📝 (Standard) Write health check route

Author: Arlevoy

ops/health-check-route.s.md

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+# [Standard] Writing a health check route
+
+## Owner: Arthur Levoyer
+
+# Why
+
+In order to monitor correctly their environments, some Cloud services require a HealthCheck route which returns a status depending on how the API is running.
+
+## Checks
+
+- [ ] Do a call to every DB used by the API
+- [ ] Send a 2xx status code status in case of API running correctly and 5xx status code if not
+- [ ] Do the less data usage DB calls
+- [ ] Include a timestamp in order not to reduce the number of successive calls
+
+## Examples
+
+In the examples below the API is concentrating calls to one database RDS and one DynamoDB
+
+### Example 1: Bad example
+
+```javascript
+app.get("/health-check", (req, res) => {
+  res.status(200).send({ status: "OK" });
+});
+```
+
+- There is no call to any of the two databases
+- There is no 5xx status code if the API is not running
+- There is not timestamp hence the route may be called successively every seconds
+
+### Example 2: Bad example
+
+```javascript
+app.get("/health", async (req, res) => {
+  try {
+    await findAllDataCollectors(); //  DataCollectors ~ 100 entries
+    res.status(200).send({ status: "OK" });
+  } catch (error) {
+    res.status(503).send({ status: "KO" });
+  }
+});
+```
+
+- There is a call to one of the database but not the other
+- The call is using too much data
+- There is no timestamp
+- There is a 503 if the DB is down
+
+### Example 3: Good example
+
+```javascript
+app.get("/health", async (req, res) => {
+  try {
+    if (nextFetchingDate && nextFetchingDate > Date.now()) {
+      return res.status(200).send({ status: 200, message: "OK" });
+    }
+
+    nextFetchingDate = Date.now() + TIME_CHECKING_INTERVAL;
+    await Promise.all([AppsDynamoRepo.getDynamoHealth(), getHealth()]);
+
+    res.status(200).send({ status: 200, message: "OK" });
+  } catch (error) {
+    nextFetchingDate = Date.now();
+    handleError(res, error, { status: 503, error: error.message, message: "KO" });
+  }
+});
+```