# Advanced Topics
source: https://developer.mastercard.com/open-finance-europe/documentation/unlicensed/insights/api-basics/advanced-topics/index.md

## Concurrent requests {#concurrent-requests}

We do support aggressive concurrency between parallel sessions (subject to rate limits).

We do not support concurrent requests within a single session. As an example, this means that you should not fetch transactions in parallel between multiple accounts for the same session.

## Rate limits {#rate-limits}

All API are subject to rate limiting. Mastercard does not publish its rate limiting methodology, as rate limits may be temporarily adjusted to protect internal and provider infrastructure. `HTTP 429 Too Many Requests` is returned when rate limit traffic is applied. See also the [error message](https://developer.mastercard.com/open-finance-europe/documentation/unlicensed/insights/code-formats/index.md).