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ElectrumX - Reimplementation of Electrum-server

Licence: MIT Licence
Author: Neil Booth
Language: Python (>=3.5)

Motivation

For privacy and other reasons, I have long wanted to run my own Electrum server, but for reasons I cannot remember I struggled to set it up or get it to work on my DragonFlyBSD system, and I lost interest for over a year.

More recently I heard that Electrum server databases were around 35GB in size when gzipped, and had sync times from Genesis of over a week (and sufficiently painful that no one seems to have done one for a long time) and got curious about improvements. After taking a look at the existing server code I decided to try a different approach.

I prefer Python3 over Python2, and the fact that Electrum is stuck on Python2 has been frustrating for a while. It's easier to change the server to Python3 than the client.

It also seemed like a good way to learn about asyncio, which is a wonderful and powerful feature of Python from 3.4 onwards. Incidentally asyncio would also make a much better way to implement the Electrum client.

Finally though no fan of most altcoins I wanted to write a codebase that could easily be reused for those alts that are reasonably compatible with Bitcoin. Such an abstraction is also useful for testnets, of course.

Implementation

ElectrumX does not currently do any pruning. With luck it may never become necessary. So how does it achieve a much more compact database than Electrum server, which prunes a lot of hisory, and also sync faster?

All of the following likely play a part:

• aggressive caching and batching of DB writes
• more compact representation of UTXOs, the mp address index, and history. Electrum server stores full transaction hash and height for all UTXOs. In its pruned history it does the same. ElectrumX just stores the transaction number in the linear history of transactions, and it looks like that for at least 5 years that will fit in a 4-byte integer. ElectrumX calculates the height from a simple lookup in a linear array which is stored on disk. ElectrumX also stores transaction hashes in a linear array on disk.
• storing static append-only metadata which is indexed by position on disk rather than in levelDB. It would be nice to do this for histories but I cannot think how they could be easily indexable on a filesystem.
• avoiding unnecessary or redundant computations
• more efficient memory usage
• asyncio and asynchronous prefetch of blocks. With luck ElectrumX will have no need of threads or locking primitives
• because it prunes electrum-server needs to store undo information, ElectrumX should does not need to store undo information for blockchain reorganisations (note blockchain reorgs are not yet implemented in ElectrumX)

Roadmap

• test a few more performance improvement ideas
• handle blockchain reorgs
• handle client connections
• potentially move some functionality to C or C++

Once I get round to writing the server part, I will add DoS protections if necessary to defend against requests for large histories. However with asyncio it would not surprise me if ElectrumX could smoothly serve the whole history of the biggest Satoshi dice address with minimal negative impact on other connections; we shall have to see. If the requestor is running Electrum client I am confident that it would collapse under the load far more quickly that the server would; it is very inefficient at handling large wallets and histories.

Database Format

The database and metadata formats of ElectrumX are very likely to change in the future which will render old DBs unusable. For now I do not intend to provide converters as the rate of flux is high.

Miscellany

As I've been researching where the time is going during block chain indexing and how various cache sizes and hardware choices affect it, I'd appreciate it if anyone trying to synchronize could tell me:

- their O/S and filesystem
- their hardware (CPU name and speed, RAM, and disk kind)
- whether their daemon was on the same host or not
- whatever stats about sync height vs time they can provide (the
  logs give it all in wall time)
- the network they synced

Neil Booth [email protected] https://github.com/kyuupichan 1BWwXJH3q6PRsizBkSGm2Uw4Sz1urZ5sCj