erigon-pulse/docs/programmers_guide/read_db.md

How to read DB directly - not by Json-RPC/Graphql:

There are 2 options exist:

1. call --private.api.addr there is grpc interface with low-level data access methods - can read any data in any order, etc... Interface is here: https://github.com/ledgerwatch/interfaces/blob/master/remote/kv.proto Go/C++/Rust libs already exist. Names of buckets and their format you can find in bucket.go You can do such calls by network.
2. Read Erigon's db while Erigon is running - it's also ok - just need be careful - do not run too long read transactions (long read transactions do block free space in DB). Then your app will share with Erigon same OS-level PageCache where hot part of db stored. It may be great - if you read hot data (for example do incremental update of graph node) - then your reads will be super fast and almost never touch disk. But if you wanna read cold data - then your app will load cold data to PageCache and maybe evict some Erigon's hot data. Probably it will not be very dangerous - because your reads will happen once while Erigon will touch hot data often and OS's built-in LRU will understand which data is more Hot and keep it in RAM.

this 2 options ^ are exactly how RPCDaemon works with flags --private.api.addr and --datadir. One by using grpc interface, another by opening Erigon's db in read-only mode while Erigon running. But both this options are using RoKV (stands for read-only) kv_abstract.go interface. Option 1 using kv_remote.go to implement RoKV, option 2 using - kv_mdbx.go

Erigon using MDBX database. But any articles in internet about LMDB are also valid for MDBX.

We have Go, Rust and C++ implementations of RoKV interface.

Rationale and Architecture of DB interface: ./../../ethdb/Readme.md

MDBX docs: erthink.github.io/libmdbx/ and https://github.com/erthink/libmdbx/blob/master/mdbx.h