Assigning storage responsibility in a distributed data storage system with replication

A data location table master system generates a master data location table storing associations of tokens with storage nodes for varying responsibility levels. When the master data location table is updated, the data location table master system updates storage nodes affected by the update as well as other storage nodes and application nodes in the system. Then, the storage nodes and the application nodes store a copy of the master data location table. A token migration and synchronization process reallocates data object storage among the storage nodes based on the updated master data location table.

BACKGROUND

1. Technical Field

The present invention generally relates to the field of data storage and, in particular, to assigning storage responsibility in a distributed data storage system with replication.

2. Background Information

Consider a distributed data storage system with replication where the system synchronizes multiple application nodes and storage nodes regarding data object location information. Each storage node has different capabilities for storing data objects, and all storage nodes need to be synchronized with the same data object location information. If a storage node comes online or goes offline, the system needs to reallocate storage responsibility for various data objects among the storage nodes in a manner that is efficient and meets all the specified capabilities of the storage nodes. Reallocating storage responsibility may affect performance of storage nodes and, therefore, application nodes.

SUMMARY

The above and other issues are addressed by a method, non-transitory computer readable storage medium, and system for assigning storage responsibility in a distributed data storage system with replication, wherein the storage system includes a plurality of storage nodes. Each storage node has one or more partitions and each partition is associated with a unique token using a 1:1 mapping. A data object is associated with a token and is stored in a partition whose token matches the data object's token. An embodiment of the method comprises calculating a first number of tokens assigned to a first storage node in the plurality of storage nodes for primary level of responsibility. The method further comprises assigning the first number of tokens to the first storage node for primary level of responsibility. The method further comprises calculating a second number of tokens assigned to a second storage node in the plurality of storage nodes for primary level of responsibility. The method further comprises assigning the second number of tokens to the second storage node for primary responsibility. The method further comprises calculating a third number of tokens assigned to the first storage node for secondary level of responsibility where the second storage node has primary level of responsibility. The method further comprises assigning the third number of tokens to the first storage node for secondary responsibility.

An embodiment of the medium stores computer program modules executable to perform steps. The steps comprise calculating a first number of tokens assigned to a first storage node in the plurality of storage nodes for primary level of responsibility. The steps further comprise assigning the first number of tokens to the first storage node for primary level of responsibility. The steps further comprise calculating a second number of tokens assigned to a second storage node in the plurality of storage nodes for primary level of responsibility. The steps further comprise assigning the second number of tokens to the second storage node for primary responsibility. The steps further comprise calculating a third number of tokens assigned to the first storage node for secondary level of responsibility where the second storage node has primary level of responsibility. The steps further comprise assigning the third number of tokens to the first storage node for secondary responsibility.

An embodiment of the system comprises a non-transitory computer-readable storage medium storing computer program modules executable to perform steps. The steps comprise calculating a first number of tokens assigned to a first storage node in the plurality of storage nodes for primary level of responsibility. The steps further comprise assigning the first number of tokens to the first storage node for primary level of responsibility. The steps further comprise calculating a second number of tokens assigned to a second storage node in the plurality of storage nodes for primary level of responsibility. The steps further comprise assigning the second number of tokens to the second storage node for primary responsibility. The steps further comprise calculating a third number of tokens assigned to the first storage node for secondary level of responsibility where the second storage node has primary level of responsibility. The steps further comprise assigning the third number of tokens to the first storage node for secondary responsibility.

The above and other issues are addressed by a method, non-transitory computer readable storage medium, and system for updating storage responsibility in a distributed data storage system with replication in response to the storage system initially including a first set of storage nodes and then being modified to include a second set of storage nodes. Each storage node has one or more partitions and each partition is associated with a unique token using a 1:1 mapping. A data object is associated with a token and is stored in a partition whose token matches the data object's token. An embodiment of the method comprises calculating a number of tokens assigned to various storage nodes in the second set of storage nodes for various levels of responsibility. The method further comprises calculating differences between numbers of tokens assigned to various storage nodes in the first set of storage nodes and numbers of tokens assigned to various storage nodes in the second set of storage nodes. The method further comprises transferring tokens between the first set of storage nodes and the second set of storage nodes based on the calculated differences.

An embodiment of the medium stores computer program modules executable to perform steps. The steps comprise calculating a number of tokens assigned to various storage nodes in the second set of storage nodes for various levels of responsibility. The steps further comprise calculating differences between numbers of tokens assigned to various storage nodes in the first set of storage nodes and numbers of tokens assigned to various storage nodes in the second set of storage nodes. The steps further comprise transferring tokens between the first set of storage nodes and the second set of storage nodes based on the calculated differences.

An embodiment of the system comprises a non-transitory computer-readable storage medium storing computer program modules executable to perform steps. The steps comprise calculating a number of tokens assigned to various storage nodes in the second set of storage nodes for various levels of responsibility. The steps further comprise calculating differences between numbers of tokens assigned to various storage nodes in the first set of storage nodes and numbers of tokens assigned to various storage nodes in the second set of storage nodes. The steps further comprise transferring tokens between the first set of storage nodes and the second set of storage nodes based on the calculated differences.

DETAILED DESCRIPTION

FIG. 1Ais a high-level block diagram illustrating an environment100for assigning storage responsibility in a distributed data storage system with replication, according to one embodiment. The environment100may be maintained by an enterprise that enables data to be stored in a distributed manner with replication, such as a corporation, university, or government agency. As shown, the environment100includes a network110, multiple application nodes120, multiple storage nodes130, and a data location table (DLT) master system140. While two application nodes120and two storage nodes130are shown in the embodiment depicted inFIG. 1A, other embodiments can have different numbers of application nodes120and/or storage nodes130.

The network110represents the communication pathway between the application nodes120, the storage nodes130, and the DLT master system140. In one embodiment, the network110uses standard wireless and wired communications technologies and protocols and can include the Internet and associated protocols. In another embodiment, the entities on the network110can use custom and/or dedicated data communications technologies.

An application node120is a computer (or set of computers) that provides standard application functionality and data services that support that functionality. For example, the application node120is a server that executes applications that work with stored data. The application node120includes an application module123and a hypervisor module125. The application module123provides standard application functionality such as serving web pages, archiving data, or data backup/disaster recovery. In order to provide this standard functionality, the application module123issues write requests (i.e., requests to store data) and read requests (i.e., requests to retrieve data). The hypervisor module125handles these application data requests (e.g., write requests and read requests) received from the application module123by communicating with the storage nodes130. The hypervisor module125determines which storage node130to communicate with based on a hypervisor data location table (DLT)340, further described below with reference toFIG. 3.

A storage node130is a computer (or set of computers) that stores data. The storage node130can include one or more types of storage, such as hard disk, optical disk, flash memory, and cloud. For example, a storage node130handles data requests received from an application node120(specifically, from the hypervisor module125), moves data objects, and stores data objects. The storage node130includes a data object repository133and a storage node module135. The data object repository133stores data objects in partitions (equally-sized address space regions). Each partition is associated with a token and has a one-to-one (1:1) mapping with the token. The total number of partitions (tokens) is a configurable parameter of the environment100. A data object is associated with a token, and multiple data objects can be associated with the same token. A data object is stored in the partition whose token matches the data object's token. So, a token is used to virtualize and locate a data object across multiple partitions (and, therefore, across multiple storage nodes130).

The storage node module135handles data requests that are received via the network110from the hypervisor module125(e.g., hypervisor write requests and hypervisor read requests) and moves data objects within or between the storage nodes130. The storage node module135moves data objects between storage nodes130in response to receiving a data location table from the DLT master system140. The DLT provides information regarding allocations of tokens to partitions in the storage nodes130. The storage node module135is further described below with reference toFIG. 4.

The DLT master system140initializes a master data location table (mDLT)530and updates the mDLT530as necessary. For example, the DLT master system140updates the mDLT530when one or more storage nodes130come online, when one or more storage nodes130go offline, when weights of one or more storage nodes130change, or any combination thereof. Then, the DLT master system140sends the updated mDLT530to the application nodes120and the storage nodes130. The storage nodes130use the updated mDLT530to reallocate tokens to partitions in the storage nodes130. In one embodiment, the number of tokens that are transferred between storage nodes130is the minimum number that is required in order to maintain appropriate distribution of primary tokens and replica tokens. (Replica tokens are tokens assigned to storage nodes with responsibility levels that are not primary responsibility levels.) Since a token transfer between two storage nodes results in migrating the data of the corresponding partition between the storage nodes, transferring more tokens than necessary causes unnecessary load on the system.

The DLT master system140deterministically allocates secondary responsibility for a storage node's partitions to remaining storage nodes based on their percent of remaining capability and deterministically allocates further levels of responsibility for a storage node group's partitions to remaining storage nodes based on their percent of remaining capability. Specifically, each partition is associated with a token, and primary responsibility for a token is allocated to storage nodes proportional to their relative capability. Secondary responsibility for a storage node's tokens is allocated to remaining storage nodes based on their percent of remaining capability, where each such node pair is called a secondary-level node group. Thus, k-level responsibility for a (k−1)-node group's tokens is allocated to remaining storage nodes (all storage nodes except the storage nodes in the k−1 node group) based on their percent of remaining capability. In one embodiment, in the event of storage node addition, removal, or change in capability, the tokens are transferred between the storage nodes such that optimal primary token distribution and secondary token dispersion is ensured. For each other (k>2) level, tokens are transferred until dispersion at this level cannot be improved.

FIG. 1Bis a data location table (DLT) of tokens and responsibility levels of storage nodes for storing data objects corresponding to those tokens, according to one embodiment. A responsibility level is referred to herein using the variable a and indicates a level of responsibility for a token, referred to herein using the variable b. The responsibility levels are described as primary, secondary, tertiary, and so on for a=1, a=2, a=3, and so on, respectively. Henceforth, primary level of responsibility will be referred to as a “higher” responsibility level than secondary, tertiary, etc. responsibility levels. Thus, secondary responsibility level has a lower level of responsibility than primary level of responsibility and a higher level of responsibility than tertiary, etc. responsibility levels. The number of responsibility levels is referred to herein using the variable R and represents a replication factor, which is a configurable parameter of the environment100. DLT[a,b] stores a storage node identifier (ID) associated with a storage node that holds a-level responsibility for token b. Further, DLT row a stores IDs of storage nodes130that have a-level responsibility for the various tokens. DLT column b stores IDs of storage nodes130that have various levels of responsibility for token b. A DLT is stored in the hypervisor data location table (hDLT)340of an application node120, the storage node data location table (SN DLT)430of a storage node130, and the mDLT530of the DLT master system140, further described below inFIGS. 3-5. The hDLT340, the SN DLT430, and the mDLT530are identical (unless they are in the process of being updated).

FIG. 2is a high-level block diagram illustrating an example of a computer200for use as one or more of the entities illustrated inFIG. 1A, according to one embodiment. Illustrated are at least one processor202coupled to a chipset204. The chipset204includes a memory controller hub220and an input/output (I/O) controller hub222. A memory206and a graphics adapter212are coupled to the memory controller hub220, and a display device218is coupled to the graphics adapter212. A storage device208, keyboard210, pointing device214, and network adapter216are coupled to the I/O controller hub222. Other embodiments of the computer200have different architectures. For example, the memory206is directly coupled to the processor202in some embodiments.

The storage device208includes one or more non-transitory computer-readable storage media such as a hard drive, compact disk read-only memory (CD-ROM), DVD, or a solid-state memory device. The memory206holds instructions and data used by the processor202. The pointing device214is used in combination with the keyboard210to input data into the computer system200. The graphics adapter212displays images and other information on the display device218. In some embodiments, the display device218includes a touch screen capability for receiving user input and selections. The network adapter216couples the computer system200to the network110. Some embodiments of the computer200have different and/or other components than those shown inFIG. 2. For example, the application node120, the storage node130, and/or the DLT master system140can be formed of multiple blade servers and lack a display device, keyboard, and other components.

FIG. 3is a high-level block diagram illustrating the hypervisor module125fromFIG. 1A, according to one embodiment. The hypervisor module125includes a repository300, a DOID generation module310, and a hypervisor data location table (hDLT) update module320. The repository300stores a virtual volume catalog330and a hypervisor data location table (hDLT)340.

The virtual volume catalog330stores 1:1 mappings between application data identifiers and data object identifiers (DOIDs). The application data identifier is the identifier used by the application module123to refer to the data within the application. Example application data identifiers include a file name, an object name, or a range of blocks. The DOID is a unique address that is used as the primary reference for placement and retrieval of a data object (DO). In one embodiment, the DOID is a 16-byte value, and the various bytes are used as follows:

Bytes 0-3 (collectively referred to as a “token”) are used by the hypervisor module125for data object routing and location with respect to various storage nodes. Since the token portion of the DOID is used for routing, the DOID is said to support “implicit content routing.” Bytes 4-5 can be used by the storage node module135for data object placement acceleration within a storage node130(e.g., across individual disks) in a similar manner to the data object distribution model used across the storage nodes. Bytes 6-15 are used as a unique identifier for the data object.

The hDLT340stores data object placement information, such as mappings between DOIDs (or portions thereof, such as tokens) and storage nodes130. In one embodiment, one token is mapped to one or more storage nodes130(indicated by storage node identifiers). A storage node identifier is, for example, an IP address or another identifier that can be directly associated with an IP address. In one embodiment, the mappings are stored in a relational database to enable rapid access.

For a particular token, the identified storage nodes130indicate where a data object (DO) (corresponding to the token) is stored or retrieved. In one embodiment, a token is a four-byte value that can range from [00 00 00 00] to [FF FF FF FF], which provides more than 429 million individual data object locations. Since the environment100will generally include fewer than 1000 storage nodes, a storage node would be allocated many (e.g., thousands of) tokens to provide a good degree of granularity. In general, more tokens are allocated to a storage node130that has a larger capacity, and fewer tokens are allocated to a storage node130that has a smaller capacity.

The DOID generation module310takes as input a data object (DO), generates a data object identifier (DOID) for that object, and outputs the generated DOID. In one embodiment, the DOID generation module310generates the DOID by executing a specific hash function on the DO and using the hash value as the DOID. In general, the hash algorithm is fast, consumes minimal CPU resources for processing, and generates a good distribution of hash values (e.g., hash values where the individual bit values are evenly distributed). The hash function need not be secure. In one embodiment, the hash algorithm is MurmurHash3, which generates a 128-bit value.

Note that the DOID is “content specific,” that is, the value of the DOID is based on the data object (DO) itself. Thus, identical files or data sets will always generate the same DOID (and, therefore, the same token). Since data objects (DOs) are automatically distributed across individual storage nodes130based on their tokens, and tokens are content-specific, then duplicate DOs (which, by definition, have the same token) are always sent to the same storage node130. Therefore, two independent application modules123on two different application nodes120that store the same file will have that file stored on exactly the same storage node130(because the DOIDs of the data objects, and therefore the tokens, match). Since the same file is sought to be stored twice on the same storage node130(once by each application module123), that storage node130has the opportunity to minimize the storage footprint through the consolidation or deduplication of the redundant data (without affecting performance or the protection of the data).

The hypervisor DLT update module320receives a DLT from the DLT master system140and stores the received DLT in the hypervisor DLT340. For example, the DLT received from the DLT master system140is an updated mDLT, and the hDLT340is updated to store the received DLT, thereby replacing the previous hDLT with the updated mDLT.

FIG. 4is a high-level block diagram illustrating the storage node (SN) module135fromFIG. 1A, according to one embodiment. The storage node module135includes a repository400and a storage node data location table (SN DLT) update module410. The repository400stores a SN catalog420and a SN DLT430.

The SN catalog420stores 1:1 mappings between data object identifiers (DOIDs) and actual storage locations (e.g., on hard disk, optical disk, flash memory, and cloud). For a particular DOID, the data object (DO) associated with the DOID is stored at the actual storage location.

The SN DLT430stores data object placement information such as mappings between DOIDs (or portions thereof, such as tokens) and storage nodes130. In one embodiment, one token is mapped to one or more storage nodes130(indicated by storage node identifiers). A storage node identifier is, for example, an IP address or another identifier that can be directly associated with an IP address. In one embodiment, the mappings are stored in a relational database to enable rapid access.

The SN DLT update module410receives a DLT from the DLT master system140and calculates the difference between the received DLT and the SN DLT430. For example, the received DLT is an updated mDLT providing current allocation information of tokens among partitions in the storage nodes130. The difference between the received DLT and the SN DLT430results in a different set of tokens allocated to the storage node (specifically, the node's partitions) associated with the SN DLT430. For the differing tokens, the SN DLT update module410initiates a token migration process. The token migration process is a bulk transfer of all data objects that are associated with a particular token from another storage node130which currently stores the data objects. After the token migration is done, the SN DLT update module410initiates a token synchronization process to synchronize any data objects that were newly added or existing data objects that changed during the token migration. Then, the SN DLT update module410stores the received DLT in the SN DLT430. The token migration and synchronization process is further described below inFIG. 8. Thus, the previous DLT stored in the SN DLT430is replaced with the received DLT. Then, the SN DLT update module410notifies the DLT master system140that the SN module's update process is complete (further described below inFIG. 10), and the DLT master system140sends the updated mDLT to the application nodes120and remaining storage nodes130.

FIG. 5is a high-level block diagram illustrating the DLT master system140fromFIG. 1A, according to one embodiment. The DLT master system140includes a repository500and a processing server510. The repository500stores a token state repository520and a mDLT530. The processing server520includes an initialization module540and an update module550.

The token state repository520stores a total number of storage nodes130(N), a replication factor (R), a total number of tokens (equal to a total number of data partitions across all storage nodes130, TotalTokens), a weight for each storage node i (Weight(i)), and a number (e.g., exact and/or integer) of tokens assigned to each storage node i for various levels of responsibility (Tokens(i), IntTokens(i), Tokens(i, j), IntTokens(i, j), Tokens(i, j, k), IntTokens(i, j, k), etc.). The token state repository520can also store a sum of weights of all the storage nodes130(TotalWeight).

The replication factor R indicates a number of replicas of a DO in the storage nodes130. For example, for R=1, there is 1 primary and no replicas of the DO. For R=2, there is 1 primary and 1 secondary replica. The total number of tokens (equal to a total number of data partitions across all storage nodes130, TotalTokens) is also equal to the number of columns in the DLT (e.g., hDLT340, SN DLT430, and mDLT530). TotalTokens is a configurable parameter of the environment100. The number of tokens assigned to various storage nodes130is further described below in conjunction withFIG. 6and the operations of the initialization module540.

A storage node130is assigned a weight based on the storage node's performance capability, the storage node's storage capacity, or both. In a system with homogeneous storage nodes130, the weight of each node can be assigned a constant number (e.g., 1, 10, or 100).

The mDLT530stores data object placement information such as mappings between tokens and storage nodes130. One token is mapped to one or more storage nodes (indicated by storage node identifiers). A storage node identifier is, for example, an IP address or another identifier that can be directly associated with an IP address. In one embodiment, the mappings are stored in a relational database to enable rapid access. The mappings of the DLT stored in the mDLT530are sent to storage nodes130and application nodes120and used to update the SN DLT430and the hDLT340, respectively.

The initialization module540calculates an exact number of tokens assigned to each storage node i for various levels of responsibility. Based on the exact number, the initialization module540calculates an integer number of tokens assigned to each storage node i for various levels of responsibility. Then, the initialization module540assigns the tokens to storage nodes based on the integer number of tokens and records the assignment of the tokens in the mDLT530. The initialization module540is further described inFIG. 6.

When storage nodes130are added to or removed from the environment100, the overall environment's100capacity and performance increase or decrease, respectively. The update module550calculates, for an updated set of storage nodes, integer numbers of tokens assigned to each storage node i for various levels of responsibility. The update module550calculates the difference between integer numbers for the previous set of storage nodes and the updated set of storage nodes and transfers tokens as necessary in the mDLT530(e.g., the mDLT is updated). The update module550sends the updated mDLT530to an affected storage node130. The update module550receives notification from the affected storage node130once the affected storage node's130update process is complete. Then, the update module550sends the updated mDLT530to the application nodes120and the remaining storage nodes130. Note that the existing storage nodes130will continue to operate properly using the older version of the SN DLT430until the affected storage node's update process is complete. This proper operation enables the overall DLT update process to happen over time while the environment100remains fully operational.

In one embodiment, the update module550also insures that a subsequent failure or removal of a storage node130will not cause any other storage nodes to become overwhelmed. This is achieved by insuring that the alternate/redundant data (i.e., replica DOs) from a given storage node130is also distributed across the remaining storage nodes. The update module550is further described below in conjunction withFIG. 8.

FIG. 6is a flowchart illustrating a method600of initializing the mDLT530fromFIG. 5, according to one embodiment. In one embodiment, the method600is performed by the initialization module540when the environment100is initially configured. Other embodiments can perform the steps in different orders and can include different and/or additional steps. In addition, some or all of the steps can be performed by entities other than those shown inFIG. 5.

In step610, the initialization module540calculates an exact number of tokens assigned to each storage node i for various levels of responsibility. The number of tokens assigned to each storage node i for various levels of responsibility can be calculated with the following equations. For node i, where node i has primary responsibility for the tokens,

Tokens⁡(i)=Weight⁡(i)TotalWeight*TotalTokens.
Thus, the number of tokens assigned to node i for primary responsibility (Tokens(i)), is based on node i's weight normalized by the total weight of all the storage nodes.

For node j, where node j has secondary responsibility for tokens for which node i has primary responsibility,

Tokens⁡(i,j)=Tokens⁡(j)TotalTokens-Tokens⁡(i)*Tokens⁡(i).
Storage nodes i and j will be referred to as a “node group” (e.g., also called node group (i, j)), where a node group is a set of nodes that have been assigned particular tokens, and each node holds a different level of responsibility for the particular tokens. The first node in the list is the primary node, the second node in the list is the secondary node, and so on. Tokens(i, j) can also be determined using the following equation:

Tokens⁡(i,j)=Weight⁡(j)TotalWeight-Weight⁡(i)*Tokens⁡(i).
The tokens assigned to node j with secondary responsibility (and node i with primary responsibility) (Tokens(i, j)) are a percentage of the number of tokens assigned to node i with primary responsibility (Tokens(i)).

For node k, where node k has tertiary level responsibility for tokens that are allocated for node group (i, j),

Tokens⁡(i,j,k)=Tokens⁡(k)TotalTokens-Tokens⁡(i)-Tokens⁡(j)*Tokens⁡(i,j).
Tokens(i, j, k) can also be determined using the following equation:

Tokens⁡(i,j,k)=Weight⁡(k)TotalWeight-Weight⁡(i)-Weight⁡(j)*Tokens⁡(i,j).
Thus, the number of tokens assigned to node k with tertiary responsibility (and to node j with secondary responsibility and node i with primary responsibility) (Tokens(i, j, k)) is a percentage of tokens assigned to node group (i, j), where node j has a higher level of responsibility than node k.

The same pattern can be used to determine, for node l, where node l has l-level responsibility for tokens allocated to node group (i, j, . . . , m, l),

Tokens⁡(i,j,…⁢,m,l)=Weight⁡(l)TotalWeight-Weight⁡(i)-Weight⁡(j)⁢⁢…-Weight⁡(m)*Tokens⁡(i,j,…⁢⁢m).
Again, the number of tokens assigned to node l with l-level responsibility (Tokens(i, j, . . . , m, l)) is a percentage of the number of tokens assigned to node group (i, j, . . . , m), where node m has a higher responsibility level than node l.

In step620, the initialization module540calculates an integer number of tokens assigned to each storage node i for various levels of responsibility. The integer numbers, herein denoted IntTokens, are based on the determined values Tokens(node(s)). For example, IntTokens(i) can be Tokens(i) rounded up or down to the nearest integer. However, for N nodes, the sum of IntTokens(i) for i=1 to N must equal TotalTokens. In one embodiment, the first Q storage nodes get IntegerTokens(i)=Tokens(i) rounded down to the nearest integer+1, where Q is the difference between TotalTokens and rounded down Tokens(k). The remaining storage nodes get IntegerTokens(i), which is Tokens(i) rounded down to the nearest integer. Similarly, IntTokens(i, j) can be Tokens(i, j) rounded up or down to the nearest integer. However, the sum of IntTokens(i, j) for j=1 to N (excluding node i) must be equal to IntTokens(i). The same pattern applies to Tokens(i, j, . . . , m, l), where IntTokens(i, j, . . . , m, l) can be Tokens(i, j, . . . , m, l) rounded up or down to the nearest integer. However, the sum of IntTokens(i, j, . . . , m, l) for l=1 to N (excluding storage nodes i, j, . . . , m) must be equal to IntTokens(i, j, . . . , m). In one embodiment, the integer numbers of tokens assigned to each storage node i is stored in the token state repository520.

In step630, the initialization module540assigns the tokens to storage nodes. Thus, the initialization module540populates the mDLT530. The assignment of tokens to storage nodes is based on the calculated integer numbers of tokens. The assignment of tokens to storage nodes is stored in a mDLT530. Step630is further described below in conjunction withFIG. 7.

FIG. 7is a flowchart illustrating a method630of populating the mDLT530fromFIG. 5, according to one embodiment. The method630inFIG. 7corresponds to the step630inFIG. 6and is performed by the initialization module540. Other embodiments can perform the steps in different orders and can include different and/or additional steps. In addition, some or all of the steps can be performed by entities other than those shown inFIG. 5.

The row of the mDLT associated with primary responsibility level is selected720. In one embodiment, the row associated with primary responsibility level is the first row. In other embodiments, the row with primary responsibility level is any other suitable row. As described previously in conjunction withFIG. 1B, a DLT has R number of rows and TotalTokens number of columns and stores IDs of storage nodes.

An integer number of tokens assigned to a particular storage node is accessed725. The integer number of tokens assigned to the particular storage node represents how many tokens for which the particular storage node has primary responsibility level. The sum of the integer numbers of tokens assigned to all of the storage nodes is equal to TotalTokens (which is equal to the total number of columns in the mDLT). The integer number of tokens (which was calculated in step620) can be accessed from the token state repository520.

An equivalent integer number of entries in the selected row is populated730with an ID of the particular storage node. Therefore, in the row associated with primary responsibility, if there is a storage node associated with 10 tokens, then 10 entries in the row are populated with the storage node's ID. Steps725and730are performed for all storage nodes.

A row of the mDLT associated with the next highest responsibility level is selected735. Thus, if the previous row was associated with primary responsibility level, then the next row is the row associated with secondary responsibility level.

A determination is made in step740regarding whether the responsibility level of the selected row is less than or equal to R. If the responsibility level of the selected row is not less than or equal to R, then the mDLT has been fully populated, and the method630ends770. If the responsibility level of the selected row is less than or equal to R, then the mDLT has not been fully populated, and the method630proceeds to step745.

A set of responsibility level-node groups is determined745for a particular column in the row. (Recall that a particular column corresponds to a particular token.) The set of responsibility level-node groups is associated with the particular column based on the previously-populated rows of the particular column. For example, if the selected row is associated with secondary responsibility level, then the set of responsibility level-node groups includes the storage node ID located in the same column in the row associated with a higher responsibility level (e.g., primary for secondary). In another example, if the selected row is associated with an R-responsibility level, then the set of responsibility level-node groups includes the storage node ID located in the same column in another row associated with an R−1 responsibility level, another row associated with an R−2 responsibility level, etc. Thus, all storage node IDs in rows with lower responsibility levels than the selected row are in the set of responsibility level-node groups associated with the column.

A unique storage node is determined750for the particular column based on the set of responsibility-level node groups. For example, if the particular column is in a row associated with secondary responsibility level and another row associated with primary responsibility in the same column has node ID of node i, then the unique storage node is at least not node i. Following the previous example, the unique storage node in the set of responsibility-level node groups associated with a row associated with R-level responsibility is not a storage node associated with any responsibility level higher than R (for the same token).

An integer number of tokens assigned to the unique storage node is accessed755. For example, the integer number of tokens (which was calculated previously in step620) can be accessed from the token state repository520.

An equivalent integer number of entries (including the entry associated with the particular column) are populated760in the row with an ID of the unique storage node. Each of the row entries is associated with the set of responsibility level-node groups. For example, if the row is associated with secondary responsibility and the unique storage node is associated with 5 tokens, then 5 entries of the DLT in the row are filled with the storage node ID of the unique storage node, and the 5 entries are associated with the same set of responsibility level-node groups (e.g., have the same storage node associated with primary responsibility).

A determination is made in step765of whether another unique storage node exists. If another unique storage node does exist, then the row entries associated with the set of responsibility level-node groups are not fully populated, and the method630returns to step750. If another unique storage node does not exist, then the row entries are fully populated, and the method returns to step735.

FIG. 8is a flowchart illustrating a method800of updating the mDLT530fromFIG. 5and distributing the updated mDLT to application nodes120and storage nodes130, according to one embodiment. In one embodiment, the method800is performed by the update module550when a storage node is added to or removed from the environment100, when prompted by the DLT master system140, when weights of one or more storage nodes130change, an occurrence of another event, or any combination thereof. Other embodiments can perform the steps in different orders and can include different and/or additional steps. In addition, some or all of the steps can be performed by entities other than those shown inFIG. 5.

In step810, the update module550calculates, for an updated set of storage nodes, an integer number of tokens assigned to each storage node i for various levels of responsibility. The integer number of tokens for each updated storage node can be calculated using the equations previously described in conjunction withFIG. 6and stored in the token state repository520.

In step820, the update module550calculates differences between integer numbers for a previous set of storage nodes and the updated set of storage nodes. The differences between the integer numbers for the previous set of storage nodes and the updated set of storage nodes can be calculated as:
TokenDiff(node)=IntTokens(node in previous set)−IntTokens(node in updated set)
resulting in TokenDiff(node)>0 (meaning that there are more tokens assigned to the storage node than there should be given the updated set of storage nodes) or TokenDiff(node)<0 (meaning that there are fewer tokens assigned to the storage node than there should be). Other methods can be used to calculate the differences that result in similar indications for whether the storage node is associated with more or fewer tokens than the storage node should be.

In step830, the update module550transfers tokens to create an updated mDLT. Based on the TokenDiff calculations, the tokens are transferred among the various storage nodes by modifying the storage node IDs in the different entries within each row of the DLT. Tokens assigned to storage nodes that have responsibility for more tokens than they should be are transferred to storage nodes that have responsibility for fewer tokens than they should be. For example, if node A previously was handling 10 tokens and now should handle7, and node B previously was handling 5 tokens and now should handle8, three tokens previously associated with node A are transferred to node B. Transferring tokens is further described inFIG. 9.

In step840, the update module550sends the updated mDLT530to one or more affected storage nodes130. An affected storage node is a storage node whose node ID was added to or removed from the previous mDLT during the update process (i.e., step830).

In step850, the update module550receives notification from the affected storage node(s)130that the update process is complete. The update process is complete if the SN DLT430of the affected storage node130matches the updated mDLT530.

In step860, the update module550sends the updated mDLT530to application nodes120and any remaining storage nodes130so each has the updated mDLT530.

FIG. 9is a flowchart illustrating a method830of updating the mDLT530fromFIG. 5, according to one embodiment. The method830inFIG. 9corresponds to the step830inFIG. 8and is performed by the update module550. Other embodiments can perform the steps in different orders and can include different and/or additional steps. In addition, some or all of the steps can be performed by entities other than those shown inFIG. 5. In one embodiment, the method830is performed once for each storage node130.

The method830starts905and, for a particular storage node with particular responsibility level (e.g., primary responsibility level), a token difference is determined910for the particular storage node between a previous set of storage nodes and an updated set of storage nodes (see step820).

In step915, a determination is made regarding whether the token difference is indicative of the particular storage node having responsibility for more tokens than it should at that particular responsibility level, given the updated set of storage nodes130. If the token difference is not indicative of the particular storage node having responsibility for more tokens than it should, then the method830ends950. If the token difference is indicative of the particular storage node having responsibility for more tokens than it should, then the method830proceeds to step920.

In step920, another storage node with the same particular responsibility level as the particular storage node is selected. In this embodiment, if there is a particular storage node with a token difference indicative of the storage node having responsibility for too many or too few tokens, then there exists another storage node with a token difference indicative of the another storage node having responsibility for too few or too many tokens, respectively.

In step925, a determination is made regarding whether the token difference of the another storage node is indicative of the another storage node having responsibility for fewer tokens than it should at that responsibility level. If the token difference is not indicative of the another storage node having responsibility for too few tokens, the method830proceeds to step920. If the token difference is indicative of the another storage node having responsibility for too many tokens, the method830proceeds to step930.

In step930, a shared unique storage node of the particular storage node and the another storage node is found, where the shared unique storage node has a lower responsibility level than the particular responsibility level. For example, the particular storage node is storage node i, and the another storage node is storage node j, and both have primary responsibility for a number of tokens. Then, a shared unique storage node of storage node i and storage node j can be storage node k where storage node k has secondary responsibility for tokens with storage nodes i and j assigned with primary responsibility. If the storage node and the another storage node have R responsibility level, then the method830proceeds to step940(not shown).

In step935, a determination is made regarding whether the token difference of the shared unique storage node indicates that the shared unique storage node has 1) responsibility for too many tokens than it should where the particular storage node has the particular responsibility level and 2) responsibility for too few tokens where the another storage node has the particular responsibility level. If the token difference does not indicate that the shared unique storage node has 1) responsibility for too many tokens where the particular storage node has the particular responsibility level and 2) responsibility for too few tokens where the another storage node has the particular responsibility level, the method830proceeds to step945because, following the previous example, tokens have to be assigned from storage node i to storage node j, and storage node k does not have responsibility for more tokens (with storage node i assigned with primary responsibility) than it should. If the token difference does indicate that the shared unique storage node has 1) responsibility for too many tokens where the particular storage node has the particular responsibility level and 2) responsibility for too few tokens where the another storage node has the particular responsibility level, the method830proceeds to step940because tokens have to be assigned from storage node i to storage node j, and storage node k also has responsibility for more tokens (with storage node i assigned with primary responsibility) than it should.

In step940, one or more tokens are reassigned from the another storage node to the particular storage node. The number of tokens that are reassigned is less than or equal to the token difference of the particular storage node for the particular responsibility level and less than or equal to the token difference of the another storage node for the particular responsibility level.

In step945, a determination is made regarding whether all shared unique storage nodes of the particular storage node and the another storage node have been searched. If all shared unique storage nodes have not been searched, then the method830proceeds to step930. If all shared unique storage nodes have been searched, the method830proceeds to step920.

FIG. 10is a flowchart illustrating a method1000of updating the SN DLT430fromFIG. 4, according to one embodiment. In one embodiment, the method1000is performed by the SN DLT update module410when prompted by the DLT master system140with a DLT update or an occurrence of another event. Other embodiments can perform the steps in different orders and can include different and/or additional steps. In addition, some or all of the steps can be performed by entities other than those shown inFIG. 4.

In step1010, the SN DLT update module410receives the new DLT (or the updated mDLT). The new DLT is received from the DLT master system140or, more specifically, from the update module550of the DLT master system140.

In step1020, the SN DLT update module410calculates a difference between the new DLT and the previously-stored DLT. The difference between the new DLT and the previously-stored DLT is a difference in storage locations for tokens (and their associated data objects) for various levels of responsibility according to the previously-stored DLT versus the new DLT. For example, token A was previously stored on storage node X for primary responsibility, as indicated by the previously-stored DLT. The new DLT indicates that token A is now stored on storage node Y for primary responsibility. Thus, the calculated difference includes information that data objects associated with token A should now be stored on storage node Y for primary responsibility, not storage node X. The difference can be calculated by comparing, for each token, a column associated with the token in the previously-stored DLT with a column associated with the token in the new DLT. The difference between the two columns can be computed as a set difference in one embodiment.

In step1030, the SN DLT update module410migrates tokens. For a token whose previously-stored DLT column differs from its new DLT column, data objects associated with that token are transferred between the relevant storage nodes130for various levels of responsibility. For example, the data objects are transferred from the storage node130of the SN DLT update module410to another storage node or are transferred from another storage node (on which the data objects were previously stored) to the storage node of the SN DLT update module. Following the example from before, token A (and its associated data objects) is transferred from storage node X for primary responsibility to storage node Y. In an alternative embodiment, where the difference between two columns is calculated as a set difference, a token that has a non-NULL difference set causes a “migrate token” command to be sent to an affected storage node130. Following the example from before, token A has a non-NULL difference set that causes a migrate token command to be sent to storage node X. In one embodiment, the DLT master system140sends a migrate token command to the SN DLT update module410associated with an affected storage node130. In another embodiment, a token-destination storage node130sends a migrate token command to a token-origination storage node130. Following the example from before, storage node Y (token-destination for token A) sends a migrate token command to storage node X (token-origination for token A).

In step1040, the SN DLT update module410synchronizes tokens. During synchronization, the newly-transferred data objects are stored in the storage node130associated with the SN DLT update module410, for example, in the place of a data object whose token is no longer associated with the storage node130(e.g., will be or has been migrated during the token migration process). Optionally, data objects that were newly-added or existing data objects that were changed during the migration process (step1030) are synchronized. In the alternative embodiment where a migrate token command is sent to an affected storage node130, the SN DLT update module410of the affected storage node moves data objects from that storage node to another storage node as indicated by the new DLT.

In step1050, the SN DLT update module410stores the new DLT in the SN DLT. The new DLT reflects the storage of data objects in the storage node130of the SN DLT update module410.

In step1060, the SN DLT update module410notifies the DLT master system140that the update process is complete. The update process can include storing the new DLT in the SN DLT430.