Turbo NSS
---------

Turbonss is a plugin for GNU Name Service Switch ([NSS][nsswitch])
functionality of GNU C Library (glibc). Turbonss implements lookup for `user`
and `passwd` database entries (i.e. system users, groups, and group
memberships). It's main goal is to run [`id(1)`][id] as fast as possible.

Turbonss is optimized for reading. If the data changes in any way, the whole
file will need to be regenerated. Therefore, it was created, and best suited,
for environments that have a central user & group database which then needs to
be distributed to many servers/services, and the data does not change very
often.

This is the fastest known NSS passwd/group implementation for *reads*. On my
2018-era laptop a corpus with 10k users, 10k groups and 500 average members per
group, `id` takes 17 seconds with the glibc default implementation, 10-17
milliseconds with a pre-cached `nscd`, ~8 milliseconds with uncached
`turbonss`.

Due to the nature of being built with Zig, this will work on glibc versions as
old as 2.16 (may work with even older ones, I did not test beyond that).

Project goals
-------------

- Make it as fast as possible. Especially optimize for the `id` command.
- Small database size (helps making it fast).
- No runtime, no GC, as little as possible overhead.
- Easy to compile for ancient glibc versions (comes out of the box with Zig).


Dependencies
------------

1. zig around `0.11.0-dev.3735+a72d634b7`.
2. [cmph][cmph]: bundled with this repository.

Building
--------

Clone, compile and run tests first:

    $ git clone --recursive https://git.jakstys.lt/motiejus/turbonss
    $ zig build test
    $ zig build -Dtarget=x86_64-linux-gnu.2.16 -Doptimize=ReleaseSafe

One may choose different options, depending on requirements. Here are some
hints:

1. `-Dcpu=<...>` for the CPU [microarchitecture][mcpu].
2. `-Dstrip=true` to strip debug symbols.

For reference, size of the shared library and helper binaries when compiled
with `zig build -Dstrip=true -Doptimize=ReleaseSmall`:

    $ ls -h1s zig-out/{bin/*,lib/libnss_turbo.so.2.0.0}
     24K zig-out/bin/turbonss-analyze
     20K zig-out/bin/turbonss-getent
     24K zig-out/bin/turbonss-makecorpus
    136K zig-out/bin/turbonss-unix2db
     20K zig-out/lib/libnss_turbo.so.2.0.0

Many thanks to Ulrich Drepper for [teaching how to link it properly][dso].

Demo
----

turbonss is best tested, of course, with many users and groups. The guide below
will show how to synthesize 10k users, 10k groups with an avereage membership
of 1k users per group, and test ubernss with such corpus.

1. Synthesize some users and groups to `passwd` and `group` in the current directory:

   ```
   $ zig-out/bin/turbonss-makecorpus
   wrote users=10000 groups=10000 max-members=1000 to .
   $ ls -1hs passwd group
    48M group
   668K passwd
   ```

2. Convert the generated `passwd` and `group` to the turbonss database. Note
   the `db.turbo` database is more than 4 times smaller than the textual one:

   ```
   $ zig-out/bin/turbonss-unix2db --group group --passwd passwd
   total 10968064 bytes. groups=10000 users=10000
   $ ls -1hs db.turbo
   11M db.turbo
   ```

3. Optional: inspect the freshly created database:

   ```
   $ zig-out/bin/turbonss-analyze db.turbo
   File:               db.turbo
   Size:               10,968,064 bytes
   Version:            0
   Endian:             little
   Pointer size:       8 bytes
   getgr buffer size:  18000
   getpw buffer size:  57
   Users:              10000
   Groups:             10000
   Shells:             4
   Most memberships:   u_1000000 (501)
   Sections:
       Name                 Begin    End          Size bytes
       header               00000000 00000080            128
       bdz_gid              00000080 00000e40          3,520
       bdz_groupname        00000e40 00001c00          3,520
       bdz_uid              00001c00 000029c0          3,520
       bdz_username         000029c0 00003780          3,520
       idx_gid2group        00003780 0000d3c0         40,000
       idx_groupname2group  0000d3c0 00017000         40,000
       idx_uid2user         00017000 00020c40         40,000
       idx_name2user        00020c40 0002a880         40,000
       shell_index          0002a880 0002a8c0             64
       shell_blob           0002a8c0 0002a900             64
       groups               0002a900 00065280        240,000
       users                00065280 000da580        480,000
       groupmembers         000da580 005a69c0      5,030,976
       additional_gids      005a69c0 00a75c00      5,042,752
   $ zig-out/bin/turbonss-getent --db db.turbo passwd u_1000000
   u_1000000:x:1000000:1000000:User 1000000:/home/u_1000000:/bin/bash
   $ zig-out/bin/turbonss-getent --db db.turbo group g_1000003
   g_1000003:x:1000003:u_1000002,u_1000003,u_1000004
   ```

4. Now since we will be messing with the system, run all following commands in
   a container:

   ```
   $ docker run -ti --rm -v `pwd`:/etc/turbonss -w /etc/turbonss debian:bullseye
   # cp zig-out/lib/libnss_turbo.so.2 /lib/x86_64-linux-gnu/
   ```

5. Instruct `nsswitch.conf` to use both turbonss and the standard resolver:

   ```
   # sed -i '/passwd\|group/ s/files/turbo files/' /etc/nsswitch.conf
   # time id u_1000000
   <...>
   real    0m0.006s
   user    0m0.000s
   sys     0m0.008s
   ```

  The `id` call resolved `u_1000000` from `db.turbo`.

6. Compare the performance to plain `files` (that is, without turbonss):

   ```
   # sed -i '/passwd\|group/ s/turbo files/files/' /etc/nsswitch.conf
   # cat passwd >> /etc/passwd
   # cat group >> /etc/group
   # time id u_1000000
   <...>
   real    0m17.164s
   user    0m13.288s
   sys     0m3.876s
   ```

   Over 2500x difference.

More Documentation
------------------

- Architecture is detailed in `docs/architecture.md`
- Development notes are in `docs/development.md`

Project status and known deficiencies
-------------------------------------

Turbonss works, but, to the author's knowledge, was not deployed to production.
If you want to use turbonss instead of a battle-tested, albeit slower nscd,
keep the following in mind:
- turbonss has not been fuzz-tested, so it will crash a program on invalid
  database file. Please compile with `ReleaseSafe`. It is plenty fast with this
  mode, but an invalid database will lead to defined behavior (i.e. crash with
  a stack trace) instead of overwriting memory wherever.
- if the database file was replaced while the program has been running,
  turbonss will not re-read the file (it holds to the previous file
  descriptor).
- requires a nightly version of zig (that will change with 0.11).

The license is permissive, so feel free to fork and implement the above (I
would appreciate if you told me, but surely you don't have to). I am also
available for [consulting][consulting] if that's your preference instead.

[nsswitch]: https://linux.die.net/man/5/nsswitch.conf
[id]: https://linux.die.net/man/1/id
[cmph]: http://cmph.sourceforge.net/
[dso]: https://akkadia.org/drepper/dsohowto.pdf
[mcpu]: https://en.wikipedia.org/wiki/X86-64#Microarchitecture_levels
[consulting]: https://jakstys.lt/contact
