Each object is associated with one root metadata record and one or more version-specific metadata records, each corresponding to a version of object data. A conditional command may be conditioned upon whether an object is stored in a storage system. Whether the condition is satisfied is determined based on whether a root metadata record of the object exists. If the condition is satisfied, then metadata is updated to reflect execution of the conditional command. A conditional command may be conditioned upon whether a data version identifier or a metadata version identifier equals a particular value. A conditional command execution engine retrieves the relevant version identifier from a version-specific metadata record, and performs a check on whether the condition is satisfied. If the condition is satisfied, the engine ensures that the version-specific record has not been modified during the check, and updates metadata to reflect execution of the conditional command.