16. A system comprising: a processor; a memory; an event module configured to control the processor to determine one or more events performed on a first data content object of a file, the one or more events being performed by a first user on a first client device using a data-content-object processing application, the file being stored in a data store, the first data content object being a defined portion less than all of the file; a history buffer module configured to control the processor to store one or more event indications about the one or more events, each of the one or more event indications including event metadata indicating the one or more events performed by the first user on the first data content object; an event notification module configured to control the processor to retrieve a notification threshold condition associated with a second user, wherein the notification threshold condition is based on a notification frequency determined by the second user, the notification threshold condition defining first criteria for determining which of the one or more event indications are relevant to the second user and second criteria for determining based on at least one of the one or more events indications being relevant to the second user whether to generate and send a notification to the second user, the first criteria defining that a particular event indication is relevant to the second user based on, at least in part, whether the second user has collaboratively worked with another user that has viewed the first data content object of the file, the second criteria defining whether to generate and send the notification to the second user based on, at least in part, second user preferences configurable by the second user and defining threshold quality of change to the first data content object before the generating and sending of the notification; and a criterion module configured to control the processor to use the first criteria to determine which of the one or more event indications are relevant to the second user and to use the second criteria to determine whether the at least one of the one or more event indications being relevant to the second user satisfies the threshold quality of change to the first data content object, the event notification module further being configured to generate and send the notification for the second user if the second criteria is satisfied.
16. A system comprising: a processor; a memory; an event module configured to control the processor to determine one or more events performed on a first data content object of a file, the one or more events being performed by a first user on a first client device using a data-content-object processing application, the file being stored in a data store, the first data content object being a defined portion less than all of the file; a history buffer module configured to control the processor to store one or more event indications about the one or more events, each of the one or more event indications including event metadata indicating the one or more events performed by the first user on the first data content object; an event notification module configured to control the processor to retrieve a notification threshold condition associated with a second user, wherein the notification threshold condition is based on a notification frequency determined by the second user, the notification threshold condition defining first criteria for determining which of the one or more event indications are relevant to the second user and second criteria for determining based on at least one of the one or more events indications being relevant to the second user whether to generate and send a notification to the second user, the first criteria defining that a particular event indication is relevant to the second user based on, at least in part, whether the second user has collaboratively worked with another user that has viewed the first data content object of the file, the second criteria defining whether to generate and send the notification to the second user based on, at least in part, second user preferences configurable by the second user and defining threshold quality of change to the first data content object before the generating and sending of the notification; and a criterion module configured to control the processor to use the first criteria to determine which of the one or more event indications are relevant to the second user and to use the second criteria to determine whether the at least one of the one or more event indications being relevant to the second user satisfies the threshold quality of change to the first data content object, the event notification module further being configured to generate and send the notification for the second user if the second criteria is satisfied. 18. The system of claim 16 , wherein the history buffer is dedicated to the first data content object.