A computer-implemented method, operable on a device in a content delivery network, wherein said content delivery network delivers content on behalf of at least one content provider, and wherein the content associated with said at least one content provider comprises at least one property, said device implementing a content delivery service, the method comprising, by said service on said device: maintaining property invalidation information on said content delivery service, said property invalidation information comprising, for each property handled by said content delivery service, a list of one or more group invalidation commands, each of said one or more group invalidation commands specifying a group of zero or more resources; in response to receipt by said content delivery service of at least one new group invalidation command for a particular property, attempting to add said new group invalidation command to said property invalidation information by: if a size of said list of one or more group invalidation commands for said particular property does not exceed a particular threshold, then inserting said new group invalidation command into said list; otherwise, if a size of said list of one or more group invalidation commands for said particular property exceeds a particular threshold, then (i) determining a plurality of merge candidates, where each of the plurality of merge candidates comprise two or more group invalidation commands which have a prefix in common between the two or more group invalidation commands, (ii) selecting a merge candidate from the plurality of merge candidates based on said merge candidate having the smallest performance impact on valid resources amongst the plurality of merge candidates, (iii) merging the two or more group invalidation commands of the selected merge candidate to form a merged invalidation command, (iv) inserting said merged invalidation command into said list, and (v) removing one or more group invalidation commands used in said merging from said list. maintaining property invalidation information on said content delivery service, said property invalidation information comprising, for each property handled by said content delivery service, a list of one or more group invalidation commands, each of said one or more group invalidation commands specifying a group of zero or more resources; in response to receipt by said content delivery service of at least one new group invalidation command for a particular property, attempting to add said new group invalidation command to said property invalidation information by: if a size of said list of one or more group invalidation commands for said particular property does not exceed a particular threshold, then inserting said new group invalidation command into said list; otherwise, if a size of said list of one or more group invalidation commands for said particular property exceeds a particular threshold, then (i) determining a plurality of merge candidates, where each of the plurality of merge candidates comprise two or more group invalidation commands which have a prefix in common between the two or more group invalidation commands, (ii) selecting a merge candidate from the plurality of merge candidates based on said merge candidate having the smallest performance impact on valid resources amongst the plurality of merge candidates, (iii) merging the two or more group invalidation commands of the selected merge candidate to form a merged invalidation command, (iv) inserting said merged invalidation command into said list, and (v) removing one or more group invalidation commands used in said merging from said list. if a size of said list of one or more group invalidation commands for said particular property does not exceed a particular threshold, then inserting said new group invalidation command into said list; otherwise, if a size of said list of one or more group invalidation commands for said particular property exceeds a particular threshold, then (i) determining a plurality of merge candidates, where each of the plurality of merge candidates comprise two or more group invalidation commands which have a prefix in common between the two or more group invalidation commands, (ii) selecting a merge candidate from the plurality of merge candidates based on said merge candidate having the smallest performance impact on valid resources amongst the plurality of merge candidates, (iii) merging the two or more group invalidation commands of the selected merge candidate to form a merged invalidation command, (iv) inserting said merged invalidation command into said list, and (v) removing one or more group invalidation commands used in said merging from said list.