1. An invalidation tracker system for tracking messages in a caching architecture of a pricing and shopping platform, wherein the caching architecture includes a number of levels, in which each level comprises one or more servers, in which invalidation messages are communicated from one level to another in order to send invalidation messages to all servers in the caching architecture, and wherein the invalidation tracker system receives data from provider databases to be communicated to the servers in the caching architecture; wherein the invalidation tracker system includes: a recording module for recording all invalidation messages communicated to the servers in the caching architecture to form a set of sent invalidation messages; an analysing module for determining the invalidation messages received at each server in the caching architecture and comparing this with the set of sent invalidation messages to identify one or more undelivered invalidation messages; and a replay module for resending the one or more identified undelivered invalidation messages to an appropriate server in the caching architecture.