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1. create "TaskQueue" database table with following columns:
   • Id
   • QueuedOn - when task was queued
   • QueuedById - who queued the task
   • ScheduledOn - when task needs to be processed (set at queuing time)
   • TaskClass - which PHP class is responsible for processing this task queue record
   • TaskData - data, that is needed for task execution (e.g. e-mail recipient, IDs of records to be processed)
   • LastStatus - status from last attempt of this queue record processing - {scheduled (default), processing, success, failed}
   • MaxRetries - if task fails specified number of times (5 by default), then don't retry it
   • FailedRetries - failed retry count (number is reset, when task execution was successful)
2. create "TaskQueueDetails" database table with following columns:
   • Id
   • TaskQueueId - associated task queue record
   • StartedOn - when task was started executing
   • FinishedOn - when task was finished executing
   • Status - {processing (default), success, error, timeout} - create record right before execution start; error - when known error happened during processing; timeout - when status wasn't created within 1 day
   • ErrorCode - non-empty when known error happened
   • ErrorMessage - non-empty when known error happened
3. probably there won't be any UI for this functionality, because it's too general to be usable by user, but specialized sections (e.g. "E-mail Queue") can read data from these tables to keep user updated
4. task can be created by whoever needs it, e.g. user presses a button, scheduled task decides to offload some work, etc.
5. a scheduled task needs to be created to process records from "TaskQueue" table:
   1. pick records with "Status = scheduled" and "ScheduledOn < NOW()"
   2. create class instance (from "TaskClass" field) responsible for task processing
   3. if class isn't found, then:
      1. create record in "TaskQueueDetails" table with "error" status and error message set to "Task class not found"
      2. set "FailedRetries" to "MaxRetries"
   4. if class found:
      1. create record in "TaskQueueDetails" table with "processing" status
      2. let class to process task
      3. update record in "TaskQueueDetails" table by setting "success" status (if no errors happened), "error" status (and ErrorCode/ErrorMessage fields) when error happened
6. the "task-queue-detail" unit would automatically update associated "task-queue" record to indicate last status and such fields
7. delete data about successfully executed tasks after 1 month (configurable as log rotation) since task was executed

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