You are presented with multiple paper abstracts, alongside the associated literature review and chapter titles. Given the provided information, you are going to do a literature review. Please generate a coherent literature review that synthesizes the content, draws connections between different works, highlights major findings, and identifies any gaps in the existing research.

Literature Review Title: A Survey on Content Placement Algorithms for Cloud-Based Content Delivery Networks 
Chapter Title:  2) PUSH VS. PULL 
1. Abstract of Cited Paper (BIB001):  This paper investigates the optimal replication of data objects at hierarchical and transparent Web proxies. By transparent, we mean the proxies are capable of intercepting users' requests and forwarding the requests to a higher level proxy if the requested data are not present in their local cache. Two cases of data replication at proxies are studied: 1) proxies having unlimited storage capacities and 2) proxies having limited storage capacities. For the former case, an efficient algorithm for computing the optimal result is proposed. For the latter case, we prove the problem is NP-hard, and propose two heuristic algorithms. Extensive simulations have been conducted and the simulation results have demonstrated significant performance gain by using the proposed data replication algorithms and also shown the proposed algorithms out-perform the standard Web caching algorithm (LRU threshold method). 
2. Abstract of Cited Paper (BIB002):  The addition of storage capacity in network nodes for the caching or replication of popular data objects results in reduced end-user delay, reduced network traffic, and improved scalability. The problem of allocating an available storage budget to the nodes of a hierarchical content distribution system is formulated; optimal algorithms, as well as fast/efficient heuristics, are developed for its solution. An innovative aspect of the presented approach is that it combines all relevant subproblems, concerning node locations, node sizes, and object placement, and solves them jointly in a single optimization step. The developed algorithms may be utilized in content distribution networks that employ either replication or caching/replacement. In addition to reducing the average fetch distance for the requested content, they also cater to load balancing and workload constraints on a given node. Strictly hierarchical, as well as hierarchical with peering, request routing models are considered. 
3. Abstract of Cited Paper (BIB003):  Geographically distributed cloud platforms enable an attractive approach to large-scale content delivery. Storage at various sites can be dynamically acquired from (and released back to) the cloud ... 
4. Abstract of Cited Paper (BIB004):  Cloud computing and 'Storage As A Service' (SaaS) are experiencing a momentous popularity increase due to its flexible, and scalable access to resources. Especially, cloud storage is becoming an economical alternative to traditional content delivery networks (CDNs) such as Akamai and Limelight Networks for moderate-size content providers. Previous research on content distribution mainly focuses on reducing latency experienced by content customers. A few recent studies address the issue of bandwidth usage in CDNs, as the bandwidth consumption is an important issue due to its relevance to the cost of content providers. However, few researches consider both bandwidth consumption and delay performance for the content providers that use cloud storages with limited budgets, which is the focus of this paper. We develop an efficient light-weight approximation algorithm toward the joint optimization problem of content placement. We also conduct the analysis of its theoretical complexities. The performance bound of the proposed approximation algorithm exhibits a much better worst case than those in previous studies. We further extend the approximate algorithm into a distributed version that allows it to promptly react to dynamic changes in users' interests. The extensive results from both simulations and Planetlab experiments exhibit that the performance is near optimal for most of the practical conditions.

Literature Review:
Despite advances in CP algorithms for CCDNs, various limitations are present for future research. Markedly, a crucial objective includes finding the correct niche for caching in the evolving CP algorithms. They have shown promising results in traditional CDNs BIB001 and in today's CCDNs BIB004 . Jia et al. BIB002 propose a hybrid between push-and pullbased CP that can tremendously improve the performance of CP algorithms for client-side web-server proxies. Furthermore, Guan and Choi BIB005 show the benefits of a hybrid approach by pushing some content pro-actively while pulling others on demand to optimize content delivery using the storage clouds. Therefore, it is important to realize the two approaches are complementary for content placement in CDNs BIB003 . The key research challenges include analyzing (i) the effect of caching on QoS, (ii) the effect of pushing or prepositioning content on operational cost and (iii) the net gain from pull-based or push-based content placement algorithms.