It is desired to try to increase the security of a computing system running computer applications that may access data in a data storage system. In some embodiments, a token associates a user with a task that is being executed by a computing node. It may therefore be possible to determine which user executed which tasks. In some embodiments, the validity of a token is tied to the lifespan of a task associated with the token, rather than to a fixed amount of time. Therefore, if the task associated with the token is complete, the token may become invalid, rather than remaining valid for a duration of time that possibly exceeds the lifespan of the associated task. In some embodiments, a token is used to enforce data access control, e.g. to deny certain users access to certain data in the data storage system.