refactor: don't fetch bandits if current model is up-to-date

[ci skip]

Author: rasendubi

src/configuration-requestor.ts

@@ -1,9 +1,9 @@
-import { Configuration } from './configuration';
+import { BanditsConfig, Configuration, FlagsConfig } from './configuration';
 import { ConfigurationStore } from './configuration-store';
 import { IHttpClient } from './http-client';
 
 export type ConfigurationRequestorOptions = {
-  wantsBandits?: boolean;
+  wantsBandits: boolean;
 };
 
 // Requests AND stores flag configurations
@@ -22,21 +22,14 @@ export default class ConfigurationRequestor {
   }
 
   async fetchConfiguration(): Promise<Configuration | null> {
-    const configResponse = await this.httpClient.getUniversalFlagConfiguration();
-    if (!configResponse?.response.flags) {
+    const flags = await this.httpClient.getUniversalFlagConfiguration();
+    if (!flags?.response.flags) {
       return null;
     }
 
-    const needsBandits =
-      this.options.wantsBandits &&
-      Object.keys(configResponse.response.banditReferences ?? {}).length > 0;
-
-    const banditsConfig = needsBandits ? await this.httpClient.getBanditParameters() : undefined;
+    const bandits = await this.getBanditsFor(flags);
 
-    return Configuration.fromResponses({
-      flags: configResponse,
-      bandits: banditsConfig,
-    });
+    return Configuration.fromResponses({ flags, bandits });
   }
 
   async fetchAndStoreConfigurations(): Promise<void> {
@@ -45,4 +38,40 @@ export default class ConfigurationRequestor {
       this.configurationStore.setConfiguration(configuration);
     }
   }
+
+  /**
+   * Get bandits configuration matching the flags configuration.
+   *
+   * This function does not fetch bandits if the client does not want
+   * them (`ConfigurationRequestorOptions.wantsBandits === false`) or
+   * we we can reuse bandit models from `ConfigurationStore`.
+   */
+  private async getBanditsFor(flags: FlagsConfig): Promise<BanditsConfig | undefined> {
+    const needsBandits =
+      this.options.wantsBandits && Object.keys(flags.response.banditReferences ?? {}).length > 0;
+    if (!needsBandits) {
+      return undefined;
+    }
+
+    const prevBandits = this.configurationStore.getConfiguration().getBanditConfiguration();
+    const canReuseBandits = banditsUpToDate(flags, prevBandits);
+    if (canReuseBandits) {
+      return prevBandits;
+    }
+
+    return await this.httpClient.getBanditParameters();
+  }
 }
+
+/**
+ * Checks that bandits configuration matches the flags
+ * configuration. This is done by checking that bandits configuration
+ * has proper versions for all bandits references in flags
+ * configuration.
+ */
+const banditsUpToDate = (flags: FlagsConfig, bandits: BanditsConfig | undefined): boolean => {
+  const banditParams = bandits?.response.bandits ?? {};
+  return Object.entries(flags.response.banditReferences ?? {}).every(
+    ([banditKey, reference]) => reference.modelVersion === banditParams[banditKey]?.modelVersion,
+  );
+};