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Documenting recording rules per-tenant WAL (#4566)
* WIP Signed-off-by: Danny Kopping <danny.kopping@grafana.com> * Document ruler config options Signed-off-by: Danny Kopping <danny.kopping@grafana.com> * Self-review updates Signed-off-by: Danny Kopping <danny.kopping@grafana.com> * Clarifying WAL benefit in response to review Signed-off-by: Danny Kopping <danny.kopping@grafana.com> * Adding dashboard link Signed-off-by: Danny Kopping <danny.kopping@grafana.com> * Update docs/sources/configuration/_index.md Co-authored-by: Owen Diehl <ow.diehl@gmail.com> * Ensuring ruler and ingester default WAL paths do not conflict Signed-off-by: Danny Kopping <danny.kopping@grafana.com> Co-authored-by: Owen Diehl <ow.diehl@gmail.com>
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title: Recording Rules | ||
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# Recording Rules | ||
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Recording rules are evaluated by the `ruler` component. Each `ruler` acts as its own `querier`, in the sense that it | ||
executes queries against the store without using the `query-frontend` or `querier` components. It will respect all query | ||
[limits](https://grafana.com/docs/loki/latest/configuration/#limits_config) put in place for the `querier`. | ||
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Loki's implementation of recording rules largely reuses Prometheus' code. | ||
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Samples generated by recording rules are sent to Prometheus using Prometheus' **remote-write** feature. | ||
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## Write-Ahead Log (WAL) | ||
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All samples generated by recording rules are written to a WAL. The WAL's main benefit is that it persists the samples | ||
generated by recording rules to disk, which means that if your `ruler` crashes, you won't lose any data. | ||
We are trading off extra memory usage and slower start-up times for this functionality. | ||
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A WAL is created per tenant; this is done to prevent cross-tenant interactions. If all samples were to be written | ||
to a single WAL, this would increase the chances that one tenant could cause data-loss for others. A typical scenario here | ||
is that Prometheus will, for example, reject a remote-write request with 100 samples if just 1 of those samples is invalid in some way. | ||
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### Start-up | ||
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When the `ruler` starts up, it will load the WALs for the tenants who have recording rules. These WAL files are stored | ||
on disk and are loaded into memory. | ||
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Note: WALs are loaded one at a time upon start-up. This is a current limitation of the Cortex Ruler which Loki inherits. | ||
For this reason, it is adviseable that the number of rule groups serviced by a ruler be kept to a reasonable size, since | ||
_no rule evaluation occurs while WAL replay is in progress (this includes alerting rules)_. | ||
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### Truncation | ||
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WAL files are regularly truncated to reduce their size on disk. | ||
[This guide](https://ganeshvernekar.com/blog/prometheus-tsdb-wal-and-checkpoint/#wal-truncation-and-checkpointing) | ||
from one of the Prometheus maintainers (Ganesh Vernekar) gives an excellent overview of the truncation, checkpointing, | ||
and replaying of the WAL. | ||
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### Cleaner | ||
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<span style="background-color:#f3f973;">WAL Cleaner is an experimental feature.</span> | ||
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The WAL Cleaner watches for abandoned WALs (tenants who no longer have recording rules associated) and deletes them. | ||
Enable this feature only if you are running into storage concerns with WALs that are too large. WALs should not grow | ||
excessively large due to truncation. | ||
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## Scaling | ||
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Loki's `ruler` component is based on Cortex's `ruler`. | ||
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See Cortex's guide for [horizontally scaling the `ruler`](https://cortexmetrics.io/docs/guides/ruler-sharding/) using the ring. | ||
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Note: the `ruler` shards by rule _group_, not by individual rules. This is an artifact of the fact that Prometheus | ||
recording rules need to run in order since one recording rule can reuse another - but this is not possible in Loki. | ||
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## Deployment | ||
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The `ruler` needs to persist its WAL files to disk, and it incurs a bit of a start-up cost by reading these WALs into memory. | ||
As such, it is recommended that you try to minimize churn of individual `ruler` instances since rule evaluation is blocked | ||
while the WALs are being read from disk. | ||
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### Kubernetes | ||
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It is recommended that you run the `rulers` using `StatefulSets`. The `ruler` will write its WAL files to persistent storage, | ||
so a `Persistent Volume` should be utilised. | ||
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## Remote-Write | ||
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### Per-Tenant Limits | ||
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Remote-write can be configured at a global level in the base configuration, and certain parameters tuned specifically on | ||
a per-tenant basis. Most of the configuration options [defined here](../configuration/#ruler_config) | ||
have [override options](../configuration/#limits_config) (which can be also applied at runtime!). | ||
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### Tuning | ||
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Remote-write can be tuned if the default configuration is insufficient (see [Failure Modes](#failure-modes) below). | ||
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There is a [guide](https://prometheus.io/docs/practices/remote_write/) on the Prometheus website, all of which applies to Loki, too. | ||
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## Observability | ||
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Since Loki reuses the Prometheus code for recording rules and WALs, it also gains all of Prometheus' observability. | ||
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Prometheus exposes a number of metrics for its WAL implementation, and these have all been prefixed with `loki_ruler_wal_`. | ||
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For example: `prometheus_remote_storage_bytes_total` → `loki_ruler_wal_prometheus_remote_storage_bytes_total` | ||
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Additional metrics are exposed, also with the prefix `loki_ruler_wal_`. All per-tenant metrics contain a `tenant` | ||
label, so be aware that cardinality could begin to be a concern if the number of tenants grows sufficiently large. | ||
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Some key metrics to note are: | ||
- `loki_ruler_wal_appender_ready`: whether a WAL appender is ready to accept samples (1) or not (0) | ||
- `loki_ruler_wal_prometheus_remote_storage_samples_total`: number of samples sent per tenant to remote storage | ||
- `loki_ruler_wal_prometheus_remote_storage_samples...` | ||
- `loki_ruler_wal_prometheus_remote_storage_samples_pending_total`: samples buffered in memory, waiting to be sent to remote storage | ||
- `loki_ruler_wal_prometheus_remote_storage_samples_failed_total`: samples that failed when sent to remote storage | ||
- `loki_ruler_wal_prometheus_remote_storage_samples_dropped_total`: samples dropped by relabel configurations | ||
- `loki_ruler_wal_prometheus_remote_storage_samples_retried_total`: samples re-resent to remote storage | ||
- `loki_ruler_wal_prometheus_remote_storage_highest_timestamp_in_seconds`: highest timestamp of sample appended to WAL | ||
- `loki_ruler_wal_prometheus_remote_storage_queue_highest_sent_timestamp_seconds`: highest timestamp of sample sent to remote storage. | ||
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We've created a basic [dashboard in our loki-mixin](https://github.com/grafana/loki/tree/main/production/loki-mixin/dashboards/recording-rules.libsonnet) | ||
which you can use to administer recording rules. | ||
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## Failure Modes | ||
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### Remote-Write Lagging | ||
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Remote-write can lag behind for many reasons: | ||
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1. Remote-write storage (Prometheus) is temporarily unavailable | ||
2. A tenant is producing samples too quickly from a recording rule | ||
3. Remote-write is tuned too low, creating backpressure | ||
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It can be determined by subtracting | ||
`loki_ruler_wal_prometheus_remote_storage_queue_highest_sent_timestamp_seconds` from | ||
`loki_ruler_wal_prometheus_remote_storage_highest_timestamp_in_seconds`. | ||
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In case 1, the `ruler` will continue to retry sending these samples until the remote storage becomes available again. Be | ||
aware that if the remote storage is down for longer than `ruler.wal.max-age`, data loss may occur after truncation occurs. | ||
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In cases 2 & 3, you should consider [tuning](#tuning) remote-write appropriately. | ||
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Further reading: see [this blog post](https://grafana.com/blog/2021/04/12/how-to-troubleshoot-remote-write-issues-in-prometheus/) | ||
by Prometheus maintainer Callum Styan. | ||
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### Appender Not Ready | ||
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Each tenant's WAL has an "appender" internally; this appender is used to _append_ samples to the WAL. The appender is marked | ||
as _not ready_ until the WAL replay is complete upon startup. If the WAL is corrupted for some reason, or is taking a long | ||
time to replay, you can determine this by alerting on `loki_ruler_wal_appender_ready < 1`. | ||
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### Corrupt WAL | ||
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If a disk fails or the `ruler` does not terminate correctly, there's a chance one or more tenant WALs can become corrupted. | ||
A mechanism exists for automatically repairing the WAL, but this cannot handle every conceivable scenario. In this case, | ||
the `loki_ruler_wal_corruptions_repair_failed_total` metric will be incremented. | ||
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### Found another failure mode? | ||
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Please open an [issue](https://github.com/grafana/loki/issues) and tell us about it! |
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