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# plop database
The plop database is a distributed, single-master, append-only
database suitable for transparency systems like Certificate
Transparency.
Data entries are stored together with three attributes:
- index
integer; the first entry has index 0, the next one 1 and so on
- entry hash
the hash over the entry, used for duplicate detection
- leaf hash
hash over specific parts of the entry, usually together with a
timestamp for use in a merkle tree
## Storage in a file system
Two files (catlfish names in parentheses):
- treesize (treesize)
filename is static, file contains one line -- the number of entries
in the part of the database up to and including the last entry in
the last published tree, like a "current pointer"
- index (index)
filename is static, file contains one line per entry -- the leafhash
Three directories, "bucketed" in three levels, one file per database
entry (catlfish names in parentheses):
- entry (certentries)
filename=leafhash, content=the actual data of the entry
- entryhash (entryhash)
filename=entryhash, content=leafhash
- indexforhash (certindex)
filename=leafhash, content=index
## Distributed
TODO: describe distribution
## Erlang code in src/
- db.erl
public interface for adding entries and getting entries by index,
leaf hash and entry hash
- index.erl
file-based storage for ordered append-only lists of fixed-sized
entries, retrievable by index
- perm.erl
reading and writing of files
- atomic.erl
atomic file operations
- util.erl
helper functions for lower level file handling
- fsyncport.erl
interface to C implementation for fsync(2) syscall
## C code in c_src/
- net_read_write.c
read and write to/from a file descriptor, using fsync(2) to increase
probability that data lands on disk
- fsynchelper.c
erlang port for net_read_write
- erlport.c
glue
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