Add experimental feature to reduce database writes for channel data

src/channel-storage/dbstore.js

@@ -1,10 +1,19 @@
 import Promise from 'bluebird';
 import { ChannelStateSizeError } from '../errors';
 import db from '../database';
+import { Counter } from 'prom-client';
 
 const LOGGER = require('@calzoneman/jsli')('dbstore');
 const SIZE_LIMIT = 1048576;
 const QUERY_CHANNEL_DATA = 'SELECT `key`, `value` FROM channel_data WHERE channel_id = ?';
+const loadRowcount = new Counter({
+    name: 'cytube_channel_db_load_rows_total',
+    help: 'Total rows loaded from the channel_data table'
+});
+const saveRowcount = new Counter({
+    name: 'cytube_channel_db_save_rows_total',
+    help: 'Total rows saved in the channel_data table'
+});
 
 function queryAsync(query, substitutions) {
     return new Promise((resolve, reject) => {
@@ -39,6 +48,8 @@ export class DatabaseStore {
         }
 
         return queryAsync(QUERY_CHANNEL_DATA, [id]).then(rows => {
+            loadRowcount.inc(rows.length);
+
             const data = {};
             rows.forEach(row => {
                 try {
@@ -53,35 +64,47 @@ export class DatabaseStore {
         });
     }
 
-    save(id, channelName, data) {
+    async save(id, channelName, data) {
         if (!id || id === 0) {
-            return Promise.reject(new Error(`Cannot save state for [${channelName}]: ` +
-                                            `id was passed as [${id}]`));
+            throw new Error(
+                `Cannot save state for [${channelName}]: ` +
+                `id was passed as [${id}]`
+            );
         }
 
         let totalSize = 0;
         let rowCount = 0;
         const substitutions = [];
+
         for (const key in data) {
             if (typeof data[key] === 'undefined') {
                 continue;
             }
+
             rowCount++;
+
             const value = JSON.stringify(data[key]);
             totalSize += value.length;
+
             substitutions.push(id);
             substitutions.push(key);
             substitutions.push(value);
         }
 
+        if (rowCount === 0) {
+            return;
+        }
+
+        saveRowcount.inc(rowCount);
+
         if (totalSize > SIZE_LIMIT) {
-            return Promise.reject(new ChannelStateSizeError(
+            throw new ChannelStateSizeError(
                     'Channel state size is too large', {
                 limit: SIZE_LIMIT,
                 actual: totalSize
-            }));
+            });
         }
 
-        return queryAsync(buildUpdateQuery(rowCount), substitutions);
+        return await queryAsync(buildUpdateQuery(rowCount), substitutions);
     }
 }