Function: cache()
Build and use a set of values across the records in a stream set.
The first parameter is the cache key expression. This is evaluated and the result used as the key value for the cache. Before the subsequent expressions are evaluated the key value is used to retrieve the current value from the cache associated with that key. This retrieved value is then assigned to the internal variable _cacheValue. If there was no value in the cache for this key then _cacheValue is set to _NULL.
This cache key expression is followed by a number of additional expressions. These are evaluated in turn, and the value returned by the final expression is placed into the cache against the original key value.
The final cache value is returned by the expression.
Please note that the internal variable _cacheValue should not be used outside of the cache function.
Snychronisation
The cache function "synchronises" itself across each call. In PhixFlow, to make best use of modern computer architectures with parallel processing capabilities, when you run analysis the work to be done is divided up into parcels. These may be processed in any order. However, to make sure that the results from the cache function remain consistent, PhixFlow ensures that calls to the cache function are "synchronised" across all these parcels of work.
It is only necessary to be concerned about synchronisation if the same key will be used across multiple parcels of work. However, this consideration can become complex, so as a general rule, try and use a single call to the cache function in the stream configuration.
Example
You have a calculate stream processing, say, 10,000 records - and you want to accumulate totals based on a key value, of which there are around 100 across the data set. For each key value, many records will be added to the total. To do this, you should use an call to the cache function such as:
...
cache ( _out.KeyVal,
if (_cacheValue == _NULL, $currentValue = 0, $currentValue = _cacheValue,
$currentValue = $currentValue + _out.CashValue,
$currentValue
)
...
and not like:
... $currentValue = cache(_out.KeyVal), if ($currentValue == _NULL, cache(_out.KeyVal, _out.CashValue), cache(_out.KeyVal, $currentValue + _out.CashValue), ...
Syntax
cache(cache key, expression1, expression2, [,..., expressionN])
| Argument | Type | Description |
|---|---|---|
| cache key | Expression | Any valid Expression |
| expressionN | Expression | Any valid Expression |
Examples
$result = cache("accountRef", _cacheValue + 1)
Get the cache value for key value "accountRef", add one to this and write this back to the cache. Finally return the new cache value associated with this key and assign it to the variable $result.
cache("accoutRef")
Return the cache value for key value "accountRef".