Capture Output Via Sidechannel

Sidechannel Output Capture

When the orchestrator's log retrieval is missing, broken, or asynchronous, don't fight it — make the task persist its own output to somewhere you can query.

When to use

• Running a one-shot container in a system with no log-fetch API (managed PaaS, certain CI systems)
• Job runner that streams logs to a UI but offers no programmatic export
• Long-running batch job where logs roll over before you can grep them
• Headless agent contexts where shell access to the runtime host is unavailable

When NOT to use

• You can just run docker logs <id> or kubectl logs <pod> — do that
• The runtime emits a structured exit signal you actually trust (HTTP webhook, exit code only)
• Output contains secrets you must NOT persist beyond the run

The Pattern

1. Capture stdout+stderr inside the runner with tee, preserving the real exit code.
2. Encode the captured file (base64) to dodge SQL-quoting hell.
3. Insert as a single row/key in a data store you already have credentials for.
4. Exit with the original exit code (preserves up-stream pass/fail signaling).
5. Read from the data store afterwards.

Reference: bash + PostgreSQL

#!/bin/bash
set -e

# 1. Run the actual work, capture, preserve exit code
set +e
your_real_command 2>&1 | tee /tmp/run.log
EXITCODE=${PIPESTATUS[0]}
set -e

# 2. Ensure target table exists (idempotent)
export PGPASSWORD="$DB_PASSWORD"
psql -h "$DB_HOST" -U "$DB_USER" -d "$DB_NAME" -v <<'SQL'
CREATE TABLE IF NOT EXISTS _run_log (
  id SERIAL PRIMARY KEY,
  job TEXT,
  exit_code INT,
  output TEXT,
  created_at TIMESTAMPTZ DEFAULT NOW()
);
SQL

# 3. Encode + insert (base64 dodges quoting)
B64=$(base64 -w0 /tmp/run.log)
psql -h "$DB_HOST" -U "$DB_USER" -d "$DB_NAME" -v \
  -c "INSERT INTO _run_log(job, exit_code, output)
      VALUES ('$JOB_NAME', $EXITCODE,
              convert_from(decode('$B64', 'base64'), 'UTF8'))"

# 4. Exit with the real code
exit $EXITCODE

Read it back later with SELECT output FROM _run_log ORDER BY id DESC LIMIT 1.

Reference: Redis variant (when you don't have SQL)

B64=$(base64 -w0 /tmp/run.log)
redis-cli -h "$REDIS_HOST" -a "$REDIS_PASSWORD" \
  SET "run-log:$JOB_NAME:$(date +%s)" "exit=$EXITCODE;$B64" EX 86400

Reference: Public webhook (when you have neither)

curl -sS -X POST "https://ntfy.sh/<your-unique-topic>" \
  -H "Title: $JOB_NAME exit=$EXITCODE" \
  --data-binary @/tmp/run.log

Then curl "https://ntfy.sh/<topic>/json?poll=1" to retrieve. Warning: ntfy/webhook.site are public — never send secrets, tokens, or PII.

Why base64 instead of dollar-quoting

-- Naive: breaks if output contains '$$', single quotes, backticks, etc.
INSERT INTO log VALUES ($$REPORT_PLACEHOLDER$$);

Output from real programs contains arbitrary bytes — terminal escape codes, JSON with quotes, stack traces with $. Base64 makes the payload opaque to the SQL parser. The convert_from(decode(..., 'base64'), 'UTF8') decodes back to text on the server side.

Critical: preserve the exit code

The whole point of running a task is the pass/fail signal. Naive piping breaks it:

# WRONG: exit code is from `tee`, always 0
your_command 2>&1 | tee log.txt
# upstream sees success even when your_command failed

# RIGHT: capture exit before tee mangles it
set +e
your_command 2>&1 | tee log.txt
EXITCODE=${PIPESTATUS[0]}
set -e

Then exit $EXITCODE at the end so the orchestrator can still see failure.

Common Mistakes