Configure Log Aggregation

Impl centralized log collection, parsing, querying w/ Loki/Promtail or ELK stack → operational visibility.

Use When

• Consolidate logs from multi services/hosts → searchable system
• Replace local log files w/ centralized, queryable log storage
• Correlate logs w/ metrics + traces for full observability
• Impl structured logging w/ label extraction from unstructured logs
• Set retention policies for log data by storage + compliance needs
• Troubleshoot prod incidents requiring log analysis across services

In

• Required: Log sources (app logs, sys logs, container logs)
• Required: Log format patterns (JSON, plaintext, syslog, etc.)
• Optional: Label extraction rules for structured querying
• Optional: Retention + compression policies
• Optional: Existing log shipper config (Fluentd, Filebeat, Promtail)

Do

See Extended Examples for complete config files + templates.

Step 1: Choose Log Aggregation Stack

Select Loki (Prometheus-style) or ELK (Elasticsearch-based) by req's.

Loki advantages:

• Lightweight, designed for K8s + cloud-native envs
• Label-based indexing (like Prometheus) → low storage overhead
• Native Grafana integration for unified dashboards
• Horizontal scalability w/ object storage (S3, GCS)
• Lower resource consumption vs. Elasticsearch

ELK advantages:

• Full-text search across all log content (not just labels)
• Rich query DSL + aggregations
• Mature ecosystem w/ beats, logstash plugins
• Better for compliance/audit logs requiring deep historical search

For this guide → focus on Loki + Promtail (rec'd for most modern setups).

Decision criteria:

Use Loki if:
- You want label-based queries similar to Prometheus
- Storage costs are a concern (Loki indexes only labels)
- You already use Grafana for metrics
- Kubernetes/container-native deployment

Use ELK if:
- You need full-text search across all log content
- You have complex log parsing and enrichment requirements
- You require advanced analytics and aggregations
- Legacy systems with existing Logstash pipelines

→ Clear choice made by req's, team downloads appropriate install artifacts.

If err:

• Benchmark storage req's: Loki ~10x less than Elasticsearch for same logs
• Eval query patterns: full-text search needs vs. label filtering
• Consider operational overhead: ELK requires more tuning + resources

Step 2: Deploy Loki

Install + configure Loki w/ appropriate storage backend.

Docker Compose deployment (

docker-compose.yml

):

version: '3.8'

services:
  loki:
    image: grafana/loki:2.9.0
    ports:
      - "3100:3100"
    volumes:
      - ./loki-config.yml:/etc/loki/local-config.yaml
      - loki-data:/loki
    command: -config.file=/etc/loki/local-config.yaml
    restart: unless-stopped

  promtail:
    image: grafana/promtail:2.9.0
    volumes:
      - ./promtail-config.yml:/etc/promtail/config.yml
      - /var/log:/var/log:ro
      - /var/lib/docker/containers:/var/lib/docker/containers:ro
    command: -config.file=/etc/promtail/config.yml
    restart: unless-stopped
    depends_on:
      - loki

volumes:
  loki-data:

Loki config (

loki-config.yml

):

auth_enabled: false

server:
  http_listen_port: 3100
  grpc_listen_port: 9096

# ... (see EXAMPLES.md for complete configuration)

For prod w/ S3 storage:

storage_config:
  aws:
    s3: s3://us-east-1/my-loki-bucket
    s3forcepathstyle: true
  boltdb_shipper:
    active_index_directory: /loki/index
    cache_location: /loki/cache
    shared_store: s3

→ Loki starts successfully, health check passes at

http://localhost:3100/ready

, logs stored per retention policy.

If err:

• Check Loki logs:

  docker logs loki

• Valid. storage dirs exist + writable
• Test config syntax:

  docker run grafana/loki:2.9.0 -config.file=/etc/loki/local-config.yaml -verify-config

• Ensure retention settings don't exceed disk capacity
• S3: valid. IAM perms + bucket access

Step 3: Configure Promtail for Log Shipping

Set up Promtail to scrape logs + forward to Loki w/ label extraction.

Promtail config (

promtail-config.yml

):

server:
  http_listen_port: 9080
  grpc_listen_port: 0

positions:
  filename: /tmp/positions.yaml
# ... (see EXAMPLES.md for complete configuration)

Key Promtail concepts:

• Scrape configs: Define log sources + how to discover them
• Pipeline stages: Transform + label logs before sending to Loki
• Relabel configs: Dynamic labeling by metadata
• Positions file: Tracks read offsets → avoid re-processing logs

→ Promtail scrapes configured log files, labels applied correct, logs visible in Loki via LogQL queries.

If err:

• Check Promtail logs:

  docker logs promtail

• Valid. file paths accessible:

  docker exec promtail ls /var/log

• Test regex patterns independently w/ sample log lines
• Monitor Promtail metrics:

  curl http://localhost:9080/metrics | grep promtail

• Check positions file for progress:

  cat /tmp/positions.yaml

Step 4: Query Logs with LogQL

Learn LogQL syntax for filtering + aggregating logs.

Basic queries:

# All logs from a job
{job="app"}

# Logs with specific label values
{job="app", level="error"}

# Regex filter on log line content
{job="app"} |~ "authentication failed"

# Case-insensitive regex
{job="app"} |~ "(?i)error"

# Line filter (doesn't parse, just includes/excludes)
{job="app"} |= "user"  # Contains "user"
{job="app"} != "debug" # Doesn't contain "debug"

Parsing + filtering:

# JSON parsing
{job="app"} | json | level="error"

# Regex parsing with named groups
{job="app"} | regexp "user_id=(?P<user_id>\\d+)" | user_id="12345"

# Logfmt parsing (key=value format)
{job="app"} | logfmt | level="error", service="auth"

# Pattern parsing
{job="nginx"} | pattern `<ip> - <user> [<timestamp>] "<method> <path> <protocol>" <status> <size>` | status >= 500

Aggregations (metrics from logs):

# Count log lines per level
sum by (level) (count_over_time({job="app"}[5m]))

# Rate of error logs
rate({job="app", level="error"}[5m])

# Bytes processed per service
sum by (service) (bytes_over_time({job="app"}[1h]))

# Average request duration from logs
avg_over_time({job="app"} | json | unwrap duration [5m])

# Top 10 error messages
topk(10, sum by (message) (count_over_time({level="error"} [1h])))

Filter by extracted fields:

# Find specific trace in logs
{job="app"} | json | trace_id="abc123def456"

# HTTP 5xx errors from nginx
{job="nginx"} | pattern `<_> "<_> <_> <_>" <status> <_>` | status >= 500

# Failed authentication attempts
{job="app"} | json | message=~"authentication failed" | user_id != ""

Create Grafana explore queries or dashboard panels using these patterns.

→ Queries return expected log lines, filtering works correct, aggregations produce metrics from logs.

If err:

• Use Grafana Explore → debug queries interactive
• Check label names:

  curl http://localhost:3100/loki/api/v1/labels

• Valid. label values:

  curl http://localhost:3100/loki/api/v1/label/{label_name}/values

• Simplify query: start w/ basic label selector, add filters incrementally
• Check time range: logs might not exist in selected window

Step 5: Integrate Logs with Metrics + Traces

Correlate logs w/ Prometheus metrics + distributed traces → unified observability.

Add trace IDs to logs (app instrumentation):

# Python with OpenTelemetry
import logging
from opentelemetry import trace

logger = logging.getLogger(__name__)

def handle_request():
    span = trace.get_current_span()
    trace_id = span.get_span_context().trace_id

    logger.info(
        "Processing request",
        extra={"trace_id": format(trace_id, "032x")}
    )

// Go with OpenTelemetry
import (
    "go.opentelemetry.io/otel/trace"
    "go.uber.org/zap"
)

func handleRequest(ctx context.Context) {
    span := trace.SpanFromContext(ctx)
    traceID := span.SpanContext().TraceID().String()

    logger.Info("Processing request",
        zap.String("trace_id", traceID),
    )
}

Configure Grafana data links from metrics to logs:

In Prometheus panel field config:

{
  "fieldConfig": {
    "defaults": {
      "links": [
        {
          "title": "View Logs",
          "url": "/explore?left={\"datasource\":\"Loki\",\"queries\":[{\"refId\":\"A\",\"expr\":\"{job=\\\"app\\\",instance=\\\"${__field.labels.instance}\\\"} |= `${__field.labels.trace_id}`\"}],\"range\":{\"from\":\"${__from}\",\"to\":\"${__to}\"}}",
          "targetBlank": false
        }
      ]
    }
  }
}

Configure Grafana data links from logs to traces:

In Loki datasource config:

datasources:
  - name: Loki
    type: loki
    url: http://loki:3100
    jsonData:
      derivedFields:
        - datasourceName: Tempo
          matcherRegex: "trace_id=(\\w+)"
          name: TraceID
          url: "$${__value.raw}"

Correlate logs in Grafana Explore:

1. Query metrics in Prometheus
2. Click on data point
3. Select "View Logs" from context menu
4. Loki query auto-pop'd w/ relevant labels + time range
5. Click trace ID in logs
6. Tempo trace view opens w/ full distributed trace

→ Clicking metrics opens related logs, trace IDs in logs link to trace viewer, single pane for metrics/logs/traces navigation.

If err:

• Valid. trace ID format matches regex in derived fields
• Check trace_id label extracted by Promtail pipeline
• Ensure Tempo datasource config'd in Grafana
• Test URL encoding for complex filter exprs
• Valid. data link URLs in incognito/private browser window

Step 6: Set Up Log Retention + Compaction

Configure retention policies + compaction → manage storage costs.

Retention by stream (in Loki config):

limits_config:
  retention_period: 720h  # Global default: 30 days

  # Per-tenant retention (requires multi-tenancy enabled)
  per_tenant_override_config: /etc/loki/overrides.yaml

# overrides.yaml
overrides:
  production:
    retention_period: 2160h  # 90 days for production
  staging:
    retention_period: 360h   # 15 days for staging
  development:
    retention_period: 168h   # 7 days for dev

Retention by stream labels (requires compactor):

compactor:
  working_directory: /loki/compactor
  shared_store: filesystem
  compaction_interval: 10m
  retention_enabled: true
  retention_delete_delay: 2h
# ... (see EXAMPLES.md for complete configuration)

Priority determines which rule applies when multi match (lower number = higher priority).

Compression settings:

chunk_store_config:
  chunk_cache_config:
    enable_fifocache: true
    fifocache:
      max_size_bytes: 1GB
      ttl: 24h
# ... (see EXAMPLES.md for complete configuration)

Monitor retention:

# Check chunk stats
curl http://localhost:3100/loki/api/v1/status/chunks | jq

# Check compactor metrics
curl http://localhost:3100/metrics | grep loki_compactor

# Verify deleted chunks
curl http://localhost:3100/metrics | grep loki_boltdb_shipper_retention_deleted

→ Old logs auto deleted per retention policy, storage usage stabilizes, compaction cuts index size.

If err:

• Enable compactor in Loki config if retention not working
• Check compactor logs:

  docker logs loki | grep compactor

• Valid. retention_enabled: true + retention_deletes_enabled: true
• Monitor disk usage:

  du -sh /loki/

• S3: check bucket lifecycle policies don't conflict w/ Loki retention

Check

• Loki API health check returns 200:

  curl http://localhost:3100/ready

• Promtail successfully scraping logs from all config'd sources
• Labels extracted correct from log lines (visible in Grafana Explore)
• LogQL queries return expected results w/ proper filtering
• Log retention policy enforced (old logs deleted after retention period)
• Logs accessible from Grafana dashboards + Explore view
• Trace IDs from logs link to Tempo trace viewer
• Metrics panels have data links to relevant logs
• Compaction running + cutting storage overhead
• Storage usage w/in allocated disk/S3 budget

Traps

• High cardinality labels: Unbounded label values (user IDs, req IDs) → index explosion. Use fixed labels (level, service, env) + put variables in log lines.
• Missing log parsing: Raw logs w/o label extraction limits query capabilities. Always parse structured logs (JSON, logfmt) or use regex for unstructured.
• Incorrect time parsing: Mismatched timestamp formats → logs out of order or rejected. Test timestamp parsing w/ sample logs.
• Retention not working: Compactor must be enabled for retention to delete old data. Check

  retention_enabled: true

  +

  retention_deletes_enabled: true

  .
• Ingestion rate limits: Default limits (10MB/s) may be too low for high-volume systems. Adjust

  ingestion_rate_mb

  +

  ingestion_burst_size_mb

  .
• Query timeouts: Broad queries over long time ranges can timeout. Use more specific label selectors + shorter time windows.
• Log duplication: Multi Promtail instances scraping same logs create dupes. Use unique labels or positions file coordination.

→

• correlate-observability-signals

  - Unified debugging across metrics, logs, traces using trace IDs

• build-grafana-dashboards

  - Visualize log-derived metrics + create log panels in dashboards

• setup-prometheus-monitoring

  - Metrics provide context for when to query logs during incidents

• instrument-distributed-tracing

  - Add trace IDs to logs for correlation w/ distributed traces