Logging features in Tezos allow to monitor its execution and be informed in real time about events of interest, such as errors, completion of certain steps, etc. This is why various software components emit events throughout the codebase (see Using The Event Logging Framework), the logging framework dispatches them to an arbitrary number of (active) sinks which can filter print, store, or otherwise handle events.

Events have:

  • a name

  • a section; a hierarchical event classification mechanism, i.e. a path-like list of strings, sometimes the name is appended to make a fully-qualified name,

  • a level quantifying the relative importance of the event (Debug, Info, Notice, Warning, Error, and Fatal — inspired from the Syslog standard severity levels),

  • and, as contents, arbitrary structured data. They can be serialized to any format supported by the data-encoding library (e.g. JSON) and pretty-printed for human readers.

Events are by default “lazy”: if no active sink accepts their section and level, they are not evaluated at all (i.e. in OCaml, the unit -> event argument of Event.emit is never called). This means that outputting more events implies more CPU and memory usage.

“Legacy events” are the events that use the old & deprecated logging API; their contents all have the same structure: they are just human-readable text. The API makes that these events are evaluated even if they are not consumed by a sink; i.e. they are not lazily evaluated. The codebase is in the process of getting rid of them.

Sink Configuration

The logging framework refers to sinks with URIs (a.k.a. structured strings of the form schema://path?query): the schema of the URI is used to find the appropriate sink-implementation, the rest of the URI is used to configure the particular instance. There can be as many instances of a given implementation as needed, except for the (legacy) Lwt-Log Sink which should be activated at most once.

(Legacy) Lwt-Log Sink

This sink is the one currently activated by default:

  • Schema: lwt-log

  • URI-Configuration: none; (for now) can only use the legacy TEZOS_LOG environment variable or the specific section in the node’s configuration file (see below).

This sink can only output pretty-printed versions of the events.

File-Descriptor Sinks

This is configurable and duplicatable replacement for the lwt-log-sink; it allows to output events to regular Unix file-descriptors. It is actually a family of three URI-schemes:

  • file-descriptor-path:// to output to a file,

  • file-descriptor-stdout:// to output to stdout / 1, or

  • file-descriptor-stderr:// to output to stderr / 2.

Note: -stdout and -stderr schemes are there for convenience and for making the API consistent; most Unix-ish systems nowadays have /dev/std{out,err} and /dev/fd/[0-9]+ special files which can be used with file-descriptor-path (with some precautions).

The path of the URI is used by file-descriptor-path to choose the path to write to and ignored for the other two.

The query of the URI is used to further configure the sink instance.

Common options:

  • level-at-least=<loglevel> the minimal log-level that the sink will output.

  • section-prefix can be given many times and defines a list of pairs <section-prefix>:<level-threshold> which can be used to setup more precise filters. level-at-least=info can be understood as section-prefix=:info, the empty section prefix matches all sections.

  • format=<value> the output format used; acceptable values are:

    • one-per-line (the default): output JSON objects, one per line,

    • netstring: use the Netstring format (cf. Wikipedia) to separate JSON records,

    • pp to output the events pretty-printed, one per line, using a format compatible with RFC-5424 (or Syslog).

Options available only for the file-descriptor-path:// case:

  • with-pid=<bool> when true adds the current process-id to the file path provided (for instance, useful for the node when not running in --singleprocess mode).

  • fresh=<bool> when true smashes the content of the file if it already exists instead of appending to it.

  • chmod=<int> sets the access-rights of the file at creation time (default is 0o600, provided Umask allows it).


  • file-descriptor-path:///the/path/to/write.log?format=one-per-line&level-at-least=notice&with-pid=true&chmod=0o640 → Executables will write all log events of level at least Notice to a file /the/path/to/write-XXXX.log where XXXX is the PID, the file will be also readable by the user’s group (0o640).

  • file-descriptor-stderr://?format=netstring → Executables will write to stderr JSON blobs “packetized” as Netstrings.

  • file-descriptor-path:///dev/fd/4?section-prefix=rpc:debug → Executables will write to the file-descriptor 4 likely opened by a parent monitoring process. The reader will only receive the logs from the section rpc (but all of them including Debug).

The format of the events is (usually minified):

  {"hostname": <host-name>,
   "time_stamp": <float-seconds-since-epoch>,
   "section":[ <list-of-strings> ],
     <event-specific-json> } }

Additionally, the "hostname" field can be customized with environment variable TEZOS_EVENT_HOSTNAME; Its default value is the hostname of the device the node is running on.

File-Tree Sink

This is a sink that dumps events as JSON files (same format as above) in a directory structure guided by the section of the events. It can be useful for testing the logging framework itself, or for off-line post-mortem analysis for instance.

The URI scheme is unix-files, the path is the top-level directory in which the JSON files will be written.

The query of the URI allows one to filter the events early on.

  • level-at-least=<loglevel> the minimal log-level that the sink will output.

  • name-matches=<regexps> comma-separated-list of POSIX regular expressions on the name of the events.

  • name=<names> comma-separated-list of event names matched exactly.

  • section=<sections> comma-separated-list of event sections matched exactly.

  • no-section=<bool> when true only catch the events that have an empty section.

Example: unix-files:///the/path/to/write?level-at-least=info (the path should be inexistent or already a directory).

The directory structure is as follows: <section-dirname>/<event-name>/<YYYYMMDD>/<HHMMSS-MMMMMM>/<YYYYMMDD-HHMMSS-MMMMMM-xxxx.json> where <section-dirname> is either no-section or section-<section-name>.

Global Defaults

By default, only the lwt-log:// sinks are activated and configured to output events of level at least Notice.

JSON Configuration Format

A configuration JSON blob, is an object with one field "active_sinks" which contains a list of URIs:

  "active_sinks": [ <list-of-sink-URIs> ]

The URIs are discriminated among the sink implementations above using their schemes and activated.

It is used in various places: node configuration file, logging-configuration RPC, etc.

Environment Variables

The logging framework can be configured with environment variables before starting the node. Those variables work on all the code using the tezos-stdlib-unix library as long as Internal_event_unix.init is called; this should include all the regular tezos-* binaries.

  • TEZOS_EVENTS_CONFIG must be a whitespace-separated list of URIs:

    • URIs that have a schema are activated.

    • URIs without a schema, i.e. simple paths, are understood as paths to configuration JSON files (format above) to load (which themselves activate sinks).

  • TEZOS_LOG and LWT_LOG (with lower priority) contain “rules” to configure the lwt-log:// sink. The rules are expressed with a DSL documented at Lwt_log_core:

    • rules are separated by semi-colons ;,

    • each rule has the form pattern -> level,

    • a pattern is a minimalist glob-expression on the section.name of the event, e.g. rpc* for all events whose section.name starts with rpc,

    • rules are ordered, i.e., the first pattern that matches, from left to right, fires the corresponding rule.

  • TEZOS_EVENT_HOSTNAME is used by the file-descriptor-sink to tweak the JSON

    output (see above).

Node-Specific Configuration

Configuration File

See tezos-node config --help for the full schema of the node’s JSON configuration file.

In particular the fields:

  • "internal-events" contains a configuration of the sinks (format above).

  • "log" is an object which defines the configuration of the lwt-log:// sink; one can redirect the output to a file, set the rules, and change the formatting template.

Command Line Options

See tezos-node run --help, the lwt-log:// sink configuration can be also changed with 2 options:

  • -v / -vv: set the global log level to Info or Debug respectively.

  • --log-output: set the output file.

RPC /config/logging

The node exposes an administrative PUT endpoint: /config/logging.

The input schema is the JSON configuration of the sinks. It deactivates all current sinks and activates the ones provided except the lwt-log:// sink that is left untouched.

Example: (assuming the lwt-log:// is active not to miss other events) this call adds a sink to suddenly start pretty-printing all rpc events to a /tmp/rpclogs file:

tezos-client rpc post /config/logging with \
  '{ "active_sinks": [ "file-descriptor-path:///tmp/rpclogs?section-prefix=rpc:debug&format=pp&fresh=true" ] }'

Client and Baking Daemons

For now, tezos-client, tezos-{baker,endorser,accuser}-*, etc. can only be configured using the environment variables.

There is one common option --log-requests which can be used to trace all the interactions with the node (but it does not use the logging framework).

Processing Structured Events

This is work-in-progress, see:

  • tezos-admin-client show event-logging outputs the configuration currently understood by tezos-admin-client (hence through the TEZOS_EVENTS_CONFIG variable) and lists all the events it knows about.

  • tezos-admin-client output schema of <Event-Name> to <File-path> get the JSON-Schema for an event.

Example: tezos-admin-client output schema of block-seen-alpha to block-seen-alpha.json