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manual:088:2_basics.0_principles_of_operation [2011/01/10 23:39]
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manual:088:2_basics.0_principles_of_operation [2019/10/29 15:16] (current)
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 ===== Principles of Operation ===== ===== Principles of Operation =====
  
-The image below shows the building blocks of a Cacti infrastructure:\\ +The image below shows the building blocks of a Cacti infrastructure:
 {{:manual:087:cacti_schema_small.png|Functional Scheme}} {{:manual:087:cacti_schema_small.png|Functional Scheme}}
  
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 | **Graph Template Item**\\ as a part of a Graph Template | graph element | This is a complex one. Each item will create parts of an RRDTool graph statement. Typically, this will include <html><ul><li>the (reference to the) DEF needed,</li><li>LINEx/AREA/STACK along with a color for graph elements or </li><li>GPRINTs for legends, </li><li>(reference to a ) CDEF and </li><li> textual elements</li></ul></html> | | **Graph Template Item**\\ as a part of a Graph Template | graph element | This is a complex one. Each item will create parts of an RRDTool graph statement. Typically, this will include <html><ul><li>the (reference to the) DEF needed,</li><li>LINEx/AREA/STACK along with a color for graph elements or </li><li>GPRINTs for legends, </li><li>(reference to a ) CDEF and </li><li> textual elements</li></ul></html> |
 | **Graph**\\ as a real RRDTool graph statement, created when applying a Data Template to a Device | RRDTool graph statement | Whole statement, including all options and graph elements | | **Graph**\\ as a real RRDTool graph statement, created when applying a Data Template to a Device | RRDTool graph statement | Whole statement, including all options and graph elements |
 +\\ 
 +Some words on rrdtool lingo
 +^keyword^meaning^
 +|**step** | A time-variable data in intervals of a certain length.|
 +|**primary data point (PDP)** | The value for a specific step, that has been interpolated|
 +|**consolidation function (CF)** | Used to conolidate multiple primary data points (PDPs). Typical consolidation functions are average, minimum, maximum.|
 +|**consolidated data point (CDP)** | The result of multiple primary data points (PDPs) after having a consolidation function (CF) applied to them.|
 +|**round-robin archive (RRA)** | Stores a fixed amount of CDPs and specifies how many PDPs should be consolidated in to one CDP and which CF to use.|
 +|**round-robin database (RRD)** | A database that contains multiple RRAs.|
 +\\ 
 +
 +== rrdtool inner workings ==
 +\\ Cacti graphs are stored in a //round-robin database (RRD)//, also known as a circular buffer, thus the system storage footprint remains constant over time.
 +
 +RRDtool assumes time-variable data in intervals of a certain length. This interval, usually named //step//, is specified upon creation of an RRD file and cannot be changed afterwards. Because data may not always be available at just the right time, RRDtool will automatically interpolate any submitted data to fit its internal time-steps.
 +
 +The value for a specific step, that has been interpolated, is named a //primary data point (PDP)//. Multiple primary data points may be consolidated according to a //consolidation function (CF)// to form a //consolidated data point (CDP)//. Typical consolidation functions are average, minimum, maximum and last.
 +
 +After the data has been consolidated, the resulting CDP is stored in a //round-robin archive (RRA)//. A round-robin archive stores a fixed amount of CDPs and specifies how many PDPs should be consolidated into one CDP and which CF to use.
 +
 +* In other words: Multiple primary data points (PDPs) can be consolidated using a consolidation function (CF) such as average, minimum or maximum. Once the CF has been performed, the result is known as a consolidated data point (CDP). This information is stored in an round-robin archive (RRA).
 +
 +After this time the archive will “wrap around”: the next insertion will overwrite the oldest entry. This behavior is sometimes referred to as “round-robin” and is the reason for the program's name.
  
-You may be put off by all those template stuffIf you like a more practical approach, just skip to Why Templates?.+To cover several timespans and/or use several consolidation functions, an RRD file may contain multiple RRAsThe data retrieval function of RRDtool automatically selects the archive with the highest resolution that still covers the requested timespan. This mechanism is also used by RRDtool's graphing subsystem.
  
 ==== Graph Overview ==== ==== Graph Overview ====





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