The Tracking Log Reader demon reads from the shared memory log of a running instance of Varnish, aggregates data logged in a specific format for requests and ESI subrequests, and forwards the data to a messaging system (such as Kafka).


Tracking Log Reader demon

Author: Geoffrey Simmons
Date: 2017-11-19
Version: trunk
Manual section: 1


trackrdrd [[-n varnish_name] | [-f varnish_binlog]]
[-c config_file] [-u user] [-P pid_file]
[[-l log_file] | [-y syslog_facility]]
[-L tx_limit] [-T tx_timeout]
[-D] [-d] [-V] [-h]


The trackrdrd demon reads from the shared memory log of a running instance of Varnish, aggregates data logged in a specific format for requests and ESI subrequests, and forwards the data to a messaging system, such as Kafka.

trackrdrd reads data from VCL_Log entries that are displayed in this format by the varnishlog tool for client request transactions:

VCL_Log        track <DATA>
  • DATA: data to be logged

The VCL_Log entries may also specify a sharding key for the message brokers, in this format:

VCL_Log        track key <KEY>
  • KEY: the sharding key

VCL_Log entries result from use of the log() function provided by the standard vmod std distributed with Varnish. The log() call must include the prefix track and the data to be logged, or a sharding key. Note that DATA entries cannot begin with the word "key" followed by a space; these will be interpreted as KEY entries.

These log entries can be created with VCL code such as:

import std;

sub vcl_recv {
    /* ... */
    std.log("track url=" + req.url);
    std.log("track http_Host=" + req.http.Host);
    std.log("track key " + req.http.X-Key);
    /* ... */

Thus the data to be logged can be any information available in VCL. In this example, the data to be forwarded includes the URL and the Host header, and the sharding key was obtained from a request header X-Key.

trackrdrd collects all of the data logged for a request and its ESI subrequests, combining their data fields with the ampersand character (&). The data record is then buffered and ready to be forwarded to the messaging system, using the sharding key if required by the system. trackrdrd comprises a reader thread, which reads from the shared memory log and buffers data, and one or more worker threads, which read from the buffers and send data to message brokers.

In addition to the data obtained from the VCL_Log payloads, trackrdrd prepends a field XID=<xid> to the data, where <xid> is the VXID of the parent request (that includes any other ESI subrequests). It also appends a field req_endt=<t> containing the epoch time at which request processing ended, obtained from the Timestamp entry with the prefix Resp: (see vsl(3)). When there is more than one such Timestamp entry in a group of requests (for example in the logs of the ESI subrequests), the latest time is used.

The interface to the messaging system is implemented by a messaging plugin -- a shared object that provides definitions for the functions declared in the MQ interface in include/mq.h. See mq.h for documentation of the interface.

The source distribution for trackrdrd includes implementations of the MQ interface for Kafka and for file output (the latter for testing and debugging); see libtrackrdr-kafka(3) and libtrackrdr-file(3) for details.


The data read by the tracking reader from the Varnish log corresponds to the data displayed with this varnishlog command:

$ varnishlog -c -I 'VCL_log:^track ' -I Timestamp:^Resp: -g request \\
  -q 'VCL_log ~ "^track "' -i VSL

Thus the VCL example shown above may result in log entries such as:

*   << Request  >> 591570
-   VCL_Log        track url=/index.html
-   VCL_Log        track http_Host=foo.bar.org
-   VCL_Log        track key 12345678
-   Timestamp      Resp: 1430835449.167329 0.000681 0.000352

In this case, trackrdrd sends this data to message brokers, with the sharding key 12345678:


A VSL record is synthesized by the Varnish logging API if there was an error reading from the log; when this happens, trackrdrd logs the error message or warning in the VSL payload (by default using syslog(3)).


-n varnish_name
  Same as the -n option for varnishd and other Varnish binaries; i.e. the 'varnish name' indicating the directory containing the mmap'd file used by varnishd for the shared memory log. By default, the host name is assumed (as with varnishd). Also set by the config parameter 'varnish.name'. The -n and -f options are mutually exclusive.
-c config_file Path of a configuration file. If /etc/trackrdrd.conf exists and is readable, then its values are read first. If a file is specified by the -c option, then that file is read next, and config values that it specifies override values specified in /etc/trackrdrd.conf. Finally, config values specified on the command line override values specified in any config file. If no config files or other command line options are set, default config values hold.
-u user Owner of the child process. By default, the child process runs as 'nobody'. Also set by the config parameter 'user'.
-P pid_file Path of a file written by the management process that contains its process ID. By default, no PID file is written. Also set by the config parameter 'pid.file'.
-l log_file Log file for status, warning, debug and error messages. If '-' is specified, then log messages are written to stdout. By default, syslog(3) is used for logging. Log levels correspond to the 'priorities' defined by syslog(3). Also set by the config parameter 'log.file'.
-y syslog_facility
  Set the syslog facility; legal values are 'user' or 'local0' through 'local7', and the default is 'local0'. Options -y and -l are mutually exclusive. Also set by the config parameter 'syslog.facility'.
-D Run as a non-demon single process (for testing and debugging). By default, trackrdrd runs as a demon with a management (parent) process and worker (child) process.
-f varnish_binlog
  A binary dump of the Varnish SHM log produced by 'varnishlog -w'. If this option is specified, trackrdrd reads from the dump instead of a live SHM log (useful for debugging and replaying traffic). The options -f and -n are mutually exclusive; -n is the default. Also set by the config parameter 'varnish.bindump'.
-L limit Sets the upper limit of incomplete transactions kept by the Varnish logging API before the oldest transaction is force completed. An error message is logged when this happens. This setting keeps an upper bound on the memory usage of running queries. Defaults to 1000 transactions. The same as the -L option for standard Varnish logging tools such as varnishlog(3).
-T seconds Sets the transaction timeout in seconds for the Varnish logging API. This defines the maximum number of seconds elapsed between the beginning and end of the log transaction. If the timeout expires, the error message from the API is logged, and the transaction is force completed. Defaults to 120 seconds. The same as the -T option for standard Varnish logging tools such as varnishlog(3).
-d Sets the log level to LOG_DEBUG. The default log level is LOG_INFO.
-V Print version and exit
-h Print usage and exit


This version of the tracking reader is compatible with Varnish since version 5.2. See the source repository for versions that are compatible with other versions of Varnish.

The messaging plugin for Kafka (libtrackrdr-kafka) requires libraries for Kafka (librdkafka) and the multi-threaded libary for Zookeeper (libzookeeper_mt):



See INSTALL.rst in the source repository.


On startup (unless the -D option is chosen), trackrdrd reads any config files specified, and then demonizes, spawning a management process that in turn spawns a worker process.

The management process runs with the privileges of the user who started trackrdrd; these privileges must be sufficient to write the PID file and log file, if required by the configuration.

The worker process is started (and may be restarted) by the management process, and runs with the privileges of the user specified by the -u option or configuration parameter user. This process does the work of reading the Varnish log, and creates the worker threads that send data to message brokers.

To stop trackrdrd, send the TERM signal to the management process (e.g. with kill(1)); the management process in turn shuts down the worker process. Other responses to signals are detailed below in SIGNALS. If the worker process stops without being directed to do so by the management process, then the management process starts another one, up to the limit defined by the config parameter restarts.

After being instructed to terminate, the child process requests the Varnish logging API to flush open log transactions (transactions that have not yet been read to the End tag), and sends all pending messages to the message broker, but does not open any new transactions. It stops when all pending data have been sent to message brokers.


The tracking reader reads and writes data asynchronously -- a reader thread reads from the Varnish log and saves messages ready for sending in buffers, while worker threads read from the buffer and send messages to brokers.

Objects in the buffer are records and chunks. A record comprises a complete message ready to be sent to brokers, made up of one or more chunks, which store the message payload in fixed-size blocks.

The maximal length of a message payload is set by the config parameter max.reclen (payloads longer than the maximum are truncated), and the chunk.size sets the fixed length of data blocks. The best choice for these parameters depends on the distribution of message lengths. If the majority of messages are shorter than the maximum, then less memory is wasted by setting a smaller chunk size. Ideally, most messages should fit into the chunk size, and if nearly all messages require the maximum length, then chunk.size can be set equal to max.reclen.

The choice constitutes a time-space tradeoff -- if the chunk size is too large, then space is wasted; it if is too small, then the tracking reader spends too much time iterating over and copying chunks.

The max.records parameter sets the maximum number of records that can be stored in the buffers; the tracking reader computes the number of chunks necessary for that many records. max.records should be large enough for the buffering necessary during load spikes, and when the delivery of messages to the brokers is slow. max.records and chunk.size together determine the memory footprint of the tracking reader.

Free entries in the buffers for records and chunks are structured in free lists. The reader and worker threads each have local free lists, and exchange data via global free lists. That is, the reader thread takes free entries from its local free lists, and gets new entries from the global lists when the local lists are exhausted. Worker threads return free data to their local free lists, and return free lists to the global free lists periodically.

If the reader thread cannot obtain free data from the buffers -- meaning that the buffers are full and the worker threads have not yet returned free data -- then the reader discards the transaction that is currently being read from the Varnish log. No data are buffered from the transaction, leading to a loss of data. To avoid that, configure the throughput of message sends and the size of the data buffers so that free space is available as needed.


As mentioned above for command-line option -c, configuration values are read in this hierarchy:

  1. /etc/trackrdrd.conf, if it exists and is readable
  2. a config file specified with the -c option
  3. config values specified with other command-line options

If the same config parameter is specified in one or more of these sources, then the value at the "higher" level is used. For example, if varnish.name is specified in both /etc/trackrdrd.conf and a -c file, then the value from the -c file is used, unless a value is specified with the -n option, in which case that value is used.

The syntax of a configuration file is simply:

# comment
<param> = <value>

The <value> is all of the data from the first non-whitespace character after the equals sign up to the last non-whitespace character on the line. Comments begin with the hash character and extend to the end of the line. There are no continuation lines.

The parameter mq.module is required (has no default value), and mq.config_file is optional (depending on whether the MQ implementation requires a configuration file). All other config parameters have default values, and some of them correspond to command-line options, as shown below.

Parameter CLI Option Description Default
varnish.name -n Like the -n option for Varnish, this is the directory containing the file that is mmap'd to the shared memory segment for the Varnish log. This parameter and varnish.bindump are mutually exclusive. default for Varnish (the host name)
mq.module   Name of the shared object implementing the MQ interface. May be an absolute path, or the SO name of a library that the dynamic linker finds according to the rules described in ld.so(8). None, this parameter is required.
mq.config_file   Path of a configuration file used by the MQ implementation None, this parameter is optional.
nworkers   Number of worker threads used to send messages to the message broker(s). 1
worker.stack   Stack size for worker threads started by trackrdrd. Note: mq modules may start additional threads to which this limit does not apply Observed actual stack sizes are <64k, so the default leaves plenty of room. Increase only if segmentation faults on stack addresses are observed


(128 KB)

max.records   The maximum number of buffered records waiting to be sent to message brokers. 1024
max.reclen   The maximum length of a data record in characters. Should be at least as large the Varnish parameter shm_reclen. 1024
chunk.size   The size of fixed data blocks to store message data, as described above. This value may not be smaller than 64. 256
maxkeylen   The maximum length of a sharding key. Keys longer than this limit are discarded, with an error message in the log. 128
idle.pause   When the reader thread encounters the end of the Varnish log, i.e. no new transactions have been added to the log since the last read, then the thread pauses for this length of time in seconds. If the pause is too short, then the reader thread may waste CPU time in a busy-wait loop. If too long, the reader may fall too far behind in the log read, running a risk of log overruns. 0.01 seconds
tx.limit -L The upper limit for incomplete transactions to be aggregated by the Varnish logging API, as explained above. default for the logging API (1000 transactions)
tx.timeout -T The transaction timeout in seconds for the logging API, as explained above. default for the logging API (120 seconds)
qlen.goal   A goal length for the internal queue from the reader thread to the worker threads. trackrdrd uses this value to determine whether a new worker thread should be started to support increasing load. max.records/2
user -u Owner of the child process nobody, or the user starting trackrdrd
pid.file -P Path to the file to which the management process writes its process ID. If the value is set to be empty (by the line pid.file=, with no value), then no PID file is written. /var/run/trackrdrd.pid
restarts   Maximum number of restarts of the child process by the management process 1
restart.pause   Seconds to pause before restarting a child process 1
thread.restarts   Maximum number of restarts of a worker thread by the child process. A thread is restarted after a message send, message system reconnect and message resend have all failed. If the restart limit for a thread is reached, then the thread goes into the state abandoned and no more restarts are attempted. If all worker threads are abandoned, then the child process stops. 1
monitor.interval   Interval in seconds at which monitoring statistics are emitted to the log. If set to 0, then no statistics are logged. 30
monitor.workers   Whether statistics about worker threads should be logged (boolean) false
log.file -l Log file for status, warning, debug and error messages, and monitoring statistics. If '-' is specified, then log messages are written to stdout. This parameter and syslog.facility are mutually exclusive. syslog(3)
syslog.facility -y See syslog(3); legal values are user or local0 through local7. This parameter and log.file are mutually exclusive. local0
varnish.bindump -f A binary dump of the Varnish shared memory log obtained from varnishlog -w. If a value is specified, trackrdrd reads from that file instead of a live Varnish log (useful for testing, debugging and replaying traffic). This parameter and varnish.name are mutually exclusive.  


By default, trackrdrd uses syslog(3) for logging with facility local0 (unless otherwise specified by configuration as shown above). In addition to informational, error and warning messages about the running processes, monitoring information is periodically emitted to the log (as configured with the parameter monitor.interval). The monitoring logs have this form (at the info log level, with additional formatting of the log lines, depending on how syslog is configured):

Data table: len=1000 occ_rec=0 occ_rec_hi=8 occ_rec_hi_this=2 occ_chunk=0 occ_chunk_hi=8 occ_chunk_hi_this=2 global_free_rec=0 global_free_chunk=0
Reader: seen=1896 submitted=1896 nodata=0 free_rec=1000 free_chunk=8000 no_free_rec=0 no_free_chunk=0 len_hi=728 key_hi=39 len_overflows=0 truncated=0 key_overflows=0 vcl_log_err=0 vsl_err=0 closed=0 overrun=0 ioerr=0 reacquire=0
Workers: active=20 running=0 waiting=20 exited=0 abandoned=0 reconnects=0 restarts=0 sent=1896 failed=0 bytes=1050591

If monitoring of worker threads is switched on, then monitoring logs such as this are emitted for each thread:

Worker 1 (waiting): seen=105 waits=85 sent=105 bytes=57664 free_rec=0 free_chunk=0 reconnects=0 restarts=0 failed_recoverable=0 failed=0

The line prefixed by Data table describes the state of the data buffers -- completed messages waiting to be forwarded by worker threads. The field len is constant; occ_rec_hi and occ_chunk_hi are monotone increasing. All other fields are gauges, expressing a current level that may rise or fall:

Field Description
len Max number of records in the data table
occ_rec Number of records currently buffered
occ_rec_hi Occupancy high watermark for records -- highest number of buffered records since startup
occ_rec_hi_this Occupancy high watermark for records in the current monitoring interval
occ_chunk Number of chunks currently buffered
occ_chunk_hi Occupancy high watermark for chunks since startup
occ_chunk_hi_this Occupancy high watermark for chunks in the current monitoring interval
global_free_rec Current length of the global free record list
global_free_chunk Current length of the global free record list

The line prefixed by Reader describes the state of the reader thread. The fields free_rec and free_chunk are gauges, and len_hi and key_hi are monotone increasing; the rest are cumulative counters:

Field Description
seen Number of log transactions read since startup, natching the filters for the tracking reader as shown above
submitted Number of records passed from the reader thread to worker threads, to be sent to message brokers
no_data Number of log transactions read with no data payloads in the VCL_Log entries
free_rec Number of records in the reader thread's local free list
free_chunk Number of chunks in the reader thread's local free list
no_free_rec How often data was discarded because no free records were available
no_free_chunk How often data was discarded because no free chunks were available
len_hi Length high watermark -- longest complete message formed since startup
key_hi Key length high watermark -- longest sharding key since startup
len_overflows How often the length of a message exceeded max.reclen
truncated How often data from the Varnish log was truncated due to the presence of a null byte. This can happen if the data was already truncated in the log, due to exceeding shm_reclen.
key_overflows How often the length of a sharding key exceeded maxkeylen
vcl_log_err How often a VCL_Log entry beginning with track could not be parsed
vsl_err Number of errors/warnings signaled by the Varnish logging API with a VSL entry in the log transaction
closed Number of times the Varnish log was closed or abandoned
overrun Number of times log reads were overrun
ioerr Number of times log reads failed due to I/O errors
reacquire Number of times the Varnish log was re-acquired

The line prefixed by Workers gives an overview of the worker threads. The field active is constant, and running and waiting are gauges; the rest are cumulative counters:

Field Description
active Number of worker threads created, equal to the config param nworkers
running Number of worker threads currently in the running state
waiting Number of threads currently in the waiting state
exited Number of threads currently in the exited state
abandoned Number of worker threads that have been abandoned due to reaching the restart limit (thread.restarts)
reconnects How often worker threads reconnected to a message broker after an unsuccessful send
restarts How often worker threads were restarted after a message send, reconnect and resend all failed
sent Total number of messages successfully sent to a message broker
failed Number of failed sends (failure after reconnect, or after non-recoverable failures of the message plugin)
bytes Total number of bytes in successfully sent messages

If worker threads are monitored, then the running state if logged for each worker thread, one of:

  • not started
  • initializing
  • running
  • waiting
  • abandoned
  • shutting down
  • exited

In normal operation, the state should be either running, when the thread is actively reading data buffers and sending them to message brokers, or waiting, when the threads have exhausted all pending records, or has not yet been awakened to handle more records.

The fields free_rec and free_chunks are gauges, and all other fields in a log line for a worker thread are cumulative counters:

Field Description
seen Number of messages read by the worker thread from the internal queue (which is filled by the reader thread)
waits How often the worker thread was in the waiting state (no new messages on the queue)
sent Number of messages successfully sent by the worker thread
bytes Total number of bytes in messages successfully sent by the worker
free_rec Number of records currently in the worker's local free list
free_chunk Number of chunks currently in the worker's local free list
reconnects How often this worker reconnected to a message broker after an unsuccessful send
restarts How often this worker was restarted after a message send, reconnect and resend all failed, or after non-recoverable message failures
failed_recoverable How often this worker had recoverable message failures (failures that do not corrupt the state of the message plugin and do not require thread restart)
failed Number of non-recoverable message failures, requiring a thread restart


The management and child process respond to the following signals (all other signals have the default handlers):

Signal Parent Child
TERM Shutdown Shutdown
INT Shutdown Shutdown
HUP Graceful restart Flush transactions
USR1 Graceful restart Dump data table to log
USR2 Ignore Ignore
ABRT Abort with stacktrace Abort with stacktrace
SEGV Abort with stacktrace Abort with stacktrace
BUS Abort with stacktrace Abort with stacktrace

Shutdown proceeds as described above in STARTUP AND SHUTDOWN.

When signaled for graceful restart, the management process stops the running worker process and starts another one. This has the effect that the first process finishes reading data for open log transactions, and the second one begins reading data for new requests, so that as few records as possible are lost. The new process reads the same config files as the original worker process, and retains any command-line configuration, unless these values are overridden by config files. This allows for configuration changes "on-the-fly".

On receiving signal USR1, the worker process writes the contents of all buffered data as well as the current configuration to the log (syslog, or log file specified by config), for troubleshooting or debugging.

On receivng the HUP signal, the worker process requests the Varnish log API to flush all transactions that it is currently aggregating, even if they are not yet complete (to the End tag). These are consumed by the reader thread and processed normally (although data may be missing).

Where "abort with stacktrace" is specified above, a process write a stack trace to the log (syslog or otherwise) before aborting execution; in addition, the worker process executes the following actions:

  • dump the current contents of the data table (as for the USR1 signal)
  • emit the monitoring stats to the log


Both the management and worker processes return 0 on normal termination, and non-zero on error. When the worker process stops, the management process records its return value in the log, as well as any signal the worker process may have received.



This document is distributed under the same terms as the trackrdrd project, see LICENSE.