WedgeDB: Transaction Processing for Edge Databases

Wide-area Edge Database (WedgeDB) span globally and store data closer to the users. We term this the Global Edge Data management problem. In such an environment, aspects such as data storage, retrieval, transaction processing, and protection from malicious actors need to be addressed by any data management system that aims to consider itself a viable solution. Although blockchain technology (both permissioned and permissionless) has provided a way to address these concerns, transaction processing in these environments is still challenging. WedgeDB is an attempt to address these security problems in edge-cloud data systems [2]. The main goals of WedgeDB are to support distributed transaction processing coupled with secure transaction execution. Data stored in WedgeDB is partitioned into clusters with each cluster handling a unique set of keys. WedgeDB is a collection of such clusters. This is shown in figure 1. In figure 1, each cluster Cx stores a unique set of keys. This partitioning scheme allows us to build a distributed transaction model which runs transactions on a subset of clusters in the network. Transactions in WedgeDB are serializable. In WedgeDB, clients perform read operations as part of a transaction via read requests which can be sent to any of the WedgeDB nodes. The read operations are added to the transaction's history. Write operations are cached by the client until a commit is called which sends the transaction object containing the read history and write operations to a WedgeDB node to be committed. Transactions are processed in batches called Epochs. Each cluster maintains a leader which receives transactions and groups them into epochs. A cluster in WedgeDB contains 3f + 1 nodes (where f is the number of tolerable faulty nodes) and PBFT[1] is used to attain consensus among the nodes when executing transactions. Keys modified during an epoch are added to a Merkle tree which is used to verify changes to keys handled by the cluster. During transaction execution, proof of transaction execution is generated in the form of signed data blocks by the nodes in the cluster. At least f + 1 signed messages must be gathered before an epoch can be committed. These data blocks along with the root of the Merkle tree are stored in an SMR log where each entry in the SMR log corresponds to an epoch. Transactions that contain keys from different clusters are executed via two-phase commit. During the prepare phase a remote cluster executes PBFT within its local cluster and checks for dependency violations before moving ahead with the commit phase. Committed epoch may not have committed transactions and therefore an additional parameter is used to indicate the last committed epoch. This parameter combined with the dependency vector help in finding out serializability violations and abort transactions. With WedgeDB, transactions that affect only a few clusters do not require global consensus to commit. Transactions that read keys from a number of clusters do not require consensus at the server nodes and allow data to be read from a consistent snapshot of the WedgeDB network. We thus, present WedgeDB as a viable solution to the problem of global edge data management.