Write HealthCheck route

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, backend services require a HealthCheck route which returns a status depending on how the API is running.
+
+## Checks
+
+- [ ] Make a call to every database used by the API
+- [ ] Send a 2xx status code status in case of API running correctly and 5xx status code if not
+- [ ] Make the less data usage database calls: the health check route is likely to be called very often in short period of time
+
+## 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
+
+### Example 2: Bad example
+
+```javascript
+app.get("/health", async (req, res) => {
+  try {
+    await RDS.getAllEntries();
+    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 a 503 if the DB is down
+
+### Example 3: Good example
+
+```javascript
+app.get("/health", async (req, res) => {
+  try {
+    await Promise.all([DynamoDB.getDynamoHealth(), RDS.getHealth()]);
+    res.status(200).send({ status: 200, message: "OK" });
+  } catch (error) {
+    handleError(res, error, { status: 503, error: error.message, message: "KO" });
+  }
+});
+
+RDS.getHealth = async () => {
+  await knex.raw("select 1+1 as result");
+};
+
+DynamoDB.getDynamoHealth = (): Promise<Array<Object>> => {
+  return new Promise((resolve, reject) => {
+    dynamodb.describeTable({ TableName: dynamodbTableName }, (error, data) => {
+      if (error) {
+        return reject(error);
+      }
+      resolve(data);
+    });
+  });
+};
+```