Logging

On this page

    If you want to inspect what is going on during query execution, you can enable a logger in Comunica.

    This guide focuses on configuring logging levels and printing output. Click here if you want to learn more about invoking a logger from within an actor implementation.

    Logging on the command line

    Using Comunica SPARQL on the command line, logging can be enabled via the -l option. For example, printing debug-level logs can be done as follows:

    $ comunica-sparql https://fragments.dbpedia.org/2016-04/en \
    "SELECT * WHERE { ?s ?p ?o } LIMIT 100" \
    -l debug

    [2020-07-31T11:38:47.632Z]  INFO: Requesting https://fragments.dbpedia.org/2016-04/en {
  actor: 'https://linkedsoftwaredependencies.org/bundles/npm/@comunica/actor-init-sparql/^1.0.0/config/sets/http.json#myHttpFetcher'
}
[2020-07-31T11:38:47.775Z]  INFO: Identified as qpf source: https://fragments.dbpedia.org/2016-04/en {
  actor: 'https://linkedsoftwaredependencies.org/bundles/npm/@comunica/actor-init-sparql/^1.0.0/config/sets/resolve-hypermedia.json#myHypermediaQpfResolver'
}
[2020-07-31T11:38:47.777Z]  INFO: Requesting https://fragments.dbpedia.org/2016-04/en {
  actor: 'https://linkedsoftwaredependencies.org/bundles/npm/@comunica/actor-init-sparql/^1.0.0/config/sets/http.json#myHttpFetcher'
}

    All log messages will be printed to standard error (stderr).

    If you only want to print the logs, you can void all query results as follows:

    $ comunica-sparql https://fragments.dbpedia.org/2016-04/en \
    "SELECT * WHERE { ?s ?p ?o } LIMIT 100" \
    -l debug > /dev/null

    If you want to redirect all logs to a file, you can forward them like this:

    $ comunica-sparql https://fragments.dbpedia.org/2016-04/en \
    "SELECT * WHERE { ?s ?p ?o } LIMIT 100" \
    -l debug 2> /path/to/log.txt

    Logging levels

    The following logging levels are available in Comunica:

    • trace
    • debug
    • info
    • warn
    • error
    • fatal
    When enabling a level, all levels below are also enabled. For example, when enabling error, then fatal will also be enabled.

    Logging in an application

    Using the log context entry, you can enable logging in a JavaScript application that uses Comunica:

    import {LoggerPretty} from "@comunica/logger-pretty";

const result = await myEngine.query('SELECT * WHERE { ?s ?p ?o }', {
  sources: ['http://fragments.dbpedia.org/2015/en'],
  log: new LoggerPretty({ level: 'debug' }),
});

    This logger makes use of LoggerPretty, which will print everything to standard error (stderr), just like Comunica SPARQL on the command line.

    Alternatively, more advanced logging can be achieved by making use of @comunica/logger-bunyan, or by implementing your own logger that implements the Logger interface.