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Cache expression – remembering a value for a while

The cache expression evaluates another expression once and hands out the stored result until it expires. It changes nothing about the value – only how often it is computed. Use it around expressions that are expensive or call other systems, when a slightly outdated value is acceptable.

1. At a glance
Purpose Stores the value of another expression for a number of minutes
Typical use Entra profile pictures in grids and app bars, results of external HTTP calls, expensive sub queries
Returns The value of the input expression, with its type
Related Image, Sub Query

2. Settings

Setting Meaning Default
"Input" The expression whose value is cached – (required)
"Cache Key" An expression whose result distinguishes the cached values. Every distinct key has its own value. empty – one single value for the whole expression
"User Scoped" Every user has a cache of their own off – all users share the cache
"Expiration (Minutes)" How long a value is kept before the input is evaluated again 60; 0 or less switches caching off

3. Behaviour

  1. nexxcite evaluates "Cache Key" and looks for a stored value for this expression, this key and – with "User Scoped" – this user.
  2. If a value is stored and has not expired, it is returned. "Input" is not evaluated.

  3. Otherwise "Input" is evaluated, the result is stored and returned.

  4. A change to the cache expression itself makes earlier stored values unusable; the next evaluation computes anew.

  5. The cache is kept in the memory of each server and is empty after a restart. With several servers, each server has its own cache, so two users can briefly see different values.
  6. In background processing without a signed-in user, "User Scoped" values are kept apart from all users.

Choosing the key and the scope

The two settings decide whether the cache is correct:

  • "Cache Key" must contain everything the input depends on. A profile picture cached without the user as key shows the first user's picture for everybody; a value per record needs the record id (system function EntitySetRecId) in the key.
  • "User Scoped" is required as soon as the input depends on who evaluates it – the current user, their permissions, what their queries return. A shared cache would show one user's result to the next. Leave it off when the value is the same for everybody; a shared cache saves the most work.

4. Example

Profile pictures in a grid: Input = an Image expression with the source "Entra Profile Picture" and the user from the field "E-mail"; Cache Key = the field "E-mail"; User Scoped off; Expiration = 240. Each picture is read from Microsoft at most once every four hours per server.

5. Limitations

  • Do not cache values that must be current the moment they change – a status, a stock level, a permission check.
  • There is no way to empty the cache by hand other than changing the expression or restarting the application.