private String generateUrlKey(String appId, String api) {
    StringBuilder builder = new StringBuilder();
    builder.append(appId).append(":").append(api);
    return builder.toString();
  }
Generate unique key for every appID and api pattern.

@param appId the app ID.
@param api the api pattern.
@return the unique key.
src/main/java/com/eudemon/ratelimiter/AbstractUrlRateLimiter.java