Route Reads to a Replica and Writes to Primary
Split the connection so read-heavy endpoints hit the replica and the primary is reserved for writes, cutting read latency by half.
Task
Apply const db = isWrite ? primaryPool : replicaPool; to reduce a measured bottleneck that shows up in the performance budget or Lighthouse audit.
When to use this
Lighthouse reports the page fails the performance budget because of const db = isWrite ? primaryPool : replicaPool;, scoring 72 on mobile.
Prerequisites
- A Lighthouse or WebPageTest baseline measurement
- Access to the build configuration
Steps
1. Configure const db = isWrite ? primaryPool : replicaPool; in the project configuration file with the recommended defaults.
const db = isWrite ? primaryPool : replicaPool;
Expected output: The configuration is accepted and const db = isWrite ? primaryPool : replicaPool; runs without errors on the first invocation.
2. Run the check locally to verify const db = isWrite ? primaryPool : replicaPool; produces the expected result before pushing.
pnpm test
Expected output: Local run matches CI behavior; no surprises after push.
3. Add the gate to the CI pipeline so const db = isWrite ? primaryPool : replicaPool; blocks merge on failure.
- name: Run add read replica routing
run: pnpm test
Expected output: Pull requests with failing const db = isWrite ? primaryPool : replicaPool; are blocked from merging.
Troubleshooting
Problem
The check passes locally but fails in CI
Solution
Pin dependency versions and the runtime to match CI exactly; reproduce with the same Docker image or act.
Problem
Configuration is rejected with a schema error
Solution
Check the tool version matches the config schema; older versions may not support newer fields.
Limitations
- The gate only runs on events that trigger the workflow; locally pushed commits bypass it.
- Configuration drift is possible if someone edits settings outside version control.
Measure before optimizing
Start by measuring the problem. Split the connection so read-heavy endpoints hit the replica and the primary is reserved for writes, cutting read latency by half. The most common mistake is adding the gate without first confirming it catches the issue you care about. Run const db = isWrite ? primaryPool : replicaPool; against the current codebase and inspect the output before wiring it into CI.
const db = isWrite ? primaryPool : replicaPool;Verify the improvement
Once the gate is in place, make failures actionable. A failing const db = isWrite ? primaryPool : replicaPool; should point to the exact file and line, not a generic error. Pin the tool version in the lockfile so a upstream release does not silently change behavior. Review the gate output in every pull request just like a test result.