Rate Limiting and Throttling - SLA-Based Policies | MuleSoft Documentation
# Rate Limiting and Throttling - SLA-Based Policies
The Rate Limiting and Throttling - SLA-Based policies are client ID-based policies that use the ID as a reference to impose limits on the number of requests that each application can make within a period of time. To use these policies, create at least one SLA tier to define request limits as described in the tutorial. Require users to register an application through the API portal for engaging API users. The registration process grants the user a client ID and a client Secret.
You can configure an SLA tier to require manual approval of requests from users who want to register an app to call your API.
## Rate Limiting - SLA-Based Policy
Rate limiting (not SLA-based) restricts the number of requests the API accepts from all applications within a certain time period. After reaching the limit, requests are rejected. Users don’t need to register applications or send Client IDs. SLA-based rate limiting restricts the number of requests by application to your API based on the configuration of an SLA tier. By default, the Client ID and token credentials are expected in the form of query parameters named client_id and client_secret. You can alter the Mule expression that points to this, and obtain these values from elsewhere in the HTTP message.
## Throttling - SLA-Based Policy
SLA-based throttling works much like SLA-based rate limiting, except requests that exceed the SLA limits aren’t rejected, but instead are queued for later reprocessing. These requests are retried periodically according to the configuration until they are either accepted or rejected because they were retried the maximum amount of times.
You can configure the interval between attempts during which requests are not accepted and you can limit the number of attempts.
## SLA-Based Application Registration
If you create at least one SLA tier for an API, a user who requests access from a new application must choose an SLA Tier. You can offer multiple SLA tier choices based on different rate limits and throttling.
## Managing API Access Requests
If the selected SLA Tier is set to automatic, then API Manager instantly approves all requests for API access, and users can immediately send authenticated requests to the API. If the selected SLA Tier was configured with manual approval, an admin of your organization has to approve it before a user can start to send valid requests to your API.
To approve API access requests, go to the API version page, and select Applications.
There you can view details about your pending and processed requests and manage them.
## Creating an SLA Tier or Layered SLAs
First, you create an SLA tier. If you choose manual approval, any application requests for access must be manually approved by an admin in your organization.
Multiple SLA limits are supported under the following conditions:
  * API Manager manages the API
  * Mule Gateway runtime 2.1 or later manages the API


When an SLA tier having more than one limit is used for an API that runs on an Mule Gateway runtime earlier than 2.1, only the first defined limit is enforced.
You can specify multiple throughput limits for a single SLA tier.
  1. In the Add SLA Tier dialog, in Limits, set the first set of limits.
  2. Click the plus icon, and set the next set of limits.
For example:
# of Reqs | Time Period | Time Unit  
---|---|---  
  3. Click Add.


A single SLA tier named gold can offer a limit of 100 requests per second as well as a limit of 10,000 requests per day. This ensures that applications registered on a gold tier don’t exceed a bursting limit of 100 request per second. Traffic bottlenecks are avoided, while ensuring that registered applications get the advertised SLA of 10,000 request per day. By unchecking the visible flag, the first limit of 100 requests is not exposed to application developers at registration time. When you configure only one limit, that limit must be visible.
### Response Headers
Three headers are included in request responses that inform users about the SLA restrictions and inform them when nearing the threshold. When the SLA enforces multiple policies that limit request throughput, a single set of headers pertaining to the most restrictive of the policies provides this information.
For example, a user of your API may receive a response that includes these headers:
```
X-RateLimit-Limit: 20
X-RateLimit-Remaining: 14
X-RateLimit-Reset: 19100
```

Within the next 19100 milliseconds, only 14 more requests are allowed by the SLA, which is set to allow 20 within this time-window.
## Rate Limiting and Throttling in a Mule cluster
The guidelines for rate limiting and throttling in a cluster applies to rate limiting and throttling for SLA-based policies.
## See Also
  * Choosing Rate Limiting for a Cluster
  * Client ID-based Policies