Time to Live (TTL) Value

TTL Value Definition

Time-to-live (TTL) is a numerical value used for data validity and expiration. A TTL stands for the amount of time data should remain valid and available before a computing system discards it. For example, it could represent the number of seconds certain data, or a certain data record, is retained or is considered valid. TTLs are used all across the computing industry, from databases and event streaming systems, to content delivery networks (CDNs), to networking devices such as routers. Each TTL implementation will vary depending on its design goals, pragmatic purpose, and use case.

A TTL differs from an explicit timeout setting in that it is relative. For instance, a TTL could be set to “86400,” representing the number of seconds in a day. This would be very different than setting a specific date and time timeout of “2022-12-03 17:00:00.000+0500.” Consider TTL more of a countdown timer. As a relative value, it also is easier to reset. For example, a trigger could reset a TTL to a specific value rather than having to compute at that moment the new exact date and time for expiration.

Here are a range of examples to show the wide variations possible:

• In Domain Name System (DNS) servers, TTL sets the length of time a DNS cache server can serve a DNS record before getting new data from the authoritative DNS server. Content Delivery Networks (CDNs) use TTL values to determine how long to cache data and serve it from a CDN edge server before retrieving updated information from an origin server. Ensuring an optimal amount of time between pulls from the origin server enables a CDN to reduce required bandwidth from the origin while efficiently serving content closer to users.
• Content Delivery Networks (CDNs) use TTL values to determine how long to cache data and serve it from a CDN edge server before retrieving updated information from an origin server. Ensuring an optimal amount of time between pulls from the origin server enables a CDN to reduce required bandwidth from the origin while efficiently serving content closer to users.
• In the World Wide Web’s hypertext transfer protocol (HTTP), TTL value represents how many seconds the web server can return cached content before it must check again to ensure the content is fresh. Web server settings define a default TTL value.
• In data networking, TTLs are applied to packets so that the packet is discarded entirely if it exists too long in a network without reaching its destination. TTL in IPv4 was supposed to represent the number of seconds remaining in the valid lifespan of a packet. However, due to the high speed of networking equipment these days, it is often used as a simple “hop count,” decremented by one for each router the packet passes through.
• In databases, TTLs provide the functionality to automatically delete expired data. The TTL can be set when defining a Table, or when using the INSERT and UPDATE queries. It’s measured in seconds. If the field is not updated within the TTL, it is deleted. The expiration works at the individual column level, while a default for an entire table or row can also be used. This provides a lot of flexibility.