$txid variable for nginx - a sortable unique id in 20 case-insensitive characters
C Makefile Perl Perl6
Latest commit f1c197c Feb 22, 2015 @streadway Merge pull request #6 from GUI/fix-fd-leak
Fix file descriptor leak on /dev/urandom each time nginx reloads



ngx_txid is a module that exposes $txid: a cached, request scoped, 20 character, base32hex encoded, temporally and lexically sortable, case insensitive, 96 bit identifier.

Use $txid to correlate logs or upstream requests.


  • Configure and nginx with --add-module=path/to/ngx_txid


Assuming you want to to store your logs in column format. You'll need a join key for each request even across connections regardless of how accurate the scheduler is from your timer_resolution directive. The remaining columns can be specific per data logged.

    log_format agent     '$txid $http_user_agent';
    log_format referrer  '$txid $http_referer';
    log_format peer      '$txid $remote_addr';
    log_format status    '$txid $status';
    log_format request   '$txid $request';
    log_format conn      '$txid conn:$connection reqs:$connection_requests pipe:$pipe';

    server {
        listen       80;
        server_name  example.com;
        access_log   logs/example.com/conns.log conn;
        access_log   logs/example.com/agents.log agent;

        location / {
            remove_header    X-Request-Id;
            add_header       X-Request-Id $txid;
            proxy_set_header X-Request-Id $txid;
            proxy_pass       http://localhost:8080;


The design of this transaction ID should meet the following requirements:

  • Be roughly numerically temporally sortable with ~second granularity.
  • Have a representation that is roughly lexically sortable with ~second granularity.
  • Have a probability of less than 1e-9 for collision at 1 million transactions per second.
  • Be efficient and easy to decode into fixed size C types
  • Always be available at the risk of higher collision probability
  • Use as few bytes as possible
  • Work with IPv4 and IPv6 networks


Use a monotonic millisecond resolution clock in the high 42 bits and system entropy for the low 54 bits. Use enough entropy bits to satisfy a collision probability at a desired global request rate.

+------------- 64 bits------------+--- 32 bits ----+
+------ 42 bits ------+--22 bits--|----------------+
| msec since 1970-1-1 | random    | random         |

A request rate of 1 million per second across all servers means 1000 random values per millisecond. Estimating the collision probability using the birthday paradox can be done with this formula: 1 - e^(-((m^2)/(2*n))) where m is the number of ids and n is the number of random values possible.

When using 54 bits of entropy:

1mil req/s  = 1 - exp(-((1000^2) /(2*2^54))) = 2.775558e-11
10mil req/s = 1 - exp(-((10000^2)/(2*2^54))) = 2.775558e-09

The odds of collision are small even at 10 million requests per second.

Nginx keeps track of the current clock in increments of the configuration directive timer_resolution. The clock resolution for $txid is 1ms, so a timer resolution greater than 1ms means that the probability of collision will increase. If you have a timer_resolution of 10ms, 1 million requests per second would require 10,000 random values per second in the worst case.


Base32hex is used with a lower case alphabet and without padding characters is chosen for the following reasons:

  • lexically sort order equivalent to numeric sort order
  • case insensitive equality
  • lower case is easer for visual compares
  • denser than hex encoding by 4 bytes

Other techniques

  • snowflake: uses time(41) + unique id(10) + sequence(12).

    • Pro: guaranteed unique sequences
    • Pro: fits in 63 bits
    • Cons: requires unique id coordination for each server - 16 workers processes per host means a limit of 64 instances of nginx
    • Cons: only 11 bits available for unique id, needs monitoring
    • Cons: total ordering only possible in the same process
    • Cons: service interruption possible when clocks lose synchronization
  • flake: uses time + mac id + sequence.

    • Pro: guaranteed unique sequences
    • Cons: uses 128 bits
    • Cons: wastes 22 bits of timestamp data
    • Cons: only a single process per host can generate ids - needs to synchronize access to the sequence from each worker process
    • Cons: service interruption possible when clocks lose synchronization
    • Cons: needs cross platform MAC Address lookup.
  • UUIDv4: 122 bits of entropy

    • Pro: very low probability of collision
    • Cons: unsortable
  • UUID with timestamp: 48 bits of time + 74 bits entropy

    • Pro: very low probability of collision
    • Cons: string representation is not temporally local
  • httpd mod_unique_id: host ip(32) + pid(32) + time(32) + sequence (16) + thread id (32)

    • Pro: deterministic
    • Cons: uses 144 bits
    • Cons: assumes unique IPv4 for the hostnamme's interface
    • Cons: unsortable case-sensitive custom representation - base64 with a custom alphabet
    • Cons: hard limit of 65535 ids per second per pid - small tolerance for clock steps


This module uses the perl Test::Nginx module with tests in the t directory with a specialized build of nginx of the version specified in Makefile. A local install of dependent CPAN modules will be downloaded with cpanminus into t/ext. Sudo/root access is not required.

make test

Memory profiling of the tests can be performed when valgrind is installed.

make grind