Patent Publication Number: US-11645233-B2

Title: Distributed file cache

Description:
BACKGROUND OF THE INVENTION 
     The present disclosure generally relates to distributed system management, and more specifically, to file management in a distributed file cache system. 
     A distributed cache is an extension of the traditional concept of the cache used in a single locale. The distributed cache is a method of configuring a cache to span multiple nodes (e.g., physical machines or virtual machines) in a distributed system, so as to enable quick file retrieval. Some traditional schemes implement a distributed file cache system that includes a storage node for storing files persistently and a plurality of client nodes, which are coupled to the storage node for caching files. 
     SUMMARY 
     In a first aspect, embodiments of the present disclosure provide a method of file management. The method comprises, in response to determining that at least one client node is obtaining a file of a first version stored at a storage node, generating contact information indicating that the file of the first version is accessible from the storage node and the at least one client node. The method further comprises recording the contact information into a distributed hash table associated with a plurality of nodes including the storage node and the at least one client node. The method further comprises generating first version information indicating that the file is of the first version. In addition, the method comprises recording the first version information into a blockchain associated with the plurality of nodes. 
     In a second aspect, embodiments of the present disclosure provide a method of file management. The method comprises receiving a first request to access a file from a user at a client node. The method further comprises obtaining first version information about the file from a blockchain associated with a plurality of nodes including the client node, the first version information indicating that the file is of a first version. The method further comprises determining a set of nodes from which the file of the first version is accessible based on a distributed hash table associated with the plurality of nodes, the set of nodes being included in the plurality of nodes. In addition, the method comprises obtaining the file of the first version from at least one node in the set of nodes. 
     It is to be understood that the summary is not intended to identify key or essential features of embodiments of the present invention, nor is it intended to be used to limit the scope of the present embodiment. Other features of the present embodiment will become easily comprehensible through the description below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Through the more detailed description of some embodiments of the present embodiment in the accompanying drawings, objects, features, and advantages of the present embodiment will become more apparent, wherein the same reference generally refers to the same components in the embodiments of the present embodiment. 
         FIG.  1    depicts a cloud computing node, in accordance with an embodiment of the present invention. 
         FIG.  2    depicts a cloud computing environment, in accordance with embodiments of the present invention. 
         FIG.  3    depicts abstraction model layers, in accordance with embodiments of the present invention. 
         FIG.  4    depicts an example data processing environment in which embodiments of the present invention can be implemented, in accordance with embodiments of the present invention. 
         FIG.  5 A  depicts an example diagram for updating a file, in accordance with embodiments of the present invention. 
         FIG.  5 B  depicts an example diagram for updating a file, in accordance with embodiments of the present invention. 
         FIG.  5 C  depicts an example diagram for updating a file, in accordance with embodiments of the present invention. 
         FIG.  6    depicts a flowchart of an example method for joining a distributed file cache system, in accordance with embodiments of the present invention. 
         FIG.  7    depicts an example diagram for caching a file in the distributed file cache system, in accordance with embodiments of the present invention. 
         FIG.  8    depicts a flowchart of an example method for caching a file in the distributed file cache system, in accordance with embodiments of the present invention. 
         FIG.  9    depicts an example diagram for accessing a file cached in the distributed file cache system, in accordance with embodiments of the present invention. 
         FIG.  10    depicts a flowchart of an example method of searching for file contact information in the distributed hash table, in accordance with embodiments of the present invention. 
         FIG.  11    depicts an example diagram for reading a file cached in the distributed file cache system, in accordance with embodiments of the present invention. 
         FIG.  12    depicts an example diagram for updating a file cached in the distributed file cache system, in accordance with embodiments of the present invention. 
         FIG.  13    depicts a flowchart of an example method for accessing a file cached in the distributed file cache system, in accordance with embodiments of the present invention. 
         FIG.  14    depicts a flowchart of an example method for reading a file cached in the distributed file cache system, in accordance with embodiments of the present invention. 
         FIG.  15    depicts a flowchart of an example method for updating a file cached in the distributed file cache system, in accordance with embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Some embodiments will be described in more detail with reference to the accompanying drawings, in which the embodiments of the present disclosure have been illustrated. However, the present disclosure can be implemented in various manners, and thus should not be construed to be limited to the embodiments disclosed herein. 
     It is to be understood that although this disclosure includes a detailed description on cloud computing, implementation of the teachings recited herein are not limited to a cloud computing environment. Rather, embodiments of the present invention are capable of being implemented in conjunction with any other type of computing environment now known or later developed. 
     Cloud computing is a model of service delivery for enabling convenient, on-demand network access to a shared pool of configurable computing resources (e.g. networks, network bandwidth, servers, processing, memory, storage, applications, virtual machines, and services) that can be rapidly provisioned and released with minimal management effort or interaction with a provider of the service. This cloud model may include at least five characteristics, at least three service models, and at least four deployment models. 
     Characteristics are as follows: 
     On-demand self-service: a cloud consumer can unilaterally provision computing capabilities, such as server time and network storage, as needed automatically without requiring human interaction with the service&#39;s provider. 
     Broad network access: capabilities are available over a network and accessed through standard mechanisms that promote use by heterogeneous thin or thick client platforms (e.g., mobile phones, laptops, and PDAs). 
     Resource pooling: the provider&#39;s computing resources are pooled to serve multiple consumers using a multi-tenant model, with different physical and virtual resources dynamically assigned and reassigned according to demand. There is a sense of location independence in that the consumer generally has no control or knowledge over the exact location of the provided resources but may be able to specify location at a higher level of abstraction (e.g., country, state, or datacenter). 
     Rapid elasticity: capabilities can be rapidly and elastically provisioned, in some cases automatically, to quickly scale out and rapidly released to quickly scale in. To the consumer, the capabilities available for provisioning often appear to be unlimited and can be purchased in any quantity at any time. 
     Measured service: cloud systems automatically control and optimize resource use by leveraging a metering capability at some level of abstraction appropriate to the type of service (e.g., storage, processing, bandwidth, and active user accounts). Resource usage can be monitored, controlled, and reported providing transparency for both the provider and consumer of the utilized service. 
     Service Models are as follows: 
     Software as a Service (SaaS): the capability provided to the consumer is to use the provider&#39;s applications running on a cloud infrastructure. The applications are accessible from various client devices through a thin client interface such as a web browser (e.g., web-based e-mail). The consumer does not manage or control the underlying cloud infrastructure including network, servers, operating systems, storage, or even individual application capabilities, with the possible exception of limited user-specific application configuration settings. 
     Platform as a Service (PaaS): the capability provided to the consumer is to deploy onto the cloud infrastructure consumer-created or acquired applications created using programming languages and tools supported by the provider. The consumer does not manage or control the underlying cloud infrastructure including networks, servers, operating systems, or storage, but has control over the deployed applications and possibly application hosting environment configurations. 
     Infrastructure as a Service (IaaS): the capability provided to the consumer is to provision processing, storage, networks, and other fundamental computing resources where the consumer is able to deploy and run arbitrary software, which can include operating systems and applications. The consumer does not manage or control the underlying cloud infrastructure but has control over operating systems, storage, deployed applications, and possibly limited control of select networking components (e.g., host firewalls). 
     Deployment Models are as follows: 
     Private cloud: the cloud infrastructure is operated solely for an organization. It may be managed by the organization or a third party and may exist on-premises or off-premises. 
     Community cloud: the cloud infrastructure is shared by several organizations and supports a specific community that has shared concerns (e.g., mission, security requirements, policy, and compliance considerations). It may be managed by the organizations or a third party and may exist on-premises or off-premises. 
     Public cloud: the cloud infrastructure is made available to the general public or a large industry group and is owned by an organization selling cloud services. 
     Hybrid cloud: the cloud infrastructure is a composition of two or more clouds (private, community, or public) that remain unique entities but are bound together by standardized or proprietary technology that enables data and application portability (e.g., cloud bursting for load-balancing between clouds). 
     A cloud computing environment is service oriented with a focus on statelessness, low coupling, modularity, and semantic interoperability. At the heart of cloud computing is an infrastructure that includes a network of interconnected nodes. 
     Referring now to  FIG.  1   , a schematic of an example of a cloud computing node is shown. Cloud computing node  10  is only one example of a suitable cloud computing node and is not intended to suggest any limitation as to the scope of use or functionality of embodiments of the invention described herein. Regardless, cloud computing node  10  is capable of being implemented and/or performing any of the functionality set forth hereinabove. 
     In cloud computing node  10  there is a computer system/server  12  or a portable electronic device such as a communication device, which is operational with numerous other general purpose or special purpose computing system environments or configurations. Examples of well-known computing systems, environments, and/or configurations that may be suitable for use with computer system/server  12  include, but are not limited to, personal computer systems, server computer systems, thin clients, thick clients, hand-held or laptop devices, multiprocessor systems, microprocessor-based systems, set top boxes, programmable consumer electronics, network PCs, minicomputer systems, mainframe computer systems, and distributed cloud computing environments that include any of the above systems or devices, and the like. 
     Computer system/server  12  may be described in the general context of computer system-executable instructions, such as program modules, being executed by a computer system. Generally, program modules may include routines, programs, objects, components, logic, data structures, and so on that perform particular tasks or implement particular abstract data types. Computer system/server  12  may be practiced in distributed cloud computing environments where tasks are performed by remote processing devices that are linked through a communications network. In a distributed cloud computing environment, program modules may be located in both local and remote computer system storage media including memory storage devices. 
     As shown in  FIG.  1   , computer system/server  12  in cloud computing node  10  is shown in the form of a general-purpose computing device. The components of computer system/server  12  may include, but are not limited to, one or more processors or processing units  16 , a system memory  28 , and a bus  18  that couples various system components including system memory  28  to processing unit  16 . 
     Bus  18  represents one or more of any of several types of bus structures, including a memory bus or memory controller, a peripheral bus, an accelerated graphics port, and a processor or local bus using any of a variety of bus architectures. By way of example, and not limitation, such architectures include Industry Standard Architecture (ISA) bus, Micro Channel Architecture (MCA) bus, Enhanced ISA (EISA) bus, Video Electronics Standards Association (VESA) local bus, and Peripheral Component Interconnect (PCI) bus. 
     Computer system/server  12  typically includes a variety of computer system readable media. Such media may be any available media that is accessible by computer system/server  12 , and it includes both volatile and non-volatile media, removable and non-removable media. 
     System memory  28  can include computer system readable media in the form of volatile memory, such as random-access memory (RAM)  30  and/or cache memory  32 . Computer system/server  12  may further include other removable/non-removable, volatile/non-volatile computer system storage media. By way of example only, storage system  34  can be provided for reading from and writing to a non-removable, non-volatile magnetic media (not shown and typically called a “hard drive”). Although not shown, a magnetic disk drive for reading from and writing to a removable, non-volatile magnetic disk (e.g., a “floppy disk”), and an optical disk drive for reading from or writing to a removable, non-volatile optical disk such as a CD-ROM, DVD-ROM or other optical media can be provided. In such instances, each can be connected to bus  18  by one or more data media interfaces. As will be further depicted and described below, memory  28  may include at least one program product having a set (e.g., at least one) of program modules that are configured to carry out the functions of embodiments of the invention. 
     Program/utility  40 , having a set (at least one) of program modules  42 , may be stored in memory  28  by way of example, and not limitation, as well as an operating system, one or more application programs, other program modules, and program data. Each of the operating system, one or more application programs, other program modules, and program data or some combination thereof, may include an implementation of a networking environment. Program modules  42  generally carry out the functions and/or methodologies of embodiments of the invention as described herein. 
     Computer system/server  12  may also communicate with one or more external devices  14  such as a keyboard, a pointing device, a display  24 , etc.; one or more devices that enable a user to interact with computer system/server  12 ; and/or any devices (e.g., network card, modem, etc.) that enable computer system/server  12  to communicate with one or more other computing devices. Such communication can occur via Input/Output (I/O) interfaces  22 . Still yet, computer system/server  12  can communicate with one or more networks such as a local area network (LAN), a general wide area network (WAN), and/or a public network (e.g., the Internet) via network adapter  20 . As depicted, network adapter  20  communicates with the other components of computer system/server  12  via bus  18 . It should be understood that although not shown, other hardware and/or software components could be used in conjunction with computer system/server  12 . Examples include, but are not limited to: microcode, device drivers, redundant processing units, external disk drive arrays, RAID systems, tape drives, and data archival storage systems, etc. 
     Referring now to  FIG.  2   , illustrative cloud computing environment  50  is depicted. As shown, cloud computing environment  50  includes one or more cloud computing nodes  10  with which local computing devices used by cloud consumers, such as, for example, personal digital assistant (PDA) or cellular telephone  54 A, desktop computer  54 B, laptop computer  54 C, and/or automobile computer system  54 N may communicate. Nodes  10  may communicate with one another. They may be grouped (not shown) physically or virtually, in one or more networks, such as Private, Community, Public, or Hybrid clouds as described hereinabove, or a combination thereof. This allows cloud computing environment  50  to offer infrastructure, platforms and/or software as services for which a cloud consumer does not need to maintain resources on a local computing device. It is understood that the types of computing devices  54 A-N shown in  FIG.  2    are intended to be illustrative only and that computing nodes  10  and cloud computing environment  50  can communicate with any type of computerized device over any type of network and/or network addressable connection (e.g., using a web browser). 
     Referring now to  FIG.  3   , a set of functional abstraction layers provided by cloud computing environment  50  ( FIG.  2   ) is shown. It should be understood in advance that the components, layers, and functions shown in  FIG.  3    are intended to be illustrative only and embodiments of the invention are not limited thereto. As depicted, the following layers and corresponding functions are provided: 
     Hardware and software layer  60  includes hardware and software components. Examples of hardware components include: mainframes  61 ; RISC (Reduced Instruction Set Computer) architecture based servers  62 ; servers  63 ; blade servers  64 ; storage devices  65 ; and networks and networking components  66 . In some embodiments, software components include network application server software  67  and database software  68 . 
     Virtualization layer  70  provides an abstraction layer from which the following examples of virtual entities may be provided: virtual servers  71 ; virtual storage  72 ; virtual networks  73 , including virtual private networks; virtual applications and operating systems  74 ; and virtual clients  75 . 
     In one example, management layer  80  may provide the functions described below. Resource provisioning  81  provides dynamic procurement of computing resources and other resources that are utilized to perform tasks within the cloud computing environment. Metering and Pricing  82  provide cost tracking as resources are utilized within the cloud computing environment, and billing or invoicing for consumption of these resources. In one example, these resources may include application software licenses. Security provides identity verification for cloud consumers and tasks, as well as protection for data and other resources. User portal  83  provides access to the cloud computing environment for consumers and system administrators. Service level management  84  provides cloud computing resource allocation and management such that required service levels are met. Service Level Agreement (SLA) planning and fulfillment  85  provide pre-arrangement for, and procurement of, cloud computing resources for which a future requirement is anticipated in accordance with an SLA. 
     Workloads layer  90  provides examples of functionality for which the cloud computing environment may be utilized. Examples of workloads and functions which may be provided from this layer include: mapping and navigation  91 ; software development and lifecycle management  92 ; virtual classroom education delivery  93 ; data analytics processing  94 ; transaction processing  95 ; and distributed cache processing  96 . Hereinafter, reference will be made to  FIG.  4    to  FIG.  15    to describe details of the distributed cache processing  96 . 
     As described above, some traditional schemes implement a distributed cache system comprising a storage node for storing data persistently and a plurality of client nodes coupled to the storage node. The plurality of client nodes may be used for caching data. However, the aforementioned schemes may have various limitations and/or drawbacks. 
     For example, some traditional schemes rely on a local read-only cache of each client node to enable caching of data. In these schemes, there is no data sharing between client nodes, and thus a new client node can only get data from the storage node. Some traditional schemes organize the plurality of client nodes into a cache cluster and enable communication between the cache cluster and the storage node, via a gateway node. However, there is usually no distance consideration for data sharing among nodes within the cache cluster. Therefore, nearby data access cannot be achieved. Additionally, some traditional schemes enable data sharing over a peer-to-peer network. However, data consistency among peer nodes and data updating are usually not taken into account in these schemes. 
     In order to at least partially solve the above and other potential problems, embodiments of the present invention provide a new solution for file management in a distributed file cache system. According to embodiments of the present disclosure, in response to a file of a first version stored at a storage node being obtained by at least one client node, contact information indicating that the file of the first version is accessible from the storage node and the at least one client node is generated and recorded into a distributed hash table. By exploiting the distributed hash table to store the contact information, nearby file access can be achieved. According to embodiments of the present disclosure, version information indicating that the file is of the first version is generated and recorded into a blockchain associated with the plurality of nodes. Since the blockchain prevents from tampering with the stored data, file consistency can be ensured during file update. In addition, embodiments of the present disclosure also provide a solution for managing file slicing and slice versions. As such, unchanged slices can be reused among different versions of the file, thereby implementing fine-grained data sharing among different nodes in the distributed system. 
     With reference now to  FIG.  4   , an environment  400  in which embodiments of the present disclosure can be implemented is shown. It is to be understood that the structure and functionality of the environment  400  are described only for the purpose of illustration without suggesting any limitations as to the scope of the present disclosure. The embodiments of the present disclosure can be embodied with a different structure and/or functionality. 
     In  FIG.  4   , the environment  400  is shown to include a plurality of nodes  410 - 1 ,  410 - 2 ,  410 - 3 ,  410 - 4 , and  410 - 5  (collectively referred to as “nodes” or individually referred to as a “node” within environment  400 ). The number of nodes in  FIG.  4    is merely for the purpose of illustration, without suggesting any limitation to the number of nodes in the environment  400 . In some embodiments, a node within environment  400  may be implemented by a physical device or a virtual machine. For example, a node within environment  400  may be implemented by computer system/server  12  as discussed with reference to  FIG.  1   . The plurality of nodes within environment  400  may be interconnected with each other by any suitable wired and/or wireless mechanism, for example, via a network  430  such as Internet. 
     The environment  400  may be a distributed file cache system. As shown, the node  410 - 5  may act as a storage node for storing files persistently. The nodes  410 - 1 ,  410 - 2 ,  410 - 3 , and  410 - 4  may act as client nodes which read and/or write files stored at the storage node  410 - 5  in response to requests from respective users  420 - 1 ,  420 - 2 ,  420 - 3 , and  420 - 4 . In the distributed file cache system of environment  400 , files may be cached and shared among the client nodes  410 - 1 ,  410 - 2 ,  410 - 3 , and  410 - 4 . In particular, if a file stored at the storage node  410 - 5  is accessed by a plurality of client nodes, the storage node  410 - 5  may identify the file as a hot file and cache the file at the plurality of client nodes which have actual contents of the file. For example, if the user  420 - 3  wants to read a file which has been cached by the client node  410 - 2  near the client node  410 - 3 , the client node  410 - 3  may read the file from the client node  410 - 2 , instead of from the storage node  440 . In this way, embodiments of the present invention provide fast file retrieval. 
     In some embodiments, in order to implement distributed file caching, the plurality of nodes within environment  400  may be organized into a distributed hash table (DHT) network, such as Coral DHT, Kademlia DHT, etc. A DHT is a class of a decentralized distributed system that provides a lookup service similar to a hash table: (key, value) pairs are stored in a DHT, and any participating node can efficiently retrieve the value associated with a given key. In the following, Coral DHT will be taken as an example of the DHT network into which the plurality of nodes  410  are organized. It is to be understood that this is merely for the purpose of illustration, without suggesting any limitations as to the scope of the present disclosure. Embodiments of the present disclosure are also applicable to other distributed networks. 
     In Coral DHT, a key may be the hash of Uniformed Resource Locator (URL) of a file, which may be 160 bits, for example. Each participating node in Coral DHT may be assigned with an identifier (ID). The ID of a node may be the hash of an IP address of the node, which may be also 160 bits, for example. As such, the key and the node IDs are uniformly distributed in the same 160-bit ID space. In Coral DHT, the value associated with a key may be a list of addresses of nodes which have actual contents of the file indicated by the key. Generally speaking, Coral DHT provides two operations: a “put” operation and a “get” operation. For example, when a file is cached by a node within environment  400 , the node may perform a “put” operation to record contact information about the file into Coral DHT. The contact information may indicate that the file is accessible from the node  410 . When another node  410  wants to access a file in response to a request from the user, the other node  410  may perform a “get” operation to search the Coral DHT for the contact information about the file, so as to obtain a list of addresses of nodes which have actual contents of the file to be accessed. 
     In Coral DHT, nodes may be organized into different levels of clusters according to round-trip time (RTT) between two nodes in each level of cluster. For example, in some embodiments, three levels of clusters can be implemented in Coral DHT, in which the lowest-level cluster (also referred to as “Level-2 cluster”) may be a regional-wide cluster within which the RTT between any two nodes is below 30 milliseconds, a higher-level cluster (also referred to as “Level-1 cluster”) may be a continental-wide cluster within which the RTT between any two nodes is below 100 milliseconds, and the highest-level cluster (also referred to as “Level-0 cluster”) may be a global-wide cluster with unlimited RTT thresholds. When a node within environment  400  initiates a “get” operation to search for file contact information about a file, the search may start from the Level-2 cluster. If the file contact information cannot be found in the Level-2 cluster, the search will be then performed in the Level-1 cluster, followed by the Level-0 cluster. It can be seen that Coral DHT creates self-organizing clusters of nodes that fetch information from each other to avoid communicating with more distant or heavily-loaded nodes. 
     In some embodiments, in order to enable file updating in the distributed file cache system of environment  400 , a blockchain associated with the plurality of nodes within environment  400  may be leveraged to manage versions of a file. For example, when a new file is cached by a client node, version information about the file may be recorded in a newly generated block of the blockchain. The version information may indicate the URL of the file as well as the version of the file. The URL of the file may be used to generate a key in the DHT. When the file is updated, new version information about the file indicating the URL of the file as well as the new version of the file may be recorded in another newly generated block of the blockchain. Since the blockchain prevents from tampering with the stored data, file consistency can be ensured during file update. 
     In some embodiments, in order to implement fine-grained data sharing among the plurality of nodes within environment  400 , a file cached by a client node may be divided into a plurality of slices, and slice information about the plurality of slices may be generated and stored at the client node which caches the file. In an example embodiment, for each of the plurality of slices in a file, the slice information may record at least the following: a version of the slice, a start position and an end position of the slice in the file. In some embodiments, for example, the slice information may be recorded in an example data structure as illustrated in Table 1 as below: 
                     TABLE 1                  Slice Information                                     Slice No.   Version   Start Position   End Position                                                 1   0   0   1000           2   0   1001   2000           3   0   2001   3000                        
Additionally, in some embodiments, for each of the plurality of slices in a file, the slice information may also record one or more checksums for ensuring data integrity and accuracy during data transmissions. For example, a slice may include a plurality of binaries. A checksum for the slice can be determined by summing the plurality of binaries. In some embodiments, for example, the slice information including one or more checksums may be recorded in an example data structure as illustrated in Table 2 as below:
 
                     TABLE 2                  Slice Information                                     Slice No.   Version   Start Position   End Position   Weak Checksum   Strong Checksum                                             1   0   0   1000   8A74E65A   8A74E65A4085F1F57E5AF210                           53E29070       2   0   1001   2000   F06AFC5D   F06AFC5D6E6C633827B5F646                           89277A4B       3   0   2001   3000   D926D7BB   D926D7BB9CCF46FC04A61B                           D65D87B9B3                    
In Table 2, for example, the strong checksum may be determined as a sum of the plurality of binaries included in a slice, while the weak checksum may be determined as the highest predetermined number of bits in the strong checksum, so as to enable quick verification of data integrity.
 
     In some embodiments, when updating the file cached by the client node, some slices may be reused among different versions of the file. That is, only a part of the plurality of slices may need to be updated. In this way, file updating can be implemented in a flexible and cost-effective manner. The slice information for the file can be updated accordingly. As such, data sharing among the plurality of nodes within environment  400  can be performed based on slices of a file, instead of based on an entire file. 
       FIGS.  5 A- 5 C  depict example diagrams for updating a file in accordance with some embodiments of the present disclosure. 
       FIG.  5 A  depict a file of version 1, which is denoted as file  500 - 1 . For example, the file  500 - 1  includes three slices  510 ,  520  and  530 . In this example, the version of each of the three slices is 1. For example, the slice information for the file  500 - 1  is shown in Table 3 as below: 
                     TABLE 3                  Slice Information for the file 500-1                                     Slice No.   Version   Start Position   End Position   Weak Checksum   Strong Checksum                                             510   1   0   1000   8A74E65A   8A74E65A4085F1F57E5AF210                           53E29070       520   1   1001   2000   F06AFC5D   F06AFC5D6E6C633827B5F646                           89277A4B       530   1   2001   3000   D926D7BB   D926D7BB9CCF46FC04A61B                           D65D87B9B3                      FIG.  5 A  also depicts a file of version 2 (denoted as file  500 - 2 ) into which the file  500 - 1  is modified. In an example embodiment, compared with the file  500 - 1 , a part  511  in the slice  510  and a part  531  in the slice  530  are modified, while another part  512  is added into the slice  510 . For example, the slice information for the file  500 - 2  is shown in Table 4 as below:
 
                     TABLE 4                  Slice Information for the file 500-2                                     Slice No.   Version   Start Position   End Position   Weak Checksum   Strong Checksum                                             510   2   0   1100   C16024C6   C16024C65856B196A93490386                           154F98D       520   1   1101   2100   F06AFC5D   F06AFC5D6E6C633827B5F646                           89277A4B       530   2   2101   3100   A6B14A31   A6B14A3123D327627A887A6                           A442D427F                    
Compared with Table 3, the version, the end position and the checksums of the slice  510  are changed in Table 4, since the contents in the slice  510  as well as the size of the slice  510  have been changed. The version and the checksums of the slice  520  remain unchanged in Table 4, since the contents in the slice  520  remain unchanged. However, since the end position of the slice  510  is changed, the start position and the end position of the slice  520  are changed in Table 4. Likewise, the version, the start and end positions as well as the checksums of the slice  530  are changed in Table 4. Specifically, in Table 4, the versions of the slices  510  and  530  may be updated with the latest version of the file (i.e., version 2).
 
       FIG.  5 B  depicts a file of version 3 (denoted as file  500 - 3 ) into which the file  500 - 2  is modified. In an example embodiment, compared with the file  500 - 2 , a part  521  is added into the slice  520 , a part  522  in the slice  520  is modified, a part  523  is deleted from the slice  520  and a part  524  in the slice  520  remains unchanged. For example, the slice information for the file  500 - 3  is shown in Table 5 as below: 
                     TABLE 5                  Slice Information for the file 500-3                                     Slice No.   Version   Start Position   End Position   Weak Checksum   Strong Checksum                                             510   2   0   1100   C16024C6   C16024C65856B196A93490386                           154F98D       520   3   1101   1800   B0DB73FC   B0DB73FC81EAC0A110D3B40                           C41FC43A5       530   2   1801   2800   A6B14A31   A6B14A3123D327627A887A6                           A442D427F                    
Compared with Table 4, the information about the slice  510  remains unchanged in Table 5, since the contents in the slice  510  remain unchanged. The version, the end position and the checksums of the slice  520  are changed in Table 5, since the contents in the slice  520  as well as the size of the slice  520  have been changed. Since the end position of the slice  520  is changed, the start and end positions of the slice  530  are changed in Table 5. However, the version and the checksums of the slice  530  remain unchanged in Table 5, since the contents of the slice  530  remain unchanged. Specifically, in Table 5, the version of the slice  520  may be updated with the latest version of the file (i.e., version 3).
 
       FIG.  5 C  depicts a file of version 4 (denoted as file  500 - 4 ) into which the file  500 - 3  is modified. In an example embodiment, compared with the file  500 - 3 , a new slice  540  is added in front of the slice  510 , and a new slice  550  is inserted between the slices  520  and  530 . The slices  510 ,  520  and  530  remain unchanged. For example, the slice information for the file  500 - 4  is shown in Table 6 as below: 
     
       
         
           
               
             
               
                 TABLE 6 
               
             
            
               
                   
               
               
                 Slice Information for the file 500-4 
               
            
           
           
               
               
               
               
               
               
            
               
                 Slice No. 
                 Version 
                 Start Position 
                 End Position 
                 Weak Checksum 
                 Strong Checksum 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 510 
                 2 
                 201 
                 1300 
                 C16024C6 
                 C16024C65856B196A93490386 
               
               
                   
                   
                   
                   
                   
                 154F98D 
               
               
                 520 
                 3 
                 1301 
                 2000 
                 B0DB73FC 
                 B0DB73FC81EAC0A110D3B40 
               
               
                   
                   
                   
                   
                   
                 C41FC43A5 
               
               
                 530 
                 2 
                 3301 
                 4300 
                 A6B14A31 
                 A6B14A3123D327627A887A6 
               
               
                   
                   
                   
                   
                   
                 A442D427F 
               
               
                 540 
                 4 
                 0 
                 200 
                 F95ADBCE 
                 F95 ADBCE0A51589CB 6E8711 
               
               
                   
                   
                   
                   
                   
                 2EB6BECD4 
               
               
                 550 
                 4 
                 2001 
                 3300 
                 EC443E36 
                 EC443E361BE4CEE5BDA5A02 
               
               
                   
                   
                   
                   
                   
                 6B1335BE8 
               
               
                   
               
            
           
         
       
     
     Compared with Table 5, the start and end positions of the slices  510 ,  520  and  530  are changed in table 6, while other information about the slices  510 ,  520  and  530  remains unchanged. Slice information about the new slices  540  and  550  is generated and recorded in Table 5. Specifically, the versions of the slices  540  and  550  may reuse the latest version of the file (i.e., version 4). 
     In this way, fine-grained file sharing and file updating can be achieved by file slicing as described with reference to  FIGS.  5 A- 5 C . In the following, more details about file caching, file access and file slicing in the distributed file cache system of environment  400  will be described in detail with reference to  FIGS.  6 - 12   . 
     As described above, in order to enable distributed file caching and file updating among the plurality of nodes in the environment  400 , both a DHT and a blockchain associated with the plurality of nodes may be established. For example, the storage node  410 - 5  in the environment  400  may be responsible for initializing both the DHT and the blockchain. Each of the client nodes  410 - 1 ,  410 - 2 ,  410 - 3 , and  410 - 4  may perform corresponding operations to join both the DHT and the blockchain, so as to join the distributed file cache system of environment  400 . 
       FIG.  6    depicts a flowchart of a method  600  for joining the distributed file cache system  400  in accordance with embodiments of the present disclosure. For example, the method  600  may be performed by a client node which wants to join the distributed file cache system of environment  400 , such as, the client node  410 - 1 ,  410 - 2 ,  410 - 3  or  410 - 4 , in the environment  400 . It is to be understood that the method  600  may also comprise additional blocks (not shown) and/or may omit the illustrated blocks. The scope of the present disclosure described herein is not limited in this aspect. 
     At block  610 , in response to initialization of the client node and assignment of the client node with a local ID by a user, the client node may discover a list of existing nodes in the DHT. Several mechanisms may be used to discover the existing nodes, including but not limited to using IP multicasts, using some seed nodes, etc. 
     At block  620 , the client node may group the discovered nodes by respective levels of clusters, so as to obtain a list of levels of clusters in the DHT. For example, in Coral DHT, the discovered nodes may be grouped into three levels of clusters: Level-2 cluster, Level-1 cluster, and Level-0 cluster. The Level-2 cluster is the lowest-level cluster, which is a regional-wide cluster with a 30-millisecond threshold. The Level-1 cluster is a higher-level cluster, which is a continental-wide cluster with a 100-millisecond threshold. The Level-0 cluster is the highest-level cluster, which is a global-wide cluster with unlimited RTT thresholds. 
     At block  630 , the client node may retrieve a cluster of a level from the list. For example, the client node can start with retrieving the lowest-level cluster with the lowest RTT threshold (such as, the Level-2 cluster in Coral DHT). 
     At block  640 , the client node may determine RTTs from the client node to the nodes in the retrieved cluster. Then, at decision block  650 , the client node may compare the determined RTTs with the RTT threshold of the cluster (also referred to as “a diameter of the cluster”). For example, in Level-2 cluster in Coral DHT, the client nodes may compare the determined RTTs with 30 milliseconds. 
     If the RTTs to a predetermined proportion (for example, 90%) of the nodes in the cluster are below the diameter of the cluster, at block  660 , the client node may join the cluster. Otherwise, the method  600  proceeds to block  630 , where the client node may retrieve from the list a next lowest-level cluster (such as, the Level-1 cluster in Coral DHT). 
     At block  670 , upon join of the cluster, the client node may insert itself into higher-level clusters. For example, if the client node joins the Level-2 cluster in Coral DHT at block  660 , it may also join the Level-1 and Level-0 clusters at block  670 . 
     At block  680 , the client node may join the blockchain. For example, the client node may exploit any mechanism currently known or to be developed in the future to join the blockchain, which will not be further described in detail herein. 
     In this way, a new client node can join the distributed file cache system of environment  400  by joining both the DHT and blockchain. 
     With reference now to  FIG.  7   , an example diagram for caching a file in the distributed file cache system  400  in accordance with embodiments of the present disclosure is shown. In  FIG.  7   , a DHT  710  and a blockchain  720  associated with the plurality of nodes  410 - 1 ,  410 - 2 , through  410 - 5  are shown.  FIG.  7    also shows a set of client nodes  730 , which may include one or more of the plurality of client nodes  410 - 1 ,  410 - 2 ,  410 - 3 , and  410 - 4  in the environment  400 . 
     As shown in  FIG.  7   , the set of client nodes  730  (such as, more than one client nodes) may obtain  701  a same file from the storage node  410 - 5 . For example, the file stored at the storage node  410 - 5  may be of a first version. In response to the file of the first version being obtained by the client nodes  730 , the storage node  410 - 5  may identify the file of the first version as a hot file to be cached. 
     In some embodiments, in response to determining that the file of the first version stored at the storage node  410 - 5  is to be cached, the storage node  410 - 5  may divide  702  the file of the first version into a plurality of slices. In addition, the storage node  410 - 5  may generate  703  slice information based on the plurality of slices. For example, with respect to each of the plurality of slices, the slice information may indicate at least one of the following: a version of the slice corresponding to the first version, start and end positions of the slice in the file of the first version. The storage node  410 - 5  may record  703  the slice information by itself and record  704  the slice information at the client nodes  730 . 
     In some embodiments, in response to determining that the file of the first version stored at the storage node  410 - 5  is to be cached, the storage node  410 - 5  may generate  705  contact information for the file of the first version. For example, the generated contact information may indicate that the file of the first version is accessible from the client nodes  730  and the storage node  410 - 5 . The storage node  410 - 5  may record  706  the contact information into the DHT. 
     In some embodiments, each node associated with the DHT may have a respective route table for recording nearby nodes. For example, the storage node  410 - 5  may check a respective route table to determine one or more nearby nodes of the storage node  410 - 5 . Then, the storage node  410 - 5  may record the contact information at the determined one or more nearby nodes. In some embodiments, if the DHT  710  is Coral DHT, the storage node  410 - 5  may perform a “put” operation to record the contact information about the file of the first version into Coral DHT. For example, the storage node  410 - 5  may generate the hash of the URL of the file as a key in Coral DHT and store the contact information as a value associated with the key. In this way, the contact information can be detected by other nodes in the DHT. 
     In some embodiments, the storage node  410 - 5  may also generate  707  first version information indicating that the file is of the first version. For example, the first version information may be recorded in a following data structure: {the URL of the file: the version of the file}. The URL of the file will be used to generate a key in the DHT. The storage node  410 - 5  may record  708  the first version information into the blockchain  720 . For example, the first version information may be recorded as a transaction in a newly generated block of the blockchain  720 . It is to be understood that the blockchain component  413 - 5  can exploit any mechanism currently known or to be developed in the future to record a transaction into the blockchain, which will not be further described in detail herein. Since the blockchain prevents from tampering with the stored data, file consistency can be ensured. 
       FIG.  8    depicts a flowchart of a method  800  for caching a file in the distributed file cache system in accordance with embodiments of the present disclosure. For example, the method  800  may be performed at the storage node  410 - 5 . The method  800  will be described in connection with  FIG.  7   . It is to be understood that the method  800  may also comprise additional blocks (not shown) and/or may omit the illustrated blocks. The scope of the present disclosure described herein is not limited in this aspect. 
     At block  810 , the storage node  410 - 5  determines if a file of a first version stored at the storage node  410 - 5  is obtained by at least one client node. The storage node  410 - 5  and the plurality of client nodes  410 - 1 ,  410 - 2 ,  410 - 3 , and  410 - 4  are associated with a DHT  710 . 
     In response to a set of client nodes  730  obtaining the file of the first version stored at the storage node  410 - 5 , at block  820 , the storage node  410 - 5  generates contact information indicating that the file of the first version is accessible from the storage node  410 - 5  and the set of client nodes  730 . 
     At block  830 , the storage node  410 - 5  records the contact information into the DHT  710 . In some embodiments, the storage node  410 - 5  may determine a node from the plurality of nodes  410 , such that a distance from the node to the storage node  410 - 5  is below a threshold distance. Then, the storage node  410 - 5  may record the contact information into the DHT  710  at the determined node. 
     At block  840 , the storage node  410 - 5  generates first version information indicating that the file is of the first version. Then, at block  850 , the storage node  410 - 5  records the first version information into a blockchain  720  associated with the plurality of nodes  410 . 
     In some embodiments, the storage node  410 - 5  may further divide the file of the first version into a plurality of slices, generate slice information about the plurality of slices, and record the slice information at the storage node  410 - 5  and the set of client nodes  730 . In some embodiments, the slice information may indicate at least one of the following: respective versions of the plurality of slices corresponding to the first version; respective start positions of the plurality of slices in the file of the first version; and respective end positions of the plurality of slices in the file of the first version. 
     With reference now to  FIG.  9   , an example diagram of a process  900  for accessing a file cached in the distributed file cache system  400  in accordance with embodiments of the present disclosure is shown. In  FIG.  9   , the DHT  710  and the blockchain  720  associated with the plurality of nodes  410 - 1 ,  410 - 2 , through  410 - 5  are shown.  FIG.  9    also depicts a user  910  and a client node  920  associated with the user  910 . The user  910  can be one of the users  420 - 1 ,  420 - 2 ,  420 - 3 , or  420 - 4 , and the client node  920  can be one of the client nodes  410 - 1 ,  410 - 2 ,  410 - 3 , or  410 - 4  which corresponds to the user  910 . 
     As depicted in  FIG.  9   , the client node  920  may receive  901  a request from the user  910  to access a file. For example, the user  910  may intend to read a file or update a file. In response to receiving the request from the user  910 , the client node  920  may obtain  902  version information about the file from the blockchain  720 . In some embodiments, the obtained version information (also referred to as “first version information”) may indicate the URL of the file as well as the version of the file. For example, the first version information may indicate that the file is of a first version. 
     In response to the first version information being obtained, the client node  920  may determine, from the plurality of nodes  410 , a set of nodes from which the file of the first version is accessible. In some embodiments, the client node  920  may search  903  the DHT  710  for contact information about the file of the first version. For example, the client node  920  may generate the hash of the URL of the file indicated by the first version information and use the generated hash as a key for searching the DHT  710 . In some embodiments, if the DHT  710  is Coral DHT, the client node  920  may initiate a “get” operation to search the DHT for the contact information. 
       FIG.  10    depicts a flowchart of a method  1000  of searching for file contact information in the DHT in accordance with embodiments of the present disclosure. For example, the method  1000  may be performed by a client node which wants to search for file contact information in the DHT, such as, the client node  920  as shown in  FIG.  9   . It is to be understood that the method  1000  may also comprise additional blocks (not shown) and/or may omit the illustrated blocks. The scope of the present disclosure described herein is not limited in this aspect. 
     In  FIG.  10   , the method may initiate in response to the client node  920  initiating a “get” operation to search the DHT for file contact information. At block  1010 , the client node  920  may determine a Level-N cluster from which the search starts. For example, in Coral DHT (which includes three levels of clusters), the value of N may be initialized to be 2. That is, the search may start from the lowest-level cluster with the lowest RTT threshold in the DHT. 
     At block  1020 , the client node  920  may search the Level-N cluster for the key generated from the URL of the file. For example, in Coral DHT, the client node  920  may perform an operation referred to as “find_closer_node (key)” to find some node storing a value (that is, a list of nodes) under “key.” 
     At block  1030 , the client node  920  may determine if the list of nodes (i.e., the value associated with the key) is found in the Level-N cluster. If the client node  920  determines that the list of nodes associated with the key is found in the Level-N cluster, at block  1040 , the client node  920  may further check if the found list of nodes includes an entry associated with the first version. If so, the contact information about the file is found, which indicates a list of nodes which have actual contents of the file indicated by the key. 
     If the client node  920  determines that the list of nodes associated with the key is absent in the Level-N cluster, or if there is no entry associated with the first version in the found list of nodes, at block  1050 , the client node  920  may decrement the value of N by 1. At block  1060 , the client node  920  may determine if the value of N is below 0. If the value of N is greater than or equal to 0, the method proceeds to block  1020 , where the client node  920  may continue the search on a higher-level cluster (such as, the Level-1 cluster). If the value of N is below 0, it means that the search on all of the levels of the clusters in the DHT has been performed but no file contact information is found. In this event, the client node  920  may determine that the contact information about the file is absent in the DHT. 
     Referring back to  FIG.  9   , in some embodiments, if the contact information about the file being found in the DHT  710 , the client node  920  may determine, based on the contact information, a set of nodes  930  from which the file of the first version is accessible. Otherwise, the client node  920  may add the storage node  410 - 5  to the set of nodes  930 . That is, if the contact information about the file is absent in the DHT  710 , the client node  920  may obtain the file of the first version from the storage node  410 - 5 . 
     In some embodiments, in response to determining the set of nodes  930  from which the file of the first version is accessible, the client node  920  may obtain  904  the file of the first version from one or more of the set of nodes  930 . 
     In some embodiments, in order to obtain the file of the first version, the client node  920  may obtain slice information about the file from each of the set of nodes  930 . Here, it is assumed that the set of nodes  930  include three different nodes: node A, node B and node C. For example, slice information A on the node A, slice information B on the node B and slice information C on the node C are shown in Tables 7-9 as below: 
     
       
         
           
               
             
               
                 TABLE 7 
               
             
            
               
                   
               
               
                 Slice Information A on the node A 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Slice No. 
                 Version 
                 Start Position 
                 End Position 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 1 
                 0 
                 1 
                 1000 
               
               
                   
                 2 
                 0 
                 1001 
                 2000 
               
               
                   
                 3 
                 0 
                 2001 
                 3000 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 8 
               
             
            
               
                   
               
               
                 Slice Information B on the node B 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Slice No. 
                 Version 
                 Start Position 
                 End Position 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 1 
                 0 
                 1 
                 1000 
               
               
                   
                 2 
                 1 
                 1001 
                 2000 
               
               
                   
                 3 
                 0 
                 2001 
                 3000 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 9 
               
             
            
               
                   
               
               
                 Slice Information C on the node C 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Slice No. 
                 Version 
                 Start Position 
                 End Position 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 1 
                 0 
                 1 
                 1000 
               
               
                   
                 2 
                 2 
                 1001 
                 2000 
               
               
                   
                 3 
                 0 
                 2601 
                 3600 
               
               
                   
                 4 
                 2 
                 2001 
                 2600 
               
               
                   
                   
               
            
           
         
       
     
     The client node  920  may obtain the slice information A, B, and C from the three different nodes A, B, and C, respectively. In some embodiments, in response to the slice information A, B and C being obtained, the client node  920  may determine the latest slice information which contains the latest slice version. For example, if the client node  920  wants to access the file of version 2, the client node  920  may determine that the slice information C indicating slices of the latest version 2 is the latest. The client node  920  may further check if local slice information about the file is present. 
     In some embodiments, if there is no local slice information about the file to be accessed at the client node  920 , the client node  920  may traverse the slice #1 to the slice #4 in the latest slice information C one by one. If a slice has a version which is below 2, it means that the slice can be reused from a historical version of the slice. For example, the slice #1 can be retrieved from any of the nodes A, B and C. However, if a slice has a version which is equal to 2, it means that the slice can only be retrieved from the node having the latest file, such as, the node C. 
     Alternatively, in some embodiments, if there is local slice information about the file to be accessed at the client node  920 , the client node  920  may traverse the slice #1 to the slice #4 in the latest slice information C one by one. If a slice has a version which is equal to the local slice version, no action is required. Otherwise, the slice should be retrieved from the node having the latest file, such as, the node C. For example, the local slice information at the client node  920  is shown in Table 10 as below: 
                     TABLE 10                  Local Slice Information on the node 920                                     Slice No.   Version   Start Position   End Position                                                 1   0   1   1000           2   1   1001   2000           3   0   2601   3600                        
According to Table 10, for example, the slices #1 and #3 at the client node  920  can be reused. That is, there is no need to retrieve the slices #1 and #3 from other nodes.
 
     In some embodiments, in response to obtaining all of the plurality of slices included in the file of the first version, the client node  920  may sort the plurality of slices based on their start and end positions, so as to get the whole file. 
     As described above, in  FIG.  9   , the request received from the user  910  may be intended to read the file or modify the file. If the request is to read the file, some corresponding operations need to be performed by the client node  920 .  FIG.  11    shows an example diagram of a process  1100  for reading a file cached in the distributed file cache system  400  in accordance with embodiments of the present disclosure. 
     As depicted in  FIG.  11   , if the request sent from the user  910  to the client node  920  is intended to read the file, the process  900  may be performed so as to obtain the file of the latest version (such as, the first version). 
     In response to the file of the first version being obtained, the client node  920  may return  1101  the obtained file of the first version to the user  910 . Additionally, in some embodiments, the client node  920  may update  1102  the slice information based on the plurality of slices included in the obtained file of the first version and record the updated slice information at the client node  920 . The updating of the slice information has been described with reference to  FIGS.  5 A- 5 C , which will not be repeated here. 
     Additionally, in some embodiments, the client node  920  may generate  1103  contact information indicating that the file of the first version is accessible from the client node  920 , and record  1104  the generated contact information into the DHT  710 . The recording of the contact information has been described in further detail above with reference to  FIG.  7   . 
     Alternatively, in some embodiments, in  FIG.  9   , the request received from the user  910  may be intended to modify the file.  FIG.  12    depicts an example diagram of a process  1200  for updating a file cached in the distributed file cache system  400  in accordance with embodiments of the present disclosure. 
     As depicted in  FIG.  12   , if the request (also referred to as “first request” in the following) sent from the user  910  to the client node  920  is intended to modify a file, the process  900  may be performed so as to obtain the file of the latest version (such as, the first version). 
     In response to the file of the first version being obtained, the client node  920  may generate  1201  a copy of the file of the first version and modify  1202  the copy locally based on the first request. For example, the client node  920  may determine, from the plurality of slices in the copy, one or more slices that need to be modified, and only modify the determined one or more slices based on the first request. Additionally, the client node  920  may transmit  1203 , to the storage node  410 - 5 , a second request to modify the file at the storage node  410 - 5  from the first version to a second version corresponding to the modified copy. 
     In response to receiving the second request from the client node  920 , the storage node  410 - 5  may modify  1204  the file locally from the first version to the second version. The modification at the storage node  410 - 5  may fail if a conflict occurs. For example, if the storage node  410 - 5  receives two requests at the same time to modify the file from a same original version, the modification at the storage node  410 - 5  may fail. In some embodiments, in response to failure of the modification at the storage node  410 - 5 , the storage node  410 - 5  may transmit  1205 , to the client node  920 , a notification indicating that the second request is rejected. In response to receiving the notification from the storage node  410 - 5 , the client node  920  may remove  1206  the modified copy from the client node  920 . Alternatively, or in addition, the client node  920  may also return to the user  910  a response indicating that the first request to modify the file is rejected (not shown in  FIG.  12   ). It can be seen that the temporary copy maintained in the client node  920  enables reverting back to the original state if the modification is rejected by the storage node. Further, the file of the old version can still be shared when being modified. 
     Alternatively, in some embodiments, in response to success of the modification at the storage node  410 - 5  (i.e., no conflict occurs), the storage node  410 - 5  may generate  1207  version information (also referred to as “second version information”) indicating that the file is modified from the first version to the second version by the client node  920 . Then, the storage node  410 - 5  may record  1208  the second version information into the blockchain  720 . 
     In some embodiments, the second version information may be recorded as a new transaction. For example, the transaction may include the following information: the URL of the file, the first and second versions of the file, a timestamp at which the modification was made, and/or the signature of the client node  920  which modifies the file. The new transaction may be broadcasted to all of the nodes  410  associated with the blockchain  720 . When a node in the blockchain  720  discovers the new transaction, a verification contract will be triggered at the node to verify whether blocks in the blockchain  720  have conflicts on the same file, such as, whether there are two or more transactions that update the file from a same original version at the same time. 
     If the verification result at each node in the blockchain  720  indicates no conflict, the new transaction will be recorded into a newly generated block of the blockchain  720 . As such, the second version information can be recorded into the blockchain  720 . If the verification result at some node indicates that there is a conflict, the new transaction (that is, the second version information) will not be recorded into the blockchain  720 . 
     Referring to  FIG.  12   , the client node  920  may determine  1209  if the second version information indicating that the file is modified from the first version to the second version by the client node  920  is recorded into the blockchain. 
     If the second version information is recorded into the blockchain successfully (i.e., the verification result indicates no conflict), the client node  920  may overwrite  1210  the file of the first version with the modified copy. Then, the client node  920  may update  1211  the slice information based on the modified copy and record the updated slice information at the client node  920 . The updating of the slice information has been previously described in further detail with reference to  FIGS.  5 A- 5 C . Further, the client node  920  may also generate  1211  contact information indicating that the file of the second version is accessible from the client node  920 , and record  1212  the generated contact information into the DHT  710 . 
     Alternatively, if the client node  920  determines  1209  that there is a conflict e.g., there are two or more transactions that update the file from a same original version at the same time), the client node  920  may remove  1213  the modified copy from the client node  920 . Alternatively, or in addition, the client node  920  may also return to the user  910  a response indicating that the first request to modify the file is rejected (not shown in  FIG.  12   ). 
       FIG.  13    depicts a flowchart of a method  1300  for accessing a file cached in the distributed file cache system in accordance with embodiments of the present disclosure. For example, the method  1300  may be performed at a client node (such as, the client node  920  shown in  FIG.  9   ). The method  1300  will be described in connection with  FIG.  9   . It is to be understood that the method  1300  may also comprise additional blocks (not shown) and/or may omit the illustrated blocks. The scope of the present disclosure described herein is not limited in this aspect. 
     At block  1310 , the client node  920  receives, from a user, a request to access a file. 
     At block  1320 , the client node  920  obtains first version information about the file from a blockchain associated with a plurality of nodes including the client node  920 . The first version information indicates that the file is of a first version. 
     At block  1330 , the client node  920  determines, from the plurality of nodes, a set of nodes from which the file of the first version is accessible based on the DHT associated with the plurality of nodes. 
     In some embodiments, the client node  920  may search the DHT for first contact information about the file, the first contact information indicating the set of nodes from which the file of the first version is accessible. In response to the first contact information being found in the DHT, the client node  920  may determine the set of nodes based on the first contact information. 
     In some embodiments, the plurality of nodes include a storage node for storing the file. In response to the first contact information being absent in the DHT, the client node  920  may add the storage node to the set of nodes. 
     At block  1340 , the client node  920  obtains the file of the first version from at least one node in the set of nodes. 
     In some embodiments, the file of the first version includes a plurality of slices. The client node  920  may obtain a set of slice information about the file from the set of nodes, the slice information obtained from a node in the set of nodes indicating information about a set of slices included in the file at the node. The client node  920  may determine, from the set of nodes, the at least one node storing the plurality of slices based on the set of slice information and the first version. The client node  920  may obtain the plurality of slices from the at least one node. 
     In some embodiments, the first request is to read the file.  FIG.  14    depicts a flowchart of a method  1400  for reading a file cached in the distributed file cache system in accordance with embodiments of the present disclosure. For example, the method  1400  may be performed by the client node  920  subsequent to the method  1300 . The method  1400  will be described in connection with  FIG.  10   . It is to be understood that the method  1400  may also comprise additional blocks (not shown) and/or may omit the illustrated blocks. The scope of the present disclosure described herein is not limited in this aspect. 
     At block  1410 , in response to the file of the first version being obtained, the client node  920  returns the file of the first version to the user. 
     At block  1420 , in response to the file of the first version including a plurality of slices being obtained, the client node  920  generates slice information about the plurality of slices. 
     At block  1430 , the client node  920  records the slice information at the client node  920 . 
     At block  1440 , the client node  920  generates second contact information indicating that the file of the first version is accessible from the client node  920 . 
     At block  1450 , the client node  920  records the second contact information into the DHT. 
     Alternatively, in some embodiments, the first request is to modify the file.  FIG.  15    depicts a flowchart of a method  1500  for modifying a file cached in the distributed file cache system in accordance with embodiments of the present disclosure. For example, the method  1500  may be performed by the client node  920  subsequent to the method  1300 . The method  1500  will be described in connection with  FIG.  11   . It is to be understood that the method  1500  may also comprise additional blocks (not shown) and/or may omit the illustrated blocks. The scope of the present disclosure described herein is not limited in this aspect. 
     At block  1510 , in response to the file of the first version being obtained, the client node  920  generates a copy of the file of the first version. 
     At block  1520 , the client node  920  modifies the copy. In some embodiments, the file of the first version includes a plurality of slices. The client node  920  may determine, one or more slices to be modified from the plurality of slices in the copy, and then modified the one or more slices. 
     At block  1530 , the client node  920  transmits, to a storage node for storing the file, a second request to modify the file from the first version to a second version corresponding to the modified copy. 
     In some embodiments, in response to receiving the second request from the client node  920 , the storage node may modify the file at the storage node from the first version to the second version corresponding to the modified copy. In response to failure of the modification at the storage node, the storage node transmits a notification indicating that the second request is rejected to the client node  920 . In response to success of the modification at the storage node, the storage node may generate second version information indicating that the file is modified from the first version to the second version by the client node  920 . The storage node may record the second version information into the blockchain. 
     At block  1540 , the client node  920  determines if the second request is rejected by the storage node. 
     In response to receiving from the storage node the notification indicating that the second request is rejected, at block  1550 , the client node  920  returns, to the user, a response indicating that the first request is rejected. Alternatively, or in addition, at block  1550 , the client node  920  may also remove the modified copy from the client node  920 . 
     At block  1560 , the client node  920  determines if the second version information indicating that the file is modified from the first version to the second version by the client node is recorded into the blockchain. 
     In response to the second version information being recorded into the blockchain, at block  1570 , the client node  920  overwrites the file of the first version with the modified copy. Otherwise, the method  1500  proceeds to block  1550 . 
     At block  1580 , the client node  920  updates the slice information. In some embodiments, the client node  920  store first slice information about a plurality of slices in the file of the first version. In response to the file of the first version being overwritten with the modified copy, the client node  920  may generate second slice information based on the modified copy and replace the first slice information with the second slice information. 
     The present invention may be a system, a method, and/or a computer program product at any possible technical detail level of integration. The computer program product may include a computer readable storage medium (or media) having computer readable program instructions thereon for causing a processor to carry out aspects of the present invention. 
     The computer readable storage medium can be a tangible device that can retain and store instructions for use by an instruction execution device. The computer readable storage medium may be, for example, but is not limited to, an electronic storage device, a magnetic storage device, an optical storage device, an electromagnetic storage device, a semiconductor storage device, or any suitable combination of the foregoing. A non-exhaustive list of more specific examples of the computer readable storage medium includes the following: a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), a static random access memory (SRAM), a portable compact disc read-only memory (CD-ROM), a digital versatile disk (DVD), a memory stick, a floppy disk, a mechanically encoded device such as punch-cards or raised structures in a groove having instructions recorded thereon, and any suitable combination of the foregoing. A computer readable storage medium, as used herein, is not to be construed as being transitory signals per se, such as radio waves or other freely propagating electromagnetic waves, electromagnetic waves propagating through a waveguide or other transmission media (e.g., light pulses passing through a fiber-optic cable), or electrical signals transmitted through a wire. 
     Computer readable program instructions described herein can be downloaded to respective computing/processing devices from a computer readable storage medium or to an external computer or external storage device via a network, for example, the Internet, a local area network, a wide area network and/or a wireless network. The network may comprise copper transmission cables, optical transmission fibers, wireless transmission, routers, firewalls, switches, gateway computers and/or edge servers. A network adapter card or network interface in each computing/processing device receives computer readable program instructions from the network and forwards the computer readable program instructions for storage in a computer readable storage medium within the respective computing/processing device. 
     Computer readable program instructions for carrying out operations of the present invention may be assembler instructions, instruction-set-architecture (ISA) instructions, machine instructions, machine dependent instructions, microcode, firmware instructions, state-setting data, configuration data for integrated circuitry, or either source code or object code written in any combination of one or more programming languages, including an object oriented programming language such as Smalltalk, C++, or the like, and procedural programming languages, such as the “C” programming language or similar programming languages. The computer readable program instructions may execute entirely on the user&#39;s computer, partly on the user&#39;s computer, as a stand-alone software package, partly on the user&#39;s computer and partly on a remote computer or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user&#39;s computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider). In some embodiments, electronic circuitry including, for example, programmable logic circuitry, field-programmable gate arrays (FPGA), or programmable logic arrays (PLA) may execute the computer readable program instructions by utilizing state information of the computer readable program instructions to personalize the electronic circuitry, in order to perform aspects of the present invention. 
     Aspects of the present invention are described herein with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer readable program instructions. 
     These computer readable program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. These computer readable program instructions may also be stored in a computer readable storage medium that can direct a computer, a programmable data processing apparatus, and/or other devices to function in a particular manner, such that the computer readable storage medium having instructions stored therein comprises an article of manufacture including instructions which implement aspects of the function/act specified in the flowchart and/or block diagram block or blocks. 
     The computer readable program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other device to cause a series of operational steps to be performed on the computer, other programmable apparatus or other device to produce a computer implemented process, such that the instructions which execute on the computer, other programmable apparatus, or other device implement the functions/acts specified in the flowchart and/or block diagram block or blocks. 
     The flowchart and block diagrams in the Figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and computer program products according to various embodiments of the present invention. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of instructions, which comprises one or more executable instructions for implementing the specified logical function(s). In some alternative implementations, the functions noted in the blocks may occur out of the order noted in the Figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts or carry out combinations of special purpose hardware and computer instructions.