Purging is instructing the CDN to no longer serve a file from cache.
By doing a purge for /images/logo.png, you instruct the CDN to invalidate that cached image on all edge servers globally.
After the purge has completed and a user requests the purged file, the CDN will send a (conditional) request to your origin.
Your origin will then respond by sending the newer version of the file which the CDN will cache and serve to users going forward.
Tip: unrelated to purge, your origin should always sends objects to the CDN with the Last-Modified (or ETag) header. Without one of these headers, the CDN cannot do conditional requests and will always fetch the complete object from origin to satisfy a user request for an expired object, resulting in slow cache miss responses and unnecessary bytes over the wire.
Sending the Last-Modified (or ETag) header speeds up communication between CDN and origin.
CDNs and Purge
Some CDNs don't do invalidation but delete the object from cache. A few CDNs can do both (you choose).
The table below shows differences between CDNs related to purging: speed, capabilities, assurance and more.
Many CDNs nowadays do the purge (near) instantly but on some CDNs it may take several minutes.
Some CDNs (eg. CloudFront) charge for purging if you go over a certain number of purges per month.
Is the object invalidated (expires in cache) or deleted (removed from cache)?
Purge per file, purge all files, purge per directory (recursive or not), purge by file type or file extension, purge by tag or key. Do not assume every CDN has all these capabilities.
When has the purging of objects completed on all CDN POPs? Some CDNs provide a way to know, for every purge request, when the purge was actually completed.
SpeedIs purging (near) instant?
FreeIs purging free with no limits?
TypeInvalidate or delete?
AssuranceCDN provides a way to know when purge completed?
CDNs missing in this table? That is because we don't have the relevant info
More info per CDN
AdvancedHosting CDN for Static Content should soon have purging complete withing a few seconds. VideoCDN purging takes a few minutes.
BelugaCDN exposes completion times and information on how many assets were invalidated by the purge request via the API with real-time feedback in the UI. They support full regular expressions on purge request paths. More info in the docs.
Object invalidations typically take from 10 to 100 seconds to complete. You can check the status of an invalidation by viewing your distribution from the CloudFront console (source) The first 1,000 invalidation paths that you submit per month are free; you pay for each invalidation path over 1,000 in a month. AWS Documentation: Invalidating Objects (Web Distributions Only).
CDN Planets measures real-world purge speed of CloudFront every hour from multiple global locations.
View the CloudFront Purge Speed data.
Purge by tag/key is in development.
According to Edgio, purging happens pretty fast (10 - 30 seconds) most of the time, but it happens regularly that purging is not complete within 60 seconds. It's handy they provide a way to know when the purge has completed.
Edgio supports purging by file extension, e.g. /dir/*.css
CDN Planets measures real-world purge speed of Edgio every hour from multiple global locations.
View the Edgio Purge Speed data.
Single object purges and surrogate key purges are completed across the entire global edge cache in around 150ms. Purge-all operations take longer, and are comparable to the time required to deploy a service configuration, around 10-20 seconds. Fastly's API docs cover all capabilities. To get started you can probably best read the Fastly Core Concepts doc about Purging. Fastly customers may run into the API rate limiting.
A request for purge will be formed within one minute. Then it will be sent to the CDN servers. Purging time depends on the number of files/paths and usually does not take longer than 15 minutes More info in their Purge: purge some or all assets from CDN resource cache article.
CDN Planets measures real-world purge speed of Gcore every hour from multiple global locations.
View the Gcore Purge Speed data.
The average time is 40s, and the completion rate is 99.97% within 5 minutes. For URL purge request, the daily upper limit is 1000. For directory purge request, the daily upper limit is 100.
StackPath states a purge takes place immediately. However, "when StackPath experiences a high load, a purge may take 1 to 2 minutes to complete.". Learn more in the StackPath Help doc at Purge Files from the CDN, including about purge rate limits.
The distribution of the purge request across the network takes approximately 5 minutes. Custom purges such as tag, key or header require Tencent DevOps involvement. 10000 files and 100 folder purges are the default daily values which can be adjusted on special request. Each purge request and status is logged. Logs data is available through the console.