Deprecation Note

We published the last version of Graylog Documentation before the release of Graylog 4.2. Now, all documentation and help content for Graylog products are available at

There will be no further updates to these pages as of October 2021.

Do you have questions about our documentation? You may place comments or start discussions about documentation here:

Indexer failures

Every Graylog node is constantly keeping track about every indexing operation it performs. This is important for making sure that you are not silently losing any messages. The web interface can show you a number of write operations that failed and also a list of failed operations. Like any other information in the web interface this is also available via the REST APIs so you can hook it into your own monitoring systems.


Information about the indexing failure is stored in a capped MongoDB collection that is limited in size. A lot (many tens of thousands) of failure messages should fit in there but it should not be considered a complete collection of all errors ever thrown.

Common indexer failure reasons

There are some common failures that can occur under certain circumstances. Those are explained here:


An error message would look like this:

MapperParsingException[failed to parse [failure]]; nested: NumberFormatException[For input string: "some string value"];

You tried to write a string into a numeric field of the index. The indexer tried to convert it to a number, but failed because the string did contain characters that could not be converted.

This can be triggered by for example sending GELF messages with different field types or extractors trying to write strings without converting them to numeric values first. The recommended solution is to actively decide on field types. If you sent in a field like http_response_code with a numeric value then you should never change that type in the future.

The same can happen with all other field types like for example booleans.

Note that index cycling is something to keep in mind here. The first type written to a field per index wins. If the Graylog index cycles then the field types are starting from scratch for that index. If the first message written to that index has the http_response_code set as string then it will be a string until the index cycles the next time. Take a look at Index model for more information.