How Fast Is Zold?

Zold is a non-blockchain cryptocurrency without a central ledger. Each Zold wallet has its own list of transactions, both positive (coming in) and negative (coming out). Two wallets take participation in each payment. The first wallet gets a money spending transaction and the second one gets a money receiving transaction. In order to spread the knowledge about a new payment both wallets get distributed to as many nodes as possible. The question is how long will it take for the entire network to accept new payments, if they are coming in at high speed? What will be the so called “confirmation time”?

Nodes are randomly connected to each other. Each node maintains a list of R neighbour nodes. R may unpredictably differ from zero up to N, the total number of nodes in the network.

When a wallet modification is pushed to a node, the node pulls the wallet from its neighbours, merges the changes, and pushes the wallet back to its visible neighbours. Each of them will do the same, until the entire network has a new version of the wallet. How many hops will the wallet have to jump through until the process is complete?

It seems that H, the approximate number of hops, will be equal to the logarithm of N to base R:

Now we have to find out how fast each node will be able to do those pull, merge, and push operations for each wallet. It seems logical to assume that the speed of the entire update will depend on two factors: how fast a neighbour node responds and how many of them are there. With the current Ruby software, utilizing 4 parallel threads, an average response time P is 400ms (which is pretty high and has to be decreased down to 50ms in future versions). With this response time and the current average R of 40 nodes, the speed of update S is 50 seconds. We can assume that the dependency is linear:

Thus, the network of N=1000 nodes, where each node is connected to R=16 neighbours, can accept a modification of a single wallet and spread it over all nodes in approximately 48 seconds (confirmation time):

The amount of transactions a single modification contains doesn’t seriously affect the result of the formula, since the most time consuming part of the update operation is the transfer of data between nodes. With 1,000,000 nodes in the network and the same R of 16, the confirmation time will be 90 second. Pretty fast, huh? As you see, the size of the network has almost no effect to the speed of data propagation.

What will indeed slow down the propagation process is the length of the queue of wallets each node maintains. When a node receives a new version of a wallet, it doesn’t start working with it immediately, but places it into the queue. The longer the queue, the bigger the value of S, which means slower processing of each update.

To resolve this issue each node accumulates wallets in the queue and pushes them to the neighbours in packages. Also, it pulls packages of wallets from its neighbours. The length of the package L depends on F, the intensity of the wallets flow:

With the current S of 50 seconds and the insentity F of a thousand wallets per second, the approximate length of the package will be 50,000 wallets:

Obviously, the length of the package L will affect its processing time S. The larger the package, the longer it takes to process its content and to deliver it over the network. The delivery process is the largest time consumer. The existing Ruby software spends up to 10μs out of the entire 50 seconds of S for the merge of the wallet, which approximately is 0.00002% of S. The rest of the time is spent on the communication with its neighbours over the HTTP protocol.

The time it takes to deliver a package over HTTP consists of 1) connection time, 2) data transfer time, and 3) HTTP processing time on the node. The connection takes 20-50ms. The data transfer takes 10μs per transaction, if the speed of the network is 100Mb/s and an average size of a single transaction is 1Kb. The largest time consumer is the HTTP processing code inside each node, which doesn’t change significantly, no matter how large is the package.

Ergo, the value of P for 50,000 wallet modifications with a single transaction in each one, would be equal to:

To summarize, even though the confirmation time depends on the amount of nodes in the network and the intensity of modificiations made by wallet owners, the dependency is not linear. The confirmation time does grow, but even with a million nodes in the network and a thousand transactions per second its value jumps from 20 to 200 seconds. With a million transactions per second and a million nodes in the network the estimated confirmation time will be equal to an hour.

To compare, VISA’s peak volume was 47,000 tps in 2013, while its regular volume is 1,700 tps. This analysis demonstrates how fast some other cryptocurrencies are.

We are developing a set of stress tests at the moment, in order to confirm the numbers and formulas presented above.