query to return an empty result set, so we can
assert on that. If a failure occurs for any reason, the healthcheck will
return false.
An SRV RR contains a priority value for each of the SRV targets that
are present, ranging from 0 - 65535. When caching SRV records we may want to
filter out any targets above or below a particular threshold.
This change adds support for specifying a lower and/or upper bound on
target priorities for any SRV RRs. Any targets returned when resolving
the SRV RR whose priority does not fall between the lower and upper
thresholds are ignored.
For example: Let's say we are running two Redis servers, a primary and
cold server as a backup (but not a replica). Both servers would pass health
checks, but clearly the primary should be preferred over the backup
server. In this case, we could configure our SRV RR with the primary
target as priority 1 and backup target as priority 10. The
`DISCOURSE_REDIS_HOST_SRV_LE` could then be set to 1 and the target with
priority 10 would be ignored.
See /t/66045.
This removes the option to override the sleep time between caching of
DNS records. The override was invalid because `''.to_i` is 0 in Ruby,
causing a tight loop calling the `run` method.
For Redis connections that operate over TLS, we need to ensure that we
are setting the correct arguments for the Redis client. We can utilise
the existing environment variable `DISCOURSE_REDIS_USE_SSL` to toggle
this behaviour.
No SSL verification is performed for two reasons:
- the Discourse application will perform a verification against any FQDN
as specified for the Redis host
- the healthcheck is run against the _resolved_ IP address for the Redis
hostname, and any SSL verification will always fail against a direct
IP address
If no SSL arguments are provided, the IP address is never cached against
the hostname as no healthy address is ever found in the HealthyCache.
Modify the cache_critical_dns script for SRV RR awareness. The new
behaviour is only enabled when one or more of the following environment
variables are present (and only for a host where the `DISCOURSE_*_HOST_SRV`
variable is present):
- `DISCOURSE_DB_HOST_SRV`
- `DISCOURSE_DB_REPLICA_HOST_SRV`
- `DISCOURSE_REDIS_HOST_SRV`
- `DISCOURSE_REDIS_REPLICA_HOST_SRV`
Some minor changes in refactor to original script behaviour:
- add Name and SRVName classes for storing resolved addresses for a hostname
- pass DNS client into main run loop instead of creating inside the loop
- ensure all times are UTC
- add environment override for system hosts file path and time between DNS
checks mainly for testing purposes
The environment variable for `BUNDLE_GEMFILE` is set to enables Ruby to
load gems that are installed and vendored via the project's Gemfile.
This script is usually not run from the project directory as it is
configured as a system service (see
71ba9fb7b5/templates/cache-dns.template.yml (L19))
and therefore cannot load gems like `pg` or `redis` from the default
load paths. Setting this environment variable configures bundler to look
in the correct project directory during it's setup phase.
When a `DISCOURSE_*_HOST_SRV` environment variable is present, the
decision for which target to cache is as follows:
- resolve the SRV targets for the provided hostname
- lookup the addresses for all of the resolved SRV targets via the
A and AAAA RRs for the target's hostname
- perform a protocol-aware healthcheck (PostgreSQL or Redis pings)
- pick the newest target that passes the healthcheck
From there, the resolved address for the SRV target is cached against
the hostname as specified by the original form of the environment
variable.
For example: The hostname specified by the `DISCOURSE_DB_HOST` record
is `database.example.com`, and the `DISCOURSE_DB_HOST_SRV` record is
`database._postgresql._tcp.sd.example.com`. An SRV RR lookup will return
zero or more targets. Each of the targets will be queried for A and AAAA
RRs. For each of the addresses returned, the newest address that passes
a protocol-aware healthcheck will be cached. This address is cached so
that if any newer address for the SRV target appears we can perform a
health check and prefer the newer address if the check passes.
All resolved SRV targets are cached for a minimum of 30 minutes in memory
so that we can prefer newer hosts over older hosts when more than one target
is returned. Any host in the cache that hasn't been seen for more than 30
minutes is purged.
See /t/61485.
This is useful for cases where you want to add resiliency to DNS lookups
for redis and postgres, so they will continue to work even if there is
a DNS outage
