Item value preprocessing allows to define and execute transformation rules for the received item values.
Preprocessing is managed by a preprocessing manager process, which was added in Zabbix 3.4, along with preprocessing workers that perform the preprocessing steps. All values (with or without preprocessing) from different data gatherers pass through the preprocessing manager before being added to the history cache. Socket-based IPC communication is used between data gatherers (pollers, trappers, etc) and the preprocessing process. Either Zabbix server or Zabbix proxy (for items monitored by the proxy) is performing preprocessing steps.
To visualize the data flow from data source to the Zabbix database, we can use the following simplified diagram:
The diagram above shows only processes, objects and actions related to item value processing in a simplified form. The diagram does not show conditional direction changes, error handling or loops. Local data cache of preprocessing manager is not shown either because it doesn't affect data flow directly. The aim of this diagram is to show processes involved in item value processing and the way they interact.
To visualize the data preprocessing process, we can use the following simplified diagram:
The diagram above shows only processes, objects and main actions related to item value preprocessing in a simplified form. The diagram does not show conditional direction changes, error handling or loops. Only one preprocessing worker is shown on this diagram (multiple preprocessing workers can be used in real-life scenarios), only one item value is being processed and we assume that this item requires to execute at least one preprocessing step. The aim of this diagram is to show the idea behind item value preprocessing pipeline.
Item value processing is executed in multiple steps (or phases) by multiple processes. This can cause:
UINT, (trapper item can be used), dependent item has value type
As a result, dependent item receives a value, while master item changes its state to NOT SUPPORTED.
CHARtype is used for master item, then master item value will be truncated at the history synchronization phase, while dependent items will receive their value from the initial (not truncated) value of master item.
Preprocessing queue is a FIFO data structure that stores values preserving the order in which values are revieved by preprocessing manager. There are multiple exceptions to FIFO logic:
To visualize the logic of preprocessing queue, we can use the following diagram:
Values from the preprocessing queue are flushed from the beginning of the queue to the first unprocessed value. So, for example, preprocessing manager will flush values 1, 2 and 3, but will not flush value 5 as value 4 is not processed yet:
Only two values will be left in queue (4 and 5) after flushing, values are added into local data cache of preprocessing manager and then values are transferred from local cache into history cache. Preprocessing manager can flush values from local data cache in single item mode or in bulk mode (used for dependent items and values received in bulk).
Zabbix server configuration file allows users to set count of preprocessing worker processes. StartPreprocessors configuration parameter should be used to set number of pre-forked instances of preprocessing workers. Optimal number of preprocessing workers can be determined by many factors, including the count of “preprocessable” items (items that require to execute any preprocessing steps), count of data gathering processes, average step count for item preprocessing, etc.
But assuming that there is no heavy preprocessing operations like parsing of large XML / JSON chunks, number of preprocessing workers can match total number of data gatherers. This way, there will mostly (except for the cases when data from gatherer comes in bulk) be at least one unoccupied preprocessing worker for collected data.