In the Linux operating system, the messages.log (or syslog on some systems) is a critical file that stores messages from the system kernel, security-related messages, and messages related to various system services. It is an important resource for troubleshooting, monitoring, and understanding the system's operation.

With respect to Neo4j, there are some events or errors that may happen at the OS level that never make it into the neo4j.log or debug.log files, and in these cases the only evidence of the issue may be in the messages log.
 

Locating the Log File

In most Linux distributions, the messages.log file is located in the /var/log/ directory. You can use the cd command to navigate to this directory and the ls command to list the files within it. Once in the directory, use a text editor like vim, nano, or a utility like less or more to view the file.

bash
cd /var/log/
ls
less messages.log

Reading the Log File

When you open the messages.log file, you'll see several lines of information. Each line generally contains the following:

1. Date and Time: This is the timestamp of when the log entry was created.

2. Hostname: This is the name of the system where the log was created.

3. Service/Program Name: This is the name of the service or program that created the log entry.

4. PID: This is the Process ID of the process that generated the log.

5. Message: This is the actual log message. It contains information about what happened in the system at that point in time.

Here's an example of a line from messages.log and below breaks down each section in the entry.

Jun 28 13:05:37 myhostname kernel: [264434.334353] usb 1-1: new high-speed USB device number 6 using ehci-pci

Timestamp Section

Jun 28 13:05:37

System Name Section

myhostname

Service/Program Name Section

kernel

Kernel Timestamp Section 

This is the timestamp used by the kernel, measured in seconds since the system started

[264434.334353]

Log Message Section

usb 1-1: new high-speed USB device number 6 using ehci-pci

 

Searching through the Log File

You can use grep to search through the messages.log file. For example, if you want to find entries related to usb, you can use:

grep usb /var/log/messages.log  

This will display all lines that contain "usb".

 

Understanding the Messages

The messages can be highly specific to the system and the programs running on it. Understanding them often involves some familiarity with the system and the ability to look up error messages and codes online.

Here are a few common types of messages you might see:

Boot and Shutdown Logs: You can see logs from the system booting up or shutting down, including the services starting or stopping.

Hardware Logs: Messages related to hardware, such as USB devices being connected or disconnected, or hard disk errors.

Security Messages: Information related to security, such as failed login attempts or changes to user accounts.

Service and System Errors: Errors from system services or applications, which can be critical for troubleshooting purposes.

OOM Killer & the Neo4j process

In a scenario where the Linux system is running low on memory resources, the OOM Killer might decide to terminate a memory-consuming process like Neo4j.

In the case that Neo4j is terminating abnormally, but giving no evidence for such in its log files, check for these kinds of events in the messages.log, as this may be the only clue as to what is going on.

This event is logged into the system and could appear something like this:

[123456.789012] Out of memory: Kill process 98765 (neo4j) score 258 or sacrifice child
[123456.789012] Killed process 98765 (neo4j) total-vm:7654321kB, anon-rss:6543210kB, file-rss:0kB, shmem-rss:0kB
[123456.789012] oom_reaper: reaped process 98765 (neo4j), now anon-rss:0kB, file-rss:0kB, shmem-rss:0kB

In this log excerpt, '98765' is the Process ID (PID) for the Neo4j process that was killed to free up memory. total-vm, anon-rss, file-rss, and shmem-rss represent the memory usage details of the process at the time of its termination.

An occurrence such as this indicates that the system was critically low on memory and the OOM Killer chose to terminate the Neo4j process to maintain system stability.

Conclusion

The messages.log file is a valuable tool for troubleshooting and monitoring in Linux. Understanding how to read and interpret this file can provide you with important insights into the system's operation and help you identify and address issues. Remember to check it regularly and always investigate any unusual or unexpected entries.