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QlikView has a very efficient, patented caching algorithm that effectively eliminates the calculation time for calculations that have been made before. In other words, if you use the “back” button in the toolbar, or if you happen to make a selection that you have made before, you usually get the result immediately. No calculation is necessary.

But how does it work? What is used as lookup ID?

For each object or combination of data set and selection or data sub-set and expression QlikView calculates a digital fingerprint that identifies the context. This is used as lookup ID and stored in the cache together with the result of the calculation.

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Here "calculation" means both the Logical Inference and Chart calculation - or in fact, any expression anywhere. This means that both intermediate and final results of a selection are stored.

There are some peculiarities you need to know about the cache…

  • The cache is global. It is used for all users and all documents. A cache entry does not belong to one specific document or one user only. So, if a user makes a selection that another user already has made, the cache is used. And if you have the same data in two different apps, one single cache entry can be used for both documents.
  • Memory is not returned, when the document is unloaded. Cache entries will usually not be purged until the RAM usage is close to or has reached the lower working set limit. QlikView will then purge some entries and re-use the memory for other cache entries. This behavior sometimes makes people believe there is a memory leak in the product. But have no fear – it should be this way. So, you do not need to restart the service to clear the cache.
  • The oldest cache entries are not purged first. Instead several factors are used to calculate a priority for each cache entry; factors like RAM usage, cost to calculate it again and time since the most recent usage. Entries with a combined low priority will be purged when needed. Hence, an entry that is cheap to calculate again will easily be purged, also if it recently was used. And another value that is expensive to recalculate or just uses a small amount of RAM will be kept for a much longer time.
  • The cache is not cleared when running macros which I have seen some people claim.
  • You need to write your expression exactly right. If the same expression is used in several places, it should be written exactly the same way – Capitalization, same number of spaces, etc. – otherwise it will not be considered to be the same expression. If you do, there should be no big performance difference between repeating the formula, referring to a different expression using the label of the expression or using the Column() function.

The cache efficiently speeds up QlikView. Basically it is a way to trade memory against CPU-time: If you put more memory in your server, you will be able to re-use more calculations and thus use less CPU-time.

HIC

Further reading on the Qlik engine internals:

Symbol Tables and Bit-Stuffed Pointers

Colors, States and State vectors

Logical Inference and Aggregations

72 Comments

It is exactly the same in both Qlik Sense and QlikView.

HIC

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Creator
Creator

Hy Henric

How can I find out that an expression doesn't use cache?

Thank you very much

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Employee
Employee

It might be that I'm wrong, but I'm pretty sure that everything is cached, with very few exceptions. The question is just for how long (and possibly to what extent).

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Creator
Creator

Let's state it in another way.

I have a qv app with section access. And I want to put in place precaching.

How can I check if it's really working? How can I check that when a user open the app, the Qv server is (or is not) using the precached expressions?

And is there a way to monitor the content of the cache?

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Employee
Employee

There is no (straightforward) way to do this today, but I would stay tuned on this area in the near future, that's all I can say.

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Employee
Employee

Cache is generated by users viewing sheet objects (which will render them, and hence calculate the data in them). "Precaching", or Pre-Loading, which is a more correct term, an application in the server, or even (only) opening the app, will only pre-load the data model, and will not generate any object cache, unless the user that opens the app for the first time happen to be on a sheet that contain objects. And only those objects visible/on that sheet will be calculated and subsequently cached.

So, you need a user, of any kind, to "view" the sheet object for the relevant object cache to be generated. Otherwise, you only have the data model loaded into memory.

Also, if you have a data model with section access, I absolutely assume that will affect the cache in the sense that you will only "share" cache with users who have section access permissions that intersects enough with your own for them to "overlap" in the sense of what you can/are allowed to see in the data model, and hence what is cached.

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