Accounts may be pre-configured with a set of thresholds (or limits) to manage capacity and prevent abuse of the system. The system recognizes two kinds of limits: rate limits and absolute limits. Rate limits are thresholds that are reset after a certain amount of time passes. Absolute limits are fixed. Limits are configured by operators and may differ from one deployment of the OpenStack Compute service to another. Please contact your provider to determine the limits that apply to your account. Your provider may be able to adjust your account’s limits if they are too low. Also see the API Reference for *Limits*.
Rate limits are specified in terms of both a human-readable wild-card URI and a machine-processable regular expression. The human-readable limit is intended for displaying in graphical user interfaces. The machine-processable form is intended to be used directly by client applications.
The regular expression boundary matcher “^” for the rate limit takes effect after the root URI path. For example, the regular expression ^/servers would match the bolded portion of the following URI: https://servers.api.openstack.org/v2/3542812/servers.
Table: Sample rate limits
| Verb | URI | RegEx | Default |
| POST | * | .* | 120/min |
| POST | */servers | ^/servers | 120/min |
| PUT | * | .* | 120/min |
| GET | *changes-since* | .*changes-since.* | 120/min |
| DELETE | * | .* | 120/min |
| GET | */os-fping* | ^/os-fping | 12/min |
Rate limits are applied in order relative to the verb, going from least to most specific.
In the event a request exceeds the thresholds established for your account, a 413 HTTP response is returned with a Retry-After header to notify the client when they can attempt to try again.
Absolute limits are specified as name/value pairs. The name of the absolute limit uniquely identifies the limit within a deployment. Please consult your provider for an exhaustive list of absolute value names. An absolute limit value is always specified as an integer. The name of the absolute limit determines the unit type of the integer value. For example, the name maxServerMeta implies that the value is in terms of server metadata items.
Table: Sample absolute limits
| Name | Value | Description |
| maxTotalRAMSize | 51200 | Maximum total amount of RAM (MB) |
| maxServerMeta | 5 | Maximum number of metadata items associated with a server. |
| maxImageMeta | 5 | Maximum number of metadata items associated with an image. |
| maxPersonality | 5 | The maximum number of file path/content pairs that can be supplied on server build. |
| maxPersonalitySize | 10240 | The maximum size, in bytes, for each personality file. |