# Logging

# 1. Functional Specification

# I. Summary

As a user of the MeiliSearch binary, I want to be able to know what is happening in the engine at different levels of granularity depending on my needs.

# II. Motivation

Keeping track of the behavior of a system is useful for those who test, develop and use it in production. The purpose of this specification is to indicate the logging behavior of the search engine.

# III. Additional Materials

# Algolia

Algolia offers an API endpoint dedicated to retrieving the logs of search and indexing operations.

A call on the getLogs endpoint affect the algolia quota but it is not counted as a log.

The logs are kept for a period of 7 days. After this period, they are no longer accessible from the API.

Algolia gives parameters to modify the request according to the user's needs.

A query performed without parameters returns the last 10 records by default.

The maximum number of logs that can be returned per request is 1000.

It is possible to use the offset and length parameters to search the log entries.

A type parameter is also provided to select the type of log to retrieve.

Value Description
all All the logs
query Exclusively the queries
build Exclusively the build operations
error Exclusively the errors

Here is the information that Algolia chooses to return:

Key Description
timestamp Timestamp in ISO-8601 format
method Rest type of the method
answer_code HTTP response code
query_body Request body. Limited to 1000 characters
answer Answer body. Limited to 1000 characters
url Request URL
ip Client ip of the call
query_headers Request Headers (API Key is obfuscated)
sha1 Id of log entry
nb_api_calls Number of API calls
processing_time_ms Processing time for the query (Does not include network time)
query_nb_hits Number of hits returned for the query
exhaustive Exhaustive flags used during the query
index Index name of the log
inner_queries Contains an object for each performed query with the indexName, queryID, offset, and userToken

Source: Algolia documentation (opens new window)

# TypeSense

TypeSense makes no mention of logs in its documentation. However, the tracing policy seems to be in verbose mode and gives a lot of more or less relevant information.

typesense_1     | I20210403 01:06:33.688689     1 typesense_server_utils.cpp:301] Starting Typesense 0.19.0
typesense_1     | I20210403 01:06:33.690609     1 typesense_server_utils.cpp:304] Typesense is using jemalloc.
typesense_1     | I20210403 01:06:33.730222     1 typesense_server_utils.cpp:405] Starting API service...
typesense_1     | I20210403 01:06:33.730813    82 typesense_server_utils.cpp:210] Since no --nodes argument is provided, starting a single node Typesense cluster.
typesense_1     | I20210403 01:06:33.734120     1 http_server.cpp:189] Typesense has started listening on port 8108
typesense_1     | I20210403 01:06:33.744827    82 server.cpp:1045] Server[braft::RaftStatImpl+braft::FileServiceImpl+braft::RaftServiceImpl+braft::CliServiceImpl] is serving on port=8107.
typesense_1     | I20210403 01:06:33.744860    82 server.cpp:1048] Check out http://1c913ea63cb1:8107 in web browser.
typesense_1     | I20210403 01:06:33.747742    82 log.cpp:674] Use crc32c as the checksum type of appending entries
typesense_1     | I20210403 01:06:33.747857    82 log.cpp:1158] log load_meta /data/state/log/log_meta first_log_index: 1 time: 56
typesense_1     | I20210403 01:06:33.748559    82 log.cpp:1098] load open segment, path: /data/state/log first_index: 1
typesense_1     | I20210403 01:06:33.748966    82 raft_meta.cpp:521] Loaded single stable meta, path /data/state/meta term 3 votedfor time: 193

It also gives the stack trace in case of exception.

# ElasticSearch

Elasticsearch is probably the most versatile search engine when it comes to logging. See the documentation (opens new window).

It is possible to define a precise rolling log strategy.

A rolling log strategy permits to:

  • Facilitate the administration of systems that generate a large number of logs.
  • automates swapping, compressing, deleting, and sending logs. It assists in keeping the logging system within the specified file system space limits.
  • Defines an interval over which log analysis can be performed
  • Gives an efficient way to identify log files that are no longer used so that an automated process can clean up and compress the log directory and run log analysis programs.

See more information here about log rotation strategy (opens new window)

This is an important thing to have when systems generate a lot of logs or are under heavy load. It's also a good practice to populate a monitoring dashboard at certain times and, through that, provide an easy way to analyze and understand what happened in the system in a human-readable way.

Internally, Elasticsearch uses log4j2 to track events and configure the logging policy. Logging levels can be configured on a per-package basis, giving the precision needed to monitor a specific feature or time in a search engine's lifecycle.

A log level can be: debug, info, warn, error, fatal, or unknown.

Elasticsearch's SaaS platform, like Algolia, offers a fairly similar endpoint for browsing system-generated logs.

However, it is possible to disable log storage, change the retention period and also filter the logs with more details.

image Elastic App Search's Log Retention settings view.

It is also possible to do that from a Log Settings API endpoint.

API Logs track requests and responses at the engine level, while Analytics Logs track requests, clicks and counts.

# IV. Explanation

# Current Logging behaviour of MeiliSearch (0.20)

MeiliSearch uses the rust lib env_logger to output logs based on the log level fetched from the RUST_LOG environment variable. env_logger allows to define a specific log level per module if necessary. See more here (opens new window).

# Logging behaviour for Milli (0.21)

We have decided to keep using the rust lib env_logger for Milli/Transplant. However, we will make some changes to make the logging more consistent and versatile.

Instead of using RUST_LOG, we create a MEILI_LOG_LEVEL environment variable that corresponds to RUST_LOG in the binary to improve its meaning and clarity to users. A CLI option --log-level will also be added to follow our configuration convention.

# Log Levels
Level Description
ERROR Everything related to an error. A non blocking error has occured.
WARN Used to warn about an exceptional non-blocking event occurring.
INFO Default Log Level. It displays high level informations of events occuring in the search engine.
DEBUG Used for debugging and development purposes. More verbose than INFO.
TRACE Display everything happening at engine level. Can be useful for rust developers dealing with complex issues
OFF Disable the logs.
# Log Format
[2021-03-02T20:33:09Z INFO actix_web::middleware::logger] "POST /indexes/indexUID/documents HTTP/1.1" 202 14 "-" "PostmanRuntime/7.26.10" 0.023529
# Mandatory log format part. E.g [TIME_FORMAT LOG_LEVEL MODULE] part.
  • Time when the request was started to process (in rfc3339 format)
  • Log levels are ERROR, WARN, INFO, DEBUG, TRACE, OFF.
  • The module part gives information about the module that records the log.
# HTTP Call

Transplant uses actix_web::middleware::logger to record information about the API endpoints that receive calls.

Given "POST /indexes/indexUID/documents HTTP/1.1" 202 14 "-" "PostmanRuntime/7.26.10" 0.023529

  • Peer IP address (or IP address of reverse proxy if used)
  • First line of request (Example: GET /test HTTP/1.1)
  • Response status code
  • Size of response in bytes, including HTTP headers
  • User-Agent
  • Time taken to serve the request, in seconds to 6 decimal places

At DEBUG log level, the search endpoint must log the request body and the response body.

# V. Impact on Documentation

The documentation only mentions the logging behavior for the development env on the MEILI_ENV part.

We should explain how to specify the logging level using the MEILI_LOG_LEVEL environment variable and display the logging level table as information in a dedicated section. It should also mention the usage of the cli flag --log-level.

# VI. Impact on SDKs


# 2. Technical Aspects


# 3. Future Possibilities

  • Store logs on filesystem (give us future possibilites of rolling strategy). We will keep an eye on https://roadmap.meilisearch.com/c/81-specify-log-path, Github issues and, Slack Community messages. Keep in mind that it is possible to send logs to files using syslog or systemd journalctl.
  • Develop an API endpoint to search for logged events and configure the logging policy for the instance (SaaS feature in mind).
  • Add syntactic sugar helper flag like -v, -vv, -vvv that can be translated to a MEILI_LOG_LEVEL value.
  • Use the rust clone of log4j2, log4rs (opens new window) instead of env_logger to provide the same kind of options that Elasticsearch propose.

# 4. Planned Changes

# 0.21

# Core

  • Use a consistent method to log (relative to internal implementation).
  • Log output should start with the mandatory log format part.
  • The HTTP logs should be logged as described in this spec.
  • The users should be able to choose the log level by filling the MEILI_LOG_LEVEL environment variable or using the CLI option --log-level.
  • If log level is set to DEBUG, the /search endpoint should output request parameters and body response as a log output.
  • Logs should be displayed in production environment as in development environment, e.g. the default log level is INFO
  • Milli only display logs from DEBUG to TRACE log level.

# Documentation

  • Add a dedicated logging section in the documentation.
  • Add a link to this dedicated section on the environment section (opens new window).
  • We should add the default level for the production environment on the environment section, by default its INFO.