# A Survey of Ledger Technology-Based Databases
## 5 Summary  
The discussed ledger databases are summarised in Tables 1-3. Key-value and document-oriented databases are dominant in ledger databases; however, only some approaches provide SQL databases. The differences in performance and architecture between centralised and decentralised structures are easily observed in the tables. While CLD does not require a consensus mechanism, decentralised permissioned blockchain-based database necessitates the consensus mechanism to reach agreement between participants in the network. The other observable divergence between the two structures is the implementation of smart contracts. The API interfaces in the centralised structure is generally more easily modified and expanded then the decentralised structure, where necessary software updates must be completed separately on every node, which are maintained by given enterprises. As a result, smart contracts provide a better approach due to their ability to be efficiently deployed on blockchain. Thus, similarly to an API interface update, they are accessible by all participants. As a result of the mentioned differences, summarising these technologies into CLDs and decentralised permissioned blockchain-based databases categories is logical. The information found in the table relates to the setup of experiments corresponding to the given transactions per second. Some mentioned information is adjustable depending on the specification of the given technology. The transactions per second measurement is a median value to be able to order the performance of technologies. Transaction throughput depends on the type and size of transaction, the use of smart contracts, the bandwidth, and the discussed variations in architectural structure. The data found in the transactions per second row are calculated based on numerous sources from articles [54; 55] and research [21; 32; 42; 45; 46]. During experiments, transactions included both read and write operations. The configurations for each discussed ledger database's experiment:  
* **BigchainDB:** four nodes were in the same Azure data centre. Each node had 2 nd Generation Intel Xeon Platinum 8272 CL processor and Intel Xeon Platinum 8168 processor and 4 GB RAM. Three different types of test were done with one million and 16.0000 transactions. * **BlockchainDB:** four nodes were in the same Azure data center with 16 vcpus and 32 GB memory. All experiments used an Ethereum backend. During experiments 4000 and 8000 transactions were sent. Experiments were run for online verification, offline verification and no verification. \begin{table}
\begin{tabular}{c c} \hline \hline
**Name** & **LedgerDB** \\ \hline
**Database type** & key-value \\
**Transaction processing** & Execute-commit-index \\
**Smart contract supported** & not supported \\
**Transactions per second** & 100k \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Overview of discussed Centralised Ledger Technology. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Name** & **Blockchain** & **BigchainDB** & **FalconDB** \\ \hline
**Database type** & PostgreSQL & MongoDB & MySQL and IntegriDB \\
**Replication** & Txn-based & Txn-based & Storage-based \\
**Transaction processing** & Execute and & Order-then-commit & Order-then-execute \\
**Consensus** & & & \\
**mechanism** & Kafka & Tendermint & Tendermint \\
**Smart contract** & procedures & not supported & Underlying Blockchain \\
**supported** & as smart contract & not supported & required \\
**Transactions** & & & \\
**per second** & 1.5k & 600 & 2k \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Overview of discussed permissioned blockchain technologies.