Indexing in depth

For general operation of EventStoreDB the following information is not critical but useful for developers wanting to make changes in the indexing subsystem and for understanding crash recovery and tuning scenarios.

Index map files

Indexmap files are text files made up of line delimited values. The line delimiter varies based on operating system, so while you can consider indexmap files valid when transferred between operating systems, if you make changes to fix an issue (for example, disk corruption) it is best to make them on the same operating system as the cluster.

The indexmap structure is as follows:

  • hash - an md5 hash of the rest of the file
  • version - the version of the indexmap file
  • checkpoint - the maximum prepare/commit position of the persisted ptables
  • maxAutoMergeLevel - either the value of MaxAutoMergeLevel or int32.MaxValue if it was not set. This is primarily used to detect increases in MaxAutoMergeLevel, which is not supported.
  • ptable,level,index- List of all the ptables used by this index map with the level of the ptable and it's order.

EventStoreDB writes indexmap files to a temporary file and then deletes the original and renames the temporary file. EventStoreDB attempts this 5 times before failing. Because of the order, this operation can only fail if there is an issue with the underlying file system or the disk is full. This is a 2 phase process, and in the unlikely event of a crash during this process, EventStoreDB recovers by rebuilding the indexes using the same process used if it detects corrupted files during startup.

Writing and merging of index files

Merging ptables, updating the indexmap and persisting memtable operations happen on a background thread. These operations are performed on a single thread with any additional operations queued and completed later. EventStoreDB runs these operations on a thread pool thread rather than a dedicated thread. Generally there is only one operation queued at a time, but if merging to ptables at one level causes 2 tables to be available at the next level, then the next merge operation is immediately queued. While merge operations are in progress, if EventStoreDB is appending large numbers of events, it may queue 1 or more memtables for persistence. The number of pending operations is logged.

For safety ptables EventStoreDB is currently merging are only deleted after the new ptable has persisted and the indexmap updated. In the event of a crash, EventStoreDB recovers by deleting any files not in the indexmap and reindexing from the prepare/commit position stored in the indexmap file.

Manual Merging

If you have set the maximum level (MaxAutoMergeIndexLevel) for automatically merging indexes, then you need to trigger merging indexes above this level manually by using the /admin/mergeindexes endpoint, or the es-cli tool that is available with commercial support.

Triggering a manual merge causes EventStoreDB to merge all tables that have a level equal to the maximum merge level or above into a single table. If there is only 1 table at the maximum level or above, no merge is performed.